Method for dividing formation water of tight sandstone gas reservoir based on mercury injection and online nuclear magnetic resonance

Abstract

The application provides a method for dividing compacted sandstone gas reservoir formation water based on mercury injection and online nuclear magnetic resonance, and relates to the field of gas reservoir formation water production, and specifically comprises the following steps: obtaining compacted sandstone core standard rock samples. The core is cut into two sections, and the first section of the core is subjected to high-pressure mercury injection experiment. The nuclear magnetic resonance signal T2 spectrum curve of the core in a fully saturated water state and the area of the nuclear magnetic resonance T2 spectrum curve are scanned. Gas displacement and nuclear magnetic scanning experiment are performed. The displacement pressure difference is sequentially increased to obtain nuclear magnetic resonance correction curves under different displacement pressure differences. A water saturation-displacement pressure difference relationship curve is drawn, and the critical pressure difference for production of different types of formation water is defined according to the curve shape and slope change rule. Data processing is performed to obtain the production rule of compacted sandstone gas reservoir formation water in different states. The technical scheme of the application overcomes the problem in the prior art that the critical production pressure difference and dynamic production rule of various types of formation water in different pore size ranges cannot be quantitatively characterized.

Description

Technical Field

[0001] This invention relates to the field of formation water production in gas reservoirs, specifically to a method for classifying formation water in tight sandstone gas reservoirs based on mercury intrusion porosimetry and online nuclear magnetic resonance. Background Technology

[0002] Tight sandstone gas reservoirs possess abundant natural gas reserves and have very broad development prospects. However, this type of reservoir has a complex pore structure, exhibiting various pore types ranging from nanometer to micrometer scale. Fluid occurrence and production mechanisms completely differ in different pore structures within the same core sample, and free water, capillary water, and bound water may coexist and interconvert, making clear definition and separation difficult. Current experimental methods all have inherent limitations, resulting in the inability to effectively distinguish between them.

[0003] When testing formation water using the drying method, all pore water is evaporated, and only the total water saturation can be obtained, completely losing the ability to classify it. The nuclear magnetic resonance T2 cutoff value method is currently a commonly used method, but it relies on empirical cutoff values. For strongly heterogeneous dense sandstone, this value varies greatly, and there is a lack of universal and accurate core-specific classification methods. The centrifugation method can separate the movable and immovable fluids in high-permeability sandstone, but the extremely high capillary pressure of dense sandstone requires ultra-high-speed centrifugation, which is difficult to conduct and cannot realistically simulate the underground displacement process. Moreover, the immovable fluids obtained in the end are actually bound water and strong capillary water, which cannot effectively classify these two types of formation water.

[0004] Therefore, there is a need for a method for classifying formation water in tight sandstone gas reservoirs based on mercury injection and online nuclear magnetic resonance, which can achieve a four-level fine classification of free water, weak capillary bound water, strong capillary bound water, and bound water, and quantitatively characterize the critical production pressure difference and dynamic production law of various types of formation water within different pore sizes. Summary of the Invention

[0005] The main objective of this invention is to provide a method for classifying formation water in tight sandstone gas reservoirs based on mercury intrusion porosimetry and online nuclear magnetic resonance, in order to solve the problem that existing technologies cannot quantitatively characterize the critical production pressure differential and dynamic production patterns of various types of formation water within different pore sizes.

[0006] To achieve the above objectives, this invention provides a method for classifying formation water in tight sandstone gas reservoirs based on mercury intrusion porosimetry and online nuclear magnetic resonance, specifically including the following steps:

[0007] S1, Obtain standard rock samples of dense sandstone cores for cleaning and drying.

[0008] S2 measures the overall length, porosity, and diameter of the core sample.

[0009] S3, the core is cut into two sections and dried again.

[0010] S4. High-pressure mercury intrusion test was conducted on the first core sample to determine the capillary pressure curve of the mercury gas and obtain the cumulative pore size distribution characteristics of the core sample.

[0011] S5, scan the nuclear magnetic resonance signal T2 spectrum curve T20 and the area A0 of the nuclear magnetic resonance T2 spectrum curve of the second core under fully saturated water state.

[0012] S6. Uniformly select and set the gas drive pressure difference between the minimum and maximum displacement pressure difference as the intermediate displacement pressure difference; first, conduct gas drive experiments with the minimum displacement pressure difference, and scan and measure the T2 spectrum curve online at intervals. After the T2 spectrum curves of the second core tend to overlap, stop the gas drive and nuclear magnetic resonance scanning experiments under the current displacement pressure difference.

[0013] S7, increase the displacement pressure difference sequentially and repeat step S6. When the gas drive pressure difference reaches the maximum displacement pressure difference, perform the last set of gas drive and online NMR scanning experiments. After the T2 spectrum curves of the second core tend to overlap, stop the gas drive and NMR scanning experiments under the displacement pressure difference to obtain the NMR correction curves under different displacement pressure differences, and finally obtain the different pore fluid distribution and production.

[0014] S8. Plot the relationship curve between water saturation and displacement pressure difference, and define the critical pressure difference for water production of different types of formations based on the curve shape and slope variation.

[0015] S9. Process the data obtained from steps S4 to S8 to obtain the formation water production patterns of tight sandstone gas reservoirs under different conditions.

[0016] Further, step S5 specifically involves: measuring the mass M0 of the dried second core segment and evacuating it until the vacuum level does not change, then pressurizing it to saturate the formation aqueous solution and weighing the weight m0 of the second core segment; placing the second core segment in the holder of the online nuclear magnetic resonance displacement device and scanning the nuclear magnetic resonance signal T2 spectrum curve T20 and the area A0 of the nuclear magnetic resonance T2 spectrum curve of the second core segment under fully saturated water conditions.

[0017] Further, in step S7, obtaining the NMR correction curves under different displacement pressure differentials specifically involves: stopping the gas drive and NMR scanning experiments under the displacement pressure differential after the T2 spectrum curves of the second core segment tend to overlap; at this point, removing the second core segment, weighing its mass mf, and then drying it again at high temperature. After complete drying, the core segment is placed back into the holder of the online NMR displacement device, and the T2 spectrum curve of the second core segment matrix is ​​scanned. f And calculate the peak area A of the nuclear magnetic resonance T2 spectrum curve. f By superimposing and offsetting the second core matrix curve, the nuclear magnetic resonance correction curves under different displacement pressure differentials are obtained, and finally the distribution and production of different pore fluids are obtained.

[0018] Furthermore, the specific steps in step S8 for plotting the relationship curve between water saturation and displacement pressure difference are as follows:

[0019] Based on the corrected T2 spectrum curves and the distribution law of mercury injection pore size, the distribution law of fluid in different pore sizes under different displacement pressure differentials was obtained. At the same time, the area A of the online NMR T2 spectrum curves under different displacement pressure differentials was calculated. n The distribution characteristics of fluids with different pore sizes under different displacement pressure differences and the relationship between water saturation and displacement pressure difference were obtained; the curves showing the relationship between water saturation and displacement pressure difference were plotted.

[0020] Furthermore, in step S8, the critical pressure difference for different types of formation water production is defined based on the curve shape and slope variation law as follows: the gas drive pressure difference corresponding to the first inflection point where the slope change amplitude is greater than 40% is defined as the boundary point between the gas drive pressure difference of free water and weak capillary water; the gas drive pressure difference corresponding to the second inflection point where the slope change amplitude is greater than 40% is defined as the boundary point between the gas drive pressure difference of weak capillary water and strong capillary water; and the gas drive pressure difference corresponding to the horizontal water saturation curve is defined as the boundary point between the gas drive pressure difference of strong capillary water and bound water.

[0021] Furthermore, step S9 specifically includes the following steps:

[0022] S9.1, the capillary force of mercury is converted to obtain the gas-water capillary force curve. The conversion relationship is as follows:

[0023] ;

[0024] in, The capillary pressure of mercury, in units ; Capillary pressure of water, in units ; The surface tension of mercury, in units Values ; The surface tension of water, in units ; The contact angle between mercury and the capillary wall of the core. The contact angle between water and the capillary wall of the core.

[0025] S9.2, Perform online nuclear magnetic gas-driven water removal operation.

[0026] S9.3, using the conversion relationship between the T2 distribution curve and the pore throat radius distribution curve, the T2 distribution curve is transformed into formation water distribution curves with different pore throat radii.

[0027] S9.4 defines the critical pressure difference boundaries for different types of formation water.

[0028] Furthermore, step S9.2 specifically includes the following steps:

[0029] S9.2.1, the second core sample in a fully water-saturated state is placed in the holder of the online nuclear magnetic resonance displacement device, the basic parameters for nuclear magnetic resonance scanning are set, the nuclear magnetic resonance T2 spectrum in the fully water-saturated state is scanned, and the results are calculated. It characterizes the formation water production pattern under 100% saturated water conditions.

[0030] S9.2.2, using a backpressure valve to set a constant backpressure in the online NMR displacement device holder, based on the minimum nitrogen displacement pressure difference ΔP1, a minimum nitrogen displacement pressure is set and constant pressure displacement is performed. The T2 spectrum curve is measured online at regular intervals. Once the T2 spectrum curves of the second core sample tend to overlap, the gas drive and NMR scanning experiments under the displacement pressure difference are stopped. The formation water distribution pattern under the current state is obtained, and the peak area A1 under the ΔP1 state is calculated. Based on the water saturation formula, the water saturation S under the minimum nitrogen displacement pressure difference ΔP1 state is calculated. w1 The formula for water saturation is:

[0031] ;

[0032] Among them, A n S represents the peak area under the nth displacement pressure difference ΔPn state. wn Let be the water saturation under the ΔPn state.

[0033] S9.2.3, keeping the back pressure constant, the gas drive pressure difference is increased sequentially to calculate the displacement pressure of different nitrogen gases; multiple online scanning tests are performed in each group of experiments. After the T2 spectrum curves of the second core sample tend to overlap, the gas drive and NMR scanning experiments under the current displacement pressure difference are stopped to obtain the formation water distribution pattern under the current condition, and A is calculated. n The water saturation S under the ΔPn state is obtained based on the water saturation calculation formula. wn .

[0034] Furthermore, the specific conversion relationship between the T2 distribution curve and the pore throat radius distribution curve in step S9.3 is as follows:

[0035] ;

[0036] in, The pore size corresponding to the core sample, in units of... ; and The aperture conversion factor is dimensionless. The relaxation time corresponding to the core sample, in units of .

[0037] Furthermore, step S9.4 specifically includes the following steps:

[0038] S9.4.1, based on the obtained i-th displacement pressure difference Water saturation under the conditions ,draw and Relationship curve.

[0039] S9.4.2, Calculate the slope of the water production curve:

[0040] ;

[0041] in, For the i-th displacement pressure difference Slope of the water production curve under the condition, in units .

[0042] S9.4.3, based on the calculated slope of the water production curves under different nitrogen displacement pressure differentials, the displacement pressure differential corresponding to the inflection point where the slope change range is greater than 40% is taken as the critical production pressure differential for weak capillary water. When the gas drive pressure differential is less than the critical production pressure differential, the produced formation water is free water. The displacement pressure differential corresponding to the inflection point where the slope change range is greater than 40% is taken as the critical production pressure differential for strong capillary water. When the gas drive pressure differential is between the critical production pressure differentials for strong and weak capillary water, the formation water produced by gas drive is weak capillary water. Finally, the maximum gas drive pressure differential obtained from the capillary force curve is the critical pressure differential for bound water. When the gas drive pressure differential is greater than the critical pressure differential for bound water, no more formation water will be produced. When the gas drive pressure differential is between the critical production pressure differential for strong capillary water and the critical pressure differential for bound water, the formation water produced by gas drive is strong capillary water.

[0043] S9.4.4, based on water saturation and The relationship curves yielded the critical pressure difference for different types of formation water production and the corresponding formation water occurrence curves, thus revealing the production characteristics and patterns of different types of formation water.

[0044] The present invention has the following beneficial effects:

[0045] This invention proposes a four-level fine classification standard for formation water in tight sandstone gas reservoirs based on high-pressure mercury intrusion porosimetry and online nuclear magnetic resonance experiments, simulating formation conditions, including free water, weak capillary bound water, strong capillary bound water, and bound water. This standard can accurately evaluate the saturation of mobile fluids under in-situ conditions and quantitatively characterize the critical production pressure difference and dynamic production law of various types of formation water within different pore size ranges. Attached Figure Description

[0046] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. In the drawings:

[0047] Figure 1 The T2 spectrum variation curves of online nuclear magnetic gas drive are shown.

[0048] Figure 2 The curves showing the variation of formation water at different pore sizes are presented.

[0049] Figure 3 The curves showing the relationship between water saturation and displacement pressure difference are presented.

[0050] Figure 4 The graph shows the formation water production of different types of formations with different aperture sizes. Detailed Implementation

[0051] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0052] Example 1

[0053] A method for classifying formation water in tight sandstone gas reservoirs based on mercury intrusion porosimetry and online nuclear magnetic resonance includes the following steps:

[0054] S1, Obtain standard rock samples of dense sandstone cores for cleaning and drying.

[0055] S2 measures the basic data of the core, such as its overall length, porosity, and diameter.

[0056] S3, the core is cut into two sections and dried again.

[0057] S4. High-pressure mercury intrusion test is performed on the first core segment to determine the capillary pressure curve of the mercury gas and obtain the cumulative pore size distribution characteristics of the core. Based on the pore size distribution characteristics obtained in this step, the core NMR scanning T2 spectrum results can be converted into different pore size distribution patterns.

[0058] S5, scan the nuclear magnetic resonance signal T2 spectrum curve T20 and the area A0 of the nuclear magnetic resonance T2 spectrum curve of the second core under fully saturated water state.

[0059] S6. When the displacement pressure of the non-wetting phase is less than the threshold pressure, fluid cannot enter the core. Therefore, based on the threshold pressure obtained from mercury intrusion porosimetry of the core, the gas drive threshold pressure is obtained according to the capillary force conversion formula. Based on this, the minimum displacement pressure difference for the online NMR gas drive experiment can be set. The formation water displaced under the minimum displacement pressure difference is the free water of the core. Based on the maximum pressure obtained from mercury intrusion porosimetry, the maximum gas drive pressure difference is obtained according to the capillary force conversion formula. Based on this, the maximum displacement pressure difference for the online NMR gas drive experiment can be set. The formation water that cannot be displaced under the maximum displacement pressure difference corresponds to bound water. The gas drive pressure difference between the minimum and maximum displacement pressure differences is uniformly selected and set as the intermediate displacement pressure difference; this completes the division and setting of the experimental displacement pressure difference. First, the gas drive experiment is carried out with the minimum displacement pressure difference. The T2 spectrum curve is measured online at intervals. After the T2 spectrum curves of the second core tend to overlap, the gas drive and NMR scanning experiments under the current displacement pressure difference are stopped.

[0060] S7. Gradually increase the displacement pressure difference and repeat step S6. When the gas drive pressure difference reaches the maximum displacement pressure difference, perform the last set of gas drive and online NMR scanning experiments. Stop the gas drive and NMR scanning experiments under the displacement pressure difference after the T2 spectrum curves of the second core segment tend to overlap. At this time, remove the second core segment, weigh the core mass mf, and dry the second core segment again at high temperature. After it is completely dried, put it back into the online NMR core holder, set the confining pressure to 25 MPa, and scan the NMR T2 spectrum curve of the second core segment matrix. f And calculate the peak area A of the nuclear magnetic resonance T2 spectrum curve. f By superimposing and offsetting the second segment of the core matrix curve, corrected nuclear magnetic resonance curves under different displacement pressure differentials are obtained, such as... Figure 1 As shown, the final results show the distribution and production of fluids in different pore sizes.

[0061] S8. Plot the relationship curve between water saturation and displacement pressure difference, and define the critical pressure difference for water production of different types of formations based on the curve shape and slope variation.

[0062] S9. Process the data obtained from steps S4 to S8 to obtain the formation water production patterns of tight sandstone gas reservoirs under different conditions.

[0063] Specifically, step S5 is as follows: measure the mass M0 of the dried second core segment, and evacuate it to 0.098 MPa until the vacuum degree does not change. Then pressurize it to 25 MPa and saturate the formation aqueous solution for 96 hours, and weigh the second core segment m0. Place the second core segment in the holder of the online nuclear magnetic displacement device, set the confining pressure to 25 MPa, and scan the nuclear magnetic resonance signal T2 spectrum curve T20 and the area A0 of the nuclear magnetic resonance T2 spectrum curve of the second core segment under fully saturated water state.

[0064] Specifically, in step S7, obtaining the corrected NMR curves under different displacement pressure differentials involves: stopping the gas drive and NMR scanning experiments under the displacement pressure differential after the T2 spectrum curves of the second core segment tend to overlap; removing the second core segment, weighing its mass mf, and then drying it again at high temperature. After complete drying, the core segment is placed back into the holder of the online NMR displacement device, the confining pressure is set to 25 MPa, and the T2 spectrum curve of the second core matrix is ​​scanned. f And calculate the peak area A of the nuclear magnetic resonance T2 spectrum curve. f By superimposing and offsetting the second core matrix curve, the nuclear magnetic resonance correction curves under different displacement pressure differentials are obtained, and finally the distribution and production of different pore fluids are obtained.

[0065] Specifically, in step S8, the curve showing the relationship between water saturation and displacement pressure difference is plotted as follows:

[0066] Based on the corrected T2 spectrum curves and the distribution law of mercury injection pore size, the distribution law of fluid in different pore sizes under different displacement pressure differences is obtained. At the same time, the area of ​​the online NMR T2 spectrum curves under different displacement pressure differences is calculated, and the distribution characteristics of fluid in different pore sizes under different displacement pressure differences and the relationship between water saturation and displacement pressure difference are obtained. The curve of the relationship between water saturation and displacement pressure difference is plotted.

[0067] Specifically, in step S8, the critical pressure differentials for different types of formation water production are defined based on the curve shape and slope variation: the gas drive pressure differential corresponding to the first inflection point where the slope change is greater than 40% is defined as the boundary between free water and weak capillary water gas drive pressure differentials; the gas drive pressure differential corresponding to the second inflection point where the slope change is greater than 40% is defined as the boundary between weak capillary water and strong capillary water gas drive pressure differentials; and the gas drive pressure differential corresponding to a horizontal water saturation curve is defined as the boundary between strong capillary water and bound water gas drive pressure differentials. Based on this classification method, the formation water production patterns within different pore sizes of different core samples and the quantitative characterization of formation water production corresponding to different critical pressure differentials can be obtained.

[0068] Specifically, step S9 includes the following steps:

[0069] S9.1, the capillary force of mercury is converted to obtain the gas-water capillary force curve. The conversion relationship is as follows:

[0070] ;

[0071] in, The capillary pressure of mercury, in units ; Capillary pressure of water, in units ; The surface tension of mercury, in units Values ; The surface tension of water, in units ; The contact angle between mercury and the capillary wall of the core is given by a value of [value missing]. ; The contact angle between water and the capillary wall of the core is taken as a value. .

[0072] S9.2, Perform online nuclear magnetic gas-driven water removal operation.

[0073] S9.3, using the conversion relationship between the T2 distribution curve and the pore throat radius distribution curve, the T2 distribution curve is transformed into formation water distribution curves with different pore throat radii.

[0074] S9.4 defines the critical pressure difference boundaries for different types of formation water.

[0075] Specifically, step S9.2 includes the following steps:

[0076] S9.2.1, the second core sample in a fully water-saturated state is placed in the holder of the online nuclear magnetic resonance displacement device. The confining pressure is set to 25 MPa, the basic parameters for nuclear magnetic resonance scanning are set, and the nuclear magnetic resonance T2 spectrum in the fully water-saturated state is scanned. Calculations are then performed. It characterizes the formation water production pattern under 100% saturated water conditions.

[0077] In step S9.2.2, the back pressure of the online NMR displacement device holder is set to a constant 15 MPa using a back pressure valve. Based on the minimum nitrogen displacement pressure difference ΔP1, a minimum nitrogen displacement pressure is set and constant pressure displacement is performed. The T2 spectrum curve is measured online at regular intervals. After the T2 spectrum curves of the second core sample tend to overlap, the gas drive and NMR scanning experiments under the displacement pressure difference are stopped. The formation water distribution pattern under the current state is obtained, and the peak area A1 under the ΔP1 state is calculated. According to the water saturation formula, the water saturation S under the minimum nitrogen displacement pressure difference ΔP1 state is calculated. w1 The formula for water saturation is:

[0078] ;

[0079] Among them, A n S represents the peak area under the nth displacement pressure difference ΔPn state. wn Let be the water saturation under the ΔPn state.

[0080] S9.2.3, keeping the back pressure constant at 15 MPa, the gas drive pressure difference was increased sequentially to calculate the displacement pressure of different nitrogen gases. Multiple online scanning tests were performed in each group of experiments. Gas drive and NMR scanning experiments under the current displacement pressure difference were stopped after the T2 spectrum curves of the second core sample tended to overlap. The formation water distribution pattern under the current state was obtained, and the peak area A under the nth displacement pressure difference ΔPn was calculated. n S is obtained based on the formula for calculating water saturation. wn .

[0081] Specifically, the conversion relationship between the T2 distribution curve and the pore throat radius distribution curve in step S9.3 is as follows:

[0082] ;

[0083] in, The pore size corresponding to the core sample, in units of... ; and The aperture conversion factor is dimensionless. The relaxation time corresponding to the core sample, in units of The NMR T2 distribution curve was transformed into... Figure 2 The curves showing the formation water variation at different pore sizes are shown.

[0084] Specifically, step S9.4 includes the following steps:

[0085] S9.4.1, based on the obtained i-th displacement pressure difference Water saturation under the conditions ,draw and Relationship curve.

[0086] S9.4.2, Calculate the slope of the water production curve:

[0087] ;

[0088] in, For the i-th displacement pressure difference Slope of the water production curve under the condition, in units .

[0089] S9.4.3, based on the calculated slopes of the water production curves under different nitrogen displacement pressure differentials, the displacement pressure differential corresponding to the inflection point where the slope change range is greater than 40% is taken as the critical production pressure differential for weak capillary water. When the gas drive pressure differential is less than the critical production pressure differential, the produced formation water is free water. The displacement pressure differential corresponding to the inflection point where the slope change range is greater than 40% is taken as the critical production pressure differential for strong capillary water. When the gas drive pressure differential is between the critical production pressure differentials for strong and weak capillary water, the formation water produced by gas drive is weak capillary water. Finally, the maximum gas drive pressure differential obtained from the capillary force curve is the critical pressure differential for bound water. When the gas drive pressure differential exceeds the critical pressure differential for bound water, no more formation water will be produced. When the gas drive pressure differential is between the critical production pressure differential for strong capillary water and the critical pressure differential for bound water, the formation water produced by gas drive is strong capillary water. The critical pressure differentials are determined based on the slope of the water production curve, resulting in... Figure 3 The curve showing the relationship between water saturation and displacement pressure difference is shown.

[0090] S9.4.4, based on water saturation and The relationship curves yielded the critical pressure differentials for different types of formation water production and the corresponding formation water occurrence curves, thus revealing the production characteristics and patterns of different types of formation water, such as... Figure 4 As shown, a fine classification of bound water, strong capillary water, weak capillary water, and free water is achieved.

[0091] Example 2

[0092] S1. A cylindrical rock sample of tight sandstone core from a gas field was drilled, cleaned, and then placed in an oven to dry at 120℃ for 24 hours.

[0093] S2, the core sample was cut at the end and the length and diameter of the dried rock sample were measured. Porosity and permeability tests were also performed to obtain a standard rock sample with a flat end, a length of 8 cm, a porosity of 6.9%, and a diameter of 2.5 cm.

[0094] S3, the core was cut into two sections, each 4cm in length, and then dried again at 120℃ for 24 hours.

[0095] S4. High-pressure mercury intrusion testing was conducted on the first core sample to determine the capillary pressure curve of the mercury gas. The maximum mercury intrusion pressure was 413 MPa. The minimum pore size of the core corresponding to the mercury intrusion was calculated to be 0.002 μm through capillary force calculation. Based on the water saturation of the mercury intrusion corresponding to different mercury intrusion pressure differences, the corresponding pore throat distribution frequency curves were plotted. Based on the pore size distribution characteristics obtained in this step, the core NMR T2 spectrum curves in subsequent steps can be converted into fluid distribution curves for different pore sizes.

[0096] S5. The mass of the second dried core sample, M0 = 51.32 g, was measured. A vacuum was then applied to the core sample to 0.098 MPa, and the vacuum level was allowed to remain constant for 48 hours. Then, a simulated formation water solution was prepared based on the mineralization of the reservoir water and added to a saturation tank using a molecular pump. The tank was pressurized to formation pressure to fully saturate the core sample with formation water for 96 hours. The weight of the saturated core sample, m0 = 53.07 g, was measured. Based on the core's porosity, a weighing method was used to ensure the core was 100% saturated with the formation water solution. The saturated core sample was placed in the holder of an online nuclear magnetic resonance (NMR) displacement device. A confining pressure of 25 MPa was set, and the T2 spectrum curve (T20) and area A0 of the T2 spectrum curve were scanned under fully saturated water conditions.

[0097] Since the pore structure of the first core sample used in the mercury intrusion porosimetry experiment was destroyed, the second core sample was used for the displacement experiment.

[0098] S6. When the displacement pressure of the non-wetting phase is less than the threshold pressure, fluid cannot enter the core. Therefore, based on the mercury injection water saturation obtained from the mercury injection pressure and the threshold pressure obtained from the mercury injection pressure, the starting pressure for core gas drive, i.e., the threshold pressure for gas drive, is obtained according to the capillary force conversion formula. Based on this, the minimum displacement pressure difference for the online NMR gas drive experiment can be set. The formation water displaced under this gas drive pressure difference is the free water in the core. Based on the maximum pressure obtained from mercury injection, the maximum gas drive pressure difference is obtained according to the capillary force conversion formula. Based on this, the maximum displacement pressure difference for the online NMR gas drive experiment can be set. The formation water that still cannot be displaced under this gas drive pressure difference corresponds to bound water. The gas drive pressure difference between the minimum and maximum displacement pressure differences is uniformly selected and set as the intermediate displacement pressure difference, thus completing the division and setting of the experimental displacement pressure difference. Then, gas drive experiments were first conducted with the minimum displacement pressure difference. The T2 spectrum curves were measured online at intervals. After the T2 spectrum curves of the core tended to overlap, the gas drive and nuclear magnetic resonance scanning experiments under the current displacement pressure difference were stopped.

[0099] S7, sequentially increase the displacement pressure difference and repeat step S6. When the gas drive pressure difference reaches the maximum displacement pressure difference, perform the last set of gas drive and online NMR scanning experiments. Stop the gas drive and NMR scanning experiments at this displacement pressure difference after the T2 spectrum curves of the core tend to overlap. At this time, take out the core, weigh the core mass mf, and dry the core again at high temperature. After it is completely dried, put it back into the online NMR core holder, set the confining pressure to 25 MPa, and scan the NMR T2 spectrum curve T2 of the core matrix. f And calculate the peak area A of the nuclear magnetic resonance T2 spectrum curve. f By superimposing and offsetting the core matrix curves, nuclear magnetic resonance correction curves under different displacement pressure differentials are obtained, and finally, the distribution and production of different pore fluids are obtained.

[0100] S8. Based on the corrected T2 spectrum curve and the distribution law of mercury injection pore size, the distribution law of fluid in different pore sizes under different displacement pressure differentials can be obtained. At the same time, the area of ​​the online NMR T2 spectrum curve under different displacement pressure differentials can be calculated. Based on this, the distribution characteristics of fluid in different pore sizes under different displacement pressure differentials and the relationship between water saturation and displacement pressure differential can be obtained. By plotting the curve of water saturation and displacement pressure differential, the critical pressure differential for different types of formation water production can be defined according to the curve shape and slope variation law. Specifically, the gas drive pressure differential corresponding to the first inflection point with a large slope change is defined as the boundary point of gas drive pressure differential between free water and weak capillary water. The gas drive pressure differential corresponding to the second inflection point with a large slope change is defined as the boundary point of gas drive pressure differential between weak capillary water and strong capillary water. At the same time, the gas drive pressure differential corresponding to the horizontal water saturation curve is defined as the boundary point of gas drive pressure differential between strong capillary water and bound water. Based on this classification method, the formation water production patterns within different core pore sizes and the quantitative characterization of formation water production corresponding to different critical pressure differences can be obtained.

[0101] S9. The data obtained from steps S4 to S8 are processed to obtain the formation water occurrence patterns of tight sandstone gas reservoirs under different conditions.

[0102] S9.1, Gas-water capillary force curve conversion:

[0103] ;

[0104] in, The capillary pressure of mercury, in units ; Capillary pressure of water, in units ; The surface tension of mercury, in units Values ; The surface tension of water, in units ; The contact angle between mercury and the capillary wall of the core is given by a value of [value missing]. ; The contact angle between water and the capillary wall of the core is taken as a value. .

[0105] Based on this conversion formula, the mercury capillary force value is divided by 5 to obtain the gas-water capillary force curve, and the threshold pressure P0, median capillary pressure P1, and maximum pressure P are recorded. 2。

[0106] S9.2, Online Nuclear Magnetic Gas Flooding Water Operation Procedure:

[0107] S9.2.1 First, place the core in the fully saturated water state in the holder of the online nuclear magnetic resonance instrument, set the confining pressure to 25 MPa, set the basic parameters for nuclear magnetic resonance scanning, scan the nuclear magnetic resonance T2 spectrum in the fully saturated water state, and calculate the peak area A0 of the nuclear magnetic resonance curve in this state. Based on this, the occurrence law of formation water in the 100% saturated water state can be characterized.

[0108] In step S9.2.2, the back pressure of the clamp is set to a constant 15 MPa using a back pressure valve. Based on the minimum nitrogen displacement pressure difference ΔP1, a minimum nitrogen displacement pressure is set and constant pressure displacement is performed. The T2 spectrum curve is measured online at regular intervals. After the T2 spectrum curves of the core tend to overlap, the gas drive and NMR scanning experiments under this displacement pressure difference are stopped. The formation water distribution pattern under this condition is obtained, and the peak area A1 is calculated. According to the water saturation formula, the water saturation S under the minimum nitrogen displacement pressure difference ΔP1 condition is calculated. w1 The formula for water saturation is:

[0109] ;

[0110] Among them, A n S represents the peak area under the nth displacement pressure difference ΔPn state. wn Let be the water saturation under the ΔPn state.

[0111] S9.2.3, keeping the back pressure constant at 15 MPa, the gas drive pressure difference was increased sequentially to calculate the displacement pressure of different nitrogen gases. Multiple online scanning tests were performed in each group of experiments. Gas drive and NMR scanning experiments under that displacement pressure difference were stopped after the T2 spectrum curves of the core samples tended to overlap. The formation water distribution pattern under this condition was obtained, and the peak area A under the nth displacement pressure difference ΔPn was calculated. n S is calculated based on water saturation. wn .

[0112] S9.3, aperture conversion of NMR T2 spectrum curve:

[0113] Relationship between T2 distribution curve and pore throat radius distribution curve:

[0114] ;

[0115] in, The pore size corresponding to the core sample, in units of... ; and The aperture conversion factor is dimensionless. The relaxation time corresponding to the core sample, in units of .

[0116] The values ​​of a and b are the conversion parameters between the T2 distribution curve and the pore throat radius distribution curve of 100% saturated water in the core. By fitting this relationship, coefficients a and b are obtained, and the T2 distribution curve is transformed into formation water distribution curves with different pore throat radii.

[0117] S9.4, Determination of the critical pressure difference boundary for different types of formation water:

[0118] S9.4.1, based on the obtained i-th displacement pressure difference Water saturation under the conditions ,draw and Relationship curve.

[0119] S9.4.2, Calculate the slope of the water production curve:

[0120] ;

[0121] in, For the i-th displacement pressure difference Slope of the water production curve under the condition, in units .

[0122] S9.4.3, calculate the slope of the water production curve under different nitrogen displacement pressure differentials for this core according to the above formula, as shown in Table 1:

[0123] Table 1: Slope of permeate curves under different nitrogen displacement pressure differentials

[0124]

[0125] The displacement pressure difference corresponding to the first point with a large slope change is taken as the critical production pressure difference for weak capillary water. The pressure difference corresponding to the first point with a large slope change in this core is 0.2 MPa. When the gas drive pressure difference is less than this pressure difference, the produced formation water is free water. The displacement pressure difference corresponding to the second point with a large slope change is taken as the critical production pressure difference for strong capillary water. The pressure difference corresponding to the second point with a large slope change in this core is 2.5 MPa. When the gas drive pressure difference is between the critical production pressure difference for strong and weak capillary water, the formation water produced by gas drive is weak capillary water. Finally, the maximum gas drive pressure difference obtained from the capillary force curve is the critical pressure difference for bound water. When the pressure difference is greater than this pressure difference, no formation water will be produced when gas drive is performed. When the gas drive pressure difference is between the critical production pressure difference for strong capillary water and the critical pressure difference for bound water, the formation water produced by gas drive is strong capillary water.

[0126] S9.4.4, based on water saturation and The relationship curves yielded the critical pressure difference for different types of formation water production and the corresponding formation water occurrence curves, thus revealing the production characteristics and patterns of different types of formation water.

[0127] In summary, this invention can combine mercury intrusion porosimetry experiments with nuclear magnetic resonance T2 spectra under centrifugation and gas drive experimental conditions to accurately analyze the occurrence patterns of formation water in tight gas reservoirs under different occurrence states within different pore size ranges, laying the foundation for the development of tight gas reservoirs.

[0128] Of course, the above description is not intended to limit the present invention, and the present invention is not limited to the examples given above. Any changes, modifications, additions or substitutions made by those skilled in the art within the scope of the present invention should also fall within the protection scope of the present invention.

Claims

1. A method for dividing formation water of tight sand gas reservoir based on mercury injection and online nuclear magnetic resonance, characterized in that, Specifically comprising the following steps: S1, obtaining a standard core sample of the tight sandstone core for cleaning and drying treatment; S2, measuring the length, porosity and diameter of the core as a whole; S3, cutting the core into two segments and drying again; S4, performing a high-pressure mercury injection experiment on the first segment of the core to measure the gas-mercury capillary pressure curve and obtain the pore size cumulative distribution characteristics of the core; S5, scanning the nuclear magnetic resonance T2 spectrum curve T20 of the second segment of the core in a fully saturated water state and the area A0 of the nuclear magnetic resonance T2 spectrum curve; S6, uniformly selecting and setting the gas displacement pressure difference between the minimum displacement pressure difference and the maximum displacement pressure difference as the intermediate displacement pressure difference; first, performing a gas displacement experiment at the minimum displacement pressure difference, and measuring the T2 spectrum curve online at intervals; stopping the gas displacement and nuclear magnetic scanning experiment at the current displacement pressure difference after the T2 spectrum curve of the second segment of the core tends to overlap; S7, sequentially increasing the displacement pressure difference and repeating step S6; when the gas displacement pressure difference reaches the maximum displacement pressure difference, performing the last set of gas displacement and online nuclear magnetic scanning experiment; stopping the gas displacement and nuclear magnetic scanning experiment at the displacement pressure difference after the T2 spectrum curve of the second segment of the core tends to overlap, obtaining the nuclear magnetic resonance correction curve under different displacement pressure differences, and finally obtaining the distribution and output of different pore fluids; S8, drawing a water saturation-displacement pressure difference relationship curve, and defining the critical pressure difference of water production of different types of formation water according to the curve shape and slope change rule; S9, processing the data obtained in steps S4 to S8 to obtain the formation water production rule of the tight sandstone gas reservoir in different states; In step S8, the critical pressure difference of water production of different types of formation water is defined according to the curve shape and slope change rule: the gas displacement pressure difference corresponding to the inflection point with a first slope change amplitude greater than 40% is defined as the dividing point of the gas displacement pressure difference of free water and weak capillary water, the gas displacement pressure difference corresponding to the inflection point with a second slope change amplitude greater than 40% is defined as the dividing point of the gas displacement pressure difference of weak capillary water and strong capillary water, and the gas displacement pressure difference corresponding to the horizontal water saturation curve is defined as the dividing point of the gas displacement pressure difference of strong capillary water and bound water; Step S9 specifically comprises the following steps: S9.1, converting the capillary force of mercury to obtain a gas-water capillary force curve, and the conversion relationship is: ； wherein is the capillary pressure of mercury, units ; is the capillary pressure of water, units ; is the surface tension of mercury, units , taking values ; is the surface tension of water, units ; is the contact angle of mercury with the capillary wall of the core; is the contact angle of water with the capillary wall of the core; S9.2, performing online nuclear magnetic gas displacement water operation; S9.3, converting the T2 distribution curve into a formation water distribution curve of different pore throat radii by using the conversion relationship between the T2 distribution curve and the pore throat radius distribution curve; S9.4, determining the critical pressure difference division limit of different types of formation water; In step S9.3, the conversion relationship between the T2 distribution curve and the pore throat radius distribution curve is: ； wherein, is the pore size of the core, unit ; and is the pore size conversion factor, dimensionless; is the relaxation time of the core, unit ; Step S9.4 specifically comprises the following steps: S9.4.1, the ith obtained displacement pressure differential water saturation under the condition , plot with relationship curve; S9.4.2, calculating the slope of the water production curve: ； wherein, the i-th displacement pressure differential the slope of the water production curve in the state ; S9.4.3, according to the slope of the calculated water production curve under different nitrogen displacement pressure differences, the inflection point corresponding to the first slope change of more than 40% is taken as the critical production pressure difference of weak capillary water, when the gas displacement pressure difference is less than the critical production pressure difference, the produced formation water is free water; the inflection point corresponding to the second slope change of more than 40% is taken as the critical production pressure difference of strong capillary water, when the gas displacement pressure difference is between the critical production pressure difference of strong and weak capillary water, the produced formation water by gas displacement is weak capillary water; finally, the maximum gas displacement pressure difference obtained from the capillary force curve is the critical pressure difference of irreducible water, and when the gas displacement pressure difference is greater than the critical pressure difference of irreducible water, no formation water will be produced, when the gas displacement pressure difference is between the critical production pressure difference of strong capillary water and the critical pressure difference of irreducible water, the produced formation water by gas displacement is strong capillary water; S9.4.4, according to water saturation With The relationship curve of the critical pressure difference of different types of formation water output and the corresponding formation water occurrence curve is obtained, and the output characteristics and rules of different types of formation water are obtained.

2. The method according to claim 1, characterized in that, The step S5 is specifically: measuring the mass M0 of the dried second core section, and vacuumizing the second core section until the vacuum degree does not change, and then pressurizing to saturated formation water solution, and weighing the weight m0 of the second core section; placing the second core section in the clamping device of the online nuclear magnetic displacement device, and scanning the nuclear magnetic resonance T2 spectrum curve T20 of the second core section in the completely saturated water state and the area A0 of the nuclear magnetic resonance T2 spectrum curve.

3. The method according to claim 1, characterized in that, The NMR correction curve under different displacement pressure differentials obtained in step S7 is as follows: after the T2 spectrum curve of the second section of core tends to overlap, the gas displacement under the displacement pressure differential and the NMR scanning experiment are stopped, at this time, the second section of core is taken out, the mass mf of the second section of core is weighed, and the second section of core is again treated by high-temperature drying, after completely drying, the second section of core is re-placed in the on-line NMR displacement device holder, and the NMR T2 spectrum curve T2 of the second section of core matrix is scanned f The peak area A of the NMR T2 spectrum curve is calculated f The NMR correction curve under different displacement pressure differentials is obtained by superimposing and canceling the second section of core matrix curve, and finally the distribution and production of different pore fluids are obtained.

4. The method according to claim 1, characterized in that, The step S8 of drawing the water saturation versus displacement pressure difference curve is specifically: According to the corrected T2 spectrum curve, the distribution law of fluid in different pore diameters under different displacement pressure differences is obtained in combination with the mercury intrusion pore size distribution law, and the area of the online nuclear magnetic resonance T2 spectrum amplitude curve under different displacement pressure differences is calculated, the fluid distribution characteristics of different pore diameters under different displacement pressure differences and the relationship between water saturation and displacement pressure difference are obtained; and the water saturation versus displacement pressure difference curve is drawn.

5. The method according to claim 1, wherein, The step S9.2 specifically includes the following steps: S9.2.1, placing the second core section in the completely saturated water state in the clamping device of the online nuclear magnetic displacement device, setting the basic parameters of nuclear magnetic resonance scanning, scanning the nuclear magnetic resonance T2 spectrum in the completely saturated water state, calculating A0, and representing the production law of formation water under the state of 100% saturated water; S9.2.2, with back pressure valve set online nuclear magnetic displacement device holder back pressure constant, based on the minimum nitrogen displacement pressure difference ΔP1, set the minimum nitrogen displacement pressure and constant pressure displacement, every interval a certain time online scanning measurement T2 spectrum curve, and the second section of the core T2 spectrum curve tend to overlap after stop displacement pressure difference under the gas drive and nuclear magnetic scanning experiment, get the current state of the formation water distribution rule, and calculate the peak area A1 under the state of ΔP1, according to the water saturation formula, the minimum nitrogen displacement pressure difference ΔP1 state water saturation S w1 ; water saturation formula is: ； wherein A n is the peak area in the nth displacement pressure differential state, S wn is the water saturation in the state of ΔPn. S9.2.3, keep the back pressure constant, increase the gas drive pressure difference in turn, calculate the displacement pressure of different nitrogen; each group of experiments are tested by multiple online scanning, after the T2 spectrum curve of the second core tends to overlap, stop the current displacement pressure difference of gas drive and nuclear magnetic scanning experiment, get the formation water distribution rule under the current state, and calculate A n , according to the water saturation calculation formula, S wn is obtained.

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