Deep shale dessert evaluation method based on high-temperature and high-pressure rheological experiment

Abstract

The invention discloses a deep shale dessert evaluation method based on a high-temperature and high-pressure rheological experiment, and relates to the technical field of petroleum and natural gas exploration and development. The invention provides a new thought about deep dessert evaluation: under the condition of simulating high temperature and high pressure of a deep stratum, macro and micro scales before and after shale rheology are compared and analyzed, and pore size distribution, adsorption and desorption characteristics and microstructure characteristics before and after a shale high-temperature and high-pressure rheology experiment are analyzed. The evolution characteristics of the pore structure in the deformation process are analyzed approximately dynamically, so that a deep shale dessert evaluation model is established preliminarily. According to the method, desserts of deep shale characteristics are developed, the source and storage of rich gas of shale gas and flow of a fracturing transformation and permeation enhancement mechanism under high temperature and high pressure are evaluated, and in other words, reservoir comprehensive evaluation before development is paid attention to in shale gas dessert evaluation or whether complex cracks are formed or not after shale fracturing is paid attention to more attention to shale gas dessert evaluation.

Description

Technical Field

[0001] The present invention relates to the technical field of oil and gas exploration and development, in particular to the technical field of shale compressibility evaluation, and more specifically to a deep shale sweet spot evaluation method based on high temperature and high pressure rheology. Background Art

[0002] Shale gas is a natural gas resource that can be mined in shale layers. my country has a large amount of recoverable shale gas reserves. Among them, deep shale gas reserves with a burial depth of more than 3500m are abundant, but due to its high temperature, ultra-high pressure, dense matrix, complex reservoir space and structure, and strong heterogeneity, it is difficult to mine and must undergo large-scale fracturing to achieve effective development. Therefore, obtaining shale compressibility evaluation parameters and quantitatively evaluating the compressibility of shale gas horizontal wells are of great significance to achieve efficient and economic development of shale gas fields.

[0003] At present, there are many evaluation indicators for shale compressibility, and the main evaluation indicators are mainly concentrated in the calculation of brittleness index. However, the brittleness index alone cannot fully evaluate the compressibility of shale. Li Qinghui (2012) proposed that the brittle characteristics of shale are related to its type, coring depth, coring orientation and loading conditions. When actually evaluating its mechanical properties, it is recommended to conduct research on specific areas, and to simulate the underground mechanical environment for experiments, so as to obtain more accurate results. Lai Fuqiang (2018) used parameters such as brittleness coefficient (BIe), brittle mineral content (BM), organic carbon content (TOC), clay mineral content (CL), fracture toughness (KC) and horizontal stress difference coefficient (Kh), and used the hierarchical analysis method to quantitatively analyze the weight of each parameter to evaluate the fracturability of shale gas reservoirs. Liu Yaowen (2022) proposed a new method for compressibility evaluation based on the perspective of crack rupture, extension and filling: key evaluation parameters were selected: brittleness index, ground stress characteristic parameters, and construction parameters for stress inversion, and a comprehensive compressibility index was obtained based on various parameters to establish a new model for mechanical parameter interpretation.

[0004] Deep shale reservoirs mainly develop micropores and mesopores with pore sizes less than 50 nm. In the high temperature and high pressure environment of deep shale (pressure is generally higher than 70 MPa, and the average formation temperature is about 130 ℃), it is difficult to form a complex fracture network after fracturing under high temperature and high pressure. At the same time, it is difficult to initiate and extend the fractures, and the difficulty of injecting proppants is relatively greater than that of the shallow and medium layers, which makes it difficult to obtain fractures with high conductivity. The flow and output of gas in micropores in shale reservoirs are the key to the development of shale gas fracturing.

[0005] After reviewing relevant literature, it was found that the current compressibility evaluation method is not perfect, especially the research on the compressibility of deep shale is insufficient. The existing compressibility evaluation mainly has the following deficiencies: (1) The existing compressibility evaluation is relatively macroscopic, and the macroscopic and microscopic characteristics of shale are not combined; (2) It fails to reflect the changes in pore structure, organic matter and mineral structure; (3) The pore and fracture evolution process of shale rheology under high temperature and high pressure is unclear; (4) The analysis conditions were relatively static, and the changes in the shale pore structure and adsorption and desorption before and after the transformation were not analyzed. Summary of the invention

[0006] In order to overcome the defects and deficiencies in the above-mentioned prior art, the present invention provides a deep shale sweet spot evaluation method based on high temperature and high pressure rheology. The purpose of the present invention is to solve the problems in the evaluation of shale compressibility in the above-mentioned prior art. The present invention provides a new idea for deep sweet spot evaluation: under the conditions of high temperature and high pressure of simulated deep formations, the macro and micro scales of shale rheology before and after are compared and analyzed, and the pore size distribution, adsorption analysis characteristics, and microstructural characteristics of shale before and after high temperature and high pressure rheology experiments are analyzed. The pore structure evolution characteristics during the deformation process are approximately dynamically analyzed to preliminarily establish a deep shale sweet spot evaluation model. The present invention develops the "sweet spot" of deep shale characteristics, evaluates the "source" and "reservoir" of shale gas rich gas, and the "flow" of the permeability enhancement mechanism of fracturing transformation under high temperature and high pressure, that is, in the evaluation of shale gas sweet spots, attention is paid to the comprehensive evaluation of reservoirs before development or whether complex fractures are formed after shale fracturing.

[0007] In order to solve the above problems existing in the prior art, the present invention is implemented through the following technical solutions.

[0008] The present invention provides a deep shale sweet spot evaluation method based on high temperature and high pressure rheology, the evaluation method comprising the following steps: S1. Take cores from the shale gas well to be evaluated and prepare shale samples to be evaluated; S2. Cutting the prepared shale samples to be evaluated according to the experimental requirements of each category of experimental samples; S3. Conduct basic experiments on the core of the upper part of the cut sample, and conduct high temperature and high pressure rheological experiments on the core of the lower part of the cut sample; S4. Cut the samples after high temperature and high pressure rheological test, select the fresh end surface for scanning electron microscope observation, analyze the microstructure characteristics of the samples after high temperature and high pressure rheological test, and at the same time, perform low temperature CO2 and low temperature N2 adsorption experiments on the samples after high temperature and high pressure rheological test, and analyze the pore size distribution characteristics after high temperature and high pressure rheological test; S5. Compare and analyze the microstructural characteristics after high temperature and high pressure rheological experiments with those obtained from basic experiments, and analyze the change characteristics of shale after high temperature and high pressure rheological experiments by combining the results of scanning electron microscopy, low temperature CO2 adsorption and low temperature N2 adsorption experiments before and after high temperature and high pressure rheological experiments; S6. Comprehensively organize the experimental results after high temperature and high pressure rheological experiments and the basic experimental results to form the deep shale recoverability evaluation index; S7. Based on the evaluation indexes of deep shale enrichment conditions and deep shale recoverability, and in accordance with the analytical structure of the target layer, condition layer and parameter layer, the analytic hierarchy process is used to evaluate the sweet spots of deep shale.

[0009] Further preferably, in step S7, the analysis structure of the target layer, condition layer and parameter layer specifically refers to that the condition layer is the primary indicator and the parameter layer is the secondary indicator; the deep shale sweet spot is the target layer, the enrichment condition of the deep shale and the recoverability of the deep shale are the condition layer, and the specific parameter indicators are the parameter layer.

[0010] More preferably, in the S7 step, the primary indicator of deep shale enrichment conditions includes the secondary indicators of TOC content, effective thickness of the reservoir, porosity and gas content; the primary indicator of deep shale recoverability includes the secondary indicators of brittle mineral content, reservoir burial depth, horizontal stress difference and post-rheological pore and fracture changes.

[0011] Further preferably, in step S3, a high temperature and high pressure rheological experiment is performed on the core of the lower part of the cut sample, and the specific steps are as follows: The end surface of the core at the lower part of the cut sample is ground on a flat glass surface with diamond sand to ensure that the end surfaces are smooth and parallel to each other, thereby obtaining a high temperature and high pressure rheological test sample; Before loading, the experimental samples were placed in a drying oven at 120°C for 24 hours and then stored in drying bottles. Place the prepared experimental sample into the core chamber of the experimental equipment; After the experimental sample is installed, it is first vacuumed for 30 minutes, and then the axial pressure and confining pressure are applied synchronously in a certain ratio. When the confining pressure reaches the set value, the displacement control is switched to the pressure control, and the heating is started after it stabilizes. When the temperature and confining pressure reach the preset values, friction correction is carried out, and then a high-temperature and high-pressure rheological experiment is carried out. The displacement rate loading method is used during loading, and the experiment is stopped after loading to failure.

[0012] Further preferably, in step S3, a high-temperature and high-pressure rheological experiment is performed on the core of the lower part of the cut sample using a gas medium high-temperature and high-pressure experimental equipment. The high-temperature and high-pressure conditions are determined according to the depth of the actual formation, and the size of the core of the lower part of the cut sample is specified according to the size of the core chamber of the high-temperature and high-pressure experimental equipment.

[0013] More preferably, the size of the lower part of the core of the cut sample is a Φ20mm×40mm columnar core.

[0014] Further preferably, the basic experiments include any one or more combinations of XRD experiments, TOC experiments, RO experiments, rock pyrolysis experiments, SEM electron microscopy experiments, low-temperature liquid nitrogen adsorption experiments and low-temperature carbon dioxide adsorption experiments.

[0015] More preferably, step S6 is specifically to comprehensively organize the experimental results after the high temperature and high pressure rheological experiment with the shale XRD experimental results and the TOC content analysis results to form a deep shale recoverability evaluation index.

[0016] Further preferably, according to the X-ray diffraction analysis method of clay minerals and common non-clay minerals in sedimentary rocks SY / T5163-2018, a shale XRD experiment is carried out using a Japanese Rigaku TTRⅢ multifunctional X-ray diffractometer to analyze the non-clay mineral components and clay mineral components of the shale under study.

[0017] More preferably, the organic carbon analysis is performed according to the determination method of total organic carbon in sedimentary rocks in GB / T19145-2003, and a LECO CS-230 carbon-sulfur analyzer is used to perform a pyrolysis experiment on the studied shale to analyze the TOC content of the shale.

[0018] More preferably, referring to SY / T 5162-1997 rock sample scanning electron microscope analysis method, a Zeiss Sigma500 field emission scanning electron microscope is used to observe the shale microstructure and analyze the shale pore and fracture microstructure.

[0019] Further preferably, according to GB / T 21650.3-2011 Mercury injection and gas adsorption method for pore size distribution and porosity of solid materials, Part 3, gas adsorption method for analyzing micropores, low-temperature CO2 and N2 adsorption experiments are carried out on shale using ASAP 2460 surface area and porosity analyzer and supporting MicroActive software to analyze the pore distribution and adsorption characteristics of shale.

[0020] Compared with the prior art, the beneficial technical effects brought by the present invention are as follows: The evaluation of shale microstructure is focused on systematization, which reduces the impact of heterogeneity on experimental results. Comprehensive microstructure experiments are carried out on the same piece of shale, and the research content is more systematic and targeted. Rheological experiments are carried out to evaluate the microstructural characteristics of shale. On the one hand, the in-situ conditions of underground shale are restored. On the other hand, high-temperature and high-pressure deformation analysis can be used to analyze the evolution of pore structure after fracturing or geological tectonic movement, shifting from static microstructure to dynamic evaluation, and then a comprehensive evaluation of shale source, storage and flow is carried out, which is conducive to guiding the industrial development of shale gas. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a schematic diagram of the process of shale sweet spot evaluation method based on high temperature and high pressure rheological experiment; Figure 2 It is a schematic diagram of sample cutting in the embodiment; Figure 3 This is a schematic diagram of the structure of the high temperature and high pressure rheological experimental device; Figure 4 This is a schematic diagram of the core chamber for high temperature and high pressure rheological experiments; Figure 5 Evaluation structure diagram for AHP dessert; Figure 6 The relationship between the comprehensive index of sweet spot evaluation and gas production is calculated for example. DETAILED DESCRIPTION

[0022] In order to enable those skilled in the art to better understand the technical solutions in this specification, the technical solutions in the embodiments of this specification will be clearly and completely described below in conjunction with the drawings in the embodiments of this specification. Obviously, the described embodiments are only part of the embodiments of this specification, not all of the embodiments. Based on the embodiments in this specification, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of this specification.

[0023] Example 1 As a preferred embodiment of the present invention, refer to the attached specification Figure 1 As shown, this embodiment discloses a deep shale sweet spot evaluation method based on high temperature and high pressure rheology, and the evaluation method comprises the following steps: S1. Take cores from the shale gas well to be evaluated and prepare shale samples to be evaluated; S2. Cutting the prepared shale samples to be evaluated according to the experimental requirements of each category of experimental samples; S3. Conduct basic experiments on the core of the upper part of the cut sample, and conduct high temperature and high pressure rheological experiments on the core of the lower part of the cut sample; S4. Cut the samples after high temperature and high pressure rheological test, select the fresh end surface for scanning electron microscope observation, analyze the microstructure characteristics of the samples after high temperature and high pressure rheological test, and at the same time, perform low temperature CO2 and low temperature N2 adsorption experiments on the samples after high temperature and high pressure rheological test, and analyze the pore size distribution characteristics after high temperature and high pressure rheological test; S5. Compare and analyze the microstructural characteristics after high temperature and high pressure rheological experiments with those obtained from basic experiments, and analyze the change characteristics of shale after high temperature and high pressure rheological experiments by combining the results of scanning electron microscopy, low temperature CO2 adsorption and low temperature N2 adsorption experiments before and after high temperature and high pressure rheological experiments; S6. Comprehensively organize the experimental results after high temperature and high pressure rheological experiments and the basic experimental results to form the deep shale recoverability evaluation index; S7. Based on the evaluation indexes of deep shale enrichment conditions and deep shale recoverability, and in accordance with the analytical structure of the target layer, condition layer and parameter layer, the analytic hierarchy process is used to evaluate the sweet spots of deep shale.

[0024] As an implementation method of this embodiment, refer to the attached Figure 5 As shown, in step S7, the analysis structure of the target layer, condition layer and parameter layer specifically refers to that the condition layer is the primary indicator and the parameter layer is the secondary indicator; the deep shale sweet spot is the target layer, the enrichment condition of the deep shale and the recoverability of the deep shale are the condition layer, and the specific parameter indicators are the parameter layer.

[0025] Furthermore, in the S7 step, the first-level indicator of deep shale enrichment conditions includes the secondary indicators of TOC content, effective thickness of the reservoir, porosity and gas content; the first-level indicator of deep shale recoverability includes the secondary indicators of brittle mineral content, reservoir burial depth, horizontal stress difference and post-rheological pore and fracture changes.

[0026] Example 2 As another preferred embodiment of the present invention, this embodiment is a further detailed supplement and elaboration of the technical solution of the present invention based on the above-mentioned embodiment 1. Figure 3 As shown, a high-temperature and high-pressure rheological experiment is carried out on the core of the lower part of the cut sample using a gas medium high-temperature and high-pressure experimental equipment. The high-temperature and high-pressure conditions are determined according to the depth of the actual formation, and the size of the core of the lower part of the cut sample is specified according to the size of the core chamber of the high-temperature and high-pressure experimental equipment.

[0027] As an example, the size of the lower core of the cut sample is a Φ20mm×40mm cylindrical core.

[0028] A high temperature and high pressure rheological experiment was carried out on the core of the lower part of the cut sample. The specific steps are as follows: The end surface of the core at the lower part of the cut sample is ground on a flat glass surface with diamond sand to ensure that the end surfaces are smooth and parallel to each other, thereby obtaining a high temperature and high pressure rheological test sample; Before loading, the experimental samples were placed in a drying oven at 120°C for 24 hours and then stored in drying bottles. Place the prepared experimental sample into the core chamber of the experimental equipment; After the experimental sample is installed, it is first vacuumed for 30 minutes, and then the axial pressure and confining pressure are applied synchronously in a certain ratio. When the confining pressure reaches the set value, the displacement control is switched to the pressure control, and the heating is started after it stabilizes. When the temperature and confining pressure reach the preset values, friction correction is performed, and then a high temperature and high pressure rheological test is performed. During loading, the displacement rate loading method is selected, and the test is stopped after loading to failure. After the experiment is completed, the sample is unloaded, photographed, and described.

[0029] Example 3 As another preferred embodiment of the present invention, this embodiment is a further detailed supplement and elaboration of the technical solution of the present invention based on the above-mentioned embodiment 1 or embodiment 2. In this embodiment, the basic experiments include any one or more combinations of XRD experiments, TOC experiments, RO experiments, rock pyrolysis experiments, SEM electron microscopy experiments, low-temperature liquid nitrogen adsorption experiments, and low-temperature carbon dioxide adsorption experiments.

[0030] Furthermore, step S6 specifically includes comprehensively organizing the experimental results after the high temperature and high pressure rheological experiment with the shale XRD experimental results and the TOC content analysis results to form a deep shale recoverability evaluation index.

[0031] As an example, according to the X-ray diffraction analysis method of clay minerals and common non-clay minerals in sedimentary rocks SY / T5163-2018, a shale XRD experiment was carried out using the Rigaku TTRⅢ multifunctional X-ray diffractometer to analyze the non-clay mineral components and clay mineral components of the shale under study.

[0032] As another example, according to the determination method of total organic carbon in sedimentary rocks in GB / T19145-2003, an organic carbon analysis was performed, and a LECO CS-230 carbon-sulfur analyzer was used to perform a pyrolysis experiment on the studied shale to analyze the TOC content of the shale.

[0033] As another example, referring to SY / T 5162-1997 rock sample scanning electron microscope analysis method, a Zeiss Sigma500 field emission scanning electron microscope was used to observe the shale microstructure and analyze the shale pore and fracture microstructure.

[0034] As another example, according to GB / T 21650.3-2011 Mercury injection and gas adsorption methods for pore size distribution and porosity of solid materials - Part 3 - Analysis of micropores by gas adsorption method, low-temperature CO2 and N2 adsorption experiments were carried out on shale using ASAP 2460 surface area and porosity analyzer and its supporting MicroActive software to analyze the pore distribution and adsorption characteristics of shale.

[0035] Example 4 As another preferred embodiment of the present invention, refer to the attached specification Figure 1 As shown, this embodiment discloses a deep shale sweet spot evaluation method based on high temperature and high pressure rheology, comprising: The entire evaluation shale sample is prepared by using the shale on the same small layer of the full-diameter core of the deep shale gas well, and coring is carried out using the wire cutting coring technology.

[0036] like Figure 2 As shown, the obtained long core is cut and designed according to the experimental requirements of each type of experimental sample. As an example, the cutting ratio can be 2:8; The upper part of the cut samples was subjected to XRD experiment, TOC experiment, Ro experiment, rock pyrolysis experiment, SEM electron microscopy experiment, low-temperature liquid nitrogen adsorption experiment, and low-temperature carbon dioxide adsorption experiment for mineral composition; A high-temperature and high-pressure deformation experiment was carried out on the cut lower part of the core, and then a high-temperature and high-pressure rheological experiment was carried out using a high-temperature and high-pressure quasi-triaxial experimental equipment; a microstructure characterization experiment of the samples after the rheological experiment was carried out, and the samples after the rheological experiment were cut and subjected to a pore structure characterization experiment; finally, the shale microstructures before and after deformation were compared to analyze the shale source, storage and flow characteristics.

[0037] Further, as shown in the attached manual Figure 3 As shown in the figure, the shale is subjected to a high temperature and high pressure test using a gas medium high temperature and high pressure test equipment, and the high temperature and high pressure conditions are determined according to the actual depth of the formation. The core of the high temperature and high pressure test needs to be made according to the size of the core chamber of the high temperature and high pressure test equipment, and the size of the sample is Φ20mm×40mm columnar core. The samples of the high temperature and high pressure test are taken to minimize the impact of sample heterogeneity on the experiment, and ensure that the pore structure, mineral content, organic matter and other characteristics of the samples are as similar as possible to those in adjacent positions.

[0038] The end faces of the prepared core samples were ground on a flat glass surface with diamond sand to ensure that the end faces were smooth and parallel to each other. The experimental samples were placed in a drying oven at 120°C for 24 hours before loading, and then placed in a drying bottle for storage. The prepared core was placed in the core chamber of the experimental equipment. After the sample was installed, it was first vacuumed for 30 minutes, and then the axial pressure and confining pressure were applied synchronously in a certain proportion. When the confining pressure setting value was reached, the displacement control was switched to pressure control, and heating began after stabilization. Constant pressure control can ensure that the expansion of the sample during heating will not change the pressure, ensuring that the sample is in a static rock pressure state during heating. When the temperature and confining pressure reach the preset values, friction correction is performed, and then high temperature and high pressure rheology is performed. The displacement rate loading method is set during loading, and the experiment is stopped after loading to failure. After the experiment is completed, the sample is unloaded, photographed, and described.

[0039] After the high temperature and high pressure experiment, the samples were further sectioned and the fresh end faces were selected for scanning electron microscopy observation to analyze the microstructural characteristics after high temperature and high pressure deformation. At the same time, low temperature CO2 and low temperature N2 adsorption experiments were carried out on some samples to analyze the pore size distribution characteristics after high temperature and high pressure rheology experiments.

[0040] Furthermore, a comparative analysis was conducted on the shale microstructure before and after the high temperature and high pressure rheological experiment. Combined with the scanning electron microscopy, low temperature CO2 adsorption and low temperature N2 adsorption experimental results before and after the experiment, the changing characteristics of shale under high temperature and high pressure were analyzed.

[0041] Furthermore, the experimental results under high temperature and high pressure are comprehensively collated with the shale XRD results and TOC content analysis results to form the deep shale recoverability evaluation index.

[0042] Furthermore, the basic experimental data and high-temperature and high-pressure rheological experimental data were combined, and the analytic hierarchy process was used to obtain the deep shale sweet spot evaluation index.

[0043] Example 5 Considering that the existing evaluation of deep shale sweet spots is limited to the macroscopic mechanical characteristics and mineral characteristics of shale, and most of the evaluations are based on static parameter evaluation results at normal temperature and pressure. There is a lack of consideration of the in-situ high temperature and high pressure conditions of deep shale and the evolution characteristics of pore fracture characteristics under high temperature and high pressure. This method provides a new idea for the evaluation of deep shale sweet spots: under the simulated high temperature and high pressure conditions of deep formations, the macro and micro scales of shale rheology before and after are compared and analyzed, and the pore size distribution, adsorption and desorption characteristics, and microstructural characteristics of shale before and after high temperature and high pressure rheological experiments are analyzed. The pore structure evolution characteristics during the deformation process under high temperature and high pressure are analyzed approximately dynamically, so as to establish a deep shale sweet spot evaluation model. The "sweet spot" evaluation of deep shale characteristics is carried out from two aspects: characterizing the "source" and "reservoir" characteristics of shale gas richness and the "flow" mechanism of fracturing transformation and permeability enhancement under high temperature and high pressure. That is, in the evaluation of shale gas sweet spots, attention is paid to the comprehensive evaluation of reservoirs before development, or more attention is paid to whether shale fracturing forms a channel conducive to shale gas flow.

[0044] Further, according to Figure 1 The deep shale sweet spot evaluation method based on high temperature and high pressure rheological experiment is carried out. First, the research samples are prepared and the samples are Figure 2 The samples were cut and subjected to basic experiments and high-temperature and high-pressure rheological experiments.

[0045] Furthermore, according to the X-ray diffraction analysis method of clay minerals and common non-clay minerals in sedimentary rocks SY / T5163-2018, a shale XRD experiment was carried out using the Rigaku TTRⅢ multifunctional X-ray diffractometer to analyze the non-clay mineral components and clay mineral components of the shale under study.

[0046] Furthermore, organic carbon analysis was performed according to the determination method of total organic carbon in sedimentary rocks in GB / T19145-2003, and a LECO CS-230 carbon-sulfur analyzer was used to perform pyrolysis experiments on the studied shale to analyze the TOC content of the shale.

[0047] Furthermore, referring to SY / T 5162-1997 rock sample scanning electron microscope analysis method, a Zeiss Sigma 500 field emission scanning electron microscope was used to observe the shale microstructure and analyze the shale pore and fracture microstructure.

[0048] Furthermore, according to GB / T 21650.3-2011 Mercury injection and gas adsorption methods for pore size distribution and porosity of solid materials - Part 3 - Analysis of micropores by gas adsorption method, low-temperature CO2 and N2 adsorption experiments were carried out on shale using ASAP 2460 surface area and porosity analyzer and the supporting MicroActive software to analyze the pore distribution and adsorption characteristics of shale.

[0049] Furthermore, a high temperature and high pressure test device with a gas medium was used to conduct a high temperature and high pressure test on shale. The schematic diagram of the test device is shown in FIG. Figure 3 , high temperature and high pressure conditions are determined according to the actual depth of the formation. The core of the high temperature and high pressure experiment needs to be formulated according to the size of the core chamber of the high temperature and high pressure experimental equipment, and the size of the sample is Φ20mm×40mm cylindrical core. The samples of the high temperature and high pressure experiment should minimize the impact of sample heterogeneity on the experiment, and ensure that the pore structure, mineral content, organic matter and other characteristics of the samples are as similar as possible to those in adjacent positions.

[0050] Furthermore, the end faces of the prepared core samples were ground on a flat glass surface with diamond sand to ensure that the end faces were smooth and parallel to each other. The experimental samples were placed in a drying oven at 120°C for 24 hours before loading and were uniformly stored in drying bottles. The prepared cores were placed in the core chamber of the experimental equipment. The core chamber structure is as follows: Figure 4 As shown. After the sample is installed, it is first vacuumed for 30 minutes, and then the axial pressure and confining pressure are applied synchronously in a certain ratio. When the confining pressure setting value is reached, the displacement control is switched to the pressure control, and the heating is started after stabilization. Constant pressure control can ensure that the expansion of the sample during heating will not change the pressure, and ensure that the sample is in a static rock pressure state during heating. When the temperature and confining pressure reach the preset values, friction correction is performed, and then high temperature and high pressure rheology is performed. The displacement rate loading method is set during loading. After loading to failure, the experiment is stopped. After the experiment is completed, the sample is unloaded, photographed, and described.

[0051] After the high temperature and high pressure experiment, the samples were further sectioned and the fresh end faces were selected for scanning electron microscopy observation to analyze the microstructural characteristics after high temperature and high pressure deformation. At the same time, low temperature CO2 and low pressure N2 adsorption experiments were carried out on some samples to analyze the pore size distribution characteristics after high temperature and high pressure rheology experiments.

[0052] Furthermore, the microstructure of shale before and after the high temperature and high pressure rheological experiment was compared and analyzed, and the changing characteristics of shale under high temperature and high pressure were analyzed by combining the scanning electron microscopy, low temperature CO2 adsorption and low temperature N2 adsorption experimental results before and after the experiment.

[0053] Furthermore, the experimental results under high temperature and high pressure are comprehensively collated with the shale XRD results and TOC content analysis results to form the deep shale recoverability evaluation index.

[0054] Finally, according to the "source" and "reservoir" characteristics of deep shale and the "flow" characteristics of shale gas that characterize the recoverability of shale, Figure 5 The analysis structure uses the analytic hierarchy process to evaluate the sweet spots in deep shale. The evaluation results are used as a reference for on-site deep shale development and guide the economic and efficient development of deep shale gas.

[0055] Furthermore, the hierarchical structure is composed of the geological characteristics and recoverable characteristics of shale gas reservoirs, which also reflects the characteristics of shale gas generation, migration and enrichment, as well as the complex relationship between the many factors studied. Through summary and analysis, this study considers dividing the factors affecting the quality of shale gas reservoirs into two evaluation indicators: geological characteristics of shale reservoirs and recoverable characteristics of shale gas exploitation. Each evaluation indicator is determined by multiple secondary factors affecting the quality of shale gas reservoirs. According to the characteristics of each parameter and the nature of the evaluation indicators, the deep shale gas sweet spot is selected as the target layer, and the geological conditions and reservoir recoverability of the reservoir are set as the first-level indicators according to the "source", "reservoir" characteristics and "flow characteristics" of the shale gas reservoir. The reservoir geological conditions include TOC content, effective thickness, porosity and gas content parameters, and the reservoir recoverability includes the burial depth of the shale reservoir, brittleness index, horizontal stress difference and pore volume change before and after rheology. Then different hierarchical structures are formed, and the elements of the same level dominate the elements of the next level, and are dominated by the elements of the previous level. This dominance relationship from top to bottom forms a hierarchical hierarchy, with the top level being the result of the sweet spot selection, the middle level being the conditional level, and the lowest level being the evaluation parameters of the sweet spot evaluation area.

[0056] After the hierarchical structure is established, the subordinate relationship between the upper and lower layers is determined. According to the hierarchical relationship, the target layer, condition layer and parameters are coded and weighted.

[0057] Table 1 Evaluation indexes of deep shale The weight assignment is performed by comparing two elements on a scale of 1 to 9. The meaning of the scale is shown in Table 2.

[0058] Table 2 Meaning of scale The Delphi method is used to quantitatively calculate the weight of each evaluation factor. The basic idea is that if the influence of factor Bi on target factor A is different, the factors Bi are used to form a pairwise comparison discriminant matrix C: ; Among them, C must satisfy: a, C ii =1; b, C ij =1 / C ji ; The weights of all elements in the target A can be obtained by solving the characteristic value of the discriminant function C. The random consistency ratio CR proposed by T. LSaaty is used to check the consistency of the matrix. Among them, , , n is the matrix order, λMax is the maximum characteristic root. When CR<10%, the discriminant matrix inconsistency is considered acceptable. When CR>10%, it is necessary to recalculate until CR<10%.

[0059] Table 3 RI value table Using the above method, Matlab software was used to calculate the characteristic vector and maximum characteristic root of each factor in the high-yield coalbed methane area in the study area, and the reasonable weight value was obtained after consistency test. Among them, the importance scale of the two factors in the discriminant factors is taken according to Table 4 below.

[0060] Table 4 Comparison matrix and parameter values ​​of each layer Through matrix calculation and consistency test, the weights of each parameter for the optimal selection of deep shale gas sweet spot are obtained. The weight calculation results are shown in Table 5 below: Table 5 Calculation results of each parameter weight From the weight calculation results, it can be found that TOC accounts for the highest proportion of 0.3 in reservoir geological conditions, and porosity is the lowest, which is 0.17. Among the reservoir recoverability conditions that characterize the flow of reservoir fluid, the proportion of brittle mineral content (brittleness index) is the highest, reaching 0.42, while the burial depth is the lowest, which is 0.12.

[0061] In the parameter layer, each parameter is different in different shale gas wells and blocks. According to the characteristics of shale gas blocks, the parameters of shale gas target areas are divided into three levels: good, medium, and poor, represented by level I, level II, and level III, respectively, and each level is assigned 70~100 points, 30~70 points, and 0~30 points (Table 8-2). According to the scoring type, the parameters of different shale gas wells or blocks are normalized, and finally the comprehensive evaluation index is calculated based on the weight of each parameter in the evaluation of shale gas sweet spots, and the shale gas reservoir development well layer is optimized according to the index.

[0062] Table 6 Shale gas “sweet spot” indicator system and grade assignment table Therefore, according to the research, it can be determined that the comprehensive evaluation index characterizing the deep shale gas sweet spot can be expressed as: Comprehensive recoverable index = 0.215*NI(TOC)+0.181*NI(H effective thickness)+0.112*NI(porosity)+0.163*NI(gas content)+0.139*NI(brittleness index)+0.041*NI(burial depth)+0.055*NI(horizontal stress difference)+0.095*NI(pore volume increase multiple).

[0063] This study established a favorable area optimization evaluation method system and optimized favorable blocks for shale gas distribution. The optimization of shale gas "sweet spot areas" based on development involves the influence of two conditions: geology and reservoir recoverability. Based on geology and compressibility, the sweet spot areas are optimized and evaluated at three levels. The three progressive levels are target layer, condition layer, and parameter layer from top to bottom. The shale gas target area is a relatively favorable sweet spot area optimized by combining various parameters. The first-level indicators of the condition layer refer to the geological conditions and recoverability of shale, which correspond to the resource basis of shale "source" and "reservoir" and the recoverability of "flow" reservoirs. First, the good gas content of shale reservoirs is the basis for the high yield of shale reservoirs; secondly, the high brittleness and good compressibility of shale are important factors affecting the production of shale gas resources. According to the main controlling factors of shale gas enrichment described in the parameter calculation model, the shale organic matter content, shale thickness, porosity, pore development, brittle minerals, burial depth, horizontal stress difference, and the multiple of pore fracture volume increase after rheology are taken as important considerations to conduct a comprehensive evaluation of shale gas wells in the Weiyuan area of ​​the Sichuan Basin. Referring to the evaluation index weight table and the index system and grade scoring table of favorable areas, the parameters of the known wells are scored respectively, and the total score of each mineral-bearing area is obtained by summing up. The higher the score, the greater the potential for shale gas exploration and development, and the more favorable the block. The evaluation results are shown in Table 7.

[0064] Table 7 Comprehensive evaluation results According to the evaluation results, the comprehensive evaluation index of the wells in the study area is between 58.4 and 87.58, and the overall evaluation index is good. In order to verify the relationship between the evaluation index and the development effect, the relationship between the comprehensive evaluation index and the test gas production was established. The results are as follows: Figure 6 shown.

[0065] The results show that the comprehensive evaluation index is positively correlated with the test gas production. With the increase of the comprehensive index, the test gas production imaging tends to increase. The two are linearly correlated with a high degree of fit, R2=0.9166, indicating that the established evaluation model has good applicability to this area.

Claims

1. A deep shale sweet spot evaluation method based on high temperature and high pressure rheology, characterized by: The evaluation method includes the following steps: S1. Take cores from the shale gas well to be evaluated and prepare shale samples to be evaluated; S2. Cutting the prepared shale samples to be evaluated according to the experimental requirements of each category of experimental samples; S3. Conduct basic experiments on the core of the upper part of the cut sample, and conduct high temperature and high pressure rheological experiments on the core of the lower part of the cut sample; S4. Cut the samples after high temperature and high pressure rheological test, select the fresh end surface for scanning electron microscope observation, analyze the microstructure characteristics of the samples after high temperature and high pressure rheological test, and at the same time, perform low temperature CO2 and low temperature N2 adsorption experiments on the samples after high temperature and high pressure rheological test, and analyze the pore size distribution characteristics after high temperature and high pressure rheological test; S5. Compare and analyze the microstructural characteristics after high temperature and high pressure rheological experiments with those obtained from basic experiments, and analyze the change characteristics of shale after high temperature and high pressure rheological experiments by combining the results of scanning electron microscopy, low temperature CO2 adsorption and low temperature N2 adsorption experiments before and after high temperature and high pressure rheological experiments; S6. Comprehensively organize the experimental results after high temperature and high pressure rheological experiments and the basic experimental results to form the deep shale recoverability evaluation index; S7. Based on the evaluation indexes of deep shale enrichment conditions and deep shale recoverability, and in accordance with the analytical structure of the target layer, condition layer and parameter layer, the analytic hierarchy process is used to evaluate the sweet spots of deep shale.

2. A method for evaluating deep shale sweet spots based on high temperature and high pressure rheological experiments according to claim 1, characterized in that: In step S7, the analysis structure of the target layer, condition layer and parameter layer specifically refers to that the condition layer is the primary indicator and the parameter layer is the secondary indicator; the deep shale sweet spot is the target layer, the enrichment condition of the deep shale and the recoverability of the deep shale are the condition layer, and the specific parameter indicators are the parameter layer.

3. A method for evaluating deep shale sweet spots based on high temperature and high pressure rheological experiments as claimed in claim 2, characterized in that: In the step S7, the first-level indicator of deep shale enrichment conditions includes the secondary indicators TOC content, effective thickness of the reservoir, porosity and gas content; the first-level indicator of deep shale recoverability includes the secondary indicators brittle mineral content, reservoir burial depth, horizontal stress difference and post-rheological pore and fracture changes.

4. A method for evaluating deep shale sweet spots based on high temperature and high pressure rheological experiments according to any one of claims 1 to 3, characterized in that: In step S3, a high temperature and high pressure rheological experiment is performed on the core of the lower part of the cut sample. The specific steps are as follows: The end surface of the core at the lower part of the cut sample is ground on a flat glass surface with diamond sand to ensure that the end surfaces are smooth and parallel to each other, thereby obtaining a high temperature and high pressure rheological test sample; Before loading, the experimental samples were placed in a drying oven at 120°C for 24 hours and then stored in drying bottles. Place the prepared experimental sample into the core chamber of the experimental equipment; After the experimental sample is installed, it is first vacuumed for 30 minutes, and then the axial pressure and confining pressure are applied synchronously in a certain ratio. When the confining pressure reaches the set value, the displacement control is switched to the pressure control, and the heating is started after it stabilizes. When the temperature and confining pressure reach the preset values, friction correction is carried out, and then a high-temperature and high-pressure rheological experiment is carried out. The displacement rate loading method is used during loading, and the experiment is stopped after loading to failure.

5. A method for evaluating deep shale sweet spots based on high temperature and high pressure rheological experiments according to any one of claims 1 to 3, characterized in that: In step S3, a high-temperature and high-pressure rheological experiment is performed on the core of the lower part of the cut sample using a gas medium high-temperature and high-pressure experimental equipment. The high-temperature and high-pressure conditions are determined according to the depth of the actual formation, and the size of the core of the lower part of the cut sample is specified according to the size of the core chamber of the high-temperature and high-pressure experimental equipment.

6. A method for evaluating deep shale sweet spots based on high temperature and high pressure rheological experiments as claimed in claim 5, characterized in that: The size of the core of the lower part of the cut sample is Φ20mm×40mm columnar core.

7. A method for evaluating deep shale sweet spots based on high temperature and high pressure rheological experiments according to any one of claims 1 to 3, characterized in that: The basic experiments include any one or more combinations of XRD experiments, TOC experiments, RO experiments, rock pyrolysis experiments, SEM electron microscopy experiments, low-temperature liquid nitrogen adsorption experiments and low-temperature carbon dioxide adsorption experiments.

8. The method for evaluating deep shale sweet spots based on high temperature and high pressure rheological experiments according to claim 7, characterized in that: Specifically, step S6 is to comprehensively organize the experimental results after the high-temperature and high-pressure rheological experiment with the shale XRD experimental results and the TOC content analysis results to form a deep shale recoverability evaluation index.

9. The method for evaluating deep shale sweet spots based on high temperature and high pressure rheological experiments according to claim 7, characterized in that: According to the X-ray diffraction analysis method of clay minerals and common non-clay minerals in sedimentary rocks SY / T5163-2018, the shale XRD experiment was carried out using the Rigaku TTRⅢ multifunctional X-ray diffractometer to analyze the non-clay mineral components and clay mineral components of the shale under study.

10. The method for evaluating deep shale sweet spots based on high temperature and high pressure rheological experiments according to claim 7, characterized in that: Organic carbon analysis was carried out according to the determination method of total organic carbon in sedimentary rocks in GB / T19145-2003. The shale was subjected to pyrolysis experiment using LECO CS-230 carbon-sulfur analyzer to analyze the TOC content of the shale.

11. The method for evaluating sweet spots in deep shale based on high temperature and high pressure rheological experiments according to claim 7, characterized in that: With reference to SY / T 5162-1997 rock sample scanning electron microscope analysis method, a Zeiss Sigma500 field emission scanning electron microscope was used to observe the shale microstructure and analyze the shale pore and fracture microstructure.

12. The method for evaluating sweet spots in deep shale based on high temperature and high pressure rheological experiments according to claim 7, characterized in that: According to GB / T 21650.3-2011 Determination of pore size distribution and porosity of solid materials by mercury injection and gas adsorption method, part 3, analysis method of micropores by gas adsorption method, low-temperature CO2 and N2 adsorption experiments were carried out on shale using ASAP 2460 surface area and porosity analyzer and its supporting MicroActive software to analyze the pore distribution and adsorption characteristics of shale.

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