Method for testing electrical property of full-diameter natural rock core

Abstract

The invention discloses an electrical property testing method for a full-diameter natural rock core, which comprises the following steps of: firstly, freezing and preserving an obtained loose full-diameter natural rock core, and marking point positions on two symmetrical sides along the diameter direction of a full-diameter natural rock sample after unfreezing; then drilling is carried out at the marked point positions, and the leads are inserted into the drilled holes; measuring the resistance of all symmetrical points on the two symmetrical sides of the rock sample by using a digital bridge; calculating the conductivity of the full-diameter natural rock core by using the obtained resistance; the method has important application value in accurate acquisition of the electrical parameters of the loose sand shale reservoir and gas reservoir identification, and provides a new idea for solving the problem of gas reservoir identification.

Description

Technical Field

[0001] The present invention belongs to the technical field of reservoir rock electrical property testing and fluid identification in the field of oil and gas field development, and relates to an electrical property testing method for full-diameter natural rock cores. Background Art

[0002] The Quaternary strata in the Qaidam Basin are characterized by relatively shallow burial depths, being in the early stage of unconsolidated or diagenetic processes, and are typical representatives of weak diagenesis; this geological background has led to the formation of unconsolidated sandstone and mudstone reservoirs in the Subei area; the main characteristics of such reservoirs are high shale content, mainly composed of mudstone and muddy siltstone, with thin layers of siltstone bands intercalated, small lithological differences, and unclear logging curve response characteristics, making it difficult to identify gas layers; the theoretical technology for interpreting such high-shale and rich-water gas layers is not yet mature, causing difficulties in optimizing and evaluating enrichment areas; in order to overcome these geological problems, the Subei Gas Field has achieved certain results in the development of complex gas layers in recent years, and through continuous exploration and innovation, a series of technical means have been adopted to solve the problem of gas layer identification; however, the geological characteristics of the Subei Gas Field are still very complex, and there are still challenges in accurately identifying gas layers. The electrical parameters of reservoir rocks are the key to fluid identification and fluid saturation analysis and calculation. However, due to the influence of engineering factors, the electrical response of rocks obtained by logging is inaccurate. In contrast, the electrical test results of indoor full-diameter natural rock cores are more reliable and can more accurately reflect the electrical characteristics of reservoir rocks, providing theoretical support for the analysis of influencing factors of reservoir rock electrical properties and gas layer identification. Summary of the Invention

[0003] The purpose of the present invention is to provide an electrical property testing method for full-diameter natural rock cores, which solves the problems of low accuracy and low efficiency in accurately obtaining the electrical properties of existing unconsolidated sandstone and mudstone reservoirs and gas layer identification methods.

[0004] The technical solution adopted by the present invention is an electrical property testing method for full-diameter natural rock cores, which is specifically implemented according to the following steps:

[0005] Step 1, freeze and store the taken unconsolidated full-diameter natural rock core.

[0006] Step 2, thaw the full-diameter natural rock core frozen in Step 1.

[0007] Step 3, mark points symmetrically on both sides along the diameter direction of the full-diameter natural rock sample.

[0008] Step 4, drill holes at the marked points in Step 3.

[0009] Step 5, select leads and then insert the leads into the holes obtained in Step 4.

[0010] Step 6, use a digital bridge to measure the resistance of all symmetric points on both symmetric sides of the rock sample.

[0011] Step 7: Calculate the conductivity of the full-diameter natural core using the resistance obtained in Step 6.

[0012] The features of the present invention also lie in:

[0013] In Step 1, the freezing storage temperature of the full-diameter natural core is -30°C;

[0014] In Step 2, the thawing process is as follows: First, encapsulate the full-diameter natural core with a thermoplastic tube, and place the encapsulated full-diameter natural core at room temperature of 23°C for thawing, with a thawing time of 24 - 25 h;

[0015] In Step 3, the process of marking points is specifically as follows: Determine two symmetric sides along the diameter direction of the natural rock sample. First, mark on one symmetric side, mark several points. The first point is 1 - 2 cm away from the top of the rock sample, and the last point is no more than 3 cm away from the bottom of the rock sample. Then, evenly mark the remaining points according to the length of the rock sample. Finally, mark the symmetric points on the other side of the natural rock sample according to the marked points on one side. The distance between each point is determined according to the length of the full-diameter core, following the principle of uniform distribution;

[0016] In Step 4, specifically, the drilling depth is 2 - 3 cm;

[0017] In Step 5, select a low-resistance material with a diameter of 1 - 2 mm, a length of 3 - 5 cm, and a resistance of 0.01 - 0.02 Ω or less as the lead wire, and insert the lead wire into the drilled holes on both sides of the rock sample to ensure that the lead wire is in close contact with the hole wall;

[0018] In Step 6, specifically, place the rock sample horizontally on an insulating experimental platform at room temperature of 23°C, then adjust the digital bridge to the resistance measurement gear, clamp the LD and HD electrodes of the bridge to the lead wires on both sides of the rock sample respectively. Wait for the resistance to stabilize, record the resistance reading, and measure the resistance of all symmetric points on both sides of the rock sample in the same way;

[0019] In Step 7, specifically, after measuring the resistance of the full-diameter natural core in Step 6, the conductivity of the full-diameter natural core can be calculated by Equation (1);

[0020]

[0021] Equation (1) is the resistance expression with the two end points of the diameter connected to the circuit. In the equation, R is the resistance of the natural core; σ is the conductivity of the natural core; t is the thickness of the natural core, i.e., the length of the core; a is the radius of the core, and δ is the width of the contact point, i.e., the diameter of the lead wire. Finally, convert the conductivity into resistivity, and the unit of resistivity is Ω·m.

[0022] The beneficial effects of the present invention are:

[0023] The electrical property testing method for full-diameter natural rock cores of the present invention provides the resistivity value of reservoir rocks in their natural state through indoor testing, maximizing the restoration of the true electrical properties of reservoir rocks. By comparing the measured rock resistivity with well logging data, it can be determined whether it is due to the reservoir itself or external factors such as instruments, mud, and surrounding rocks. The present invention has important application value in accurately obtaining the electrical property parameters of loose sandstone and mudstone reservoirs and gas layer identification, providing a new idea for solving the problem of gas layer identification. Brief Description of the Drawings

[0024] Figure 1 It is a schematic diagram of the point position marking in Step 3 of the electrical property testing method for full-diameter natural rock cores of the present invention;

[0025] Figure 2 It is a schematic diagram of resistance testing in Step 6 of the electrical property testing method for full-diameter natural rock cores of the present invention;

[0026] Figure 3 It is a comparison chart of the resistivity measured indoors for full-diameter rock samples and well logging resistivity in the electrical property testing method for full-diameter natural rock cores of the present invention. Detailed Embodiments

[0027] The present invention will be described in detail below in conjunction with the drawings and specific embodiments.

[0028] The present invention provides an electrical property testing method for full-diameter natural rock cores, as shown in the following embodiments:

[0029] Embodiment 1, the technical objective of the electrical property testing method for full-diameter natural rock cores of the present invention:

[0030] At present, there is no method to test the resistivity of full-diameter natural cores. In particular, it is more difficult to obtain the electrical parameters of rocks in unconsolidated sandstone and mudstone reservoirs. The present invention proposes a new method. Through indoor tests, the resistivity values of reservoir rocks in their natural state are given, maximizing the restoration of the true electrical characteristics of reservoir rocks. Although this value is not exactly the same as the resistivity of formation rocks, it is meaningful for analyzing the changes in formation resistivity and influencing factors. This is because resistivity is not only a function of various aspects such as rock composition, pore structure, formation water salinity, gas / water saturation, etc., but is also affected by external factors such as measurement methods and measurement conditions. Under the two measurement states, the pore structure, rock composition, and salinity of formation water remain unchanged, and the water saturation basically remains unchanged. Without considering temperature and pressure, the difference between these two resistivities can be investigated. By comparing the resistivity of the rock measured experimentally with well logging data, it can be determined whether it is due to the reservoir itself or external factors such as instruments, mud, and surrounding rocks. In short, the accurate acquisition of the electrical properties of unconsolidated sandstone and mudstone reservoirs and gas layer identification have always been challenging problems. Traditional identification methods have problems such as low accuracy and low efficiency, and cannot meet the requirements for accurate gas layer identification. The present invention has important application value in accurately obtaining the electrical parameters of unconsolidated sandstone and mudstone reservoirs and gas layer identification, providing a new idea for solving the problem of gas layer identification.

[0031] Example 2, the specific implementation process of the electrical test method for the full-diameter natural core of the present invention:

[0032] Step 1, place the taken unconsolidated full-diameter natural core in a freezer for preservation at a temperature of -30°C.

[0033] Step 2, since the natural rock sample is in a frozen state and is relatively loose, first encapsulate the rock sample with a thermoplastic tube and place the rock sample at room temperature of 23°C for thawing. The thawing time is about 24 hours for the need of drilling with an electric grinder.

[0034] Step 3, find the symmetric two sides along the diameter direction of the natural rock sample, and first label one side symmetrically. The first point is about 1 - 2 cm away from the top of the rock sample, then mark the following points according to the length of the rock sample, and the last point is about no more than 3 cm away from the bottom of the rock sample, and then mark the symmetric points on the other side of the rock sample.

[0035] Step 4, drill holes at the labeled positions with a small electric grinder, and the depth of the holes can be controlled at 2 - 3 cm.

[0036] Step 5, select a low-resistance material with a diameter of about 1 - 2 mm, a length of about 3 - 5 cm, and a resistance of about 0.01 - 0.02 Ω as the lead wire, and slowly insert the lead wire into the holes on both sides of the rock sample to ensure that the lead wire is in close contact with the hole wall.

[0037] Step 6, under the condition of room temperature of 23°C, horizontally place the rock sample on an insulating experimental platform. Then, adjust the digital bridge to the resistance measurement gear, clamp the LD and HD electrodes of the bridge to the leads on both sides respectively, and wait for the resistance to stabilize (about 1 - 2 minutes, with the last two digits of the resistance reading remaining unchanged), and record the resistance reading; measure the resistance of all symmetric points on both sides in the same way.

[0038] Step 7, after measuring the resistance of the full-diameter natural core, the conductivity of the full-diameter natural core can be calculated from Equation (1):

[0039]

[0040] Equation (1) is the resistance expression with the two endpoints of the diameter connected to the circuit. Among them, R is the resistance of the natural core; σ is the conductivity of the natural core; t is the thickness of the natural core, that is, the length of the core; a is the radius of the core, and δ is the width of the contact point, that is, the diameter of the lead. Finally, convert the conductivity into resistivity, and the unit of resistivity is Ω·m.

[0041] Example 3, test the resistivity of the full-diameter natural rock samples of Well T2, Well T1, and Well TQ2 under laboratory conditions;

[0042] Among them, the rock samples of Well T2 are all stored in a sealed and frozen state, maintaining the natural state under formation conditions to the greatest extent. The rock samples of Well T1 and Well TQ2 are all stored under conventional conditions and are dry rock samples. As shown in Table 2, the resistivity of the rock samples of Well T2 is between 0.662 and 3.471 Ω·m. As shown in Table 3, the resistivity of the rock samples of Well T1 and Well TQ2 is 18131 - 41173 Ω·m. Since the conductivity of rocks is mainly controlled by the fluids in them and the conductive ability of the rock skeleton is weak, the resistivity of dry rock samples is relatively large; as Figure 3 shown, after comparing the resistivity measured indoors with the well logging resistivity, it is found that the water content of silty mudstone is relatively high and the resistivity shows a low value, while the water content of muddy siltstone is relatively low and the resistivity is relatively high. This shows that water saturation and shale content are important factors leading to low resistivity of reservoir rocks. The variation trend of the resistivity of the full-diameter core indoors of silty mudstone and muddy siltstone is basically the same as that of the well logging resistivity. The indoor test value is larger than the well logging value, which may be related to external factors such as instruments, mud, and surrounding rocks. By using this method to test the electrical properties of the full-diameter natural core, on the one hand, relatively accurate electrical property parameters of reservoir rocks are obtained. On the other hand, by comparing the resistivity measured indoors with the well logging resistivity, it is further proved that the low resistivity of the reservoir is controlled by internal factors;

[0043] Table 2 Comparison of resistivity of full-diameter natural rock samples and well logging resistivity under laboratory conditions

[0044]

[0045]

[0046] Table 3 Test Results of Full-Diameter Dry Rock Samples (Pores without Water)

[0047]

[0048] The resistivity of the natural core of Well T2 was tested using the full-diameter natural core resistivity test method, and relatively real electrical parameters of the reservoir rock were obtained. The test results are shown in Table 1. The resistivity can be calculated using the resistance value of the selected rock sample at the first measurement point (11') or the fourth measurement point (44'). The test results show that since it is a natural rock sample and there is water in the rock pores, the resistivity is generally low, which also conforms to the characteristics of the internal low-resistivity gas reservoir of the reservoir. At the same time, the resistivity differences of each rock sample also reflect the differences in lithology or water saturation of different reservoir sections.

[0049] Table 1 Resistivity Calculation of Full-Diameter Natural Rock Samples in Well T2

[0050]

Claims

1. A method for testing the electrical properties of a full-diameter natural rock core, characterized in that: Please follow the steps below to implement: Step 1, freezing and preserving the obtained loose full-diameter natural rock core; Step 2, thawing the full-diameter natural core frozen in step 1; Step 3, marking points along the two symmetrical sides of the diameter direction of the full-diameter natural rock sample; Step 4, drilling holes at the points marked in step 3; Step 5, select the lead wire, and then insert the lead wire into the drilled hole obtained in step 4; Step 6, using a digital bridge to measure the resistance of all symmetrical points on both sides of the rock sample; Step 7, using the resistance obtained in step 6, calculate the conductivity of the full-diameter natural core.

2. The electrical property testing method of full-diameter natural rock core according to claim 1, characterized in that: The freezing storage temperature of the full-diameter natural core in step 1 is -30°C.

3. The electrical property testing method of full-diameter natural rock core according to claim 1, characterized in that: The thawing process of step 2 is as follows: firstly, the full-diameter natural rock core is encapsulated with a thermoplastic tube, and the encapsulated full-diameter natural rock core is placed at room temperature of 23° C. for thawing for 24 to 25 hours.

4. The electrical property testing method of full-diameter natural rock core according to claim 1, characterized in that: The process of marking the points in step 3 is specifically as follows: determine the two symmetrical sides along the diameter direction of the natural rock sample, first mark one symmetrical side, evenly mark several points, the first point is 1 to 2 cm away from the top of the rock sample, and the last point is no more than 3 cm away from the bottom of the rock sample, then evenly mark the remaining points according to the length of the rock sample, and finally mark the symmetrical points on the other side of the natural rock sample according to the points on one side that have been marked.

5. The electrical property testing method of full-diameter natural rock core according to claim 1, characterized in that: The step 4 specifically includes: the drilling depth is 2 to 3 cm.

6. The electrical property testing method of full-diameter natural rock core according to claim 1, characterized in that: In step 5, a low-resistance material with a diameter of 1 to 2 mm, a length of 3 to 5 cm, and a resistance of 0.01 to 0.02 Ω is selected as a lead, and the lead is inserted into the drilled holes on both sides of the rock sample to ensure that the lead is in close contact with the hole wall.

7. The electrical property testing method of full-diameter natural rock core according to claim 1, characterized in that: The step 6 is specifically as follows: at room temperature of 23°C, place the rock sample horizontally on an insulating experimental platform, then adjust the digital bridge to the resistance measuring position, clamp the bridge LD and HD electrodes to the leads on both sides of the rock sample respectively, wait for the resistance to stabilize, record the apparent resistance, and measure the resistance of all symmetrical points on both sides of the rock sample in the same way.

8. The electrical property testing method of full-diameter natural rock core according to claim 1, characterized in that: The step 7 is specifically as follows: after the resistance of the full-diameter natural core is measured in step 6, the conductivity of the full-diameter natural core can be calculated by formula (1); Formula (1) is the resistance expression of the circuit connected with two ends of the diameter, where R is the resistance of the natural core; σ is the conductivity of the natural core; t is the thickness of the natural core, i.e. the length of the core; a is the radius of the core, and δ is the width of the contact, i.e. the diameter of the lead wire; finally, the conductivity is converted into resistivity, and the unit of resistivity is Ω.m.

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