Circular No. 02/2011/TT-BTNMT on technical procedures for physical exploration boreholes

CIRCULAR

Technical regulations for borehole geophysical logging

MINISTER OF NATURAL RESOURCES AND ENVIRONMENT

Pursuant to the Law on Minerals and the Law Amending and Supplementing Certain Provisions of the Law on Minerals;
Pursuant to Decree No. 25/2008/NĐ-CP dated March 4, 2008 of the Government stipulating the functions, tasks, powers, and organizational structure of the Ministry of Natural Resources and Environment;
Pursuant to Decree No. 89/2010/NĐ-CP dated September 18, 2010 of the Government amending and supplementing Article 3 of Decree No. 25/2008/NĐ-CP dated March 4, 2008 of the Government stipulating the functions, tasks, powers, and organizational structure of the Ministry of Natural Resources and Environment;
Considering the proposal of the Director of the Geological Survey Department and the Head of the Legal Department,

c) Enterprises may be granted permission for no more than one block out of the total three dual-frequency blocks (FDD) B

PART I

GENERAL PROVISIONS

Article 1. Scope of Regulation

This Circular stipulates the procedures, contents, and requirements for borehole geophysical logging work in basic geological surveys on minerals, mineral exploration, engineering geological surveys, hydrogeological surveys, geological disaster investigations, and other related fields.

Article 2. Applicability

This Circular applies to units, organizations, and individuals participating in implementing tasks, projects, and programs of basic geological surveys on minerals, mineral exploration, engineering geological surveys, hydrogeological surveys, geological disaster investigations, and other related fields (referred to as specialized projects) conducting geophysical logging work.

Article 3. Explanation of Terms

Borehole geophysical logging (Borehole geophysics, Logging, Karotaz) is an activity using specialized geophysical equipment inserted into boreholes to measure, process, and interpret geological parameters of rock and mineral physical properties in the space around the borehole wall.

Article 4. Application and Requirements for Applying Borehole Geophysics

1. Borehole geophysical methods are applied in basic geological surveys on minerals, hydrogeology, engineering geology, mineral evaluation and exploration, and environmental research. Borehole geophysics determines the technical condition of the borehole, sample collection, seam opening, and monitoring mechanical changes during drilling.

2. Requirements for applying borehole geophysics:

a) Geophysical logging work can only be carried out when approved by competent authorities or permitted by other legal documents;

b) The content of the geophysical logging project must be established according to the requirements and nature of the assigned tasks:

- For independent projects, all available geological, hydrogeological, and geophysical data in the study area must be collected. In cases where data is incomplete, test measurements should be conducted in one or two boreholes to establish a combination of borehole geophysical methods and criteria for each method. The project shall be established in accordance with the regulations on project establishment and basic geological survey reports on mineral resources;

- For geophysical logging work carried out within basic geological survey projects on minerals, technical designs shall be incorporated into the general project. Based on the tasks to be resolved, geological characteristics, and existing geophysical data, a combination of methods, technical criteria, and volumes for each method shall be selected.

c) For boreholes in basic geological surveys on minerals, solid mineral evaluations, underground water, hot water, and mineral water with depths greater than 100 meters, depending on the specific requirements of the project, deviation and azimuth measurements may be conducted simultaneously during drilling at intervals of 10 meters or each drilling stage. For boreholes less than 100 meters deep, if deviation measurement is required, it must be specifically designed in the project;

d) Geophysical logging can only be conducted when all the requirements and conditions for preparing the borehole as specified in Article 33 of this Circular have been met;

đ) When conducting geophysical logging work, radiation safety regulations and current labor safety regulations must be followed.

Article 5. Organization of borehole geophysical units

1. Borehole geophysical work shall be carried out by geophysical units. Depending on the workload and operational area, geological units may establish teams, squads, or groups of borehole geophysics.

2. Borehole geophysics shall be organized according to station units. The manpower of each station shall include at least five (05) people, including:

a) A station chief with a level of senior geophysical investigator or higher, responsible for overall management, participating in fieldwork, and analyzing and processing office materials;

b) One (01) senior geophysical investigator conducting field measurements and analyzing and processing office materials;

c) One (01) senior geophysical investigator or technical maintenance investigator;

d) One (01) intermediate-level investigator operating the winch and one driver.

3. When a unit has two stations or more, a borehole geophysics squad shall be established, led by a senior geophysical investigator or higher as the squad leader.

Chapter II

METHODS AND COMBINATIONS OF BOREHOLE GEOPHYSICS METHODS

Section 1

TYPES OF BOREHOLE GEOPHYSICAL METHODS

Article 6. Borehole geophysical methods

1. Borehole geophysical measurement work includes electrical resistance, electromagnetic, magnetic, radiometric, seismic, ultrasonic, and borehole condition methods.

2. Electrical borehole methods are used to study natural and artificial electric field parameters of the surrounding rock and ore environment around the borehole, including:

a) Electrical resistance methods comprising: resistivity measurement, micro-potential measurement, deep profile measurement, and resistivity measurement of solutions;

b) Natural potential method, with two measurement methods: gradient and potential;

c) Current intensity method;

d) Polarization stimulation method;

đ) Electromagnetic methods comprising: induction measurement and electromagnetic wave propagation speed measurement (permittivity constant method);

3. Radiometric borehole methods are used to study natural and artificial radiation field parameters of the surrounding environment of the borehole, including: natural gamma measurement, artificial gamma measurement, gamma spectrum measurement, neutron measurement, gamma-neutron measurement;

4. Methods for determining borehole conditions include: temperature, diameter, curvature, azimuth, height of cement column above casing in the borehole, dip angle of strata, and longitudinal image of the borehole wall.

Article 7. Recording Scale

1. The recording scale is selected based on the principle:

a) The horizontal scale representing the value of the geophysical field should be chosen so that the measured curves clearly diverge, reflecting the object under study and not exceeding the width of the recording tape or drawing paper;

b) The vertical scale representing the depth of the borehole can vary according to different scales (1:500; 1:200; 1:50; 1:20), depending on the requirement to study the entire cross-section or detailed study of thin strata;

c) For digital recording machines, the recording scale is selected according to the above principles and requirements for processing and analyzing results using specialized software.

2. In cases where research tasks involve geological mapping, hydrogeological surveying, engineering geology, and groundwater assessment, the recording scale is specified as follows:

a) A scale of 1:500 to 1:200 is used when studying the preliminary stratigraphic column of the entire borehole cross-section;

b) A scale of 1:50 to 1:20 is used when studying the detailed stratigraphic column of the borehole;

c) The recording scale is also selected based on the depth of the borehole as follows:

- A scale of 1:200 for boreholes up to 100 meters deep;

- A scale of 1:500 for boreholes deeper than 100 meters.

3. In the evaluation and exploration of solid minerals, the recording scale is specified as follows:

a) When studying the entire cross-sectional stratigraphic column along the borehole, the recording scale is 1:200;

b) When studying individual ore bodies and strata in detail, the recording scale ranges from 1:50 to 1:20.

Article 8. Inspection Ratio and Allowable Error

1. The inspection ratio for borehole geophysical methods must ensure at least 10% of the total drilling meterage with geophysical measurements. The inspection work must be carried out before the completion of the borehole geophysical measurement.

2. The quality of borehole geophysical measurement materials is evaluated based on the comparison between the inspection measurement results of a part of the borehole slice and the measurement results. The shapes of the two curves from the measurements must be identical and the average relative error must meet the following requirements:

a) Not higher than 10% for radioactive methods;

b) Not higher than 15% for resistivity methods;

c) Not higher than 15% for natural potential method.

3. The depth measurement error is specified as follows:

+ From 0 to 50m, the allowable error is ± 0.2m;

+ From 51 to 100m, the allowable error is ± 0.5m;

+ From 101 to 200m, the allowable error is ± 0.7m;

+ From 201 to 300m, the allowable error is ± 1m;

+ From 301 to 500m, the allowable error is ± 1.2m;

+ From 501 to 1000m, the allowable error is ± 1.5m;

+ Over 1000m, the allowable error is ± 2.5m.

Section 2

ELECTRICAL METHODS FOR BOREHOLES

Article 9. Natural Potential Method

1. The natural potential (NP) method is conducted in non-water losing boreholes and in sections without metal casing. The natural potential measurement is performed when pulling the cable up according to either the potential or gradient method. Two receiving electrodes made of pure lead are used. The NP measurement is carried out with one electrode placed at the top of the borehole and the second electrode moved within the borehole.

2. The gradient measurement method is used when there is significant interference, with both receiving electrodes M, N moving within the borehole. The distance between the receiving electrodes is from 1 to 2 meters and remains constant during the measurement.

3. The cable pull speed for the natural potential method is selected within the range of 300 to 600m/h (at recording stations, the pull speed ranges from 0 to 1800m/h).

4. The recording ratio n is the value of the potential difference measured in millivolts per centimeter (mV/cm), calculated using the formula:

 

                              (1)

ΔU - The recorded potential difference value in mV;

nh cơ₀ - The movement of the pen on the tape measured in cm.

At digital recording stations, the recording ratio n represents the ratio of the representation of the potential difference value in mV/cm along the horizontal axis of the borehole geophysical chart when printed on paper or displayed on a screen.

Article 10. Resistivity Method

1. Apparent resistivity (AR) is measured using specialized equipment called the electrode system. The electrode system has two current-emitting poles marked A, B and two receiving electrodes marked M, N. Pole B is connected to a fixed ground near the top of the borehole, while the remaining electrodes are lowered into the borehole including poles A, M, and N. When using this electrode system, the apparent resistivity value is determined by the formula:

                             (2)

                         (3)

K - Is the Electrode Factor;

ΔU - Is the potential difference between the two receiving electrodes MN measured in mV;

I - Is the current intensity in the AB emitting circuit measured in mA.

2. The electrode system lowered into the borehole has two types:

a) Potential electrode system, when the distance AM << MN;

b) Gradient electrode system, when the distance AM >> MN;

c) The potential electrode system and the gradient electrode system are divided into two types:

- Forward electrode system when the position of the different pair of electrodes (receiving-emitting) is above the same pair of electrodes (receiving-receiving);

- Reverse electrode system when the position of the different pair of electrodes is below the same pair of electrodes.

d) The length of the electrode systems is defined as follows:

- The length of the potential electrode system L:

L = AM (m)

- The length of the gradient electrode system L:

L = AO (m)

Point O is the midpoint of MN.

3. The resistivity method is carried out in non-water losing boreholes and in sections of the borehole without casing when pulling the cable upwards.

The recording ratio n is the resistivity value measured in Ωm/cm, calculated using the formula:

                        (4)

nh cơ₀ - The movement of the pen on the recording tape measured in cm;

||| R₀ - Standard resistance measured in ohms;

K - Electrode factor.

At digital recording stations, the recording ratio n represents the ratio of the resistivity value corresponding to 1cm along the horizontal axis of the borehole geophysical chart when printed on paper or displayed on a screen.

The cable pull speed for the resistivity method is from 300 to 600m/h.

4. Common interference encountered during resistivity measurement includes the following signs:

a) The curve has a repetitive shape;

b) The curve fluctuates without any pattern, showing abrupt values when starting to move the cable as well as when stopping the cable;

c) There is a shift in the "zero line" of the resistivity curve;

d) When the electrode system is in a section with casing, there is noticeable fluctuation;

đ) There is a change in current at the two emitting poles A and B.

5. Measures to limit the impact of interference on measurement results include:

a) Using a winch with a cable length not exceeding the depth of the cable deployment;

b) Ensuring that the output resistance of the same-named cable cores (receiving, emitting) is equal and the resistance of the same-named cable cores does not differ by more than 1Ω;

c) Using a suitable frequency for the emitting current;

d) Electrical circuits must have good insulation;

đ) Choosing the most appropriate combined connection method.

Article 11. Method of measuring the resistivity of drilling fluid

1. Measuring the resistivity of drilling fluid in boreholes (DRSF) must comply with the following requirements:

a) The measurement of resistivity of drilling fluid shall be carried out during the lowering of equipment into the borehole;

b) The shape of electrodes and the casing of the machine shall not be changed during the measurement;

c) Liquids must flow easily within the electrode;

d) The insulation resistance of the measuring instrument shall not be less than 2 megaohms (MΩ) when working in the borehole.

2. The electrode system for measuring the resistivity of drilling fluid is a special electrode system, with very small dimensions (L about several centimeters).

 The value of resistivity of drilling fluid is determined according to the following formula:

                                 (5)

K - Electrode coefficient, determined experimentally and recorded at the time of manufacturing the electrode system;

ΔU and I - Measured voltage and current intensity.

During field construction, the Coefficient K must be checked at least once every three months. Before starting the next construction season and after repairs, the Coefficient K must be re-determined. The Coefficient K is determined according to the provisions of Section I of Appendix 2 issued together with this Circular.

3. When using data on the resistivity of drilling fluid to determine total mineralization and permeability coefficients of underground water, it is necessary to calibrate the measurements to the same temperature.

4. The cable lowering speed for the method of measuring the resistivity of drilling fluid in boreholes should be selected between 300 to 500 meters per hour.

5. The recording ratio n is the value of resistivity expressed in Ωm/cm, calculated according to the following formula:

 

                            (6)

 L₀- The movement of the pen on the recording tape measured in cm;

||| R₀- Standard resistance measured in ohms;

K - Electrode factor.

 At recording stations, the recording ratio n is the ratio representing the resistivity of drilling fluid corresponding to 1cm on the physical map when printed on paper or displayed on screen.

Article 12. Current Intensity Method

1. The current intensity method (CIM) is used in conjunction with electrical methods in coal and ore boreholes to determine boundaries and structures of coal seams and ore bodies.

2. The recording ratio n of the current intensity method is calculated in milliamperes per centimeter according to the following formula:

 

                           (7)

||| R₀ - A standard resistor is connected in series with the cable core to the A terminal (the terminal lowered into the borehole);

ΔU - Measured voltage value in millivolts;

 L - Pen movement distance in centimeters.

At recording stations, the recording ratio n is the ratio representing the current intensity value corresponding to 1cm along the horizontal axis of the physical map when printed on paper or displayed on screen.

3. Current intensity is measured when pulling the cable up at a speed from 300 meters per hour to 600 meters per hour.

4. In dry boreholes (boreholes without water), the sliding contact method is often used to determine the boundaries of ore seams. The sliding contact method uses small-sized electrode systems and slides closely along the borehole wall. When implementing the sliding contact method, one can use single, double, and triple electrode systems to observe simultaneously from all sides of the borehole wall to enhance the accuracy of the method.

Article 13. Micro-Electrode System Method

1. The micro-electrode system method (MESM) is used to study the detailed apparent resistivity near the borehole wall using small-sized electrode systems.

2. Types of micro-electrode systems:

a) Classical micro-electrode system (Micro-Log): An electrode system consisting of 3 electrodes of type N 0,025 M 0,025 A to measure two resistivity curves according to potential and gradient systems;

b) Converging micro-electrode system (Micro-Laterlog): The A, M, N electrodes have circular shapes and an additional A electrode₀ at the center of the circle to avoid the influence of drilling fluid and limit the possibility of vertical leakage current. The reflection depth of the borehole wall for micro-electrode system measurement ranges from 7 cm to 10 cm;

c) Spherically focused converging micro-electrode system (Micro-Spherically focused): In this system, in addition to A₀ at the center, the receiving electrodes M N also include a converging emitting electrode A1 to control the outermost current path. The electrodes A₀, A₁ and B at infinity will create a spherical equipotential surface.

3. The recording ratio of resistivity values in the micro-electrode system method is chosen based on the resistivity of the drilling fluid in the borehole, usually close to the resistivity of the drilling fluid, expressed in Ωm/cm.

4. The cable pulling speed for the micro-electrode system method should be between 300 meters per hour to 500 meters per hour.

5. Before field construction and after each repair, the coefficient of the micro-electrode system must be re-determined.

If the field construction period is prolonged, the micro-electrode system coefficient must be determined once a month. The determination method is specified in Section II of Appendix 2 issued together with this Circular.

6. The insulation resistance of the micro-electrode system shall not be less than 2MΩ (insulation between electrodes on the electrode base).

Article 14. Electrical insulation resistance of cables and measuring systems

1. The minimum allowable electrical insulation resistance for multi-core cables is 2MΩ when measured in drill holes and 5MΩ on the ground surface.

2. For single-core cables, the minimum electrical insulation resistance is 1MΩ when measured in drill holes and 3MΩ on the ground surface.

3. The electrical insulation resistance of the measuring system must be greater than or equal to the following limits:

a) When immersed in water: 3MΩ;

b) When immersed in solution: 1MΩ;

c) On the ground surface: 10MΩ.

Section 3

METHODS FOR DRILL HOLE GAMMA RAY LOGGING

Article 15. Objectives of drill hole gamma ray logging methods

1. To delineate boundaries between different strata of rock formations in the drill hole stratigraphic column.

2. To define boundaries and determine thicknesses of ore beds and veins within the drill hole stratigraphic column.

3. To determine richness levels of certain minerals (uranium ores, ash content of coal, clay content of sedimentary rocks).

Article 16. Techniques for drill hole gamma ray logging

1. Gamma ray logging methods are carried out during the process of pulling the receiver from the bottom of the drill hole upwards.

2. The recording ratio n is the number of pulses per minute of the standard pulse generator per centimeter expressed as:

                            (8)

L - Recording deviation of the part measured in cm;

v - Number of pulses generated by the standard pulse generator;

Article 17. Cable Pulling Speed

1. The cable pulling speed is selected to ensure that the product of the cable pulling speed (m/h) and the time constant (s) is less than or equal to 350.

2. The time constant t of natural gamma ray logging instruments currently available ranges from 1 to 5 seconds.

3. The cable pulling speed during drill hole gamma ray logging is specified as follows:

a. For total measurement methods (natural and artificial gamma rays), the cable pulling speed during measurement is less than 300m/h;

b. For gamma ray spectral methods, the cable pulling speed during measurement is less than 150m/h;

c. For new generation drill hole gamma ray logging instruments, the cable pulling speed can be chosen according to the technical specifications of the instrument.

Article 18. Natural Gamma Method

The natural gamma method (G) is used to divide drill hole cuttings, determine rock boundaries in drill holes, assess clay content of sedimentary rocks, and separate potassium salt layers and radioactive ore layers in drill hole cuttings.

Article 19. Scattering Gamma Method

1. The scattering gamma density method is a radiometric technique based on the Compton effect to determine the density of rock formations along the drill hole wall.

a) The intensity of scattered gamma radiation depends on quantities Q, L, and σ, represented by the formula:

                                               (9)

Where:

Q - is the intensity of radiation emitted by the source, measured in g

L - Length of the measuring tube from the center of the source to the center of the measuring tube, measured in cm.

σ - Rock density, measured in g/cm³.

K: is the correlation coefficient determined experimentally.

b) Rock density is determined by the formula:

                          (10)

Analog logging stations use Igg curve recordings; with digital logging stations, both Igg and density can be used.

2. The selective scattering gamma method uses a cesium (Cs 137) radiation source and equipment designed to measure low-energy radiation, known as the selective scattering gamma method.

Article 20. Natural Gamma Spectral Method

1. The natural gamma spectral method (GG) is conducted after natural gamma measurements to determine uranium (U), thorium (Th), and potassium (K) contents, and is performed while pulling the receiver from the bottom of the drill hole upwards.

2. The recording ratio n is the number of counts per minute per cm or counts per second per cm for analog instruments, or directly represents the values of K, U, Th contents for digital instruments.

Section 4

TECHNICAL STATE STUDY METHODS FOR DRILL HOLES

Article 21. Temperature Measurement Method for Drill Holes

1. The thermal properties of rock and soil are characterized by parameters such as thermal conductivity coefficient l, specific heat capacity C, and geothermal gradient.

2. Temperature measurement of drill holes (ND) is conducted when the thermal regime has not yet been fully established (drill hole is being drilled) and after the drill hole has returned to a stable state. (The time required for the drill hole to return to a stable state is the time during which the temperature at any point within the drill hole differs by no more than 1 C from the temperature of the rock and soil at that point). The average time for a drill hole to return to a stable state after washing the drill hole is specified in each project.^°3. Drill holes used to determine natural temperature are those containing water, clay slurry, and without phenomena such as gas bubbling, water spouting, or water flowing behind casing pipes.

4. The vertical recording ratio is determined according to the depth of the drill hole: a ratio of 1:200 for drill holes with a depth less than 500m; a ratio of 1:500 for drill holes with a depth of 500m or greater.

5. The recording ratio n (degrees/cm) is determined by the formula:

C - Constant of the resistance meter;

                         (11)

Where:

- Displacement of the recording pen measured in cm;

nh cơ₀- Standard resistance.

||| R₀6. The cable lowering speed during temperature measurement is selected based on the time constant of the instrument. With a time constant (seconds) of 0.5; less than or equal to 1; less than or equal to 2; less than or equal to 4; greater than 4, corresponding to the allowable cable lowering speed (m/h) of 1000; 800; 600; 400; 300.

7. The permissible absolute error for determining temperature in drill holes is ± 0.5

8. The calibration of the temperature measuring instrument is stipulated in Section III of Appendix 2 issued together with this Circular.^°- Desiccator.

Article 22. Diameter Measurement Method for Drill Holes

1. Prior to measuring the diameter of drill holes, verification should be carried out using standard rings with known diameters.

2. The allowable speed is the speed at which the diameter readings of the drill hole through various layers of rock and soil differ by no more than 5% compared to the diameter readings of the drill hole at those layers measured with the slowest cable pulling speed. Cable pulling speed ≤ 1,000m/h.

3. Based on changes in the diameter of the drill hole and the level of detailed study, the recording ratio for diameter n = 10, n = 5, n = 2cm/1cm of the scale ratio is selected.

The depth ratio of the curve for measuring the diameter of the drill hole is recorded according to the same depth ratio when measuring other well logging methods.

When using carot stations recorded by pens, the recording ratio n is determined by the formula:

C - Constant of the diameter measuring instrument;

                                      (12)

Where:

- Displacement of the recording pen measured in cm;

nh cơ_°- Standard resistance measured in Ω.

||| R_°For digital recording stations, the recording ratio n is performed according to the standard rings of the machine.

The calibration of the diameter measuring instrument is stipulated in Section IV of Appendix 2 issued together with this Circular.

4. The permissible error for determining the diameter of the drill hole is less than or equal to 1cm.

Article 23. Method for Measuring Deviation and Azimuth of Drill Holes

1. Deviation (inclination angle) φ is the angle formed between the vertical axis Z and the drill hole axis. Azimuth angle α is the angle formed between the projection of the drill hole axis onto the ground and the true north direction.

2. Inspection points need to be arranged at major measurement sections and in the following cases:

a) Every 5 to 10 points along the entire measurement length;

b) When the orientation of the drill hole suddenly changes;

c) When there is doubt about the instrument readings;

d) When continuous measurements are taken in the drill hole, at least two to three points must be measured on a previously measured section.

3. The permissible error for the deviation angle j is less than or equal to ± 0.5

, for the azimuth angle a is less than ± 5^°4. The depth position of the measurement point is calculated according to the depth indicator and verified against the meter marks on the cable.^°.

5. The calibration of the deviation measuring instrument is stipulated in Section V of Appendix 2 issued together with this Circular.

5. Calibration of deviation measuring instruments as prescribed in Section V of Annex 2 issued together with this Circular.

Article 24. Methods for Determining the Height of Cement Columns Behind Tubing

1. The height of the cement column behind tubing is determined based on the results of thermal measurement or radioactive isotope measurement and gamma-gamma logging data.

2. To determine the height of the cement column, temperature logging must be conducted immediately after all cementing equipment has been removed from the wellbore. Prior to thermal measurement, no work should be performed in the wellbore to avoid disturbing the fluid and disrupting the thermal regime. The height of the cement column is determined at the position where there is a sudden increase in the readings on the temperature logging curve.

3. When determining the height of the cement column behind tubing based on thermal measurement results, attention should be paid to the following phenomena:

a) A sudden rise in temperature due to heat released by curing cement (dependent on quality).

b) As the depth of the wellbore increases, the accuracy of using thermal measurement to delineate the cement column behind tubing decreases.

c) In cases where the cement level needs to be determined at great depths, the boundaries of the cement column behind tubing should be rechecked by comparing the thermal logging curve with the electrical well logging curve and the diameter logging curve to determine the height of the cement column.

4. Determining the height of the cement column behind tubing using radioactive isotopes

a) To determine the height of the cement column behind tubing, a certain amount of radioactive isotope must be added to the cement slurry, increasing its activity to 0.5 to 1 mCi per meter. The changes in the isotopes within the slurry are then monitored. In favorable conditions, only mixing the radioactive isotope into the initial cement slurry is required.³ b) Natural gamma ray logging of the wellbore is conducted before cementing the wellbore.

c) After removing all cementing equipment from the wellbore, natural gamma ray logging is carried out. By comparing the natural gamma ray curves before and after cementing, the height of the cement column behind tubing can be determined based on the increase in readings on the natural gamma ray curve.

d) Checking the height of the cement column behind tubing using radioactive isotopes should only be conducted when thermal measurements and other methods are ineffective or when the expected cement column is very deep and the volume of cement injected is small.

5. Determining the height of the cement column behind tubing based on gamma-gamma logging data:

a) To verify cement placement using gamma-gamma logging, a 50 cm long probe with a diameter close to that of the tubing and a Cobalt source (Co) is used. Based on the gamma-gamma curve obtained, the height of the cement column can be determined according to the small readings on the curve.

b) For gamma-gamma logging to determine the height of the cement column in wells where the fluid density exceeds 1.6, a certain amount of water must be pumped into the wellbore before logging.⁶⁰c) To increase the accuracy of determining the height of the cement column behind tubing, neutron logging should be conducted alongside gamma-gamma logging of the wellbore.

Article 25. Determining the Position of Pressurized Water-Bearing Zones and Water Movement in the Wellbore

1. Determining pressurized water-bearing zones and water movement in the wellbore uses temperature measurement, radioactive isotope measurement, and electrical resistivity logging of the wellbore fluid.

2. Temperature measurement to determine pressurized water-bearing zones involves the water-dipping method and the water-pumping method, followed by temperature logging with a thermometer.

a) The water-dipping method is applied after preliminary washing of the wellbore (when the thermal regime is not established, and there are no abnormal temperatures). Subsequently, water is dipped to lower the water level in the wellbore, causing water flow. Temperature logging is then used to determine the position of the pressurized water-bearing zone.

b) The water-pumping method is applied when the thermal regime in the wellbore is nearly established. Temperature logging is conducted both before and after pumping water.

c) Pressurized water-bearing zones are identified at positions where there is a difference in temperature before and after water compression.

3. Radioactive isotope measurement to determine pressurized water-bearing zones involves dissolving radioactive isotopes (in salt form) in the fluid and injecting it into the wellbore. Short-lived isotopes (half-life ≤ 60 days) are used, with quantities ranging from 0.25 to 2 mCi.

4. Electrical resistivity logging of the wellbore fluid to determine pressurized water-bearing zones.

a) Electrical resistivity logging to determine pressurized water-bearing zones is conducted by measuring the resistivity of the fluid in the wellbore multiple times (8 to 10 times). Typically, measurements are taken every 1 to 2 hours. Based on the results of these measurements, the position of flowing water in the wellbore can be determined, which corresponds to the location of the pressurized water-bearing zone.

b) The velocity of water flow (permeability rate) can be determined by observing the gradual dilution of saltwater in the wellbore. The concentration of salt is determined using the resistivity curve of the wellbore fluid mixed with salt. The permeability rate of the water flow is calculated using the formula:

d - Diameter of the wellbore;

 - Concentration of salt in the groundwater;

                          (13)

Where:

; C

C_° - Concentration of salt in the wellbore fluid at time t

C₁ and t₂m - Permeability coefficient of water through the wellbore wall. This coefficient differs from the permeability coefficient of water in rock formations...₁Depending on the condition of the wellbore wall, the coefficient m can vary between 1 and 4, with m = 2 for wellbores with good water-conducting properties.₂;

The value of m is determined based on previous data from surrounding wellbores and must be tested to select the appropriate value.

5. Determining the loss of drilling fluid using radioactive isotopes: To determine the loss of drilling fluid, a certain amount of radioactive isotope must be mixed into the drilling fluid, and its movement in the wellbore is monitored through electrical resistivity logging and natural radioactivity logging. Based on the results obtained, the position of the lost drilling fluid can be determined.

COMBINATION OF WELLBORE GEOPHYSICAL METHODS

5. Determining the location of lost drilling fluid using radioactive isotopes: To determine the location of lost drilling fluid, a certain amount of radioactive isotope must be mixed into the drilling fluid and its movement within the borehole monitored based on measuring the electrical resistivity of the fluid and natural radioactivity. The results obtained are used to determine the location of lost drilling fluid.

Section 5

COMBINATION OF BOREHOLE GEOPHYSICAL METHODS

Article 26. Standard boreholes for geophysical parameters

1. Each area implementing mineral assessment or large-scale mineral exploration activities shall have at least 30 boreholes for geophysical work, from which 1 to 2 characteristic boreholes shall be selected to serve as parameter boreholes to determine a reasonable combination of borehole geophysical methods, serving calibration and synchronization of borehole geophysical stations. Parameter boreholes must be specifically determined and designed in the project approved by the competent authority.

2. Parameter boreholes must meet the following requirements:

a) Having significant depth, drilling through major rock and ore types in the working area;

b) Having favorable topographical and transportation conditions in the working area;

c) Being measured using multiple geophysical methods with reliable quality data, allowing correlation between geological layers and ore beds;

d) Having high sampling rates with accurate identification of rock names and petrographic compositions.

Article 27. Principles for selecting combinations of borehole geophysical methods

1. The general tasks of the specialized project and the specific tasks of borehole geophysical work;

2. Requirements for the level of research on borehole cross-sections;

3. Geological characteristics of the area;

4. Characteristics of surface and borehole geophysical fields;

5. Physical parameters of rocks and ores;

6. Geographic conditions of the construction area;

7. Capabilities of existing borehole geophysical stations.

Article 28. Combination of borehole geophysical methods in coal assessment and exploration

1. The combination of borehole geophysical methods includes:

a) Natural gamma logging;

b) Density and photoelectric absorption logging;

c) Resistivity logging;

d) Caliper logging;

đ) Flowmeter logging;

e) Borehole imaging;

f) Sidewall coring.

2. Borehole geophysical work in coal assessment and exploration is carried out in the following steps:

a) Logging of the combined geophysical methods along the depth of the borehole at a scale of 1:200;

b) Detailed logging of sections containing coal seams at a scale of 1:50 or 1:20 to determine the thickness and structure of the coal seams.

3. Geophysical data from boreholes can be used to calculate coal reserves when meeting the requirements specified in Appendix 4 issued together with this Circular.

Article 29. Combination of borehole geophysical methods for metallic mineral assessment and exploration

1. The combination of borehole geophysical methods includes:

a) Resistivity logging;

b) Natural potential logging;

c) Natural gamma logging;

d) Borehole imaging;

e) Caliper logging.

2. Borehole geophysical work in metallic mineral assessment and exploration is carried out in the following steps:

a) Logging of the combined geophysical methods along the depth of the borehole at a scale of 1:200;

b) Detailed logging of sections containing ore bodies at a scale of 1:50 or 1:20 to determine the thickness and structure of the ore bodies.

Article 30. Combination of borehole geophysical methods for non-metallic mineral assessment and exploration

1. The combination of borehole geophysical methods includes:

a) Resistivity logging;

b) Natural potential logging;

c) Natural gamma logging;

d) Borehole imaging;

đ) Caliper logging.

2. Borehole geophysical work in non-metallic mineral assessment and exploration is carried out in the following steps:

a) Logging of the combined geophysical methods along the depth of the borehole at a scale of 1:200;

b) Detailed logging of sections containing ore bodies at a scale of 1:50 or 1:20 to determine the thickness and structure of the ore bodies.

Article 31. Combination of borehole geophysical methods for radioactive mineral assessment and exploration

1. The combination of borehole geophysical methods includes:

a) Natural gamma logging;

b) Gamma spectroscopy;

c) Resistivity logging;

d) Natural potential logging;

đ) Caliper logging.

2. Borehole geophysical work in radioactive mineral assessment and exploration is carried out in the following steps:

a) Logging of the combined geophysical methods along the depth of the borehole at a scale of 1:200;

b) Detailed logging of sections containing ore bodies at a scale of 1:50 or 1:20 to determine the thickness, structure, and concentrations of K, U, Th in the ore bodies.

3. All boreholes in the investigation, assessment, and exploration of radioactive minerals must undergo borehole geophysical logging.

4. When the investigation and exploration area has determined the Radioactive Equilibrium Factor, the results of gamma spectroscopy measurements for K, U, Th concentrations may be used to calculate forecast resources and reserves of minerals.

Article 32. Combined Geophysical Well Drilling Methods for Hydrogeological and Engineering Geological Investigations

1. Tasks of combined geophysical well drilling methods for hydrogeological and engineering geological investigations:

a) Determining the thickness of sedimentary layers over the basement rock, studying the composition and properties of the basement rock, identifying fracture zones and fault lines;

b) Conducting engineering geological studies in construction and other economic and defense sectors;

c) Determining some physical and mechanical properties of soil and rock such as Porosity Coefficient, moisture content, density, and elastic parameters of soil and rock.

2. The combined geophysical well drilling methods include:

a) Electrical resistivity method with different electrode spacings;

b) Natural potential method;

c) Natural gamma method;

d) Well diameter measurement method;

đ) Gamma scattering method;

e) Current intensity method;

f) Acoustic well logging method;

g) Seismic well logging method;

h) Well camera method.

3. The recording ratio of the methods in the combination may be selected at 1:50 or 1:20 depending on the size of the research object.

Chapter III

TECHNICAL REQUIREMENTS FOR FIELD WORK

Article 33. Preparation of Wells

1. Technical staff overseeing drilling or the drilling team leader must provide the well geophysics station leader with a stratigraphic column of the well drawn at a scale of 1:200 and annotated with sampling ratios or a stratigraphic column at a scale of 1:50 when a 1:50 ratio measurement is required.

2. The well must be ready to allow the passage of electrode systems and geophysical well logging instruments from the top to the bottom without obstruction. The well should not have any blockages or have a diameter smaller than the specified diameter of the equipment to be lowered into it.

3. The drilling fluid must be uniform throughout the entire well. Prior to geophysical measurements, the well must be flushed with drilling fluid. For radiometric methods, the duration of flushing the well with clean water must exceed two hours.

4. During the conduct of geophysical well logging measurements, the following actions must not be carried out:

a) Repairing drilling equipment;

b) Operating the drill without the consent of the well geophysics station leader;

c) Welding within a radius of less than 400 meters.

5. Geophysical well logging measurements shall not be conducted under the following circumstances:

a) The drilling fluid in the well has a viscosity exceeding 90 seconds;

b) The drilling fluid contains more than 5% sand and rock fragments;

c) The well is spouting water or bubbles, or the well is sucking water at a rate greater than 15 meters/hour.

6. The preparation of wells according to the technical requirements mentioned above must be ensured by the drilling team leader and the technical staff overseeing the well, and must be reported in writing to the well geophysics station leader before conducting geophysical measurements.

7. When conducting geophysical measurements, there must be present the technical staff overseeing the well and the drilling team leader (shift leader). Upon completion of the geophysical well logging measurements, those individuals must sign the record.

Article 34. Geophysical Well Logging Equipment

1. All geophysical well logging equipment must be calibrated according to current regulations and meet the technical specifications required by the project.

2. The stability of the measuring equipment must be checked according to technical specifications during the measurement process. Particularly, when the working time is prolonged, there are changes in temperature or pressure, or the measured data shows significant anomalies inconsistent with general patterns, these must be recorded in the well geophysics station logbook.

3. The storage of geophysical well logging equipment must comply with the manufacturer's regulations.

Chapter IV

OFFICE WORK AND REPORT PREPARATION

Section 1

GENERAL PROVISIONS

Article 35. Completing instructions on magnetic tapes and evaluating the quality of well-bore geophysical curves

1. Completing instructions for well-bore geophysical curves includes the following contents:

a) Accurately labeling the curve according to the prescribed model, recording the scale of the curve, depth values in even meters. Standardizing the "0" point of well-bore geophysical curves, marking shifted positions (if any);

b) Marking symbols for curves and values of parameters directly on the magnetic tape (recording similar and absolute values);

c) Synchronizing meter marks for each geophysical curve and stratigraphic column of the well;

d) Conducting cutting, photocopying (if the curve is recorded on magnetic tape) or printing (if the curve is recorded digitally) multiple copies as required for research;

đ) Contents already recorded on the tape (for analog recording) or absolute digital recording must not be erased or modified.

2. Quality inspection and evaluation include:

a) Checking the initial meter value and depth marked on the chart, comparing the depth of the casing and bottom of the well according to the geophysical chart and drilling documentation;

b) Checking measurement data such as: strength value, compensation value, scale, position of the "0" mark and confirming their correctness;

c) Checking the insulation resistance of the cable core, electrode system, receiving and transmitting circuits, and equipment in the well must be greater than the permitted value;

d) Checking the operating mode of the machine, recording speed, measurement limits, selection of time constants;

đ) Comparing results between measurements and verification measurements or data from different measurement times;

e) Comparing apparent resistivity values measured by different electrode systems through various rock layers. For radioactive curves, compare average indices over thick layers on curves measured at different times or with average indices over known standard layers.

Article 36. Error Evaluation

The accuracy of well-bore geophysical measurement data is evaluated based on repeated measurements and verification measurements. The accuracy at each individual location is determined by relative error (%) and calculated using the formula:

                             (14)

Where:

σ_{ANNEX I.A[31]} - Relative error;

Xi₁and Xi₂ -Measured and verification measurement values at the i-th point.

The average relative error stb of the entire well-bore cross-section is the arithmetic mean of n observation errors at each location, calculated using the following formula:

                      (15)

This average relative error must comply with the permissible error provisions stipulated in Article 8 of this Circular.

Article 37. Geological Processing and Analysis of Well-Bore Geophysical Data

1. Calibrating and linking curves with each other;

2. Determining the boundaries of rock and ore layers within the well-bore cross-section;

3. Calculating physical parameters for each rock layer, seam, ore body;

4. Establishing the lithological stratigraphic column of the well based on field conditions and physical parameters of rocks and ores.

Section 2

DETERMINING BOUNDARIES

Article 38. Principles for Determining Layer Boundaries

Determining layer boundaries is based on a combination of indicators: electrical resistivity, natural potential, radioactivity characteristics, and other indicators of each method; simultaneously synthesizing, comparing, and verifying these indicators to select appropriate results.

Article 39. Determining the boundaries of layers according to the resistivity curve measured by the gradient electrode system

1. Layer with high resistivity and medium to large thickness (h > Lv)

Hk = Hρ_kmin+ Lv/2

Hn = Hρ_kmax+ Lv/2

Thickness of the layer:

h(m) = Hn - Hk = Hρ_kmax - Hρ_kmin                           (16)

Where:

Hk and Hn are the depths of the top and bottom of the layer;

Hρ_kmax is the depth at which the resistivity has its maximum value;

Hρ_kminis the depth at which the resistivity has its minimum value;

Lv is the size of the gradient electrode system.

2. Layer with high resistivity and small thickness (h < Lv)

Hk = Hρ_kmax(NMA) + Lv (gradient electrode system NMA)

Hn = Hρ_kmax(AMN) - Lv (gradient electrode system AMN)

Thickness of the layer:

h(m) = Hn - Hk = Hρ_kmax (AMN) - Hρ_kmax(NMA)        (17)

3. For layers with low resistivity and small thickness, the thickness of the layer is determined by the depths of the top and bottom of the adjacent upper and lower high-resistivity layers using the above expressions.

Article 40. Determining the boundaries of layers according to the resistivity curve measured by the potential electrode system

The top of the layer (Hk) with thickness greater than the size of the electrode system (H > Lv) is determined at the point where the curve transitions from gentle to vertical and is higher by Lv/2, while the bottom of the layer (Hn) is based on a point lower by Lv/2 from the transition point.

Article 41. Determining the boundaries of layers according to the natural potential curve

1. The boundary of the thick layer on the natural potential curve measured by the potential method is determined at the point where there is a sudden change in the curve. At this point, it corresponds to the potential value equal to the average natural potential of the surrounding rock and the intervening layer.

2. The boundary of the layer on the natural potential gradient curve is determined at points of maximum and minimum values.

Article 42. Determining the boundaries of layers according to the curve measuring current intensity and sliding contact

The boundaries of the layers on the curve measuring current intensity and sliding contact are determined at points where there is a sudden change in the curve. These points correspond to the extreme values of the curve. For layers with low resistivity, the boundaries are the points with extreme values; for layers with high resistivity, the boundaries will shift towards the surrounding rock layers with lower resistivity.

Article 43. Determining the boundaries according to the radiometric curve

1. The top of the layer is determined at the point where the curve starts to rise (or fall), and the bottom of the layer is determined at the point where the curve starts to fall (or rise).

2. For layers with large thickness (h > 5L), the boundaries of the layers are determined at positions corresponding to half the amplitude of the maximum of the curve within the analysis section of one layer (Imax).

3. For layers with small thickness, the point corresponding to half the amplitude will shift towards the center of the layer and will shift more as the layer thickness decreases. Usually, the Imax value decreases when the layer thickness decreases and the Vt product increases. As the Vt product increases, the position of Imax shifts more in the direction of movement of the measurement device in the borehole.

4. When measuring with a borehole logging station, the results of automatic analysis of the processing program installed in the machine or carried along with the machine may be used to determine the layer boundaries.

Section 3

GEOLOGICAL ANALYSIS OF BOREHOLE GEOPHYSICAL MEASUREMENT RESULTS

Article 44. Content of geological analysis and interpretation of borehole geophysical measurement data

1. Geological analysis and interpretation of borehole geophysical measurement data include: determining the strata present in the borehole cross-section, linking data between boreholes, establishing a geological-geophysical cross-section of the borehole.

2. Determining the strata present in the borehole cross-section:

a) Stratification of the geological cross-section of the borehole based on geophysical measurements involves dividing different geological bodies with distinct physical properties and lithological components based on the physical parameters of the strata according to standard boreholes;

b) Stratification of the borehole according to the curves of the main methods, followed by supporting methods. Strata clearly differentiated on the curves are identified first, then the entire borehole;

c) When stratifying, it is necessary to rely on the results of the curves measured in standard boreholes of the region, lithological characteristics, and characteristics of the rock and ore layers;

d) Determining geological strata based on the values of the geophysical field on the geophysical measurement curves corresponding to the physical parameters of rock layers with different lithological components and related materials.

3. Linking data between boreholes:

a) Borehole linking is conducted in three groups:

- Linking boreholes between large geological regions for studying geological structure, uplift domes, and subsidence domes;

- Linking boreholes within a region or a geological block, a mining area;

- Detailed linking of boreholes within a narrow range to control ore bodies, thin interbedded layers, or geological foundation structures of engineering works.

b) The content of linking between regions includes monitoring changes in lithological composition and thickness of major strata and complex stratigraphic systems within a certain area. To achieve this, an optimal combination of borehole geophysical methods recorded at a scale of 1:200 or 1:500 must be used.

c) The content of internal linking within a region includes:

- Using a combination of borehole geophysical methods recorded at a scale of 1:200;

- Monitoring changes in lithological composition and thickness of each stratum;

- Detecting types of faults;

- Detecting seams, including productive seams;

- Determining the boundaries and thickness of seams to establish a comprehensive geological-geophysical cross-section of the borehole for the mining area;

- Determining the structural elements of the mine.

d) The content of detailed linking includes:

- Using a combination of borehole geophysical methods recorded at a scale of 1:50 or 1:20;

- Monitoring seams, interbedded layers, and ore bodies.

- Detecting changes in thickness and lithological composition of geological objects within the study area;

- Determining the structure of productive seams and ore bodies.

3. Establishing a geological-geophysical cross-section of the borehole

a) A standard geological-geophysical cross-section of the borehole is established based on geophysical borehole data of a reasonable combination of borehole geophysical methods in the standard borehole of the work area;

b) The geological-geophysical log standard drill hole cut must be established based on the following materials: stratigraphic columns according to core samples, descriptions of rock and soil components, standard electrical geophysical curves (electrical resistance curves measured by standard electrode systems and natural potential curves), appropriate radioactive curves for each research object, drill hole diameter measurement curves, and standard geophysical logging conditions for the work area.

c) The geophysical curves in the standard geological-geophysical log cut must include all essential information representative of the measuring equipment and measurement conditions in the work area.

On the curves of the standard geological-geophysical log cut, standard layers must be distinguishable, used to correlate drill holes and name these standard layers.

Section 4

REPRESENTATION OF RESULTS, REPORT PREPARATION

Article 45. Products of geophysical well logging work

1. The products of geophysical well logging work must include original documents, geophysical well logging charts, and intermediate documents.

2. Original documents include: recording tapes of geophysical curves, data discs, measurement data, or any of these document forms, and standard machine data.

3. Geophysical well logging chart: is a comprehensive drawing including all geophysical well logging curves, well stratigraphic columns divided according to the results of geophysical measurements presented in Model No. 3 of Appendix 3 issued together with this Circular.

4. Intermediate documents include: notebooks or tables of calculated physical parameters for ore layers and surrounding rocks; results of processing and analyzing geophysical well logging data using automatic processing and analysis technology on computers: identification, classification, seam correlation.

5. In addition to the three mandatory product types mentioned above, geophysical well logging charts can be presented in various formats and scales to best illustrate the research results.

Article 46. Report Preparation

1. All geophysical well logging projects and tasks must prepare a final report.

2. For independent projects:

a) The report on geophysical well logging research results must comply with and be consistent with regulations on project proposal preparation and basic geological survey reports on mineral resources;

b) The composition of the final report team includes a chief editor with a senior exploration engineer level or higher in geophysics, two geophysical exploration engineers, two intermediate-level exploration assistants specializing in geological data analysis, and depending on the research subject and scale, there may also be an additional hydrogeological exploration engineer or engineering geology exploration engineer, and an economic exploration engineer.

c) The content of the report includes: objectives and tasks set out to be resolved, volume completed, combination of methods and equipment applied and evaluation of the effectiveness of each method, geophysical and geological results achieved, appendices, maps, drawings attached, production organization and implementation costs, conclusions and recommendations, and reference materials.

Chapter V

IMPLEMENTING PROVISIONS

Article 47. Effective Date

1. This Circular takes effect from March 15, 2011.

Abolish the technical regulation on geophysical well logging methods issued by Decision No. 661/QĐ/ĐCKS-KHTC dated December 23, 2004 of the Director of the Vietnam Geological Survey and Mineral Resources Department.

2. The Director of the Vietnam Geological Survey and Mineral Resources Department, subordinate units under the Ministry of Natural Resources and Environment, organizations, and individuals implementing projects applying geophysical well logging methods are responsible for enforcing this Circular./.