Method for testing and calculating in-situ water saturation of coal reservoirs

Abstract

The present invention discloses a method for testing and calculating the in-situ water saturation of a coal reservoir, and belongs to the technical field of unconventional natural gas exploration and development. The in-situ water saturation is obtained by the steps of pressure-maintaining coring, on-site rapid determination of movable water volume, determination of bound water nuclear magnetic signal, water saturation, determination of bound water volume, helium porosity test of fragment samples under normal pressure conditions, overpressure porosity test of plunger samples and porosity attenuation coefficient fitting, and in-situ porosity calculation. The "centrifuge-test-saturated-test" low-field nuclear magnetic resonance test process proposed in the present application can effectively avoid the error of increasing the bound water test ratio; since the bound water volume of the coal reservoir is included in the water saturation calculation, and the in-situ porosity of the coal reservoir under formation conditions is determined by a comprehensive method of combining plunger samples and fragment samples for porosity testing, the water saturation calculation result is closer to the in-situ characteristics of the coal reservoir.

Description

Technical Field

[0001] The present invention relates to the technical field of unconventional natural gas exploration and development, and in particular to a testing system and a calculation method for in-situ water saturation of a coal reservoir. Background Art

[0002] The pores in coal reservoirs are occupied by both water and natural gas. At present, water saturation and gas saturation are mostly used to quantitatively characterize the relationship between the two. Water saturation is an important parameter to pay attention to in the process of surface drainage and depressurization of coalbed methane development. Water content directly affects the content of adsorbed gas, free gas, and water-soluble gas in coal reservoirs. In particular, as the current coalbed methane development depth gradually increases, the proportion of free gas in deep coal reservoirs has increased significantly. Therefore, accurate determination of the in-situ water saturation of coal reservoirs is of great significance to the estimation of coalbed methane resources and the improvement of coalbed methane development effects.

[0003] At present, there are two ways to test the water saturation of coal reservoirs: the first is to measure the water volume and pore volume in the coal seam respectively, and calculate the water saturation based on the ratio of the two; the second is to calculate the porosity and water saturation of the coal reservoir based on the characteristics of the logging curve during the drilling process.

[0004] For the first type of testing idea, the commonly used testing method is to obtain core samples from the drilling site and record the mass, transport them to the laboratory, and record the mass of the samples again by drying / centrifugation, etc., calculate the mass of lost water, and combine it with the density of water (usually 1.0g / cm 3 ) to calculate the volume of water, and determine the porosity of coal samples based on density bottle method, conversion method, helium method, low-field nuclear magnetic resonance method, etc. However, the test results obtained by this method are quite different from the in-situ characteristics of the coal reservoir, which is mainly attributed to the presence of some residual water that cannot be removed in the sample after centrifugation / drying, namely bound water, which should be the main reason for the low water production in the current deep coalbed methane development process. The simple calculation of the water volume in coal based on mass change does not take into account the influence of this part of residual bound water.

[0005] In addition, the porosity test results are also different from the in-situ conditions. At present, the coal sample porosity test usually uses fragments or plunger specifications. The fragment sample porosity test process cannot load confining pressure, and the test results are quite different from the pore characteristics under formation conditions, especially for the high temperature and high pressure environment of deep coal reservoirs, the pore compression degree is extremely high, and the calculation error has a higher impact. Although the plunger sample test can load confining pressure, the core size obtained by field coring is usually large and cannot be directly used for laboratory plunger sample testing, so sample preparation is required first. At present, the commonly used sample preparation methods are drilling or wire cutting. During the drilling process, the coal sample needs to be flushed with water to destroy the initial water content of the coal sample; although there is no external water involved in the wire cutting process, the current wire cutting cylindrical sample preparation time is relatively long (currently it usually takes 6 hours to prepare a Φ25×50mm cylindrical sample). During the long sample preparation process, due to environmental factors such as pressure reduction, the movable water in the coal is easily lost, and the test results also cannot reflect the initial water content characteristics of the sample. Although the porosity test by low-field nuclear magnetic resonance can determine the ratio of bound water to free water, such as the method based on nuclear magnetic resonance T 2 The article "Method and device for evaluating water saturation by spectrum" points out that it can calculate the water content of the part of the nuclear magnetic resonance T 2 Spectral geometric mean T 2LMw and crude oil NMR T 2 Spectral geometric mean T 2LMhc , thereby realizing the use of nuclear magnetic resonance T 2 The water saturation can be assessed by the helium spectrum, but because the medium tested in this method is water, the measured porosity is significantly lower than that of the helium method, which also fails to reflect the true pore characteristics of the coal sample. In summary, a single test method cannot obtain a more accurate test result.

[0006] For the second type of testing idea, the specific calculation process is to first prepare coal samples with different water saturation in the laboratory, select different logging parameters to establish a chart, and calculate the porosity and water saturation based on the logging results of the coal reservoir to be tested (for example, the schemes disclosed in Chinese patents CN114660128A, CN107132250A, CN111856583B, and CN107389521B). However, this type of testing method still fails to eliminate the influence of bound water in the process of establishing the chart, and the logging parameters are greatly affected by parameters such as coal rock composition. Therefore, the established charts are mostly for small-scale research areas. When the research area changes, a large number of experimental tests need to be carried out again to establish new charts. At present, there is no universal logging interpretation template for different regions.

[0007] In summary, it is still of great significance to establish a method that can accurately measure and calculate the in-situ water saturation of coal reservoirs. Summary of the invention

[0008] The purpose of the present invention is to solve the problems existing in the prior art and provide a method for testing and calculating the in-situ water saturation of a coal reservoir. The porosity test is performed by combining a plunger sample and a fragment sample to solve the problem that the confining pressure cannot be loaded in the fragment sample porosity test. The water saturation calculation includes the bound water volume of the coal reservoir, and the calculation result is closer to the actual characteristics of the in-situ coal reservoir.

[0009] In order to achieve the above technical purpose, the present invention is implemented by the following technical scheme: a method for testing and calculating the in-situ water saturation of a coal reservoir comprises the following steps:

[0010] 1) After coring the coal reservoir and lifting it to the surface, take fresh fragment samples, record the burial depth, wipe off the surface moisture and weigh the sample mass m 1 , determine the apparent density ρ of the core 1 , calculate the sample apparent volume V 1 =m 1 / ρ 1 , collect formation water samples and test the formation water density ρ 2 ;

[0011] 2) Repeat centrifugation until the mass change of the fragmented sample is less than 0.01g, and record the mass of the sample at this time m 2 , movable water volume V 可动 =(m 1 -m 2 ) / ρ 2 ;

[0012] 3) Perform low-field nuclear magnetic resonance test on the fragmented samples and record the T 2 Relaxation spectrum;

[0013] 4) Saturate the centrifuged fragments with water until the mass change is less than 0.01 g within 24 hours, and record the mass of the saturated sample m 3 ;

[0014] 5) Perform low-field nuclear magnetic resonance test on the saturated sample and record the saturated state T 2 Relaxation spectrum and NMR porosity Combined with centrifugation T 2 Relaxation spectrum calculation T 2 Cut-off value, based on saturated state T 2 Calculation of bound water ratio T from relaxation spectrum 束缚 , calculate the bound water volume

[0015] 6) Calculate the volume of the coal sample skeleton, combined with the apparent volume V 1 The pore volume is obtained and then the helium porosity of the fragment sample under normal pressure conditions is calculated

[0016] 7) Prepare the plunger sample, measure and record the mass m of the plunger sample 1 ′ and density ρ 1 ', calculate the apparent volume V of the plunger 1 ′=m 1 ′ / ρ 1 ', the plunger sample is subjected to a pressure test, the pore pressure is fixed, several confining pressures are set to increase successively, and the helium porosity test is carried out under different effective stress conditions. After the pore volume is obtained, the corresponding porosity is calculated, and the attenuation coefficient a of the sample is obtained by fitting with the effective stress as the abscissa and the porosity under different confining pressure conditions as the ordinate;

[0017] 8) Based on the porosity of the fragment sample at normal pressure The attenuation coefficient a obtained by fitting the overburden porosity of the plug sample is used to calculate the in-situ porosity of the fragment sample.

[0018] 9) In-situ water saturation calculation:

[0019] Furthermore, a nuclear magnetic resonance instrument for low-field nuclear magnetic resonance testing is provided with a helium perfusion passage connected to the inside of the resonance instrument on the basis of the original structure, a non-magnetic sample tank and a helium bottle are provided on the helium perfusion passage, a first gas valve is provided on the connecting passage between the non-magnetic sample tank and the helium bottle, a second gas valve is provided on the connecting passage between the non-magnetic sample tank and the nuclear magnetic resonance instrument, and a second temperature sensor and a third pressure sensor are provided on the non-magnetic sample tank for monitoring the temperature and pressure in the non-magnetic sample tank.

[0020] Furthermore, in step 1), the core is obtained from the coal reservoir by pressure-maintaining coring.

[0021] Furthermore, in step 1), no less than three fresh fragment samples are quickly taken from the core extracted to the ground, and the sampling distance between different samples needs to be increased when selecting samples.

[0022] Furthermore, the low-field NMR and helium porosity testing process is completed in the laboratory after the sample is sealed and sent to the laboratory, or the tester is transported to the sampling site.

[0023] Further, in step 6), the porosity of the fragmented sample is calculated The specific process is as follows:

[0024] Calculate the formation temperature corresponding to the coal sample based on borehole temperature measurement or burial depth, adjust the constant temperature zone to the formation temperature; open the first gas valve so that the third pressure sensor is 72.5psi (reference pressure, adjustable during the test), close the first gas valve, and record the temperature and pressure of the non-magnetic sample tank and the non-magnetic core holder after the pressure in the non-magnetic sample tank changes less than 0.01psi within 5 minutes. At this time, the pressure of the non-magnetic sample tank is P 0 , temperature is T 0 , the pressure inside the non-magnetic core holder is P 1 , temperature is T 1 Based on the gas state equation (1), we can get

[0025] PV=nRTZ (1)

[0026] P 0 V r =n 0 RT 0 Z 0 (2)

[0027] P 1 (V s -V 1 )=n 1 RT 1 Z 1 (3)

[0028] Where n is the number of moles of gas, R is the molar gas constant, Z is the compressibility factor, V r is the volume of the non-magnetic sample tank, cm 3 ; V s is the volume of the non-magnetic core holder, cm 3 ; V 1 is the apparent volume of the sample, cm 3 ;

[0029] Open the second air valve and wait until the pressure in the non-magnetic core holder changes less than 0.01 psi within 5 minutes. Record the pressure of the non-magnetic sample tank as P. r0 , temperature is Tr 0 , the pressure of the non-magnetic core holder is P s1 , temperature is T s1 , then we have formula (4) and formula (5):

[0030] P r0 V r =n r0 RT r0 Z r0 (4)

[0031] P s1 (V s -(V 1-V p ))=n s1 RT s1S Z s1 (5)

[0032] Where V p is the pore volume of the sample, cm 3 , V 1 -V p It is the volume of coal sample skeleton;

[0033] Combining equation (2) with equation (4), equation (3) with equation (5), we can get:

[0034]

[0035]

[0036] Since helium is a non-adsorbing gas, we have formula (8):

[0037] n 0 -n r0 =n s1 -n 1 (8)

[0038] Right now:

[0039]

[0040] Transform formula (9) into:

[0041]

[0042] The coal sample porosity is calculated by formula (11):

[0043]

[0044] Furthermore, in step 7), the constant temperature zone is adjusted to the formation temperature, the pore pressure is fixed at 72.5 psi, and 5 successively increasing confining pressures are set. The maximum confining pressure minus the pore pressure is the effective stress of the formation condition. The helium porosity test under different effective stress conditions is performed. Taking the first confining pressure test point as an example, the pore volume Vp′ under the first confining pressure condition is obtained based on the recorded temperature and pressure changes of the non-magnetic sample tank and the non-magnetic core holder. The corresponding porosity is calculated based on formula (12): The test principles for the other four confining pressure conditions are the same, and the porosity under different effective stress conditions is obtained, which is recorded as Porosity and effective stress usually have an exponential relationship (Equation 13), so the effective stress is used as the horizontal axis and the The vertical coordinate can be fitted to obtain the attenuation coefficient a of the sample;

[0045]

[0046] Where: is the porosity, %; is the porosity at normal pressure, %; a is the attenuation coefficient, MPa -1 ; σ is the effective stress, MPa.

[0047] Furthermore, in step 7), after each confining pressure condition porosity test is completed, the vacuuming process needs to be repeated to remove the residual helium in the pores.

[0048] The beneficial effects of the present invention are:

[0049] 1. The current low-field nuclear magnetic resonance test process is usually "saturation-test-centrifugation-test". For coal reservoirs with a high proportion of bound water in the pores and not yet saturated, saturation first may lead to a high test result of the bound water ratio. This application proposes to use the "centrifugation-test-saturation-test" process to test the bound water ratio, which can effectively avoid this error and avoid the problem that saturated water may cause an increase in the bound water ratio;

[0050] 2. This application takes into account the influence of residual bound water in the sample on the results when conducting in-situ water saturation testing and calculation of coal reservoirs. After pressure-maintaining coring, laboratory testing and fitting of the coal reservoir porosity under original formation conditions (temperature, effective stress), on-site rapid determination of movable water volume, and determination of bound water volume in the laboratory, the water saturation calculation results are closer to the in-situ characteristics of the coal reservoir, and the calculation results are more accurate.

[0051] 3. This application adopts a method of combining plug samples and fragment samples for porosity testing to determine the in-situ pores of coal reservoirs under formation conditions. Compared with the method of using a single sample to measure in-situ porosity, this method can solve the problem of being unable to load confining pressure in the fragment sample porosity test, so that the obtained test results are less different from the pore characteristics under formation conditions, reducing calculation errors;

[0052] 4. The on-site test of movable water volume has the characteristics of quick and convenient sampling, and only a small amount of broken samples are needed to complete the test, and the water saturation characteristics of different positions of the same coal sample can be compared;

[0053] 5. The pressure-maintaining coring method is adopted to quickly measure the movable water volume on site, thus avoiding the influence of water loss caused by environmental changes on the test results;

[0054] 6. This application connects a non-magnetic sample tank to the nuclear magnetic resonance instrument to fill it with helium, which can achieve the simultaneous testing of helium porosity and nuclear magnetic porosity;

[0055] 7. This application will test two stages, namely the on-site rapid determination of movable water volume stage and the laboratory determination of bound water volume stage. This design can avoid the time-consuming process of wire cutting sample preparation (pressure change, etc.) that causes obvious changes in the water content of coal samples;

[0056] 8. The test method disclosed in this application has universal applicability and is also suitable for in-situ water saturation testing and calculation of reservoirs such as mudstone, shale, and sandstone. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] Figure 1 It is a nuclear magnetic resonance instrument connected to a helium supply pipeline, wherein 1-constant temperature zone, 2-non-magnetic core holder (reference size Φ25×80mm), 3-first pressure sensor, 4-confining pressure loading pump, 5-magnet, 6-second pressure sensor, 7-first temperature sensor, 8-vacuum pump, 9-vent valve, 10-nuclear magnetic signal receiver, 11-computer acquisition system, 12-non-magnetic sample tank (reference size Φ25×80mm), 13-helium bottle, 14-first gas valve, 15 second gas valve, 16-second temperature sensor, 17 third pressure sensor.

[0058] Figure 2 It is a system flow chart for testing and calculating the in-situ water saturation of coal reservoirs;

[0059] Figure 3 It is T 2 Cutoff value calculation spectrum. DETAILED DESCRIPTION

[0060] The following examples further illustrate the content of the present invention, but should not be construed as limiting the present invention. Without departing from the essence of the present invention, modifications and substitutions made to the methods, steps or conditions of the present invention all fall within the scope of the present invention.

[0061] Example 1: Method for testing and calculating in-situ water saturation of coal reservoirs

[0062] The in-situ water saturation test of coal reservoirs includes three stages: coring field test, laboratory sample preparation and laboratory test.

[0063] The coring field test process mainly requires a high-precision balance (accuracy 0.0001g), a density meter and a portable centrifuge (reference model: SA-LX01 portable centrifuge).

[0064] The equipment needed for sample preparation in the laboratory includes wire cutting machine and water saturation meter.

[0065] The test process in the laboratory is mainly carried out by relying on a nuclear magnetic resonance instrument, which is mainly composed of a constant temperature zone 1, a non-magnetic core holder 2 (reference size Φ25×80mm), a first pressure sensor 3, a confining pressure loading pump 4, a magnet 5, a second pressure sensor 6, a first temperature sensor 7, a vacuum pump 8, a vent valve 9, a nuclear magnetic signal receiver 10 and a computer acquisition system 11; however, the difference from the conventional nuclear magnetic resonance instrument is that the present application adds a helium perfusion passage connected to the inside of the resonance instrument, and the helium perfusion passage is provided with a non-magnetic sample tank 12 (reference size Φ25×80mm) and a helium bottle 13, a first gas valve 14 is provided on the connection passage between the non-magnetic sample tank 12 and the helium bottle 13, a second gas valve 15 is provided on the connection passage between the non-magnetic sample tank 12 and the nuclear magnetic resonance instrument, and a second temperature sensor 16 and a third pressure sensor 17 are provided on the non-magnetic sample tank 12 for monitoring the temperature and pressure conditions in the non-magnetic sample tank 12.

[0066] The testing and calculation process mainly involves five parts: coring, movable water volume test, porosity test, bound water volume test, and water saturation calculation (see Figure 2 ), the specific steps are as follows:

[0067] 1) Sample collection and parameter recording: The coal reservoir is cored under pressure (to avoid the loss of movable water due to pressure changes). When the core is lifted to the ground, fresh samples (broken samples, recorded as SX-W) are quickly taken, and basic data such as burial depth are recorded. The surface moisture of the sample is wiped dry, and the sample mass is measured using a high-precision balance as m 1 And record, use the densitometer to measure the core apparent density ρ 1 And record, calculate the sample apparent volume V based on mass and density 1 =m 1 / ρ 1 . Formation water samples can be collected on site, the formation water density can be tested and the ρ can be recorded. 2 If formation water samples cannot be collected, ρ 2 The density of water at normal pressure is 1.0 g / cm 3 .

[0068]

[0069] From the recorded data, we can know that m 1 =14.1202g,ρ 1 =1.37g / cm 3 , apparent volume V 1 =m 1 / ρ 1 =10.3067cm 3 , ρ 2 =1.0g / cm 3 .

[0070] 2) Determination of movable water volume: Centrifuge the sample using a portable centrifuge and record the sample mass again. Repeat the centrifugation process until the mass change of the sample after centrifugation is less than 0.01g. Record the sample mass at this time and record it as m 2 , based on the change of core water loss mass and formation water density, the movable water volume V is calculated 可动 =(m 1 -m 2 ) / ρ 2 .

[0071]

[0072]

[0073] From the recorded data, we can know that m 2 =13.8277g, then V 可动 =0.2925cm 3 .

[0074] 3) Seal the sample and transport it to the laboratory. Since the bound water in the coal is bound by pores, changes in environmental factors such as pressure will not cause it to dissipate. Therefore, the sample can be brought back to the laboratory to complete the bound water ratio test. The sample is sealed and tightly wrapped, and transported back to the laboratory together with the formation water sample (if collected). The subsequent tests are mainly low-field nuclear magnetic resonance and helium porosity tests. If the conditions at the coring site allow, the tester can be transported to the site for completion.

[0075] 4) Determination of bound water nuclear magnetic resonance signal: Close all valves on the nuclear magnetic resonance instrument, place the sample in the non-magnetic core holder 2, start the nuclear magnetic signal receiver 10 to perform low-field nuclear magnetic resonance testing, and use the computer acquisition system 11 to record the centrifugal T of the sample. 2 Relaxation spectrum, the T 2 The vertical coordinate of the relaxation spectrum is the signal generated by coal sample + bound water.

[0076] 5) Water saturation: Use a water saturation meter and water (preferably layer water) to saturate the sample until the mass change of the sample within 24 hours is less than 0.01g. Record the mass of the sample after saturation, which is recorded as m 3 .

[0077]

[0078] From the recorded data, we can know that m 3 =14.2140g.

[0079] 6) Determination of bound water volume: The saturated sample is placed in the non-magnetic core holder 2 again and subjected to low-field nuclear magnetic resonance testing, and the saturated state T is recorded. 2 Relaxation spectrum and NMR porosity Test results, saturated state T 2 The vertical coordinate of the relaxation spectrum is actually the signal generated by coal sample + bound water + movable water (see Figure 3 ), combined with centrifugation T 2 Relaxation spectrum calculation T 2 cut-off value, and based on the saturated state T 2 The bound water ratio is calculated from the relaxation spectrum and is denoted as T 束缚 (It is saturated state T 2 In the relaxation spectrum, the 2 The bound water volume is calculated by combining the porosity and the apparent volume of the sample.

[0080] 7) Helium porosity test of fragmented samples under normal pressure conditions: Start the vacuum pump 8 to evacuate the fragmented samples for 24 hours (to expel the residual undesorbed gas in the pores and improve the accuracy of the porosity test), and then turn off the vacuum pump 8. Calculate the corresponding formation temperature of the coal sample based on the borehole temperature measurement or burial depth, and adjust the constant temperature zone 1 to the formation temperature. Open the first gas valve 14 so that the third pressure sensor 17 is 72.5psi (0.5MPa), and then close the first gas valve 14. After the pressure in the non-magnetic sample tank 12 is stable (the pressure change within 5 minutes is less than 0.01psi), record the temperature and pressure of the non-magnetic sample tank 12 and the non-magnetic core holder 2, open the second gas valve 15, and after the pressure in the non-magnetic core holder 2 is stable (the pressure change within 5 minutes is less than 0.01psi), record the temperature and pressure of the non-magnetic sample tank 12 and the non-magnetic core holder 2 again, and calculate the porosity based on the recorded temperature and pressure changes. Open the vent valve 9 to exhaust the helium and take out the test sample. The porosity calculation principle and method can be found in the national standard "Methods for Determination of Physical and Mechanical Properties of Coal and Rock - Part 2-4" (GB / T 23561.2-4-2009) and "Determination of Porosity of Shale by Helium Method and Permeability by Pulse Attenuation Method" (GB / T 34533-2017). This application calculates the coal sample skeleton volume based on the gas state equation (Formula 1) and the temperature and pressure values ​​in the non-magnetic sample tank 12 and the non-magnetic core holder 2 before and after gas equilibrium, and combines the apparent volume result calculated based on mass and density to obtain the pore volume and then calculate the porosity. The main calculation process is: when helium is introduced into the non-magnetic sample tank 12 and the pressure is stable, record the non-magnetic sample tank, at the same time, then there are formulas (2) and (3):

[0081] PV=nRTZ (1)

[0082] P 0 V r =n 0 RT 0 Z 0 (2)

[0083] P 1 (V s -V 1 )=n 1 RT 1 Z 1 (3)

[0084] Where n is the number of moles of gas, R is the molar gas constant, Z is the compressibility factor, V r is the volume of the non-magnetic sample tank, cm 3 ; V s is the volume of the non-magnetic core holder, cm 3 ; V 1 is the apparent volume of the sample, cm 3 .

[0085] When the gas in the non-magnetic sample tank is injected into the non-magnetic core holder and reaches pressure equilibrium, the pressure of the non-magnetic sample tank at this time is recorded as P r0 , temperature is Tr 0 At the same time, the pressure of the non-magnetic core holder is P s1 , temperature is T s1 , then we have formula (4) and formula (5):

[0086] P r0 V r =n r0 RT r0 Z r0 (4)

[0087] P s1 (V s -(V 1 -V p ))=n s1 RT s1S Z s1 (5)

[0088] Where V p is the pore volume of the sample, cm 3 , V 1 -V p This is the volume of the coal sample skeleton.

[0089] Combining equation (2) with equation (4), equation (3) with equation (5), we can get:

[0090]

[0091]

[0092] Since helium is a non-adsorbing gas, we have formula (8):

[0093] n 0 -nr0 =n s1 -n 1 (8)

[0094] Right now:

[0095]

[0096] Transform formula (9) into:

[0097]

[0098] The coal sample porosity is calculated by formula (11):

[0099]

[0100]

[0101] 8) Plug sample overburden porosity test and porosity attenuation coefficient fitting: The large core and wire cutting instrument were used to prepare the plug sample, which was recorded as SX-W-column (reference size Φ25×50mm). The mass m of the plug sample was measured and recorded using a high-precision balance and a density meter. 1 ′ and density ρ 1 ', calculate the apparent volume V of the plunger 1 ′=m 1 ′ / ρ 1 ′. Close all valves on the nuclear magnetic resonance instrument, place the sample in the non-magnetic core holder 2, start the vacuum pump 8, evacuate the sample for 24 hours, and then turn off the vacuum pump 8. Based on the actual measurement or burial depth calculation of the corresponding formation stress by the hydraulic fracturing method, the corresponding effective stress is equivalent to the corresponding effective stress = confining pressure - pore pressure during the porosity test. Different effective stresses are formed by changing the confining pressure with a fixed pore pressure. Adjust the constant temperature zone 1 to the formation temperature, fix the pore pressure to 72.5psi (0.5MPa), set 5 successively increasing confining pressures, and the maximum confining pressure - pore pressure is the effective stress of the formation conditions. Perform helium porosity tests under different effective stress conditions. After each confining pressure condition porosity test is completed, the vacuum process needs to be repeated to remove the residual helium in the pores. Taking the first confining pressure test point as an example, the pore volume Vp′ is obtained based on the recorded temperature and pressure changes of the non-magnetic sample tank, the non-magnetic sample tank and the non-magnetic core holder (refer to step 7), and the corresponding porosity is calculated based on formula (12): The process of the other four confining pressure conditions is the same, and the porosity under different effective stress conditions is obtained, which is recorded as Porosity and effective stress usually have an exponential relationship (Equation 13), so the effective stress is used as the horizontal axis and the The attenuation coefficient a of the sample can be obtained by fitting the vertical coordinate.

[0102]

[0103]

[0104] Where: is the porosity, %; is the porosity at normal pressure, %; a is the attenuation coefficient, MPa -1 ; σ is effective stress, MPa.

[0105]

[0106] 9) In-situ porosity calculation: The block sample and the column sample are taken from the same coal sample, and the porosity attenuation coefficient a with the increase of effective stress is the same, so the porosity of the block sample at normal pressure is used. The attenuation coefficient a obtained by fitting the overburden porosity of the plug sample can be calculated based on formula (13) and the effective stress of the sample formation conditions to obtain the in-situ porosity of the fragment sample:

[0107] 10) In-situ water saturation calculation: The above data only lists the test data of one fragment sample as a reference. In order to improve the accuracy of the results, three fresh samples (fragments) were quickly taken from the target coal sample during the actual sampling process. In addition, since the samples used for gas saturation testing of coal reservoirs in on-site coring are usually 30 cm long, the sampling distance was increased as much as possible when selecting the three samples to avoid errors caused by heterogeneity (since the core is 30 cm long in total, if the sampling is concentrated within 5 cm, the test results may only represent the characteristics of 5 cm, but not the characteristics of the other 25 cm. Accordingly, the calculated results cannot represent the overall characteristics of the sample. The sampling interval of the three samples can be considered to cover the entire 30 cm). The water saturation of the coal sample is the average value of the water saturation calculation results of the three fragment samples.

[0108]

[0109]

[0110] The above shows and describes the basic principles, main features and advantages of the present invention. However, the above is only a specific embodiment of the present invention, and the technical features of the present invention are not limited thereto. Any other implementation methods derived by any technician in the field without departing from the technical solution of the present invention should be included in the patent scope of the present invention.

Claims

1. A method for testing and calculating the in-situ water saturation of a coal reservoir, characterized in that: The steps include: 1) After coring the coal reservoir and lifting it to the surface, take fresh broken samples, record the burial depth, wipe off the surface moisture and weigh the sample mass m1, measure the core apparent density ρ1, calculate the sample apparent volume V1=m1 / ρ1, collect formation water samples, and test the formation water density ρ2; 2) Repeat centrifugation until the mass change of the fragmented sample is less than 0.01g, and record the sample mass m2 and movable water volume V at this time. 可动 =(m1-m2) / ρ2; 3) Performing low-field nuclear magnetic resonance testing on the fragmented samples and recording the T2 relaxation spectrum of the fragmented samples after centrifugation; 4) Saturate the centrifuged fragments with water until the mass change is less than 0.01 g within 24 hours, and record the mass of the saturated sample m3; 5) Perform low-field NMR test on the saturated sample and record the T2 relaxation spectrum and NMR porosity of the saturated state. The T2 cutoff value was calculated based on the T2 relaxation spectrum after centrifugation, and the bound water ratio T was calculated based on the T2 relaxation spectrum in the saturated state. 束缚 , calculate the bound water volume 6) Calculate the volume of the coal sample skeleton, combine it with the apparent volume V1 to get the pore volume, and then calculate the helium porosity of the fragmented sample under normal pressure conditions 7) Prepare a plunger sample, measure and record the mass m1′ and density ρ1′ of the plunger sample, calculate the apparent volume V1′=m1′ / ρ1′ of the plunger sample, perform a pressure test on the plunger sample, fix the pore pressure, set several successively increasing confining pressures, perform helium porosity tests under different effective stress conditions, calculate the corresponding porosity after obtaining the pore volume, and obtain the attenuation coefficient a of the sample by fitting with effective stress as the abscissa and porosity under different confining pressure conditions as the ordinate; 8) Based on the porosity of the fragment sample at normal pressure The attenuation coefficient a obtained by fitting the overburden porosity of the plug sample is used to calculate the in-situ porosity of the fragment sample. 9) In-situ water saturation calculation:

2. The method for testing and calculating the in-situ water saturation of a coal reservoir according to claim 1, characterized in that: A nuclear magnetic resonance instrument for low-field nuclear magnetic resonance testing is provided with a helium perfusion passage connected to the inside of the resonance instrument on the basis of the original structure. A non-magnetic sample tank and a helium bottle are provided on the helium perfusion passage. A first gas valve is provided on the connecting passage between the non-magnetic sample tank and the helium bottle. A second gas valve is provided on the connecting passage between the non-magnetic sample tank and the nuclear magnetic resonance instrument. A second temperature sensor and a third pressure sensor are provided on the non-magnetic sample tank for monitoring the temperature and pressure in the non-magnetic sample tank.

3. The method for testing and calculating the in-situ water saturation of a coal reservoir according to claim 1, characterized in that: In step 1), the core is obtained from the coal reservoir by pressure-maintaining coring.

4. The method for testing and calculating the in-situ water saturation of a coal reservoir according to claim 1, characterized in that: In step 1), no less than three fresh fragment samples are quickly taken from the core extracted to the ground, and the sampling distance between different samples needs to be increased when selecting samples.

5. The method for testing and calculating the in-situ water saturation of a coal reservoir according to claim 1, characterized in that: The low-field NMR and helium porosity testing process is completed in the laboratory after the sample is sealed and sent to the laboratory, or the tester is transported to the sampling site.

6. The method for testing and calculating the in-situ water saturation of a coal reservoir according to claim 1, characterized in that: In step 6), calculate the porosity of the fragmented sample The specific process is as follows: Based on the borehole temperature measurement or burial depth, calculate the formation temperature corresponding to the coal sample, and adjust the constant temperature zone to the formation temperature; open the first gas valve so that the third pressure sensor is 72.5psi, close the first gas valve, and wait until the pressure in the non-magnetic sample tank changes less than 0.01psi within 5 minutes. After that, record the temperature and pressure of the non-magnetic sample tank and the non-magnetic core holder. At this time, the pressure of the non-magnetic sample tank is P0, the temperature is T0, the pressure in the non-magnetic core holder is P1, and the temperature is T1. Based on the gas state equation (1), it can be obtained PV=nRTZ (1) P0V r =n0RT0Z0 (2) P1(V s -V1)=n1RT1Z1 (3) Where n is the number of moles of gas, R is the molar gas constant, Z is the compressibility factor, V r is the volume of the non-magnetic sample tank, cm 3 ; V s is the volume of the non-magnetic core holder, cm 3 ; V1 is the apparent volume of the sample, cm 3 ; Open the second air valve and wait until the pressure in the non-magnetic core holder changes less than 0.01 psi within 5 minutes. Record the pressure of the non-magnetic sample tank at this time as P. r0 , the temperature is Tr0, the pressure of the non-magnetic core holder is P s1 , temperature is T s1 , then we have formula (4) and formula (5): P r0 V r =n r0 RT r0 Z r0 (4) P s1 (V s -(V1-V p ))=n s1 RT s1S Z s1 (5) Where V p is the pore volume of the sample, cm 3 , V1-V p It is the volume of coal sample skeleton; Combining equation (2) with equation (4), equation (3) with equation (5), we can get: Since helium is a non-adsorbing gas, we have formula (8): n0-n r0 =n s1 -n1 (8) Right now: Transform formula (9) into: The coal sample porosity is calculated by formula (11):

7. The method for testing and calculating the in-situ water saturation of a coal reservoir according to claim 1, characterized in that: In step 7), the constant temperature zone is adjusted to the formation temperature, the pore pressure is fixed at 72.5 psi, and 5 successively increasing confining pressures are set. The maximum confining pressure minus the pore pressure is the effective stress of the formation condition. The helium porosity test under different effective stress conditions is carried out. Taking the first confining pressure test point as an example, the pore volume Vp′ under the first confining pressure condition is obtained based on the recorded temperature and pressure changes of the non-magnetic sample tank and the non-magnetic core holder. The corresponding porosity is calculated based on formula (12): The test principles for the other four confining pressure conditions are the same, and the porosity under different effective stress conditions is obtained, which is recorded as The porosity and effective stress have an exponential relationship, that is, formula (13). Therefore, the effective stress is used as the horizontal axis and the The vertical coordinate can be fitted to obtain the attenuation coefficient a of the sample; Where: is the porosity, %; is the porosity at normal pressure, %; a is the attenuation coefficient, MPa -1 ; σ is the effective stress, MPa.

8. The method for testing and calculating the in-situ water saturation of a coal reservoir according to claim 1, characterized in that: In step 7), after each confining pressure condition porosity test is completed, the vacuuming process needs to be repeated to remove the residual helium in the pores.

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