A method for limiting the duration of opening of natural horizontal hydraulic fractures in reservoirs

Abstract

The present invention belongs to the field of petroleum geology and fluid geochemistry, and discloses a method for limiting the duration of opening of natural horizontal hydraulic fractures in a reservoir. By determining that horizontal tensile fractures are developed in the rock, collecting the rock sample and making fluid inclusion thin sections, determining the characteristics of the massive crystals filling the fractures, combining fluid inclusion petrographic observation with laser Raman spectroscopy analysis technology, selecting a methane-rich fluid inclusion combination as the research object, conducting subsequent microthermometry analysis and PVT simulation, combining the PVT simulation results with the basin evolution history, and finally obtaining the duration of opening of natural horizontal hydraulic fractures based on the identification of the existence of ultra-static rock fluid pressure. The method of the present invention is low in cost and highly operational, and provides a certain basis for judging the enrichment and loss of shale gas and tight sandstone gas.

Description

Technical Field

[0001] The present invention belongs to the field of petroleum geology and fluid geochemistry, and in particular relates to a method for limiting the opening duration of natural horizontal hydraulic fractures in a reservoir based on combined research of methane-rich fluid inclusions. Background Art

[0002] In oil and gas basins, natural hydraulic fractures serve as important pathways and reservoirs for the migration of gaseous hydrocarbons within tight reservoirs. The persistence of their opening is crucial for controlling reservoir permeability and fluid migration and accumulation efficiency. Traditional research methods primarily rely on petrological observations, tectonic stress field simulations, and isotope dating. These methods either describe fracture morphology through core-scale and field profile observations, indirectly inferring the formation time of fractures through basin-scale paleo-tectonic stress field analysis, or accurately define the time of fracture opening through isotope dating. These fracture research methods provide strong constraints on the formation time of fractures, but cannot yet effectively analyze the dynamic opening process and duration of fractures.

[0003] The opening and closing process of cracks is often accompanied by the cementation of minerals. The lattice defects developed inside the crystals during their crystallization and growth process will capture the original fluid and form a carrier that can record the original formation fluid information, namely the fluid inclusion. As a natural carrier of formation fluid information, the combination of primary methane-rich fluid inclusions records the temperature, pressure and fluid composition during the cementation of the cracks. This information provides the possibility for studying the duration of crack opening. However, there are still certain gaps in the current research on the duration of hydraulic fracturing cracks based on the analysis of fluid inclusion combinations. To this end, the present invention attempts to establish a systematic and effective limitation method for studying the duration of the opening of natural horizontal tensile cracks in the reservoir from the perspective of the combination of methane-rich fluid inclusions. Summary of the Invention

[0004] Natural fractures serve as effective migration pathways and storage spaces for gaseous hydrocarbons within deep, low-porosity, low-permeability, tight rock reservoirs (such as shale and tight sandstone). The duration of their open state directly impacts the accumulation and dissipation of shale and tight sandstone gas. This invention provides a method for defining the duration of the opening of natural horizontal hydraulic fractures within a reservoir based on combined studies of methane-rich fluid inclusions. Through microthermometry, Raman spectroscopy analysis, and basin simulation, combined with overpressure evolution characteristics, this method dynamically matches fracture opening with cementation time. This method can accurately and effectively define the duration of the tensile opening of natural horizontal hydraulic fractures within a reservoir, achieved through the following technical solutions:

[0005] Observe and collect cores from tight rock reservoirs containing horizontal fractures; prepare thin sections of fluid inclusions (thickness 90±10μm) that are polished on both sides without glass coverslips; observe the crystal size, morphology, and structure of the minerals filling the horizontal fractures under a microscope. Combined with the morphology of both sides of the fracture, identify the opening direction of the fracture, determine the horizontal tensile fracture, and determine the cementation stage of the massive crystalline minerals in the fracture.

[0006] The petrographic characteristics of the primary fluid inclusion assemblages developed inside the above-mentioned massive crystals were observed and analyzed. According to the phase composition and volume ratio of the inclusions, combined with the identification of methane using laser Raman spectroscopy analysis technology, the types of fluid inclusions developed in different assemblages were clarified. On this basis, the methane-rich fluid inclusion assemblages required for the study were identified, namely, the fluid inclusion assemblages composed of gas-liquid two-phase gas-rich inclusions, gas-liquid two-phase liquid-rich inclusions, and single-phase inclusions.

[0007] Microscopic temperature measurement analysis was performed on the gas-liquid two-phase fluid inclusions in the methane-rich fluid inclusion combination to test the uniform temperature range of the inclusions in the combination; the Raman shift of the single-phase methane inclusions was obtained by Raman spectroscopy analysis, and the methane density inside the single-phase inclusions was calculated based on the peak shift; finally, the methane inclusion isochoric epitaxy method was used to perform PVT simulation, and the minimum uniform temperature in the above inclusion combination was used as the capture temperature to calculate the fluid pressure during the capture process, that is, the fluid temperature and pressure conditions and their evolution during the crystallization of bulk crystals.

[0008] The trapping temperatures obtained from the PVT simulations were projected onto the burial history curves of individual wells to determine the geological time range and paleo-burial depth variations corresponding to the massive crystal cementation process in natural horizontal tensile fractures within the reservoir. The temporal coupling between the massive crystal cementation process and the fracture opening process in natural horizontal fractures was identified by calculating the lithostatic pressure during massive crystal crystallization using paleo-burial depth data and comparing this lithostatic pressure with the trapping pressure obtained from fluid inclusion PVT simulations. When the fluid pressure is less than the overlying rock load, crystal crystallization occurs after the horizontal fracture opening has ceased, and the fluid inclusion assemblage formed by crystal growth cannot record the fluid information at the time of fracture opening. When the fluid pressure is greater than the lithostatic pressure, an excess lithostatic fluid pressure exists. At this time, the fluid overpressure can effectively and actively expand the horizontal fracture, forming a natural horizontal hydraulic fracture tensile fracture, indicating that the horizontal fracture opening and the massive crystal cementation process occur simultaneously.

[0009] On the basis of the above, the geological history period and time range corresponding to the existence of super-static rock fluid pressure in horizontal fractures are clarified, so as to accurately limit the duration of the opening of natural horizontal hydraulic fracturing tensile fractures in the reservoir.

[0010] According to the technical solution of the embodiment of the present invention, it is first necessary to determine at the core scale whether the rock sample contains horizontal hydraulic fractures, and prepare thin sections of fluid inclusions containing the fractures, observe the petrographic characteristics of the fluid inclusion combination, and systematically identify the various types of fluid inclusions developed in the massive diagenetic minerals in the horizontal fractures of the reservoir. During the research process, it is necessary to focus on screening two types of methane-containing target inclusions: gas-liquid two-phase inclusions and single-phase inclusions. For the target inclusions, laser Raman spectroscopy analysis and micro-temperature measurement are carried out in sequence, combined with basin simulation analysis, to clarify the reservoir fluid overpressure characteristics and its evolution, and establish a temporal dynamic correspondence between the opening of horizontal fractures and cementation, so as to accurately define the duration of the opening.

[0011] The present invention has the following beneficial effects:

[0012] 1. This study uses petrographic observations of primary fluid inclusions, combined with laser Raman spectroscopy, microthermometry, and PVT simulation techniques, to directly correlate the hyperstatic fluid pressure recorded by the primary methane-rich fluid inclusions with the fracture opening and closing process, establishing a temporally coupled relationship between the massive crystal cementation process and the fracture opening process in natural horizontal fractures.

[0013] 2. The method of the present invention is low-cost and highly operable. It can achieve dynamic matching between the opening of horizontal fractures and the cementation time during the migration and accumulation of gaseous hydrocarbons, providing technical support for limiting the opening duration of natural horizontal hydraulic fracturing tensile fractures in reservoirs, thereby providing a certain basis for judging the enrichment and loss of shale gas and tight sandstone gas. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 A schematic flow chart of a method for limiting the opening duration of natural horizontal hydraulic fractures in a reservoir based on a combined study of methane-rich fluid inclusions provided by the present invention;

[0015] Figure 2 This is a schematic diagram of a natural horizontal hydraulic fracturing tensile fracture in the present invention;

[0016] Figure 3 Schematic diagram of the combination of massive mineral crystals and methane-rich fluid inclusions studied in the present invention;

[0017] Figure 4 Schematic diagram for limiting the duration of tensile fractures in natural horizontal hydraulic fracturing. DETAILED DESCRIPTION

[0018] The present invention provides a method for limiting the opening duration of natural horizontal hydraulic fractures in tight rock reservoirs. The method focuses on the evolution of natural horizontal fractures in deep, low-porosity, low-permeability reservoirs in oil and gas basins. The method uses the combination of primary methane-rich fluid inclusions captured during the growth of massive crystals (fracture cementation) as the research object. The method uses a variety of fluid inclusion analysis experimental techniques, such as microthermometry and laser Raman spectroscopy, combined with basin simulation and evolution, to accurately limit the opening duration of natural horizontal hydraulic fractures in the reservoir.

[0019] To more clearly and thoroughly illustrate the objectives, technical solutions, and advantages of this application, the following text provides a more detailed description of the technical solutions of this application using a specific implementation case study, namely, horizontal fractures developed in the organic-rich shale of the Wufeng Formation and Longmaxi Formation of the Silurian System in the Fengdu area of the Sichuan Basin, along with accompanying drawings. It is important to emphasize that the examples listed herein are merely illustrative and are not intended to be a comprehensive coverage of all possible scenarios of the technical solutions.

[0020] Figure 1 A flow chart of a method for limiting the opening duration of natural horizontal hydraulic fractures in a reservoir based on a combined study of methane-rich fluid inclusions provided by the present invention includes:

[0021] S1. Core sample collection, thin section preparation, and identification of horizontal tensile fractures

[0022] Collect core sections containing horizontal fractures from tight rock reservoir core samples and observe and identify the fracture geometry, occurrence, and structural characteristics at a hand-scale (with the aid of a magnifying glass). Prepare thin sections of fluid inclusions, 90 ± 10 μm thick, double-sided polished, and uncovered, preferably from areas with wide fractures. Observe the crystal size, morphology, and structure of the minerals filling the horizontal fractures under a microscope. Combined with the morphological characteristics of the fracture surfaces on either side, determine the fracture opening direction. If microscopic petrographic evidence indicates that the horizontal fracture opening displacement vector is perpendicular to the fracture surface strike, the fracture is tensile.

[0023] S2. Determination of the Stages of Massive Crystal Mineral Filling in Horizontal Fractures

[0024] Polarized light microscopy and other technical systems are used to systematically observe the crystal morphology, dissolution, and replacement of massive minerals filling horizontal tensile fractures. Combined with the color alternation and zoning mutation characteristics in the cathodoluminescence image, the crystallization stages of massive crystalline minerals in the fractures are accurately determined in the order from the two sides of the horizontal fracture to the center of the fracture (foreign growth petrography). Figure 2As shown in the figure, A is a model diagram of natural horizontal hydraulic fracture tensile fractures in the reservoir and the primary massive minerals calcite and quartz filling; B and C are single polarized light photographs and corresponding cathodoluminescence images of the filling minerals in the horizontal fractures of the Wufeng-Longmaxi Formation shale in the implementation case. The massive crystal filling sequence in the horizontal fracture is calcite precipitated on the fracture walls on both sides (cathodoluminescence is bright orange-red) and quartz precipitated in the center of the fracture (no luminescence), that is, calcite crystallization occurs earlier than quartz crystallization;

[0025] S3. Identification of methane-rich fluid inclusion assemblages in massive crystals of various periods

[0026] We selected well-preserved, appropriately sized, and clearly phase-characteristic primary fluid inclusions from each period of massive crystals and analyzed their phase combination types. We then used laser Raman spectroscopy to identify the composition of the inclusions, using the Raman spectral peak characteristics to identify the internal composition of the inclusions. This allowed us to identify the research objects we needed—methane-rich fluid inclusion combinations, including gas-liquid two-phase gas-rich inclusions, gas-liquid two-phase liquid-rich inclusions, and single-phase methane inclusions. Figure 3 As shown, Figure 3 Schematic diagrams of primary methane-rich fluid inclusions captured during the precipitation of early massive calcite crystals and late massive quartz crystals in the above-mentioned implementation cases. The left side of each figure is a model diagram of the massive euhedral crystals and their primary methane-rich fluid inclusions; the right side is an example diagram of the massive crystals and their primary methane-rich fluid inclusions;

[0027] S4. Reconstruction of fluid temperature and pressure conditions during bulk mineral cementation

[0028] Based on Raman spectroscopy analysis, the Raman shift of the CH bond symmetric stretching peak of single-phase methane inclusions was obtained, and the methane density inside the single-phase inclusions was calculated. Microthermometry analysis was performed on the liquid-rich gas-liquid two-phase inclusions in the methane-rich fluid inclusion combination screened by S3. The homogenization temperature of the first fluid inclusion to be homogenized was selected as the capture temperature. The PVT simulation was performed using the methane inclusion isochoric line epitaxy method. The fluid pressure during the capture process of the fluid inclusion combination was calculated using the above-mentioned capture temperature, and the fluid temperature-pressure field during the massive crystal mineralization was reconstructed. In the implementation case, the minimum homogenization temperature of early calcite crystallization obtained from the combined test of five methane-rich fluid inclusions ranged from 89-106°C (average 97°C), which is generally higher than the minimum homogenization temperature of late quartz precipitation obtained from the combined test of six methane-rich fluid inclusions, which ranged from 78-104°C (average 85°C). The fluid pressure during early calcite precipitation obtained from PVT simulations was 93-197MPa (140MPa), and the fluid pressure during late quartz precipitation was 59-133MPa (average 88MPa).

[0029] S5. Analysis of the temporal coupling between the horizontal crack opening process and the massive crystal cementation process

[0030] The capture temperature data of the methane-rich fluid inclusion combination is combined with the basin evolution history to limit the geological time of the natural horizontal tensile fracture cementation process in the reservoir and the corresponding ancient burial depth. The gravity of the overlying rock load during the crystallization of the block crystal is calculated by the ancient burial depth. The fluid capture pressure obtained by PVT simulation is compared with the static rock pressure of the same period. When the capture pressure is greater than the static rock pressure of the same period, there is an excess static rock fluid pressure. At this time, the fluid overpressure can effectively and actively expand the horizontal fracture, which in turn causes the natural horizontal hydraulic fracture tensile fracture to continue to open. When the fluid capture pressure is less than the static rock pressure of the same period, the fluid pressure in the fracture cannot bear the overlying rock load, and the horizontal fracture expansion stops. At this time, the fluid inclusion combination formed by the crystal growth cannot be used as a record of the fluid information when the fracture continues to open. As in the implementation case, according to the above PVT simulation temperature results combined with the burial history of a single well ( Figure 4 ), respectively determine the geological time range corresponding to the early calcite and late quartz cementation processes (calcite cementation period 41-28 Ma, quartz cementation period 40-14 Ma) and their corresponding burial depths (h 41 、h 28 、h 40 and h 14 ), on this basis, the static rock pressure P is calculated respectively lithostatic =ρ rock gh, the calculation results show that the capture pressures (197MPa, 139MPa, 105MPa and 87MPa) obtained by PVT simulation are significantly higher than those of P lithostatic , that is, the fluid pressure is the superstatic rock fluid pressure, indicating that the primary fluid inclusions formed during the growth of massive calcite and quartz filling the fractures within this time range can record the fluid information when the fractures open;

[0031] S6. Limitation of the duration of hydraulic fracturing tensile crack opening

[0032] The time when the super-static rock fluid pressure exists is the time when the horizontal hydraulic fracture tension cracks are opened. By defining the geological history period and time range corresponding to the existence of super-static rock fluid pressure in the horizontal fractures through S5, the duration of the opening of the natural horizontal hydraulic fracture tension cracks in the reservoir can be accurately limited. Figure 2 and Figure 4 As shown, hyperstatic fluid pressure supported fracture opening, accompanied by early massive calcite cementation and later massive quartz growth. The calcite cementation period (41-28 Ma) and the quartz cementation period (40-14 Ma) were both periods of sustained horizontal fracture opening under hyperstatic fluid pressure. This indicates that the duration of horizontal hydraulically fractured tensile fracture opening is 27 Ma.

[0033] Through the experimental scheme provided in this explanatory example, the presence of horizontal tensile fractures in the rock was first confirmed at the core scale. Subsequently, the rock sample was collected and fluid inclusion thin sections were prepared. The size, morphology, and order of the massive crystals filling the fractures were determined using techniques such as cathodoluminescence. Combining fluid inclusion petrographic observation with laser Raman spectroscopy, a methane-rich fluid inclusion assemblage was selected as the research object. Microthermometry analysis was then conducted on the inclusions, and the minimum homogenization temperature was selected as the capture temperature of the fluid inclusion assemblage. The internal methane density of the single-phase methane inclusions was calculated based on the Raman shift of the previously obtained single-phase methane inclusions. PVT simulation was performed using the methane inclusion isochoresistive epitaxy method to obtain the capture pressure of the inclusion assemblage, that is, the fluid pressure at the time of massive crystal cementation. The PVT simulation results were combined with the basin evolution history to determine the time range of massive crystal cementation in the horizontal tensile fractures and the paleostatic load of the contemporaneous formation. By comparing the fluid pressure with the paleostatic load, the duration of the superstatic fluid pressure was finally determined, that is, the duration of the opening of the natural horizontal hydraulic fracture tensile fracture.

[0034] The method of the present invention has low cost and strong operability, and can provide a new research direction and technical support for limiting the opening duration of natural horizontal hydraulic fracturing tension cracks in reservoirs.

[0035] Those skilled in the art will understand that the discussion of the above embodiments is merely illustrative and is not intended to limit the scope of the present invention to these examples. Within the spirit and principles of the present invention, the technical features of the above embodiments or different embodiments may be combined, the steps may be implemented in any order, and many other variations exist for the various aspects of the present invention described above, which are not provided in detail for the sake of clarity. Any omissions, modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A method for limiting the duration of opening of natural horizontal hydraulic fractures in a reservoir, characterized in that: The following steps are involved: S1. Core sample collection, thin section preparation, and identification of horizontal tensile fractures Core sections with horizontal fractures were collected from tight rock reservoir core samples. The fracture geometry, occurrence, and structural characteristics were observed at the hand-scale. Double-sided polished, uncovered fluid inclusion sections were prepared from areas with wide fractures. The crystal size, morphology, and structure of the minerals filling the horizontal fractures were observed under a microscope. The fracture opening direction was determined based on the morphological characteristics of the fracture surfaces on both sides. S2. Determination of the Stages of Massive Crystal Mineral Filling in Horizontal Fractures Polarized light microscopy was used to systematically observe the crystal morphology, dissolution, and replacement of massive minerals filling horizontal tensile fractures. Combined with the color alternation and zoning mutation characteristics in cathodoluminescence images, the crystallization stages of massive crystalline minerals in the fractures were accurately determined from the sides of the horizontal fractures toward the center. S3. Identification of methane-rich fluid inclusion assemblages in massive crystals of various periods We selected well-preserved, appropriately sized, and clearly characterized primary fluid inclusions from each period of bulk crystals and analyzed their phase combinations. We then used Raman spectral peak characteristics to identify the internal components of the inclusions, selecting the methane-rich fluid inclusion combinations we were looking for, including gas-liquid two-phase gas-rich inclusions, gas-liquid two-phase liquid-rich inclusions, and single-phase methane inclusions. S4. Reconstruction of fluid temperature and pressure conditions during bulk mineral cementation Based on Raman spectroscopy analysis, the Raman shift of the CH bond symmetric stretching peak of single-phase methane inclusions was obtained, and the methane density inside the single-phase inclusions was calculated. Microthermometry analysis was performed on the liquid-rich gas-liquid two-phase inclusions within the methane-rich fluid inclusion combination screened by S3. The homogenization temperature of the first fluid inclusion to be homogenized was selected as the capture temperature. The PVT simulation was performed using the methane inclusion isochoric epitaxy method. The fluid pressure during the capture process of the fluid inclusion combination was calculated using the above capture temperature, and the fluid temperature-pressure field during the massive crystal mineralization was reconstructed. S5. Analysis of the temporal coupling between the horizontal crack opening process and the massive crystal cementation process The capture temperature data of the methane-rich fluid inclusion assemblage are combined with the basin evolution history to define the geological time of the cementation process of natural horizontal tensile fractures in the reservoir and the corresponding paleo-burial depth. The gravity of the overlying rock load during the crystallization of the massive crystals is calculated based on the paleo-burial depth. The fluid capture pressure obtained by PVT simulation is compared with the concurrent lithostatic pressure. When the capture pressure is greater than the concurrent lithostatic pressure, there is an excess lithostatic fluid pressure. At this time, the fluid overpressure effectively and actively expands the horizontal fractures, which in turn causes the continuous opening of the natural horizontal hydraulic fracture tensile fractures. When the fluid capture pressure is less than the concurrent lithostatic pressure, the fluid pressure in the fracture cannot bear the overlying rock load, and the horizontal fracture expansion stops. At this time, the fluid inclusion assemblage formed by crystal growth cannot be used to record the fluid information when the fracture is continuously open. S6. Limitation of the duration of hydraulic fracturing tensile crack opening The duration of the existence of super-static rock fluid pressure is the time when horizontal hydraulic fracture tension cracks are opened. By using S5 to clearly define the geological history period and time range corresponding to the existence of super-static rock fluid pressure in horizontal fractures, the duration of the opening of natural horizontal hydraulic fracture tension cracks in the reservoir can be accurately defined.

2. The method for limiting the opening duration of natural horizontal hydraulic fractures in a reservoir according to claim 1, characterized in that: The thickness of the fluid inclusion thin section prepared in S1 is 90±10μm.

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