A new method for water saturation correction of free gas in shale reservoirs under in situ formation conditions

Abstract

The present invention discloses a new method for correcting the water saturation of free gas in shale reservoirs under in-situ formation conditions, comprising the following steps: S1: preparing shale samples with different water saturations; S2: conducting saturated methane nuclear magnetic resonance (NMR) T2 spectrum tests under different temperature and pressure conditions to obtain saturated methane NMR T2 spectra under different temperature and pressure conditions; S3: calculating the NMR signal intensity values ​​with NMR T2 times greater than 1 ms under different temperature and pressure conditions and converting them into methane mass; S4: calculating the methane volume under landmark conditions using an equation of state; and calculating the free gas volume under different temperature and pressure conditions; S5: obtaining the correlation between water saturation and free gas volume under different temperature and pressure conditions based on the calculation results of step S4; S6: establishing a water saturation correction formula for the free gas volume in the shale reservoir and determining the fitting coefficient therein; and S7: predicting the free gas volume of the target shale reservoir based on the correction formula after determining the fitting coefficient. The present invention can obtain more accurate free gas volume under in-situ formation conditions.

Description

Technical Field

[0001] The present invention relates to the technical field of shale gas development, and in particular to a new method for correcting the water saturation of free gas in a shale reservoir under in-situ formation conditions. Background Art

[0002] Shale gas, an unconventional resource with widespread distribution and vast reserves, is a crucial successor to future oil and gas energy. After more than a decade of exploration and development, industrial shale gas flows have been discovered in multiple formations, including the Longmaxi, Dalong, Wujiaping, Shanxi, Longtan, and Qiongzhusi Formations. In particular, shale gas production in the Longmaxi Formation of the Sichuan Basin has achieved scaled-up development, and breakthrough progress has been made in the Qiongzhusi Formation, with single-well test yields reaching millions of cubic meters per day. However, 86% of the Longmaxi Formation's shale gas resources are concentrated in deep shale reservoirs at depths exceeding 3,500 meters, and the Qiongzhusi Formation's shale reservoirs are also quite deep. Due to high temperature and high pressure, the occurrence of shale gas and formation water in situ is complex and diverse, making accurate assessment of shale gas resources challenging. Exploration and development practices have confirmed that natural gas extracted from shale and coal-rock gas is primarily free methane in the formation. At the same time, free gas volume is also one of the key parameters for reserve calculation and reservoir evaluation, so it is crucial to accurately evaluate the free gas volume of shale reservoirs.

[0003] At present, there are two main methods for obtaining the free gas content of shale gas reservoirs: field test acquisition and indoor experimental test calculation. For field testing, the gas content obtained by pressure coring is considered to be the most accurate method. The average gas content of high-quality shale reservoirs obtained by pressure coring is 15.31m 3 / t. Indoor experimental tests and calculations can be divided into two categories. One is direct testing using core saturated methane nuclear magnetic resonance, which is considered to be more reliable. The other is calculation using the volumetric method:

[0004]

[0005] Where: n free is the free gas volume, cm 3 / g; P is formation pressure, MPa; T SC is the ground temperature, K; S W is water saturation, %; is the reservoir porosity, %; Z is the compressibility factor, constant; T is the formation temperature, K; P SC is the ground pressure, MPa; ρ r is the rock density, g / cm 3 .

[0006] In both laboratory testing and calculations, water saturation must be used to correct the free gas volume to obtain an accurate free gas volume. It is generally assumed that the sum of water saturation and gas saturation is 1, and the pore volume occupied by water must be deducted during free gas calculations. However, under real formation conditions, water exists in three states within the nanopores of shale reservoirs: adsorbed water, bound water, and free water. Especially in confined spaces at high temperatures and high pressures, the density of gas and water varies with the size of the nanopores. Therefore, simply dividing the mass of formation water by its density to obtain the pore volume occupied by formation water is inaccurate, resulting in large errors in the calculation of free gas volume under shale reservoir formation conditions and misestimation of resource volume. Therefore, it is urgent to establish a new method for correcting free gas volume using water saturation to obtain a more accurate free gas volume under in situ formation conditions. Summary of the Invention

[0007] In view of the above problems, the present invention aims to provide a new method for correcting the water saturation of free gas in shale reservoirs under in-situ formation conditions.

[0008] The technical solutions of the present invention are as follows:

[0009] A new method for correcting water saturation of free gas in shale reservoirs under in-situ formation conditions comprises the following steps:

[0010] S1: Preparation of shale samples with different water saturations;

[0011] S2: Perform saturated methane nuclear magnetic T2 spectrum tests on shale samples with different water saturations under different temperature and pressure conditions to obtain nuclear magnetic T2 spectra of shale samples with different water saturations under different temperature and pressure conditions;

[0012] S3: Count the nuclear magnetic signal intensity values ​​with nuclear magnetic T2 time greater than 1ms under different temperature conditions, and convert the nuclear magnetic signal intensity values ​​into methane mass using the conversion relationship between methane mass and nuclear magnetic signal intensity under each temperature condition;

[0013] S4: Based on the methane mass, using the state equation to calculate the volume of each methane mass under the landmark conditions at different temperature and pressure conditions; and dividing the volume under the landmark conditions by the mass of each shale sample to obtain the free gas volume under different temperature and pressure conditions;

[0014] S5: Based on the calculation results of step S4, a relationship diagram between water saturation and free gas content under different temperature and pressure conditions is drawn, and the correlation between water saturation and free gas content is obtained;

[0015] S6: establishing a water saturation correction formula for the free gas content of the shale reservoir based on the correlation between the water saturation and the free gas content, and determining a fitting coefficient in the water saturation correction formula for the free gas content of the shale reservoir;

[0016] S7: Predicting the free gas volume of the target shale reservoir according to the water saturation correction formula for the free gas volume of the shale reservoir after determining the fitting coefficient.

[0017] Preferably, in step S1, preparing shale samples with different water saturations specifically includes the following sub-steps:

[0018] S11: Obtain a shale sample and prepare it into a standard plug sample, dry it, weigh it and record it as m;

[0019] S12: placing the shale sample in a vacuum saturation device to saturate it with water, removing the floating water on the surface of the sample after it is completely saturated with water, and weighing it and recording it as M;

[0020] S13: The total mass M1 of the core pores is obtained by subtracting the dry weight m from the saturated weight M;

[0021] S14: Calculate different target water saturations S using the total mass M1 of the saturated core pores and the dry weight m wn The corresponding core mass m n ;

[0022] S15: Dry the shale sample until the mass is m n The water saturation S is obtained when wn shale samples.

[0023] Preferably, in step S12, the saturation conditions for water saturation are vacuum time 24 hours, pressure 25 MPa, and saturation time 48 hours.

[0024] Preferably, in step S15, the shale sample is first dried to a mass corresponding to the maximum target water saturation, and then the process proceeds to step S2 to perform saturated methane nuclear magnetic resonance T2 spectrum testing under different temperature and pressure conditions. After the test, the shale sample is further dried to a mass corresponding to the second largest target water saturation, and this process is repeated to obtain nuclear magnetic resonance T2 spectra of shale samples of each target water saturation under different temperature and pressure conditions.

[0025] Preferably, in step S2, when performing a saturated methane nuclear magnetic resonance T2 spectrum test, the pressure is increased to above 4 MPa before the experiment, and then the temperature is increased. After reaching the target temperature and target pressure required for the experiment, the nuclear magnetic resonance T2 spectrum is collected; after each experimental test is completed, the temperature is first lowered and then the pressure is reduced.

[0026] Preferably, in step S2, the nuclear magnetic T2 spectrum is tested continuously multiple times under the same conditions. When the repetition rate of the test results is higher than the target threshold, it is considered that methane has reached an equilibrium state in the core under these conditions, and the result in the equilibrium state is used as the nuclear magnetic T2 spectrum result under these conditions.

[0027] Preferably, in step S3, the conversion relationship between the methane mass and the nuclear magnetic signal intensity under the various temperature conditions is obtained by fitting the test results of step S2 on the dried shale sample.

[0028] Preferably, in step S5, the correlation between the water saturation and the free gas content is:

[0029] For shale samples with porosity less than 4.5%, there is a negative logarithmic relationship between free gas content and water saturation;

[0030] For shale samples with porosity greater than or equal to 4.5%, there is a negative linear relationship between free gas content and water saturation.

[0031] Preferably, in step S6, the water saturation correction formula for the free gas content in the shale reservoir is:

[0032]

[0033] Where: Q f is the free gas volume of shale sample, cm 3 / g; A, B, C, and D are all dimensionless fitting coefficients; S w is water saturation, %; is the reservoir porosity, %.

[0034] Preferably, each fitting coefficient is obtained by the following steps:

[0035] Analyze the correlation between fitting coefficients A, B, C, D and temperature, pressure respectively;

[0036] According to the correlation analysis results, the correlation diagrams between parameter B and pressure, and the correlation diagrams between parameter C and temperature were drawn. Nonlinear fitting was performed on A and D to obtain the following expressions for the fitting coefficients:

[0037] B=a*P+b (4)

[0038] C=c*Td (5)

[0039] A=-e+f*Tg*P+h*P 2 (6)

[0040] D=ij*Ln(T) / Tk*P (7)

[0041] Where: P is pressure, T is temperature; ak is the fitting constant;

[0042] According to the expressions of formula (4) to formula (7), the fitting coefficient values ​​under different pressure and temperature conditions are obtained, and the fitting correlation coefficient R 2 Greater than 0.9.

[0043] The beneficial effects of the present invention are:

[0044] The present invention can obtain more accurate free gas volume under in-situ formation conditions, with an accuracy improved by 12% to 63.5%. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0046] Figure 1 This is a schematic diagram of the results of saturated methane nuclear magnetic T2 spectrum under different water saturations, different temperatures, and different pressures in a specific embodiment;

[0047] Figure 2 is a schematic diagram of the correlation results between water saturation and free gas content at different temperatures and pressures in a specific embodiment;

[0048] Figure 3 Schematic diagram of the correlation results between pressure and parameter B, and between temperature and parameter C in a specific embodiment;

[0049] Figure 4 A schematic diagram showing the comparison of the free gas volume and the pressure-maintaining coring results obtained by different experimental methods in a dry state in a specific embodiment;

[0050] Figure 5 1 is a schematic diagram showing the comparison of different correction methods for water saturation of free gas under in-situ formation conditions in an experimental test of the present invention in a specific embodiment. DETAILED DESCRIPTION

[0051] The present invention is further described below with reference to the accompanying drawings and examples. It should be noted that, in the absence of conflict, the embodiments in this application and the technical features in the embodiments can be combined with each other. It should be noted that, unless otherwise specified, all technical and scientific terms used in this application have the same meanings as those commonly understood by those of ordinary skill in the art to which this application belongs. The use of similar words such as "include" or "comprising" in the present invention means that the elements or objects appearing before the word cover the elements or objects listed after the word and their equivalents, without excluding other elements or objects.

[0052] The present invention provides a new method for correcting the water saturation of free gas in a shale reservoir under in-situ formation conditions, comprising the following steps:

[0053] S1: Preparation of shale samples with different water saturations.

[0054] In a specific embodiment, preparing shale samples with different water saturations specifically includes the following sub-steps:

[0055] S11: Obtain a shale sample and prepare it into a standard plug sample, dry it, weigh it and record it as m;

[0056] S12: placing the shale sample in a vacuum saturation device for saturation with water, removing the floating water on the surface of the sample after it is completely saturated with water, weighing it and recording it as M; optionally, the saturation conditions for water saturation are vacuum time 24 hours, pressure 25 MPa, and saturation time 48 hours.

[0057] S13: The total mass M1 of the core pores is obtained by subtracting the dry weight m from the saturated weight M;

[0058] S14: Calculate different target water saturations S using the total mass M1 of the saturated core pores and the dry weight m wn The corresponding core mass m n ;

[0059] S15: Dry the shale sample until the mass is m n The water saturation S is obtained when wn shale samples.

[0060] In a specific embodiment, the shale sample is first dried to a mass corresponding to the maximum target water saturation, and then the process proceeds to step S2 to perform saturated methane nuclear magnetic resonance T2 spectrum testing under different temperature and pressure conditions. After the test, the shale sample is further dried to a mass corresponding to the second largest target water saturation, and this process is repeated to obtain the nuclear magnetic resonance T2 spectra of shale samples of each target water saturation under different temperature and pressure conditions.

[0061] S2: Saturated methane nuclear magnetic T2 spectrum tests are performed on shale samples with different water saturations under different temperature and pressure conditions to obtain nuclear magnetic T2 spectra of shale samples with different water saturations under different temperature and pressure conditions.

[0062] In one specific embodiment, when conducting a saturated methane NMR T2 spectrum test, the pressure is increased to above 4 MPa before the experiment, and then the temperature is raised. After reaching the target temperature and pressure required for the experiment, the NMR T2 spectrum is collected. In this embodiment, by first increasing the pressure, then raising the temperature, and finally reaching the target temperature and pressure before conducting the test, the water in the core is prevented from evaporating, ensuring the accuracy of the test results under the corresponding water saturation conditions.

[0063] In one specific embodiment, when performing saturated methane NMR T2 spectroscopy testing, the temperature is first lowered and then the pressure is reduced after each test. In this embodiment, by first lowering the temperature and then the pressure, water loss in the core can be prevented, preventing the shale sample from failing to reach the next target water saturation.

[0064] In a specific embodiment, multiple nuclear magnetic T2 spectra are tested continuously under the same conditions. When the repetition rate of the test results is higher than the target threshold, it is considered that methane has reached an equilibrium state in the core under these conditions, and the results in the equilibrium state are used as the nuclear magnetic T2 spectrum results under these conditions.

[0065] S3: Count the nuclear magnetic signal intensity values ​​with nuclear magnetic T2 time greater than 1ms under different temperature conditions, and convert the nuclear magnetic signal intensity values ​​into methane mass using the conversion relationship between methane mass and nuclear magnetic signal intensity under each temperature condition.

[0066] In a specific embodiment, the conversion relationship between the methane mass and the nuclear magnetic signal intensity under various temperature conditions is obtained by fitting the test results of step S2 on a dry shale sample.

[0067] S4: Based on the methane mass, the state equation is used to calculate the volume of each methane mass under the landmark conditions under different temperature and pressure conditions; and the volume under the landmark conditions is divided by the mass of each shale sample to obtain the free gas volume under different temperature and pressure conditions.

[0068] S5: Based on the calculation result of step S4, a relationship diagram between water saturation and free gas content under different temperature and pressure conditions is drawn, and the correlation between water saturation and free gas content is obtained.

[0069] In a specific embodiment, the correlation between the water saturation and the free gas content is:

[0070] For shale samples with porosity less than 4.5%, there is a negative logarithmic relationship between free gas content and water saturation;

[0071] For shale samples with porosity greater than or equal to 4.5%, there is a negative linear relationship between free gas content and water saturation.

[0072] S6: Based on the correlation between the water saturation and the free gas content, a water saturation correction formula for the free gas content of the shale reservoir is established and a fitting coefficient in the water saturation correction formula for the free gas content of the shale reservoir is determined.

[0073] In a specific embodiment, the water saturation correction formula for the free gas content in the shale reservoir is:

[0074]

[0075] Where: Q f is the free gas volume of shale sample, cm 3 / g; A, B, C, and D are all dimensionless fitting coefficients; S w is water saturation, %; is the reservoir porosity, %.

[0076] In a specific embodiment, each fitting coefficient is obtained by the following steps:

[0077] Analyze the correlation between fitting coefficients A, B, C, D and temperature, pressure respectively;

[0078] According to the correlation analysis results, the correlation diagrams between parameter B and pressure, and the correlation diagrams between parameter C and temperature were drawn. Nonlinear fitting was performed on A and D to obtain the following expressions for the fitting coefficients:

[0079] B=a*P+b (4)

[0080] C=c*Td (5)

[0081] A=-e+f*Tg*P+h*P 2 (6)

[0082] D=ij*Ln(T) / Tk*P (7)

[0083] Where: P is pressure, T is temperature; ak is the fitting constant;

[0084] According to the expressions of formula (4)-formula (7), the fitting coefficient values ​​under different pressure and temperature conditions are obtained, and the fitting correlation coefficient R 2 Greater than 0.9.

[0085] S7: Predicting the free gas volume of the target shale reservoir according to the water saturation correction formula for the free gas volume of the shale reservoir after determining the fitting coefficient.

[0086] In a specific embodiment, taking a shale reservoir as an example, the new method for correcting water saturation of free gas in a shale reservoir under in-situ formation conditions of the present invention is used to predict the free gas in the shale reservoir under in-situ formation conditions, specifically comprising the following steps:

[0087] (1) Preparation of shale samples with different water saturations;

[0088] Obtain a shale sample and prepare it into a standard plunger sample. After drying, weigh it and record it as m. Place the shale sample in a vacuum saturation device and saturate it with water (saturation conditions are vacuum time 24 hours, pressure 25 MPa, and saturation time 48 hours). After it is completely saturated with water, remove the floating water on the surface of the sample and weigh it and record it as M. Subtract the dry weight m from the saturated weight M to obtain the total mass M1 of the saturated core pores. Use the total mass M1 of the saturated core pores and the dry weight m to calculate different target water saturations S. wn (10%, 30%, 50%) corresponding core mass m n (m1, m2, m3); place the shale sample in a constant temperature box at 40°C, detect the mass change of the sample every 2 seconds, and when the mass is reduced to m3, obtain a shale sample with a water saturation of 50%, and enter step (2); after the saturated methane nuclear magnetic resonance T2 spectrum test of the shale sample with a water saturation of 50% is completed, place the sample in the constant temperature box again, and after the mass is reduced to m2, continue to complete the saturated methane nuclear magnetic resonance T2 spectrum test; samples with different water saturations only need to repeat the above operations.

[0089] (2) Shale samples with different water saturations were subjected to saturated methane nuclear magnetic T2 spectroscopy tests under different temperature and pressure conditions;

[0090] In order to prevent the evaporation of water in the core, the pressure needs to be quickly increased to above 4MPa before the experiment, and then the temperature is increased. After reaching the temperature and pressure required for the experiment, the nuclear magnetic T2 spectrum is collected. After the temperature and pressure are stable, the nuclear magnetic T2 spectrum is tested three times in succession, and the repetition rate is higher than 98%. It is believed that methane reaches equilibrium in the core under this pressure. Repeat the above operation to obtain the nuclear magnetic T2 spectra of samples with different water saturations under different temperature and pressure conditions. After each experimental test, the temperature is lowered first and then the pressure is lowered to prevent the loss of water in the core. In this embodiment, the methane nuclear magnetic T2 spectra of three water saturations (10%, 30%, 50%), three temperature points (60°C, 80°C, 100°C) and three pressure points (15MPa, 25MPa, 35MPa) are tested, and the results are as follows Figure 1 shown.

[0091] (3) Count the nuclear magnetic signal intensity values ​​with nuclear magnetic T2 time greater than 1ms under different temperature conditions, and convert the nuclear magnetic signal intensity values ​​into methane mass using the conversion relationship between methane mass and nuclear magnetic signal intensity under each temperature condition;

[0092] In this embodiment, the conversion relationship between the methane mass and the nuclear magnetic signal intensity under the various temperature conditions is obtained by fitting the test results of step (2) on the dried shale sample.

[0093] (4) Based on the methane mass, the volume of each methane mass under the landmark conditions at different temperature and pressure conditions is calculated using the state equation; and the free gas volume under the different temperature and pressure conditions is obtained by dividing the volume under the landmark conditions by the mass of each shale sample;

[0094] (5) Based on the calculation results of step (4), a relationship diagram between water saturation and free gas content under different temperature and pressure conditions is drawn. The results are as follows: Figure 2 As shown, the correlation between water saturation and free gas content was obtained;

[0095] from Figure 2 It can be seen that the correlation between water saturation and free gas content is: for shale samples with a porosity of less than 4.5%, there is a negative logarithmic relationship between free gas content and water saturation; for shale samples with a porosity of greater than or equal to 4.5%, there is a negative linear relationship between free gas content and water saturation.

[0096] (6) Based on the correlation between the water saturation and the free gas content, a water saturation correction formula for the free gas content of the shale reservoir shown in formulas (2)-(3) is established;

[0097] (7) determining a fitting coefficient of a water saturation correction formula for free gas in the shale reservoir;

[0098] For shale samples with a porosity of less than 4.5%: under the same pressure, parameter B remains unchanged. It is believed that parameter B is mainly affected by pressure, while parameter A is affected by both temperature and pressure.

[0099] For shale samples with porosity greater than 4.5%: at the same temperature, parameter C remains basically unchanged, while parameter D changes. It is believed that parameter C is mainly affected by temperature, while parameter D is affected by both temperature and pressure.

[0100] Plot the correlation between parameter B and pressure and the correlation between parameter C and temperature, and then obtain the B and C values ​​under different pressure / temperature conditions. The results are as follows Figure 3 shown.

[0101] B=0.0618*P+0.0328 (8)

[0102] C=0.0342*T-6.972 (9)

[0103] Where: P is pressure and T is temperature.

[0104] A and D are fitted nonlinearly to obtain the A and D values ​​under different pressure / temperature conditions:

[0105] A=-0.4591+0.006*T-0.0168*P+0.0001*P 2 (10)

[0106] D=1.1577-71.6746*Ln(T) / T-0.0481*P (11)

[0107] (8) predicting the free gas volume of the target shale sample using the water saturation correction formula for the free gas volume of the shale reservoir after determining the fitting coefficient;

[0108] In this embodiment, the free gas volume in the dry state measured by the experimental test method of the present invention is first compared with the calculation results of the traditional volume method and the pressure-maintaining coring results. The results are as follows: Figure 4 As shown. Figure 4 It can be seen that the free gas volume of the present invention is greater than the pressure coring result and greater than the result of the traditional free gas volume calculation method (without considering the water saturation). The dry state experimental test results of the present invention are 0.47 to 1.62 m larger than those of the pressure coring. 3 The main reason for the difference in free gas volume between the dry state and the pressure coring results is that the results are not corrected for water saturation, while the pressure coring results are based on formation conditions. The water saturation range is 14% to 40%, which is the main reason why the free gas volume in the dry state of the present invention is greater than that of the pressure coring results. The results of the traditional free gas calculation method (which does not consider water saturation) are 23% to 45% lower than those of the pressure coring results, which indirectly confirms the accuracy and reliability of the experimental testing method of the present invention.

[0109] Then, based on the test results of the free gas experimental test method of the present invention, the free gas amount was corrected using the traditional water saturation correction method and the new correction method of the present invention, and compared with the pressure-maintaining coring results. The results are as follows: Figure 5 As shown. Figure 5 It can be seen that the new correction method of the present invention is slightly smaller than the pressure coring result, and the absolute value of the difference is 0.2 to 1.69m. 3 / t, this is because the pressure core sampling results include adsorbed gas. The results of the traditional correction method are much smaller than the pressure core sampling results, with an absolute difference of 1.78 to 3.35m 3 / t, especially in low porosity and low gas content sections. The ground temperature of deep shale gas reservoirs exceeds 120℃, and the adsorbed gas volume is usually less than 2m 3 Therefore, it is believed that the new method for correcting the free gas water saturation provided by the present invention improves the prediction accuracy of free gas by 12% to 63.5%.

[0110] In summary, the present invention can more accurately obtain the free gas content of shale reservoirs under in-situ formation conditions. Compared with the existing technology, the present invention is a significant improvement.

[0111] The above description is merely a preferred embodiment of the present invention and does not constitute any form of limitation to the present invention. Although the present invention has been disclosed as a preferred embodiment as above, it is not intended to limit the present invention. Any technician familiar with this profession can make some changes or modifications to equivalent embodiments of the technical contents disclosed above without departing from the scope of the technical solution of the present invention. However, any simple modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention are still within the scope of the technical solution of the present invention.

Claims

1. A method for correcting water saturation of free gas in shale reservoirs under in-situ formation conditions, characterized in that: The following steps are involved: S1: Preparation of shale samples with different water saturations; S2: Perform saturated methane nuclear magnetic T2 spectrum tests on shale samples with different water saturations under different temperature and pressure conditions to obtain nuclear magnetic T2 spectra of shale samples with different water saturations under different temperature and pressure conditions; S3: Count the nuclear magnetic signal intensity values ​​with nuclear magnetic T2 time greater than 1ms under different temperature conditions, and convert the nuclear magnetic signal intensity values ​​into methane mass using the conversion relationship between methane mass and nuclear magnetic signal intensity under each temperature condition; S4: Based on the methane mass, using the state equation to calculate the volume of each methane mass under the landmark conditions at different temperature and pressure conditions; and dividing the volume under the landmark conditions by the mass of each shale sample to obtain the free gas volume under different temperature and pressure conditions; S5: Based on the calculation results of step S4, a relationship diagram between water saturation and free gas content under different temperature and pressure conditions is drawn, and the correlation between water saturation and free gas content is obtained; the correlation between water saturation and free gas content is: For shale samples with porosity less than 4.5%, there is a negative logarithmic relationship between free gas content and water saturation; For shale samples with porosity greater than or equal to 4.5%, there is a negative linear relationship between free gas content and water saturation; S6: establishing a water saturation correction formula for the free gas content of the shale reservoir based on the correlation between the water saturation and the free gas content, and determining a fitting coefficient in the water saturation correction formula for the free gas content of the shale reservoir; The water saturation correction formula for the free gas content in the shale reservoir is: Where: Q f is the free gas volume of shale sample, cm 3 / g; A, B, C, and D are all fitting coefficients, dimensionless; S w is water saturation, %; is the reservoir porosity, %; The fitting coefficients are obtained by analyzing the correlation between each fitting coefficient and temperature and pressure, and then fitting the parameters under different temperatures and pressures; S7: Predicting the free gas volume of the target shale reservoir according to the water saturation correction formula for the free gas volume of the shale reservoir after determining the fitting coefficient.

2. The method for correcting water saturation of free gas in shale reservoirs under in-situ formation conditions according to claim 1, characterized in that: In step S1, preparing shale samples with different water saturations specifically includes the following sub-steps: S11: Obtain a shale sample and prepare it into a standard plug sample, dry it, weigh it and record it as m; S12: placing the shale sample in a vacuum saturation device to saturate it with water, removing the floating water on the surface of the sample after it is completely saturated with water, and weighing it and recording it as M; S13: The total mass M1 of the core pores is obtained by subtracting the dry weight m from the saturated weight M; S14: Calculate different target water saturations S using the total mass M1 of the saturated core pores and the dry weight m wn The corresponding core mass m n ; S15: Dry the shale sample until the mass is m n The water saturation S is obtained when wn shale samples.

3. The method for correcting water saturation of free gas in shale reservoirs under in-situ formation conditions according to claim 2, characterized in that: In step S12, the saturation conditions for water saturation are vacuum time 24 hours, pressure 25 MPa, and saturation time 48 hours.

4. The method for correcting water saturation of free gas in shale reservoirs under in-situ formation conditions according to claim 2, characterized in that: In step S15, the shale sample is first dried to the mass corresponding to the maximum target water saturation, and then the process proceeds to step S2 to perform saturated methane nuclear magnetic resonance T2 spectrum testing under different temperature and pressure conditions. After the test is completed, the shale sample is continued to be dried to the mass corresponding to the second largest target water saturation. This process is repeated to obtain the nuclear magnetic resonance T2 spectra of shale samples of each target water saturation under different temperature and pressure conditions.

5. The method for correcting water saturation of free gas in shale reservoirs under in-situ formation conditions according to claim 1, characterized in that: In step S2, when performing a saturated methane nuclear magnetic resonance T2 spectrum test, the pressure is increased to above 4 MPa before the experiment, and then the temperature is increased. After reaching the target temperature and target pressure required for the experiment, the nuclear magnetic resonance T2 spectrum is collected; after each experimental test is completed, the temperature is first lowered and then the pressure is reduced.

6. The method for correcting water saturation of free gas in shale reservoirs under in-situ formation conditions according to claim 1, characterized in that: In step S2, the nuclear magnetic resonance T2 spectrum is tested multiple times continuously under the same conditions. When the repetition rate of the test results is higher than the target threshold, it is considered that the methane in the core has reached an equilibrium state under these conditions, and the result in the equilibrium state is used as the nuclear magnetic resonance T2 spectrum result under these conditions.

7. The method for correcting water saturation of free gas in shale reservoirs under in-situ formation conditions according to claim 1, characterized in that: In step S3, the conversion relationship between the methane mass and the nuclear magnetic signal intensity under the various temperature conditions is obtained by fitting the test results of step S2 on the dried shale sample.

8. The method for correcting water saturation of free gas in shale reservoirs under in-situ formation conditions according to claim 1, characterized in that: The fitting coefficients are obtained by the following steps: Analyze the correlation between fitting coefficients A, B, C, D and temperature, pressure respectively; According to the correlation analysis results, the correlation diagrams between parameter B and pressure, and the correlation diagrams between parameter C and temperature were drawn. Nonlinear fitting was performed on A and D to obtain the following expressions for the fitting coefficients: B=a*P+b (4) C=c*Td (5) A=-e+f*T-g * P+h*P 2 (6) D=ij*Ln(T) / Tk*P (7) Where: P is pressure, T is temperature; ak is the fitting constant; According to the expressions of formula (4) to formula (7), the fitting coefficient values ​​under different pressure and temperature conditions are obtained, and the fitting correlation coefficient R 2 Greater than 0.9.

Images