A method for characterizing the full-aperture pore size of shale based on gravimetric and NMR combined measurement

Abstract

The present invention discloses a method for characterizing the full-aperture pore size of shale based on a gravimetric method and combined NMR measurement. The method comprises: plotting a curve of the pore distribution frequency of a rock sample versus pore diameter; calculating the water-saturated porosity of the rock sample based on the weight of the rock sample after drying and the weight of the rock sample after saturation; converting the nuclear magnetic resonance signal of the nuclear magnetic resonance T2 spectrum of the rock sample after saturation into porosity components based on the water-saturated porosity of the rock sample; plotting a curve of the porosity components and the cumulative porosity value of the rock sample after saturation versus relaxation time T2 based on the porosity components; calculating a spatiotemporal conversion coefficient between the NMR relaxation time T2 value and pore size; and plotting a full-aperture pore size result diagram of the rock sample after saturation based on the curve of the porosity components and the cumulative porosity value of the rock sample after saturation versus relaxation time T2 based on the spatiotemporal conversion coefficient. The multiple combined measurement methods eliminate the phenomenon of pore size measurement range overlap, and the obtained full-aperture pore size result is more accurate.

Description

Technical Field

[0001] The present invention relates to the field of unconventional shale oil and gas reservoir exploration, and in particular to a method for characterizing the full-aperture pore size of shale based on a gravimetric method and NMR combined measurement. Background Art

[0002] As oil and gas exploration and development technologies mature, unconventional resources such as shale oil and gas are showing tremendous development potential. Characterizing pore size in shale oil and gas reservoirs plays a crucial role in shale oil exploration, development, and reserve assessment. Shale oil and gas reservoirs are characterized by widespread micro- and nano-pores, making it difficult to accurately characterize their multi-scale pore structure using traditional methods.

[0003] Currently, common methods for characterizing pore size in shale reservoirs include core observation, gas adsorption, micro-nano CT, field emission electron microscopy, mercury intrusion, and nuclear magnetic resonance. Due to limitations in testing instrumentation and measurement accuracy, a single method cannot accurately determine the full-diameter pore size distribution of shale. Existing combined measurement techniques have overlapping measurement ranges, resulting in low-precision pore size characterization results. Therefore, it is necessary to improve the combined measurement methods for shale pore size characterization. Summary of the Invention

[0004] To solve the above technical problems, the present invention proposes a method for characterizing the full-aperture pore size of shale based on the combined measurement of gravimetric method and NMR. The method can solve the technical problem that the existing combined measurement technology has overlapping measurement ranges for measuring pore size, resulting in low accuracy of the pore size characterization results given by the combined measurement.

[0005] In a first aspect, an embodiment of the present invention discloses a method for characterizing the full-aperture pore size of shale based on a combined gravimetric method and NMR measurement, comprising the following steps:

[0006] Conduct high-pressure mercury injection testing on rock samples and draw a curve showing the change of pore distribution frequency versus pore diameter.

[0007] The rock samples were dried, weighed and tested for NMR T2 spectrum;

[0008] Inject distilled water into the rock sample for saturation treatment, then weigh and test the nuclear magnetic resonance T2 spectrum of the rock sample after saturation treatment;

[0009] The water-saturated porosity of the rock sample is calculated based on the weight of the rock sample after drying and the weight after saturation treatment;

[0010] Based on the water-saturated porosity of the rock sample, converting the nuclear magnetic resonance signal of the nuclear magnetic resonance T2 spectrum after the rock sample is saturated into a porosity component;

[0011] Based on the porosity components, a curve of the porosity components and the cumulative porosity value after saturation treatment of the rock sample is drawn along with the relaxation time T2;

[0012] Calculating the time-space conversion coefficient between the nuclear magnetic resonance relaxation time T2 value and the pore size based on the curve of the pore distribution frequency of the rock sample versus the pore diameter and the nuclear magnetic resonance T2 spectrum of the rock sample after saturation treatment;

[0013] Based on the time-space conversion coefficient, a full-aperture pore size result diagram of the rock sample after saturation treatment is drawn on the basis of the curves of the porosity component and the cumulative porosity value after the rock sample is saturated with the relaxation time T2.

[0014] According to one embodiment of the present invention, the calculation formula for the water-saturated porosity of the rock sample is as follows:

[0015]

[0016] Among them, φ iw is the water-saturated porosity of the rock sample, %; m iw is the weight of the rock sample after saturation treatment, g; m i0 is the weight of the rock sample after drying, g; ρ w is the water density, g / cm 3 ;d i is the diameter of the rock sample, cm; L i is the length of the rock sample, cm.

[0017] According to one embodiment of the present invention, based on the water-saturated porosity of the rock sample, the signal amount of the nuclear magnetic resonance T2 spectrum after the rock sample saturation treatment is converted into a porosity component, and the conversion formula is as follows:

[0018]

[0019] in, is the porosity component of the rock sample after saturation treatment, %; S iw is the nuclear magnetic resonance T2 spectrum nuclear magnetic signal after rock sample saturation treatment, PU; φ iw is the water-saturated porosity of the rock sample, %; S ac,iw is the cumulative amount of T2 spectrum nuclear magnetic signal after rock sample saturation treatment, PU; S ac,id PU is the cumulative amount of nuclear magnetic signals corresponding to the first peak of the T2 spectrum after saturation treatment of the rock sample.

[0020] According to one embodiment of the present invention, the method further includes: using CT to scan rock samples with a diameter of 25 mm and a diameter of 2 mm to obtain structural characteristics of pores with a diameter greater than 1 μm, extracting data from the data body according to different pore sizes, and drawing a histogram of porosity of different pore sizes for calibrating the porosity of pores with a diameter greater than 1 μm in the full-aperture pore size result diagram after saturation treatment of the rock sample.

[0021] According to one embodiment of the present invention, the step of calibrating the porosity of pores with a diameter greater than 1 μm in the full-aperture pore size result diagram after saturation treatment of the rock sample includes: calculating the relative error between the porosity of pores of different pore sizes with a diameter greater than 1 μm in the full-aperture pore size result diagram after saturation treatment of the rock sample and the porosity of pores of different pore sizes with a diameter greater than 1 μm identified by CT scanning.

[0022] According to one embodiment of the present invention, calculating the time-space conversion coefficient between the nuclear magnetic resonance relaxation time T2 value and the pore size includes: integrating the curve of the rock sample pore distribution frequency versus pore diameter and the nuclear magnetic resonance T2 spectrum curve after saturation treatment of the rock sample in the same coordinate system, establishing a time-space conversion curve between the nuclear magnetic resonance relaxation time T2 value after saturation treatment and the pore size, and when the peak value of the nuclear magnetic signal after saturation treatment corresponds one by one to the peak value of the high-pressure mercury injection pore distribution frequency, recording the nuclear magnetic resonance relaxation time T2 and the pore diameter d value at this time to calculate the time-space conversion coefficient.

[0023] According to one embodiment of the present invention, the space-time conversion coefficient is obtained by the ratio between the nuclear magnetic resonance relaxation time T2 value and the pore diameter d value.

[0024] According to one embodiment of the present invention, injecting distilled water into the rock sample for saturation treatment includes: injecting the distilled water in the piston container into the rock sample using an ISCO pump.

[0025] According to one embodiment of the present invention, before injecting distilled water into the rock sample for saturation treatment, the rock sample is placed in a core holder and a confining pressure is applied using a hand pump.

[0026] According to one embodiment of the present invention, a hand pump is used to apply a confining pressure within a predetermined range, and then a vacuum pretreatment system is used to vacuum the rock sample for a first predetermined time.

[0027] According to one embodiment of the present invention, the predetermined range of confining pressure is 1 to 3 MPa.

[0028] According to one embodiment of the present invention, the first predetermined time for the vacuum treatment is 46 to 50 hours.

[0029] According to one embodiment of the present invention, the distilled water injection pressure and the confining pressure increase in a step-by-step manner in two adjacent time periods. In each time period, the confining pressure and the injection pressure are respectively constant and the difference between the confining pressure and the injection pressure remains constant. When the injection pressure reaches the formation pressure of the study block, the saturation process is stopped.

[0030] According to one embodiment of the present invention, the difference between the confining pressure and the injection pressure is 2 MPa.

[0031] According to one embodiment of the present invention, the saturation process is further comprised of: completing the saturation process in a constant temperature box, wherein the temperature in the constant temperature box is consistent with the formation temperature.

[0032] According to one embodiment of the present invention, the total saturation time after adding up each time period is ≥48 hours.

[0033] According to one embodiment of the present invention, the rock sample drying process includes: placing the rock sample in a vacuum oven with controllable dryness and humidity, controlling the relative humidity during drying to 40%, and drying the rock sample until the rock sample has a constant weight.

[0034] According to one embodiment of the present invention, the rock sample drying process includes: a total drying time ≤ 4 hours.

[0035] In a second aspect, an embodiment of the present invention discloses a computer-readable storage medium comprising instructions, which, when executed on a computer, cause the computer to execute the method for characterizing the full-aperture pore size of shale based on a combined gravimetric method and NMR measurement as described in any of the above embodiments, for calculating the water-saturated porosity of a rock sample;

[0036] And / or, the instructions enable the computer to perform the calculation of the porosity component in the method as described in any one of the above embodiments,

[0037] And / or, the instructions enable the computer to execute the method described in any one of the above embodiments to draw a curve of the porosity component and the cumulative porosity value of the rock sample after saturation treatment versus the relaxation time T2,

[0038] And / or, the instructions enable the computer to execute the method of calculating the spatiotemporal conversion coefficient between the nuclear magnetic resonance relaxation time T2 value and the pore size as described in any one of the above embodiments,

[0039] And / or, the instructions enable the computer to execute the method described in any one of the above embodiments to draw a full-aperture pore size result graph after the rock sample is saturated.

[0040] In a third aspect, an embodiment of the present invention discloses a system for characterizing the full-aperture pore size of shale based on a combined gravimetric method and NMR measurement, characterized in that the system comprises:

[0041] a first calculation module, which calculates the water-saturated porosity of the rock sample based on the weight of the rock sample after drying and the weight of the rock sample after saturation treatment;

[0042] a second calculation module, which converts the nuclear magnetic resonance signal of the nuclear magnetic resonance T2 spectrum of the rock sample after saturation treatment into a porosity component based on the water-saturated porosity of the rock sample;

[0043] A first construction module, wherein the first construction module is based on the porosity component to draw a curve showing a change of the porosity component and the cumulative porosity value of the rock sample after saturation treatment with a relaxation time T2;

[0044] a third calculation module, which calculates a time-space conversion coefficient between the nuclear magnetic resonance relaxation time T2 value and the pore size based on a curve showing a change in the pore distribution frequency of the rock sample as a function of the pore diameter and a nuclear magnetic resonance T2 spectrum of the rock sample after saturation treatment;

[0045] The second construction module is a construction module for drawing a full-aperture pore size result diagram after rock sample saturation treatment based on the time-space conversion coefficient and the curve of the porosity component and the cumulative porosity value after rock sample saturation treatment changing with the relaxation time T2.

[0046] By adopting the above technical solution, the present invention has at least the following beneficial effects:

[0047] The present invention provides a method for characterizing the full-aperture pore size of shale based on a gravimetric method and NMR combined measurement. The method comprises a curve of the pore distribution frequency of a rock sample tested with high-pressure mercury injection versus pore diameter, a curve of the porosity component and the cumulative porosity value versus relaxation time T2 after saturation treatment of the rock sample obtained based on the gravimetric method and NMR combined measurement, and finally, based on the spatiotemporal conversion coefficient between the shale nuclear magnetic resonance relaxation time T2 value and the pore size, a full-aperture pore size result diagram of the rock sample after saturation treatment is drawn on the basis of the curve of the porosity component and the cumulative porosity value versus relaxation time T2 after saturation treatment of the rock sample. This method achieves the characterization of the full-aperture pore size of shale, and there is no overlap in the pore size measurement ranges among the multiple combined measurement methods, so that the obtained full-aperture pore size result is more accurate. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] 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.

[0049] Figure 1 An overall schematic diagram of a rock sample used in one embodiment of the present invention;

[0050] Figure 2 A schematic diagram of an original grayscale image and a three-dimensional reconstructed image obtained by CT scanning of a rock sample disclosed in one embodiment of the present invention;

[0051] Figure 3 A histogram showing the porosity distribution of rock samples with different pore sizes disclosed in one embodiment of the present invention;

[0052] Figure 4 Schematic diagram of a curve showing the change between the mercury intrusion pore diameter and the pore distribution frequency of a rock sample disclosed in one embodiment of the present invention;

[0053] Figure 5 Schematic diagram of a curve showing the relationship between the porosity component and the cumulative value of the porosity component and the T2 relaxation time after saturation treatment of a rock sample disclosed in one embodiment of the present invention;

[0054] Figure 6 This is a schematic diagram of the integration of the curve of the change of the nuclear magnetic resonance relaxation time T2 and the nuclear magnetic signal amount of the rock sample after saturation treatment disclosed in one embodiment of the present invention and the curve of the change of the pore distribution frequency with the pore diameter in the same coordinate system;

[0055] Figure 7 A schematic diagram of the full-aperture pore size results after shale saturation treatment disclosed in one embodiment of the present invention. DETAILED DESCRIPTION

[0056] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the embodiments of the present invention are further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings.

[0057] It should be noted that all expressions using "first" and "second" in the embodiments of the present invention are for distinguishing two non-identical entities with the same name or non-identical parameters. It can be seen that "first" and "second" are only for the convenience of expression and should not be understood as limitations on the embodiments of the present invention. Subsequent embodiments will not explain this one by one.

[0058] According to a first aspect of the present invention, an embodiment of the present invention discloses a method for characterizing the full-aperture pore size of shale based on a combined gravimetric method and NMR measurement, comprising the following steps:

[0059] Conduct high-pressure mercury injection testing on rock samples and draw a curve showing the change of pore distribution frequency versus pore diameter;

[0060] The rock samples were dried, weighed and tested for NMR T2 spectrum;

[0061] Inject distilled water into the rock sample for saturation treatment, then weigh and test the nuclear magnetic resonance T2 spectrum of the rock sample after saturation treatment;

[0062] The water-saturated porosity of the rock sample is calculated based on the weight of the rock sample after drying and the weight after saturation treatment;

[0063] Based on the water-saturated porosity of the rock sample, the nuclear magnetic resonance signal of the nuclear magnetic resonance T2 spectrum after the rock sample is saturated is converted into the porosity component;

[0064] Based on the porosity components, the curves of porosity components and cumulative porosity values ​​after saturation treatment of rock samples with relaxation time T2 are drawn;

[0065] Calculating the time-space conversion coefficient between the nuclear magnetic resonance relaxation time T2 value and the pore size based on the curve of the pore distribution frequency of the rock sample versus the pore diameter and the nuclear magnetic resonance T2 spectrum of the rock sample after saturation treatment;

[0066] Based on the time-space conversion coefficient, the full-aperture pore size result diagram after rock sample saturation treatment is drawn on the basis of the curves of porosity components and cumulative porosity values ​​changing with relaxation time T2 after rock sample saturation treatment.

[0067] In embodiments of the present invention, the NMR T2 spectrum is a curve showing how the NMR signal varies with the relaxation time T2. By measuring the NMR T2 spectrum of a rock sample after drying and comparing it with the NMR T2 spectrum of the rock sample after saturation, it can be determined that the first peak in the NMR T2 spectrum of the rock sample in both conditions represents the organic matter signal peak. Therefore, when subsequently calculating the spatiotemporal conversion coefficient between the NMR relaxation time T2 value and pore size, the relaxation time T2 value corresponding to the first peak is discarded, and calculation of the spatiotemporal conversion coefficient begins with the relaxation time corresponding to the second peak.

[0068] In some embodiments, the water-saturated porosity of a rock sample is calculated as follows:

[0069]

[0070] Among them, φ iw is the water-saturated porosity of the rock sample, %; m iw is the weight of the rock sample after saturation treatment, g; m i0 is the weight of the rock sample after drying, g; ρ w is the water density, g / cm 3 ;d i is the diameter of the rock sample, cm; L i is the length of the rock sample, cm.

[0071] In some embodiments, based on the water-saturated porosity of the rock sample, the signal amount of the nuclear magnetic resonance T2 spectrum after the rock sample is saturated is converted into a porosity component. The conversion formula is as follows:

[0072]

[0073] in, is the porosity component of the rock sample after saturation treatment, %; S iw is the nuclear magnetic resonance T2 spectrum nuclear magnetic signal after rock sample saturation treatment, PU; φ iw is the water-saturated porosity of the rock sample, %; S ac,iw is the cumulative amount of T2 spectrum nuclear magnetic signal after rock sample saturation treatment, PU; S ac,id PU is the accumulated nuclear magnetic signal corresponding to the first peak of the T2 spectrum after the rock sample is saturated.

[0074] In some embodiments, the method for characterizing the full-aperture pore size of shale using a combined gravimetric and NMR method further includes: using CT to scan rock samples with diameters of 25 mm and 2 mm to obtain structural characteristics of pores with diameters greater than 1 μm, extracting data from the data volume based on different pore sizes, and plotting a histogram of porosity for different pore sizes for use in calibrating the porosity of pores with diameters greater than 1 μm in the full-aperture pore size result graph after saturation treatment of the rock sample. In this embodiment, for the 25 mm diameter rock sample, the CT scanning accuracy is set to the micrometer level; for the 2 mm diameter rock sample, the CT scanning accuracy is set to the nanometer level.

[0075] In some embodiments, the step of calibrating the porosity of pores with diameters greater than 1 μm in the full-pore pore size result graph after saturation of the rock sample includes calculating the relative error between the porosity of pores with diameters greater than 1 μm at different pore sizes in the full-pore pore size result graph after saturation of the rock sample and the porosity of pores with diameters greater than 1 μm at different pore sizes identified by CT scanning. In this embodiment, if the relative error is less than 10%, it indicates that the method for characterizing the full-pore pore size of shale based on combined gravimetric and NMR measurement disclosed in this embodiment of the present invention is highly reliable.

[0076] In the above embodiment, during high-pressure mercury injection testing, the macropore data obtained from the high-pressure mercury injection test has large errors due to the difficulty in determining the initial point of mercury injection. Therefore, the porosity of macropores (diameter > 1 μm) of different pore sizes in the full-aperture pore size result diagram after the rock sample is saturated can be corrected in combination with the CT scanning test results. Compared with NMR technology, the pore size range of CT scanning testing is limited. By using the porosity distribution data of pores of different sizes obtained by CT scanning to calibrate the porosity of macropores (diameter > 1 μm) of different pore sizes in the full-aperture pore size result diagram after the rock sample is saturated, the method for characterizing the full-aperture pore size of shale based on the combined gravimetric method and NMR measurement disclosed in the embodiment of the present invention has a self-correction function, thereby ensuring that the full-aperture pore size results of shale obtained after the combined measurement are more accurate.

[0077] In some embodiments, calculating the nuclear magnetic resonance relaxation time T2 value and the time-space conversion coefficient of the pore size includes: integrating the curve of the rock sample pore distribution frequency versus pore diameter and the nuclear magnetic resonance T2 spectrum curve after saturation treatment of the rock sample in the same coordinate system, establishing a time-space conversion curve between the nuclear magnetic resonance relaxation time T2 value after saturation treatment and the pore size, and when the peak value of the nuclear magnetic signal after saturation treatment corresponds one by one to the peak value of the high-pressure mercury injection pore distribution frequency, recording the nuclear magnetic resonance relaxation time T2 and the pore diameter d value at this time to calculate the time-space conversion coefficient.

[0078] In some embodiments, the space-time conversion coefficient is obtained by the ratio between the NMR relaxation time T2 value and the pore diameter d value.

[0079] In some embodiments, injecting distilled water into the rock sample for saturation treatment includes: injecting distilled water in a piston container into the rock sample using an ISCO pump. This embodiment facilitates better injection of distilled water into the pores of the rock sample.

[0080] In some embodiments, before injecting distilled water into the rock sample for saturation treatment, the rock sample is placed in a core holder and subjected to confining pressure using a hand pump. In this embodiment, applying confining pressure helps prevent the injected distilled water from escaping around the rock sample instead of entering the rock sample, which could result in unsatisfactory saturation treatment.

[0081] In some embodiments, a hand pump is used to apply a confining pressure within a predetermined range, and then a vacuum pretreatment system is used to vacuum the rock sample for a first predetermined time. In this embodiment, vacuuming the rock sample can improve the saturation treatment effect of the rock sample, allowing the distilled water to better and more fully penetrate the rock sample, thereby improving the accuracy of subsequent measurement data and facilitating the acquisition of more accurate full-aperture pore size results.

[0082] In some embodiments, the predetermined range of confining pressure is 1-3 MPa. In other embodiments, the predetermined range of confining pressure is preferably 2 MPa, which can achieve a better effect of saturation treatment of the rock sample.

[0083] In some embodiments, the first predetermined time for vacuum treatment is 46 to 50 hours, which can improve the effect of rock sample saturation treatment and allow distilled water to better and more fully enter the rock sample, thereby facilitating the acquisition of more accurate full-aperture pore size results.

[0084] In some embodiments, the distilled water injection pressure and the confining pressure are increased in a step-by-step manner in two adjacent time periods, and the confining pressure and the injection pressure are respectively constant in each time period and the difference between the confining pressure and the injection pressure is kept constant. When the injection pressure reaches the formation pressure of the study block, the saturation process is stopped. In this embodiment, the confining pressure and the injection pressure are respectively constant in a time period (for example, 30 minutes) and the difference between the confining pressure and the injection pressure is kept constant for a period of time for saturation treatment, and then the confining pressure and the injection pressure are increased by a certain amount in the next time period and the difference between the confining pressure and the injection pressure in the time period is also kept constant (the same as the difference in the previous time period), until the injection pressure reaches the formation pressure of the study block and the saturation process is stopped. The above method can make the distilled water enter the rock sample better and more fully, which is beneficial to the accuracy of subsequent measurement data and to obtain more accurate full-aperture pore size results.

[0085] In some embodiments, the difference between the confining pressure and the injection pressure is 2 MPa. In this embodiment, the difference between the confining pressure and the injection pressure of 2 MPa can enable the distilled water to enter the rock sample better and more fully, thereby improving the accuracy of subsequent measurement data and facilitating the acquisition of more accurate full-aperture pore size results.

[0086] In some embodiments, the method for characterizing shale full-aperture pore size using a combined gravimetric and NMR method further includes performing the saturation process in a constant temperature chamber, wherein the temperature within the constant temperature chamber is consistent with the formation temperature. This simulates the formation environment in which the rock sample resides, thereby improving the effectiveness of the sample saturation treatment and obtaining more accurate full-aperture pore size results.

[0087] In some embodiments, the total saturation time after adding up each time period is ≥ 48 hours, so as to allow the distilled water to better and more fully penetrate the rock sample, thereby improving the accuracy of subsequent measurement data and obtaining a more accurate full-aperture pore size result.

[0088] In some embodiments, the rock sample is dried by placing the rock sample in a vacuum oven with controllable relative humidity, controlling the relative humidity to 40%, and drying the rock sample until the rock sample reaches a constant weight. This embodiment facilitates sufficient drying of the rock sample.

[0089] In some embodiments, drying the rock sample includes: a total drying time ≤ 4 hours.

[0090] According to a second aspect of the present invention, an embodiment of the present invention discloses a computer-readable storage medium comprising instructions, which, when executed on a computer, cause the computer to execute the method for characterizing the full-pore size of shale based on a gravimetric method and NMR combined measurement as described in the above embodiment to calculate the water-saturated porosity of a rock sample;

[0091] and / or, the instructions enable the computer to perform the calculation of the porosity component in any of the methods of the above embodiments,

[0092] And / or, the instructions enable the computer to execute the method of any one of the above embodiments to draw a curve of the porosity component and the cumulative porosity value of the rock sample after saturation treatment versus the relaxation time T2,

[0093] And / or, the instructions enable the computer to execute the method of calculating the spatiotemporal conversion coefficient between the nuclear magnetic resonance relaxation time T2 value and the pore size as in any one of the above embodiments,

[0094] And / or, the instructions enable the computer to execute the method of any one of the above embodiments to draw a full-aperture pore size result diagram after the rock sample is saturated.

[0095] According to a third aspect of the present invention, an embodiment of the present invention discloses a system for characterizing the full-pore-size of shale based on the method for characterizing the full-pore-size of shale based on the combined measurement of gravimetric and NMR as described in the above embodiment, the system comprising:

[0096] a first calculation module, which calculates the water-saturated porosity of the rock sample based on the weight of the rock sample after drying and the weight of the rock sample after saturation treatment;

[0097] a second calculation module, which converts the nuclear magnetic resonance signal of the nuclear magnetic resonance T2 spectrum of the rock sample after saturation treatment into a porosity component based on the water-saturated porosity of the rock sample;

[0098] A first construction module, based on the porosity component, is used to draw a curve showing the porosity component and the cumulative porosity value of the rock sample after saturation treatment versus relaxation time T2;

[0099] A third calculation module is used to calculate the time-space conversion coefficient between the nuclear magnetic resonance relaxation time T2 value and the pore size based on the curve of the pore distribution frequency of the rock sample versus the pore diameter and the nuclear magnetic resonance T2 spectrum of the rock sample after saturation treatment;

[0100] The second construction module is a construction module for drawing a full-aperture pore size result diagram of the rock sample after saturation treatment based on the time-space conversion coefficient and the curve of the porosity component and the cumulative porosity value after saturation treatment of the rock sample with the relaxation time T2.

[0101] The present invention is described below with reference to specific examples. It should be noted that the examples are merely illustrative and do not limit the present invention in any way.

[0102] 1. Rock sample preparation

[0103] Two standard plunger shale samples from the same coring layer in the study block were selected and numbered 1# and 2#. The core length was 6 cm, and the 2# sample was used as a control sample to conduct the same experiment as the 1# sample. The 1# sample was cut into three cores with lengths of 1 cm, 1.5 cm and 3.5 cm using wire cutting, and numbered 1-1, 1-2 and 1-3 for standby use. A 2mm diameter sample was drilled from the 1-1 sample for CT scanning test, the 1-2 sample was used for high-pressure mercury injection test, and the 1-3 sample was used for CT scanning test and nuclear magnetic resonance test. The processing process of the 2# sample was the same as that of the 1# sample, wherein a 2mm diameter sample was drilled from the 2-1 sample for CT scanning test, the 2-2 sample was used for high-pressure mercury injection test, and the 2-3 sample was used for CT scanning test and nuclear magnetic resonance test. The whole of the experimental rock sample is attached. Figure 1 Under the conditions of the present invention, the test results of rock samples 1# and 2# are highly consistent, so the subsequent implementation process of the embodiment is described based on the results of rock sample 1#.

[0104] 2. CT scan test

[0105] The prepared shale plug samples of 1-3 and 2-3 with a diameter of 25 mm and the shale plug samples of 1-1 and 2-1 with a diameter of 2 mm were dried according to GB / T 29172-2012 "Core Analysis Methods". CT scanning of the 25 mm and 2 mm diameter samples was then used to characterize the structural characteristics of pores with a diameter greater than 1 μm in the cores of the study area. Figure 2 The original grayscale image and 3D reconstruction image obtained by CT scanning of rock samples 1-3 with a diameter of 25 mm are shown. Data are extracted from the data body according to different pore sizes, and the distribution bar graph of porosity of different pore sizes is drawn (such as Figure 3 Table 1 below is a statistical table of pore size distribution of rock samples 1-1 (2 mm plunger rock sample) and 1-3 (25 mm plunger rock sample) obtained by three-dimensional CT scanning.

[0106] Table 1 Statistics of pore size distribution of rock samples by 3D CT scanning

[0107]

[0108] 3. High-pressure mercury injection test

[0109] The prepared 1-2 and 2-2 shale samples were dried according to GB / T 29172-2012 "Core Analysis Methods". The dried samples were subjected to high-pressure mercury injection testing. The pore distribution frequency versus pore diameter curve was plotted under semi-logarithmic coordinate conditions to determine the pore distribution characteristics of the shale samples. The mercury injection pore distribution frequency of sample 1-2 is shown in the attached figure. Figure 4 shown.

[0110] 4. Gravimetric method + nuclear magnetic resonance test

[0111] 1) Shale samples 1-3 and 2-3 with a diameter of 25 mm were dried and weighed according to GB / T 29172-2012 "Core Analysis Method";

[0112] 2) Testing the nuclear magnetic resonance T2 spectrum of the dried shale sample;

[0113] 3) placing the standard rock samples 1-3 and 2-3 after step 2) in a core holder, applying a confining pressure of 2 MPa using a hand pump, and then vacuuming the standard rock samples using a vacuum pretreatment system for 48 hours;

[0114] 4) Place the distilled water for the experiment in a piston container and use an ISCO pump to inject the distilled water in the piston container into the core holder rock sample containing the 1-3 and 2-3 rock samples. During the saturation process, the distilled water injection pressure and the confining pressure are increased in a step-by-step manner in two adjacent time periods. In each time period, the confining pressure and the injection pressure are respectively constant, and the difference between the confining pressure and the injection pressure is 2 MPa. When the injection pressure reaches the formation pressure of the study block, the saturation process is stopped. The rock sample saturation process is completed in a thermostat, and the temperature of the thermostat is consistent with the formation temperature. The total saturation time is not less than 48 hours.

[0115] 5) Remove the saturated rock samples 1-3 and 2-3 from the core holder and record their masses, and measure the nuclear magnetic resonance T2 spectra of the rock samples after saturation treatment;

[0116] 6) Experimental data processing and analysis.

[0117] (1) Establishment of the calibration relationship between the nuclear magnetic resonance T2 spectrum nuclear magnetic signal and shale water-saturated porosity

[0118] ① Calculate the water-saturated porosity of shale samples 1-3 and 2-3 based on the weight method. The calculation formula is as follows:

[0119]

[0120] Among them, φ iw is the water-saturated porosity of the i# rock sample, %; m iw is the weight of the i# rock sample after saturation treatment, g; m i0 is the weight of the i#th rock sample after drying, g; ρ w is the water density, g / cm 3 ;d i is the diameter of the i# rock sample, cm; L i is the length of the i#th rock sample, cm.

[0121] Taking rock sample 1# as an example, the water-saturated porosity is calculated as 10.89% using formula 1.

[0122] ② Based on the nuclear magnetic signal of the nuclear magnetic resonance T2 spectrum after saturation treatment of the rock sample, the nuclear magnetic signal of the T2 spectrum after saturation treatment of the shale is converted into the porosity component according to formula (1). The conversion formula is as follows:

[0123]

[0124] in, is the porosity component of the i# rock sample after saturation treatment, %; S iw is the nuclear magnetic resonance T2 spectrum nuclear magnetic signal of the i# rock sample after saturation treatment, PU; φ iw is the water-saturated porosity of the i# rock sample, %; S ac,iw is the cumulative amount of T2 spectrum nuclear magnetic signal after saturation treatment of the i# rock sample, PU; S ac,id PU is the cumulative nuclear magnetic signal corresponding to the first peak of the T2 spectrum of the i#th rock sample after saturation treatment.

[0125] Taking rock sample 1# as an example, the signal-porosity conversion coefficient obtained by formula 2 is 0.005634.

[0126] ③ Based on the porosity component obtained in step ②, draw the relationship curve between the porosity component after shale saturation treatment and the cumulative value of the porosity component and the T2 relaxation time, see the attached Figure 5 .

[0127] (2) Calculation of NMR relaxation time T2 value and pore size space-time conversion coefficient

[0128] ① Extract the NMR relaxation time T2 and NMR signal data of 1-3 and 2-3 rock samples after saturation treatment, and draw the NMR signal variation curve with relaxation time T2 under semi-logarithmic coordinate conditions. Figure 6 .

[0129] ② Extract the pore diameter and pore distribution frequency data from the high-pressure mercury injection test results of rock samples 1-2 and 2-2, and draw a curve of pore distribution frequency versus pore diameter under semi-logarithmic coordinate conditions. See the attached Figure 6 .

[0130] ③ Integrate the curve data of steps ① and ② above into the same coordinate system to establish the time-space conversion curve between NMR and high-pressure mercury injection pore distribution frequency, where the bottom coordinate of the X-axis is the pore diameter, the main coordinate of the Y-axis is the pore distribution frequency; the top coordinate of the X-axis is the relaxation time T2, and the secondary coordinate of the Y-axis is the NMR signal quantity, see the attached Figure 6 .

[0131] ④ After saturation treatment of the shale sample, the first peak of the NMR T2 spectrum represents the organic matter signal. Therefore, the time-space conversion coefficient calculation starts with the relaxation time corresponding to the second peak. When the NMR signal peak value corresponds to the high-pressure mercury injection pore distribution frequency peak value one by one, the NMR relaxation time T2 and pore diameter d value at this time are recorded, and then the time-space conversion coefficient between the NMR relaxation time T2 value and the pore size is calculated.

[0132] Taking rock sample 1# as an example, the time-space conversion coefficient established by nuclear magnetic resonance and high-pressure mercury injection (i.e., the time-space conversion coefficient between the nuclear magnetic resonance relaxation time T2 value and the pore size) is 18.97nm / ms.

[0133] ⑤ Based on the time-space conversion coefficient of 18.97 nm / ms determined in step ④, the NMR porosity component curve + porosity cumulative curve (see attached Figure 5 ) on the basis of which the full-aperture pore size results of the rock sample after saturation treatment were drawn (see Appendix Figure 7 ).

[0134] Based on the full-aperture pore size results after saturation treatment of the rock sample, the total porosity of macropores (diameter > 1 μm) and the porosity values ​​of different pore sizes of macropores (diameter > 1 μm) can be determined (i.e., the porosity of different pore sizes of macropores (diameter > 1 μm) identified by NMR can be determined). By comparing the porosity of different pore sizes of macropores (diameter > 1 μm) identified by NMR with the porosity of different pore sizes of macropores (diameter > 1 μm) identified by CT, the relative error of the porosity of different pore sizes of macropores (diameter > 1 μm) determined by these two methods was calculated (as shown in Table 2 below). In the above example, the relative error is approximately 9.10%, indicating the reliability of the method for characterizing the full-aperture pore size of shale based on the combined gravimetric and NMR method established in this embodiment of the present invention.

[0135] Table 2 Statistical results of pore size distribution of rock samples identified by 3D CT scanning and NMR

[0136]

[0137]

[0138] In summary, the method for characterizing the full-aperture pore size of shale based on the gravimetric method and NMR joint measurement disclosed in the embodiment of the present invention is based on the high-pressure mercury injection test rock sample pore distribution frequency versus pore diameter curve, based on the gravimetric method and NMR joint measurement to obtain the porosity component and porosity cumulative value versus relaxation time T2 of the rock sample after saturation treatment. Finally, based on the time-space conversion coefficient between the shale nuclear magnetic resonance relaxation time T2 value and the pore size, the full-aperture pore size result diagram of the rock sample after saturation treatment is drawn on the basis of the porosity component and porosity cumulative value versus relaxation time T2 curve after saturation treatment, thereby realizing the characterization of the full-aperture pore size of shale, and there is no overlap in the pore size measurement ranges of multiple joint measurement methods, so the full-aperture pore size result obtained is more accurate.

[0139] It should be pointed out in particular that the various components or steps in the above-mentioned embodiments can be cross-linked, replaced, added, or deleted with each other. Therefore, the combinations formed by these reasonable permutations and combinations should also fall within the scope of protection of the present invention, and the scope of protection of the present invention should not be limited to the embodiments.

[0140] The above are exemplary embodiments disclosed in the present invention. The order in which the above embodiments of the present invention are disclosed is for description only and does not represent the pros and cons of the embodiments. However, it should be noted that the discussion of any of the above embodiments is only exemplary and is not intended to imply that the scope disclosed in the embodiments of the present invention (including the claims) is limited to these examples. Various changes and modifications may be made without departing from the scope defined in the claims. The functions, steps and / or actions of the method claims according to the disclosed embodiments described herein do not need to be performed in any particular order. In addition, although the elements disclosed in the embodiments of the present invention may be described or required in individual form, they may also be understood as multiple unless expressly limited to the singular.

[0141] Those skilled in the art should understand that the discussion of any of the above embodiments is merely illustrative and is not intended to imply that the scope of the disclosure of the embodiments of the present invention (including the claims) is limited to these examples. Within the spirit of the embodiments of the present invention, the technical features of the above embodiments or different embodiments may be combined, and there are many other variations of the different aspects of the embodiments of the present invention described above, which are not provided in detail for the sake of clarity. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the embodiments of the present invention should be included within the scope of protection of the embodiments of the present invention.

Claims

1. A method for characterizing the full-aperture pore size of shale based on gravimetric and NMR combined measurement, characterized in that: The steps include: Conduct high-pressure mercury injection testing on rock samples and draw a curve showing the change of pore distribution frequency versus pore diameter. The rock samples were dried, weighed and tested for NMR T2 spectrum; Inject distilled water into the rock sample for saturation treatment, then weigh and test the nuclear magnetic resonance T2 spectrum of the rock sample after saturation treatment; The water-saturated porosity of the rock sample is calculated based on the weight of the rock sample after drying and the weight after saturation treatment; Based on the water-saturated porosity of the rock sample, converting the nuclear magnetic resonance signal of the nuclear magnetic resonance T2 spectrum after the rock sample is saturated into a porosity component; Based on the porosity components, a curve of the porosity components and the cumulative porosity value after saturation treatment of the rock sample is drawn along with the relaxation time T2; Calculating the time-space conversion coefficient between the nuclear magnetic resonance relaxation time T2 value and the pore size based on the curve of the rock sample pore distribution frequency versus pore diameter and the nuclear magnetic resonance T2 spectrum of the rock sample after saturation treatment; Based on the time-space conversion coefficient, a full-aperture pore size result diagram of the rock sample after saturation treatment is drawn on the basis of the curves of the porosity component and the cumulative porosity value after the rock sample is saturated with the relaxation time T2.

2. The method according to claim 1, characterized in that The calculation formula of the water-saturated porosity of the rock sample is as follows: Among them, φ iw is the water-saturated porosity of the rock sample, %; m iw is the weight of the rock sample after saturation treatment, g; m i0 is the weight of the rock sample after drying, g; ρ w is the water density, g / cm 3 ;d i is the diameter of the rock sample, cm; L i is the length of the rock sample, cm.

3. The method according to claim 2, characterized in that Based on the water-saturated porosity of the rock sample, the signal amount of the nuclear magnetic resonance T2 spectrum after the rock sample saturation treatment is converted into a porosity component. The conversion formula is as follows: in, is the porosity component of the rock sample after saturation treatment, %; S iw is the nuclear magnetic resonance T2 spectrum nuclear magnetic signal after rock sample saturation treatment, PU; φ iw is the water-saturated porosity of the rock sample, %; S ac,iw is the cumulative amount of T2 spectrum nuclear magnetic signal after rock sample saturation treatment, PU; S ac,id PU is the accumulated nuclear magnetic signal corresponding to the first peak of the T2 spectrum after the rock sample is saturated.

4. The method according to claim 1, wherein Also includes: CT was used to scan rock samples with diameters of 25 mm and 2 mm to obtain the structural characteristics of pores with diameters greater than 1 μm. Data were extracted from the data volume according to different pore sizes, and a histogram of the porosity of different pore sizes was drawn. This was used to calibrate the porosity of pores with diameters greater than 1 μm in the full-aperture pore size result diagram after saturation treatment of the rock samples.

5. The method according to claim 4, characterized in that The step of calibrating the porosity of pores with a diameter greater than 1 μm in the full-aperture pore size result diagram after saturation treatment of the rock sample includes: calculating the relative error between the porosity of pores of different pore sizes with a diameter greater than 1 μm in the full-aperture pore size result diagram after saturation treatment of the rock sample and the porosity of pores of different pore sizes with a diameter greater than 1 μm identified by CT scanning.

6. The method according to claim 1, characterized in that Calculating the time-space conversion coefficient between the nuclear magnetic resonance relaxation time T2 value and the pore size includes: integrating the curve of the rock sample pore distribution frequency versus pore diameter and the nuclear magnetic resonance T2 spectrum curve after saturation treatment of the rock sample in the same coordinate system, establishing a time-space conversion curve between the nuclear magnetic resonance relaxation time T2 value after saturation treatment and the pore size, and when the peak value of the nuclear magnetic signal after saturation treatment corresponds one by one to the peak value of the high-pressure mercury injection pore distribution frequency, recording the nuclear magnetic resonance relaxation time T2 and the pore diameter d value at this time to calculate the time-space conversion coefficient.

7. The method according to claim 6, characterized in that The space-time conversion coefficient is obtained by the ratio between the nuclear magnetic resonance relaxation time T2 value and the pore diameter d value.

8. The method according to claim 1, characterized in that The saturation treatment of injecting distilled water into the rock sample includes injecting the distilled water in the piston container into the rock sample using an ISCO pump.

9. The method according to claim 8, characterized in that Before injecting distilled water into the rock sample for saturation treatment, the rock sample was placed in a core holder and the confining pressure was applied using a hand pump.

10. The method according to claim 9, characterized in that A hand pump is used to apply a confining pressure within a predetermined range, and then a vacuum pretreatment system is used to vacuum the rock sample for a first predetermined time.

11. The method according to claim 10, characterized in that The predetermined range of confining pressure is 1 to 3 MPa.

12. The method according to claim 10, characterized in that The first predetermined time for the vacuum treatment is 46 to 50 hours.

13. The method according to claim 9, characterized in that The distilled water injection pressure and the confining pressure increase in a step-by-step manner in two adjacent time periods. In each time period, the confining pressure and the injection pressure are respectively constant, and the difference between the confining pressure and the injection pressure remains constant. When the injection pressure reaches the formation pressure of the study block, the saturation process is stopped.

14. The method according to claim 13, characterized in that The difference between the confining pressure and the injection pressure is 2 MPa.

15. The method according to claim 13, characterized in that Also includes: The saturation process is completed in a constant temperature box, and the temperature in the constant temperature box is consistent with the formation temperature.

16. The method according to claim 13, characterized in that The total saturation time after adding up each time period is ≥48 hours.

17. The method according to claim 1, wherein The rock sample drying process includes: placing the rock sample in a vacuum oven with controllable dryness and humidity, controlling the relative humidity during drying to 40%, and drying the rock sample to a constant weight.

18. The method according to claim 14, characterized in that The drying process of rock samples includes: total drying time ≤ 4h.

19. A computer-readable storage medium comprising instructions, which, when executed on a computer, cause the computer to execute the method for characterizing the full-pore size of shale based on a gravimetric method and NMR combined measurement as claimed in any one of claims 1 to 15, for calculating the water-saturated porosity of a rock sample; And / or, the instructions enable the computer to execute the calculation of the porosity component in the method according to any one of claims 1 to 15, And / or, the instructions enable the computer to execute the method according to any one of claims 1 to 15, wherein the method comprises drawing a curve of the porosity component and the cumulative porosity value of the rock sample after saturation treatment versus the relaxation time T2. And / or, the instructions enable the computer to execute the method according to any one of claims 1 to 15 for calculating the spatiotemporal conversion coefficient between the nuclear magnetic resonance relaxation time T2 value and the pore size, And / or, the instructions enable the computer to execute the method according to any one of claims 1 to 15, which comprises drawing a full-aperture pore size result graph after saturation treatment of the rock sample.

20. A system for characterizing the full-aperture pore size of shale based on gravimetric and NMR combined measurement, characterized in that: The system comprises: a first calculation module, which calculates the water-saturated porosity of the rock sample based on the weight of the rock sample after drying and the weight of the rock sample after saturation treatment; a second calculation module, which converts the nuclear magnetic resonance signal of the nuclear magnetic resonance T2 spectrum of the rock sample after saturation treatment into a porosity component based on the water-saturated porosity of the rock sample; A first construction module, wherein the first construction module is based on the porosity component to draw a curve showing a change of the porosity component and the cumulative porosity value of the rock sample after saturation treatment with a relaxation time T2; a third calculation module, which calculates a time-space conversion coefficient between the nuclear magnetic resonance relaxation time T2 value and the pore size based on a curve of the pore distribution frequency of the rock sample versus the pore diameter and a nuclear magnetic resonance T2 spectrum of the rock sample after saturation treatment; The second construction module is a construction module for drawing a full-aperture pore size result diagram after rock sample saturation treatment based on the time-space conversion coefficient and the curve of the porosity component and the cumulative porosity value after rock sample saturation treatment changing with the relaxation time T2.

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