CN116773578B - Method for testing and calculating in-situ water saturation of coal reservoirs - Google Patents
Method for testing and calculating in-situ water saturation of coal reservoirs Download PDFInfo
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Abstract
本发明公开一种对煤储层原位含水饱和度进行测试与计算的方法，属于非常规天然气勘探与开发技术领域。通过保压取心、现场快速测定可动水体积、束缚水核磁信号测定、水饱和、束缚水体积测定、碎块样常压条件的氦气孔隙度测试、柱塞样覆压孔隙度测试及孔隙度衰减系数拟合、原位孔隙度计算这几个步骤得到原位含水饱和度。本申请提出的“离心‑测试‑饱水‑测试”低场核磁共振测试流程可有效避免束缚水测试比例升高的误差；由于含水饱和度计算中包含了煤储层束缚水体积，且综合采用柱塞样及碎块样结合进行孔隙度测试的方法确定地层条件下的煤储层原位孔隙度，因而含水饱和度计算结果与煤储层原位特征更相近。
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, plunger sample overpressure porosity test 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
The invention relates to the technical field of unconventional natural gas exploration and development, in particular to a system and a method for testing and calculating in-situ water saturation of a coal reservoir.
Background
The pores in coal reservoirs are commonly occupied by water and natural gas, and the relationship between the water saturation and the gas saturation is quantitatively characterized at present by the water saturation and the gas saturation. Water saturation is an important parameter of concern in the development of surface drainage depressurized coalbed methane. The water content directly affects the content of adsorbed gas, free gas and water-soluble gas in the coal reservoir. In particular, the current coal bed gas development depth is gradually increased, and the free gas ratio in the deep coal reservoir is obviously increased. Therefore, the accurate measurement of the in-situ water saturation of the coal reservoir has important significance for estimating the coalbed methane resources and improving the coalbed methane development effect.
At present, the coal reservoir water saturation test thought is divided into two types, namely a first type for respectively measuring the water volume and the pore volume in a coal layer and calculating the water saturation according to the ratio of the water volume to the pore volume, and a second type for calculating the porosity and the water saturation of the coal reservoir based on the logging curve characteristics in the drilling process.
Aiming at the first test thought, the current common test method is to record the mass after a core sample is obtained through a drilling site, record the mass again through a drying/centrifuging method and the like after the core sample is transported to a laboratory, calculate the mass of water loss, calculate the volume of water by combining the density of water (normally taking the value of 1.0g/cm 3), and determine the porosity of the coal sample based on a density bottle method, a conversion method, a helium method, a low-field nuclear magnetic resonance method and the like. However, the test result obtained by the method is greatly different from the in-situ characteristic of the coal reservoir, which is mainly due to the fact that part of residual water which cannot be removed, namely bound water, still exists in the sample after centrifugation/drying, and the residual water is the main reason for low water yield in the current deep coal bed methane development process. Simply calculating the water content of the coal based on the mass change does not take into account the effect of this portion of residual bound water.
In addition, the porosity test result is also different from the in-situ condition, the current coal sample porosity test usually adopts a sample with fragment or plunger specifications, confining pressure cannot be loaded in the fragment sample porosity test process, the pore characteristics of the test result and the stratum condition are greatly different, and particularly for the high-temperature and high-pressure environment of a deep coal reservoir, the pore compression degree is extremely high, and the calculation error influence is higher. Plunger sample testing can load confining pressure, but the core obtained by on-site coring is usually large in size and cannot be directly used for laboratory plunger sample testing, so that sample preparation is needed first. The current common sample preparation mode is drilling or wire cutting, the coal sample is required to be washed by water flow in the drilling process, the initial water-containing state of the coal sample is destroyed, no external water participates in the wire cutting process, but the current time for preparing a cylindrical sample by wire cutting is long (6 hours are usually required for preparing a phi 25 multiplied by 50mm cylindrical sample at present), movable water in the coal is easy to be lost due to environmental factors such as pressure reduction in the long-time sample preparation process, and the test result can not reflect the initial water-containing characteristic of the sample. Although the porosity can be determined by the low-field nuclear magnetic resonance method, for example, the method and the device for evaluating the water saturation based on the nuclear magnetic resonance T 2 spectrum disclosed in Chinese patent CN112147172A, which are pointed out in the text, can respectively calculate the partial water nuclear magnetic resonance T 2 spectrum geometric average value T 2LMw and the nuclear magnetic resonance T 2 spectrum geometric average value T 2LMhc of crude oil, thereby realizing the water saturation evaluation by using the nuclear magnetic resonance T 2 spectrum, the measured porosity is significantly lower than that of the helium method due to the fact that the medium tested in the method is water, namely, the real pore characteristics of the coal sample are not reflected. In summary, a single test mode cannot obtain a more accurate test result.
Aiming at the second type of test thought, the specific calculation process is to firstly prepare coal samples with different water saturation in a laboratory, pertinently select different logging parameters to establish a plate, and calculate the porosity and the water saturation based on the logging result of the coal reservoir to be tested (for example, the scheme disclosed in Chinese patent CN114660128A, CN107132250A, CN111856583B, CN 107389521B). However, the influence of bound water cannot be eliminated in the process of establishing a plate by the test method, and the well logging parameters are greatly influenced by parameters such as the composition of coal and rock. Therefore, the established plate is more aimed at a small-range research area, when the research area is changed, a large amount of experimental tests need to be carried out again to establish a new plate, and at present, a general logging interpretation template is not available for different areas.
In summary, it is still significant to establish a method that can accurately measure and calculate the in situ water saturation of a coal reservoir.
Disclosure of Invention
The invention aims to solve the problems in the prior art, provides a method for testing and calculating the in-situ water saturation of a coal reservoir, adopts a combination of a plunger sample and a fragment sample to carry out the porosity test, the problems that confining pressure cannot be loaded in the fragment sample porosity test are solved, the water saturation calculation comprises the bound water volume of the coal reservoir, and the calculation result is more similar to the actual characteristics of the in-situ coal reservoir.
In order to achieve the technical aim, the method for testing and calculating the in-situ water saturation of the coal reservoir comprises the following steps of:
1) After the coal reservoir is cored and lifted to the ground, a fresh fragment sample is taken, the burial depth is recorded, the surface moisture is wiped off, the sampling quality is called as m 1, the apparent density rho 1 of the rock core is measured, the apparent volume V 1＝m1/ρ1 of the sample is calculated, a stratum water sample is collected, and the stratum water density rho 2 is tested;
2) Repeatedly centrifuging the fragment sample until the mass change is less than 0.01g, and recording the sample mass m 2 at the moment and the movable water volume V Movable device ＝(m1-m2)/ρ2;
3) Performing a low-field nuclear magnetic resonance test on the fragment sample, and recording a relaxation spectrum of T 2 after centrifuging the fragment sample;
4) Carrying out water saturation on the centrifuged fragment sample until the mass change in the sample for 24 hours is less than 0.01g, and recording the mass m 3 of the sample after water saturation;
5) Performing low-field nuclear magnetic resonance test on the sample after saturation, and recording the relaxation spectrum and nuclear magnetic porosity of the saturated state T 2 Calculating a T 2 cut-off value by combining a relaxation spectrum of T 2 after centrifugation, calculating a bound water proportion T Binding based on a relaxation spectrum of a saturated water state T 2, and calculating a bound water volume
6) Calculating the skeleton volume of the coal sample, combining the apparent volume V 1 to obtain the pore volume, and further calculating the helium porosity of the fragment sample under the normal pressure condition
7) Preparing a plunger sample, measuring and recording the mass m 1 ' and the density rho 1 ' of the plunger sample, calculating the apparent volume V 1′＝m1′/ρ1 ' of the plunger sample, performing a cover pressure test on the plunger sample, fixing pore pressure, setting a plurality of confining pressures which are gradually increased, performing helium gas porosity tests under different effective stress conditions, calculating corresponding porosities after obtaining the pore volume, and fitting by taking the effective stress as an abscissa and the porosities under different confining pressures as an ordinate to obtain the attenuation coefficient a of the sample;
8) Based on normal pressure porosity of the fragment sample Calculating attenuation coefficient a obtained by plunger sample pressure-covering porosity fitting to obtain fragment sample in-situ porosity
9) In situ water saturation calculation:
Furthermore, a helium filling passage communicated with the inside of the resonance instrument is additionally arranged on the basis of the original structure of the nuclear magnetic resonance instrument for performing the low-field nuclear magnetic resonance test, a nonmagnetic sample tank and a helium bottle are arranged on the helium filling passage, a first air valve is arranged on a connecting passage of the nonmagnetic sample tank and the helium bottle, a second air valve is arranged on a connecting passage of the nonmagnetic sample tank and the nuclear magnetic resonance instrument, and a second temperature sensor and a third pressure sensor are arranged on the nonmagnetic sample tank and used for monitoring the temperature and the pressure in the nonmagnetic sample tank.
Further, in step 1), the core is obtained from the coal reservoir by pressure maintaining and coring.
Further, in step 1), at least three fresh fragments of the core are rapidly taken from the core to the ground, and the sampling distance between different samples is increased when the samples are selected.
Further, the low field nmr and helium porosity testing process is performed in the laboratory after the sample is sealed to the laboratory or the tester is transported to the sampling site.
Further, in step 6), the fragment sample porosity is calculatedThe specific process of (2) is as follows:
The temperature of the stratum corresponding to the coal sample is calculated based on the temperature measurement of the drilling hole or the burial depth, the constant temperature zone is regulated to the temperature of the stratum, a first air valve is opened to enable a third pressure sensor to be 72.5psi (reference pressure, adjustable in test), the first air valve is closed, the temperature and the pressure of the nonmagnetic sample tank and the nonmagnetic core holder are recorded after the pressure of the nonmagnetic sample tank changes less than 0.01psi within 5min, the pressure of the nonmagnetic sample tank is P 0, the temperature is T 0, the pressure of the nonmagnetic core holder is P 1, the temperature is T 1, and the method is based on a gas state equation (1)
PV=nRTZ (1)
P0Vr＝n0RT0Z0 (2)
P1(Vs-V1)＝n1RT1Z1 (3)
Wherein n is the number of moles of gas, R is the molar gas constant, Z is the compression coefficient, V r is the volume of the nonmagnetic sample tank, cm 3;Vs is the volume of the nonmagnetic core holder, cm 3;V1 is the apparent volume of the sample, cm 3;
Opening a second air valve, recording that the pressure of the nonmagnetic sample tank is P r0, the temperature is Tr 0, the pressure of the nonmagnetic core holder is P s1, the temperature is T s1 after the pressure of the nonmagnetic core holder changes by less than 0.01psi within 5min, and the nonmagnetic core holder has the following formula (4) and formula (5):
Pr0Vr＝nr0RTr0Zr0 (4)
Ps1(Vs-(V1-Vp))＝ns1RTs1SZs1 (5)
Wherein V p is the pore volume of the sample, and cm 3,V1-Vp is the skeleton volume of the coal sample;
the combination of formula (2) and formula (4), formula (3) and formula (5) can be obtained:
Since helium is a non-adsorptive gas, then there is formula (8):
n0-nr0＝ns1-n1 (8)
Namely:
Converting formula (9) into:
the porosity of the coal sample is calculated from equation (11):
Further, in step 7), the constant temperature zone is adjusted to the formation temperature, the pore pressure is fixed to 72.5psi, 5 confining pressures which are gradually increased are set, the maximum confining pressure-pore pressure is taken as the effective stress of the formation condition, helium gas porosity tests under different effective stress conditions are carried out, the first confining pressure test point is taken 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 nonmagnetic sample tank and the nonmagnetic core holder, and the corresponding porosity is calculated based on the formula (12) The testing principle of the other four confining pressure conditions is the same, and the porosity under different effective stress conditions is obtained and correspondingly recorded asThe porosity is generally exponentially related to the effective stress (equation 13), so the effective stress is taken as the abscissa toFitting the ordinate to obtain an attenuation coefficient a of the sample;
Wherein: Porosity,%; the porosity at normal pressure,% > a is the attenuation coefficient, and MPa -1;σ is the effective stress.
Further, in step 7), the evacuation process is repeated after each confining pressure condition porosity test is completed to remove residual helium in the pores.
The beneficial effects of the invention are as follows:
1. the existing low-field nuclear magnetic resonance test flow is usually a water saturation-test-centrifugal-test, and for a coal reservoir with higher bound water proportion in pores and not yet saturated, the test result of the bound water proportion is higher due to the fact that water saturation is performed firstly;
2. According to the method, the influence of residual irreducible water in a sample on a result is considered when in-situ water saturation test and calculation of the coal reservoir, and after the steps of pressure maintaining and coring, laboratory test and fitting of the porosity of the coal reservoir under the original stratum condition (temperature and effective stress), on-site rapid determination of the movable water volume and determination of the irreducible water volume in the laboratory, the water saturation calculation result is more similar to the in-situ characteristic of the coal reservoir, and the calculation result is more accurate.
3. According to the method, the in-situ porosity of the coal reservoir under the stratum condition is determined by combining the plunger sample and the fragment sample, and compared with a mode of measuring the in-situ porosity by using a single sample, the method can solve the problem that confining pressure cannot be loaded in the fragment sample porosity test, so that the difference between an obtained test result and the pore characteristics under the stratum condition is smaller, and the calculation error is reduced;
4. The movable water volume in the field test has the characteristic of quick and convenient sampling, can be tested by only needing a small amount of fragment samples, and can compare the water saturation characteristics of different positions of the same coal sample;
5. The movable water volume is rapidly measured on site by adopting a pressure maintaining and coring mode, so that the influence of water loss on a test result caused by environmental change is avoided;
6. According to the application, the non-magnetic sample tank is connected to the nuclear magnetic resonance apparatus to be filled with helium, so that synchronous test of helium porosity and nuclear magnetic porosity can be realized;
7. The application respectively carries out two stages of testing, namely a stage of rapidly measuring the volume of movable water on site and a stage of measuring the volume of bound water in a laboratory, so that obvious change of the water-containing state of the coal sample caused by long time-consuming processes (pressure change and the like) such as wire cutting preparation of samples can be avoided by design;
8. The testing method disclosed by the application has universal applicability and is also suitable for testing and calculating the in-situ water saturation of reservoirs such as mudstones, shale, sandstones and the like.
Drawings
FIG. 1 is a nuclear magnetic resonance apparatus with helium supply lines connected thereto, wherein a 1-constant temperature zone, a 2-nonmagnetic core holder (reference size Φ25×80 mm), a 3-first pressure sensor, a 4-confining pressure loading pump, a 5-magnet, a 6-second pressure sensor, a 7-first temperature sensor, an 8-vacuum pump, a 9-vent valve, a 10-nuclear magnetic signal receiver, an 11-computer acquisition system, a 12-nonmagnetic sample tank (reference size Φ25×80 mm), a 13-helium cylinder, a 14-first gas valve, a 15-second gas valve, a 16-second temperature sensor, and a 17-third pressure sensor.
FIG. 2 is a flow chart of a system for testing and calculating in situ water saturation of a coal reservoir;
FIG. 3 is a T 2 cut-off calculation plot.
Detailed Description
The following examples further illustrate the invention but are not to be construed as limiting the invention. Modifications and substitutions to methods, procedures, or conditions of the present invention without departing from the spirit of the invention are intended to be within the scope of the present invention.
Example one method for testing and calculating in situ Water saturation of a coal reservoir
The in-situ water saturation test of the coal reservoir comprises three stages of coring site test, laboratory sample preparation and laboratory test.
The coring site test process mainly requires a high-precision balance (precision 0.0001 g), a densitometer and a portable centrifuge (reference model: SA-LX01 type portable centrifuge).
The equipment needed in the laboratory sample preparation process comprises a wire-electrode cutting machine and a water saturation instrument.
The laboratory test process mainly relies on a nuclear magnetic resonance apparatus, the nuclear magnetic resonance apparatus mainly comprises a constant temperature zone 1, a nonmagnetic core holder 2 (reference size phi 25 multiplied by 80 mm), 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, but the application is different from the conventional nuclear magnetic resonance apparatus in that a helium filling passage communicated with the interior of the resonance apparatus is additionally arranged, a nonmagnetic sample tank 12 (reference size phi 25 multiplied by 80 mm) and a helium bottle 13 are arranged on the helium filling passage, a first air valve 14 is arranged on the connecting passage of the nonmagnetic sample tank 12 and the helium bottle 13, a second air valve 15 is arranged on the connecting passage of the nonmagnetic sample tank 12 and the nuclear magnetic resonance apparatus, and a second temperature sensor 16 and a third pressure sensor 17 are arranged on the nonmagnetic sample tank 12 for monitoring the temperature and the pressure conditions in the nonmagnetic sample tank 12.
The testing and calculating process mainly involves 5 parts (see fig. 2) of coring, movable water volume testing, porosity testing, constrained water volume testing, water saturation calculating and the like, and the specific steps are as follows:
1) Sample collection and parameter recording, namely, pressure maintaining coring is adopted on a coal reservoir (movable water loss caused by pressure change is avoided), after a rock core is lifted to the ground, fresh samples (fragment samples, recorded as SX-W) are quickly taken, basic data such as burial depth and the like are recorded, moisture on the surface of the samples is wiped, the mass of the samples is m 1 by using a high-precision balance and recorded, the apparent density ρ 1 of the rock core is measured by using a densitometer and recorded, and the apparent volume V 1＝m1/ρ1 of the samples is calculated based on the mass and the density. A formation water sample can be collected on site, the formation water density is tested, ρ 2 is recorded, and if no formation water sample is collected, ρ 2 can be recorded as normal pressure water density 1.0g/cm 3.
From the recorded data, m 1＝14.1202g,ρ1＝1.37g/cm3, apparent volume V 1＝m1/ρ1＝10.3067cm3,ρ2＝1.0g/cm3.
2) And (3) measuring the movable water volume, namely centrifuging the sample by using a portable centrifuge, recording the mass of the sample again, repeating the centrifuging process until the mass change of the sample after centrifuging is less than 0.01g, recording the mass of the sample at the moment, recording the mass as m 2, and calculating the movable water volume V Movable device ＝(m1-m2)/ρ2 based on the core water loss mass change and the stratum water density.
As can be seen from the recorded data, m 2 = 13.8277g, V Movable device ＝0.2925cm3.
3) The sealed sample is transported to a laboratory. Because the bound water in the coal is bound by the pores, the change of environmental factors such as pressure and the like can not cause the loss of the bound water, and therefore, the sample can be brought back to a laboratory to finish the bound water proportion test. The sample is tightly wound and transported back to the laboratory along with the formation water sample (if collected). The subsequent low-field nuclear magnetic resonance and helium gas porosity test is mainly carried out, and the tester can be transported to the site to finish the test if the coring site conditions allow.
4) And (3) determining the bound water nuclear magnetic signals, namely closing all valves on a nuclear magnetic resonance instrument, placing a sample in a non-magnetic core holder 2, starting a nuclear magnetic signal receiver 10 for low-field nuclear magnetic resonance test, and recording a relaxation spectrum of the sample after centrifugation T 2 by a computer acquisition system 11, wherein the ordinate of the relaxation spectrum of the obtained after centrifugation T 2 is a signal generated by the coal sample and the bound water.
5) Water saturation the sample is saturated with water by means of a water saturator and water (preferably formation water) until the mass change in the sample 24h is less than 0.01g, and the mass of the sample after saturation is recorded as m 3.
As can be seen from the recorded data, m 3 = 14.2140g.
6) Determining the bound water volume, namely placing the sample after saturation in the nonmagnetic rock core holder 2 again, performing low-field nuclear magnetic resonance test, and recording the relaxation spectrum of the saturated state T 2 and the nuclear magnetic porosityThe test results show that the ordinate of the saturated state T 2 relaxation spectrum is actually a signal generated by coal sample, bound water and movable water (see figure 3), the T 2 cut-off value is calculated by combining the centrifuged T 2 relaxation spectrum, the bound water proportion is calculated based on the saturated state T 2 relaxation spectrum and is recorded as T Binding (the area proportion of the saturated state T 2 relaxation spectrum which is smaller than the T 2 cut-off value), and the bound water volume is calculated by combining the porosity and the apparent volume of the sample
7) And (3) helium porosity testing under normal pressure of the fragment sample, namely starting a vacuum pump 8 to vacuumize the fragment sample for 24 hours (discharging residual non-desorbed gas in pores to improve the porosity testing accuracy), and closing the vacuum pump 8. Based on borehole temperature measurement or depth of burial calculating the temperature of the stratum corresponding to the coal sample, and regulating the constant temperature zone 1 to the formation temperature. The first valve 14 is opened such that the third pressure sensor 17 is 72.5psi (0.5 MPa) and the first valve 14 is closed. Recording the temperature and pressure of the nonmagnetic sample tank 12 and the nonmagnetic core holder 2 after the pressure in the nonmagnetic sample tank 12 is stabilized (the pressure change is less than 0.01psi in 5 min), opening the second air valve 15, recording the temperature and pressure of the nonmagnetic sample tank 12 and the nonmagnetic core holder 2 again after the pressure in the nonmagnetic core holder 2 is stabilized (the pressure change is less than 0.01psi in 5 min), and calculating the porosity based on the recorded temperature and pressure changesThe vent valve 9 is opened to vent helium and the test sample is removed. The porosity calculation principle and the method can be seen in national standard "determination method of coal and rock physical and mechanical properties-2-4 (GB/T23561.2-4-2009)" determination of porosity and pulse attenuation permeability by shale helium method "(GB/T34533-2017). The application calculates the skeleton volume of the coal sample based on the gas state equation (formula 1) and the temperature and pressure values in the nonmagnetic sample tank 12 and the nonmagnetic core holder 2 before and after gas balance, and combines the apparent volume results calculated based on mass and density to obtain the pore volume so as to calculate the porosity. The main calculation process is that when helium is introduced into the nonmagnetic sample tank 12 and the pressure is stable, the nonmagnetic sample tank is recorded, and at the same time, the nonmagnetic sample tank has the following formula (2) and formula (3):
PV=nRTZ (1)
P0Vr＝n0RT0Z0 (2)
P1(Vs-V1)＝n1RT1Z1 (3)
Wherein n is the number of moles of gas, R is the molar gas constant, Z is the compression coefficient, V r is the volume of the nonmagnetic sample tank, cm 3;Vs is the volume of the nonmagnetic core holder, cm 3;V1 is the apparent volume of the sample, and cm 3.
When the gas in the nonmagnetic sample tank is injected into the nonmagnetic core holder and the pressure balance is achieved, recording that the pressure of the nonmagnetic sample tank at the moment is P r0, the temperature is Tr 0, the pressure of the nonmagnetic core holder at the same time is P s1, the temperature is T s1, and the nonmagnetic core holder has the following formula (4) and formula (5):
Pr0Vr＝nr0RTr0Zr0 (4)
Ps1(Vs-(V1-Vp))＝ns1RTs1SZs1 (5)
Wherein V p is the pore volume of the sample, and cm 3,V1-Vp is the skeleton volume of the coal sample.
The combination of formula (2) and formula (4), formula (3) and formula (5) can be obtained:
Since helium is a non-adsorptive gas, then there is formula (8):
n0-nr0＝ns1-n1 (8)
Namely:
Converting formula (9) into:
the porosity of the coal sample is calculated from equation (11):
8) Plunger sample coverage porosity test and porosity attenuation coefficient fitting, namely preparing a plunger sample by using an acquired large rock core and a linear cutting instrument, recording as an SX-W-column (reference dimension phi 25 multiplied by 50 mm), respectively measuring and recording the mass m 1 ' and the density rho 1 ' of the plunger sample by using a high-precision balance and a densitometer, and calculating the apparent volume V 1′＝m1′/ρ1 ' of the plunger sample. And (3) closing all valves on the nuclear magnetic resonance instrument, placing the sample in the nonmagnetic core holder 2, starting the vacuum pump 8, vacuumizing the sample for 24 hours, and closing the vacuum pump 8. Corresponding stratum stress is calculated based on actual measurement or burial depth of a hydraulic fracturing method, corresponding effective stress=confining pressure-pore pressure is equivalent in the porosity test process, and different effective stresses are formed by adopting a mode of changing confining pressure by adopting fixed pore pressure. And regulating the constant temperature zone 1 to the stratum temperature, fixing the pore pressure to 72.5psi (0.5 MPa), setting 5 confining pressures which are gradually increased, taking the maximum confining pressure-pore pressure as the effective stress of stratum conditions, carrying out helium gas porosity test under different effective stress conditions, and repeating the vacuumizing process after the porosity test of each confining pressure condition is finished so as to remove residual helium gas in the pores. Taking the first confining pressure test point as an example, pore volume (refer to step 7)) Vp' is obtained based on recorded temperature and pressure changes of the nonmagnetic sample tank, the nonmagnetic core holder, and the corresponding porosity is calculated based on formula (12) The other four confining pressure conditions have the same process, and the porosity under different effective stress conditions is obtained and correspondingly recorded asThe porosity is generally exponentially related to the effective stress (equation 13), so the effective stress is taken as the abscissa toThe attenuation coefficient a of the sample can be obtained by fitting the ordinate.
Wherein: Porosity,%; The porosity is the normal pressure, the attenuation coefficient is shown as a percent, the attenuation coefficient is shown as a MPa -1, and the effective stress is shown as a sigma.
9) In-situ porosity calculation, namely taking a fragment sample and a column sample from the same coal sample, wherein the attenuation coefficient a of the porosity increasing along with effective stress is the same, so that the normal-pressure porosity of the fragment sample is obtainedThe attenuation coefficient a obtained by plunger sample overburden porosity fitting can be calculated to obtain the in-situ porosity of the fragment sample based on the formula (13) and the effective stress of the sample stratum condition
10 In situ water saturation calculation: In the above data, only test data of one fragment sample is listed as a reference, in order to improve the accuracy of the result, 3 fresh samples (fragments) are quickly taken from a target coal sample in the actual sampling process, and since the sample for gas saturation test of a coal reservoir is usually 30cm long by on-site coring, the sampling distance is increased as much as possible when 3 samples are selected to avoid errors caused by heterogeneity (the total length of a core is 30cm, if sampling is concentrated in a range of 5cm, the test result may only represent 5cm characteristics and not represent other 25cm characteristics, correspondingly, the calculation result cannot represent the overall characteristics of the sample, the sampling interval of 3 samples can be considered to cover all the 30 cm), and the water saturation of the coal sample is obtained as an average value of the water saturation calculation result of 3 fragment samples.
The foregoing has outlined and described the basic principles, features, and advantages of the present invention. However, the foregoing is merely specific examples of the present invention, and the technical features of the present invention are not limited thereto, and any other embodiments that are derived by those skilled in the art without departing from the technical solution of the present invention are included in the scope of the present invention.
Claims (8)
1. A method for testing and calculating in-situ water saturation of a coal reservoir, comprising the steps of:
1) After the coal reservoir is cored and lifted to the ground, a fresh fragment sample is taken, the burial depth is recorded, the surface moisture is wiped off, the sampling quality is called as m 1, the apparent density rho 1 of the rock core is measured, the apparent volume V 1＝m1/ρ1 of the sample is calculated, a stratum water sample is collected, and the stratum water density rho 2 is tested;
2) Repeatedly centrifuging the fragment sample until the mass change is less than 0.01g, and recording the sample mass m 2 at the moment and the movable water volume V Movable device ＝(m1-m2)/ρ2;
3) Performing a low-field nuclear magnetic resonance test on the fragment sample, and recording a relaxation spectrum of T 2 after centrifuging the fragment sample;
4) Carrying out water saturation on the centrifuged fragment sample until the mass change in the sample for 24 hours is less than 0.01g, and recording the mass m 3 of the sample after water saturation;
5) Performing low-field nuclear magnetic resonance test on the sample after saturation, and recording the relaxation spectrum and nuclear magnetic porosity of the saturated state T 2 Calculating a T 2 cut-off value by combining a relaxation spectrum of T 2 after centrifugation, calculating a bound water proportion T Binding based on a relaxation spectrum of a saturated water state T 2, and calculating a bound water volume
6) Calculating the skeleton volume of the coal sample, combining the apparent volume V 1 to obtain the pore volume, and further calculating the helium porosity of the fragment sample under the normal pressure condition
7) Preparing a plunger sample, measuring and recording the mass m 1 ' and the density rho 1 ' of the plunger sample, calculating the apparent volume V 1′＝m1′/ρ1 ' of the plunger sample, performing a cover pressure test on the plunger sample, fixing pore pressure, setting a plurality of confining pressures which are gradually increased, performing helium gas porosity tests under different effective stress conditions, calculating corresponding porosities after obtaining the pore volume, and fitting by taking the effective stress as an abscissa and the porosities under different confining pressures as an ordinate to obtain the attenuation coefficient a of the sample;
8) Based on normal pressure porosity of the fragment sample Calculating attenuation coefficient a obtained by plunger sample pressure-covering porosity fitting to obtain fragment sample in-situ porosity
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, wherein a helium filling passage communicated with the inside of the resonator is additionally arranged on the basis of the original structure of a nuclear magnetic resonance instrument for performing low-field nuclear magnetic resonance testing, a nonmagnetic sample tank and a helium bottle are arranged on the helium filling passage, a first air valve is arranged on a connecting passage of the nonmagnetic sample tank and the helium bottle, a second air valve is arranged on a connecting passage of the nonmagnetic sample tank and the nuclear magnetic resonance instrument, and a second temperature sensor and a third pressure sensor are arranged on the nonmagnetic sample tank for monitoring the temperature and the pressure in the nonmagnetic sample tank.
3. The method for testing and calculating the in-situ water saturation of a coal reservoir according to claim 1, wherein in step 1), the core is obtained by pressure maintaining and coring on the coal reservoir.
4. The method for testing and calculating the in-situ water saturation of a coal reservoir according to claim 1, wherein in step 1), at least three fresh fragments of the core are rapidly taken from the core extracted to the ground, and the sampling distance between the samples is increased when the samples are selected.
5. The method of testing and calculating in situ water saturation of a coal reservoir of claim 1, wherein the low field nmr and helium porosity testing process is performed in a laboratory after the sample is sealed to the laboratory or the tester is transported to a sampling site.
6. The method for testing and calculating in situ water saturation of a coal reservoir of claim 1, wherein in step 6), fragment sample porosity is calculatedThe specific process of (2) is as follows:
The temperature of the stratum corresponding to the coal sample is calculated based on the temperature measurement of the drilling hole or the burial depth, the constant temperature zone is regulated to the temperature of the stratum, the first air valve is opened to enable the third pressure sensor to be 72.5psi, the first air valve is closed, the temperature and the pressure of the nonmagnetic sample tank and the nonmagnetic core holder are recorded after the pressure of the nonmagnetic sample tank changes by less than 0.01psi within 5min, at the moment, the pressure of the nonmagnetic sample tank is P 0, the temperature is T 0, the pressure of the nonmagnetic core holder is P 1, the temperature is T 1, and the method is based on a gas state equation (1)
PV=nRTZ (1)
P0Vr＝n0RT0Z0 (2)
P1(Vs-V1)＝n1RT1Z1 (3)
Wherein n is the number of moles of gas, R is the molar gas constant, Z is the compression coefficient, V r is the volume of the nonmagnetic sample tank, cm 3;Vs is the volume of the nonmagnetic core holder, cm 3;V1 is the apparent volume of the sample, cm 3;
Opening a second air valve, recording that the pressure of the nonmagnetic sample tank is P r0, the temperature is Tr 0, the pressure of the nonmagnetic core holder is P s1, the temperature is T s1 after the pressure of the nonmagnetic core holder changes by less than 0.01psi within 5min, and the nonmagnetic core holder has the following formula (4) and formula (5):
Pr0Vr＝nr0RTr0Zr0 (4)
Ps1(Vs-(V1-Vp))＝ns1RTs1SZs1 (5)
Wherein V p is the pore volume of the sample, and cm 3,V1-Vp is the skeleton volume of the coal sample;
the combination of formula (2) and formula (4), formula (3) and formula (5) can be obtained:
Since helium is a non-adsorptive gas, then there is formula (8):
n0-nr0＝ns1-n1 (8)
Namely:
Converting formula (9) into:
the porosity of the coal sample is calculated from equation (11):
7. The method for testing and calculating in-situ water saturation of a coal reservoir according to claim 1, wherein in step 7), the constant temperature zone is adjusted to the formation temperature, the pore pressure is fixed at 72.5psi, 5 confining pressures are set to be gradually increased, the maximum confining pressure-pore pressure is the effective stress of the formation condition, helium gas porosities under different effective stress conditions are tested, and the corresponding porosities are calculated based on equation (12) by taking a first confining pressure test point as an example, and obtaining the pore volume Vp' under the first confining pressure condition based on recorded temperature and pressure changes of the nonmagnetic sample tank and the nonmagnetic core holder The testing principle of the other four confining pressure conditions is the same, and the porosity under different effective stress conditions is obtained and correspondingly recorded asThe porosity is exponentially related to the effective stress, i.e. formula (13), so that the effective stress is taken as an abscissa toFitting the ordinate to obtain an attenuation coefficient a of the sample;
Wherein: Porosity,%; the porosity at normal pressure,% > a is the attenuation coefficient, and MPa -1;σ is the effective stress.
8. The method of testing and calculating the in situ water saturation of a coal reservoir of claim 1, wherein in step 7), the evacuation process is repeated after each confining pressure condition porosity test is completed to remove residual helium from the pores.
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