Method for limiting opening duration of natural horizontal hydrofracture tensional fracture in reservoir

Abstract

The invention belongs to the field of petroleum geology and fluid geochemistry, and discloses a method for limiting opening duration of a natural horizontal hydraulic fracturing tensible fracture in a reservoir. The method comprises the following steps: determining that a horizontal tensional fracture is developed in rock, collecting a rock sample, making a fluid inclusion slice, determining the characteristics of filling blocky crystals in the fracture, combining fluid inclusion lithofacies observation and laser Raman spectroscopy analysis technologies, and selecting a methane-rich fluid inclusion combination as a research object. And carrying out subsequent microscopic temperature measurement analysis and PVT simulation, combining a PVT simulation result with a basin evolution history, and finally obtaining the opening duration of the natural horizontal hydraulic fracturing tensible fracture on the basis of judging the existence of the ultra-quiet rock fluid pressure. The method is low in cost and high in operability, and a certain judgment basis is provided for enrichment and loss of shale gas and tight sandstone gas.

Description

Technical Field

[0001] The present invention belongs to the fields of petroleum geology and fluid geochemistry, and particularly relates to a method for determining the opening duration of natural horizontal hydraulic fracturing tensile fractures in a reservoir based on the study of methane-rich fluid inclusion assemblages. Background Art

[0002] In hydrocarbon-bearing basins, natural hydraulic fracturing fractures are important channels and reservoirs for the migration of gaseous hydrocarbon fluids in tight reservoirs. The persistence of their opening is the key to controlling reservoir permeability and fluid migration and accumulation efficiency. Traditional research methods mainly focus on petrological observation, tectonic stress field simulation, and isotope dating and other technical means. Either the fracture morphology is described by core-scale and field profile observations, and the formation time of the fractures is indirectly inferred by combining the changes in the paleo-tectonic stress field at the basin scale; or the filling minerals in the fractures are accurately dated by isotope dating technology, and then the opening time of the fractures is determined. The above fracture research methods have strong limitations on the formation time of fractures, but they cannot effectively analyze the dynamic opening process and duration of fractures.

[0003] The opening and closing process of fractures is often accompanied by mineral cementation. During the crystal crystallization growth process, lattice defects developed inside the crystal will trap the original fluid, forming a carrier that can record the information of the original formation fluid, namely fluid inclusions. The assemblage of primary methane-rich fluid inclusions, as a natural carrier of formation fluid information, records the temperature, pressure, and fluid composition at the time of fracture cementation. These information provide the possibility for the study of the fracture opening duration process. However, there is still a certain gap in the current research on the continuous opening time of hydraulic fracturing fractures based on the analysis of fluid inclusion assemblages. Therefore, the present invention attempts to establish a systematic and effective determination method for the opening duration of natural horizontal tensile fractures in the reservoir from the perspective of methane-rich fluid inclusion assemblages. Summary of the Invention

[0004] Natural fractures are effective migration channels and reservoirs for gaseous hydrocarbons in deep low-porosity and low-permeability tight rock reservoirs (such as shale and tight sandstone). The duration of their open state directly affects the enrichment and dissipation of shale gas and tight sandstone gas. The present invention provides a method for determining the opening duration of natural horizontal hydraulic fracturing tensile fractures in a reservoir based on the study of methane-rich fluid inclusion assemblages. By means of microthermometry, Raman spectroscopy analysis and basin simulation, combined with the characteristics of overpressure evolution, the dynamic matching of fracture opening and cementation time is realized. The opening duration of natural horizontal hydraulic fracturing fractures in the reservoir can be accurately and effectively determined, and is achieved through the following technical solutions: Observe and collect cores of tight rock reservoirs with horizontal fractures; prepare fluid inclusion thin sections (with a thickness of 90±10μm) that are polished on both sides and without cover glasses; observe under the microscope the crystal size, morphology, and structure of the minerals filling the horizontal fractures, and in combination with the morphology on both sides of the fractures, identify the opening direction of the fractures, determine the horizontal tensile fractures, and define the cementation stages of the massive crystal minerals in the fractures.

[0005] Observe and analyze the petrographic characteristics of the primary fluid inclusion assemblages developed within the above massive crystals. According to the phase composition and volume ratio of the inclusions, and in combination with the identification of methane by laser Raman spectroscopy, clarify the types of fluid inclusions developed within different assemblages. On this basis, identify the methane-rich fluid inclusion assemblages required for the study, 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.

[0006] Conduct microscopic temperature measurement analysis on the gas-liquid two-phase fluid inclusions within the methane-rich fluid inclusion assemblages to measure the range of homogenization temperatures of the inclusions within the assemblage; obtain the Raman shift of the single-phase methane inclusions through Raman spectroscopy analysis, and calculate the methane density inside the single-phase inclusions based on the peak shift; finally, perform PVT simulation using the isochore extrapolation method of methane inclusions, and use the minimum homogenization temperature in the above inclusion assemblage as the trapping temperature to calculate the fluid pressure during the trapping process, that is, the fluid temperature and pressure conditions and their evolution during the crystallization process of the massive crystals.

[0007] Project the trapping temperature obtained from the above PVT simulation onto the single-well burial history curve to determine the geological time range and the change in paleo-burial depth corresponding to the massive cementation process of the natural horizontal tensile fractures in the reservoir. Identify the time coupling relationship between the massive crystal cementation process and the fracture opening process in the natural horizontal fractures: calculate the lithostatic pressure during the crystallization process of the massive crystals through the paleo-burial depth data, and compare the lithostatic pressure with the trapping pressure obtained from the PVT simulation of the fluid inclusions. When the fluid pressure is less than the overlying rock load, it indicates that the crystal crystallization occurs after the horizontal fracture opening stops, and the fluid inclusion assemblage formed by crystal growth cannot record the fluid information during fracture opening; when the fluid pressure is greater than the lithostatic pressure, that is, there is an over-lithostatic fluid pressure, at this time the fluid overpressure can effectively and actively expand the horizontal fractures to form natural horizontal hydraulic fracturing tensile fractures, that is, the opening of the horizontal fractures occurs simultaneously with the massive crystal cementation process.

[0008] On the above basis, clarify the geological historical period and time range corresponding to the existence of over-lithostatic fluid pressure in the horizontal fractures, so as to accurately define the duration of the opening of the natural horizontal hydraulic fracturing tensile fractures in the reservoir.

[0009] According to the technical solution of the embodiment of the present invention, first, it is necessary to determine that the rock sample contains horizontal hydraulic fracturing tensile fractures at the core scale, prepare a thin section of fluid inclusions containing such fractures, observe the petrographic characteristics of the fluid inclusion assemblage, and systematically identify 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 microthermometry are carried out in sequence, combined with basin simulation analysis, to clarify the overpressure characteristics and evolution of reservoir fluids, establish a time-dynamic correspondence relationship between the opening and cementation of horizontal fractures, and thus accurately define the duration of opening.

[0010] The present invention has the following beneficial effects: 1. The present invention conducts petrographic observations on primary fluid inclusions, combines laser Raman spectroscopy analysis, microthermometry technology, and PVT simulation technology, directly correlates the overpressure of the primary methane-rich fluid inclusion assemblage with the fracture opening and closing process, and establishes a time coupling relationship between the cementation process of massive crystals and the fracture opening process in natural horizontal fractures; 2. The method of the present invention has low cost and strong operability, can realize the dynamic matching of the tensile opening and cementation time of horizontal fractures during the migration and accumulation of gaseous hydrocarbons, provides technical support for determining the opening duration of natural horizontal hydraulic fracturing tensile fractures in the reservoir, and thus provides a certain basis for judging the enrichment and dissipation of shale gas and tight sandstone gas. Description of the Drawings

[0011] Figure 1 It is a schematic flow chart of a method for determining the opening duration of natural horizontal hydraulic fracturing tensile fractures in a reservoir based on the study of methane-rich fluid inclusion assemblages provided by the present invention; Figure 2 It is a schematic diagram of a natural horizontal hydraulic fracturing tensile fracture of the present invention; Figure 3 It is a schematic diagram of the massive mineral crystal and its methane-rich fluid inclusion assemblage studied by the present invention; Figure 4 It is a schematic diagram for determining the duration of natural horizontal hydraulic fracturing tensile fractures. Detailed Embodiments

[0012] The present invention provides a method for determining the opening duration of natural horizontal hydraulic fracturing tensile fractures in a tight rock reservoir. Aiming at the evolution process of natural horizontal fractures in deep low-porosity and low-permeability reservoirs in oil and gas-bearing basins, taking the primary methane-rich fluid inclusion assemblage captured during the growth process of massive crystals (fracture cementation process) as the research object, various fluid inclusion analysis experimental techniques such as microthermometry and laser Raman spectroscopy analysis technology are used, combined with basin simulation and evolution, so as to accurately determine the opening duration of natural horizontal hydraulic fracturing tensile fractures in the reservoir.

[0013] In order to clearly and thoroughly present the objectives, technical solutions adopted, and advantages of this application, the following will elaborate on the technical solutions of this application in more detail with the help of specific implementation cases, namely the horizontal fractures developed in the organic-rich shale of the Wufeng Formation of the Ordovician - Longmaxi Formation of the Silurian in the Fengdu area of the Sichuan Basin, along with the accompanying drawings. It should be emphasized here that the examples listed in the text are only for illustrative purposes, are of an exemplary nature, and do not comprehensively cover all possible situations of the technical solutions.

[0014] Figure 1 The following is a schematic flow chart of a method for determining the opening duration of natural horizontal hydraulic fracturing tensile fractures in a reservoir based on the study of methane-rich fluid inclusion assemblages provided by the present invention, including: S1. Core sample collection, thin section preparation, and identification of horizontal tensile fractures Collect core segments with horizontal fractures from tight rock reservoir core samples, and observe and identify the geometric morphology, occurrence, and structural characteristics of the fractures at the hand specimen scale (magnifying glass can be used). Preferably, prepare fluid inclusion thin sections with a thickness of 90 ± 10 μm, double-sided polished, and without a cover glass in the area where the fractures are wider. Observe the crystal size, morphology, and structure of the minerals filled in the horizontal fractures under the microscope, and combine the morphological characteristics of the two sides of the fracture surface to identify the opening direction of the fracture. If microscopic petrographic evidence shows that the opening displacement vector of the horizontal fracture is perpendicular to the strike of the fracture surface, it represents that the fracture is in a tensile opening; S2. Determination of the periods of massive crystal mineral filling in horizontal fractures Use techniques such as polarized light microscopy to systematically observe the morphological features, dissolution, and replacement phenomena of massive mineral crystals filled in horizontal tensile fractures, and combine features such as color alternation and zoning mutations in the cathodoluminescence images to accurately determine the crystallization periods of massive crystal minerals in the fractures in the order from both sides of the horizontal fracture to the center of the fracture (growth petrography towards the center). As Figure 2 shown, where A is a model diagram of natural horizontal hydraulic fracturing tensile fractures in the reservoir and the filling of primary massive minerals calcite and quartz, B and C are single polarized light photographs of the minerals filled in the horizontal fractures of the Wufeng - Longmaxi Formation shale in the implementation case and the corresponding cathodoluminescence images. The sequence of massive crystal filling in the horizontal fracture is calcite (cathodoluminescence is bright orange-red) precipitated on both sides of the fracture wall first, and quartz (no luminescence) precipitated in the center of the fracture, that is, the crystallization of calcite is earlier than that of quartz; S3. Identification of methane-rich fluid inclusion assemblages in massive crystals of each period Select well-preserved, appropriately sized, and distinct-phase native fluid inclusions from each stage of massive crystals, analyze their phase combination types, and identify the inclusion compositions using laser Raman spectroscopy analysis technology, i.e., identify the internal compositions of inclusions based on the Raman spectral peak characteristics, and screen out the research objects we need - methane-rich fluid inclusion combinations, including gas-rich gas-liquid two-phase inclusions, liquid-rich gas-liquid two-phase inclusions, and single-phase methane inclusions. As Figure 3 shown, Figure 3 Figure 4 is a schematic diagram of the native methane-rich fluid inclusion combination captured during the precipitation of early massive calcite crystals and late massive quartz crystals in the above-mentioned implementation case. On the left side of each figure is a model diagram of the massive euhedral crystal and its native methane-rich fluid inclusions; on the right side is an example diagram of the massive crystal and its native methane-rich fluid inclusions; S4. Reconstruction of the fluid temperature and pressure conditions during the cementation process of massive minerals Based on the Raman shift of the symmetric stretching peak of the C-H bond in the single-phase methane inclusion obtained from Raman spectroscopy analysis, calculate the methane density inside the single-phase inclusion. Conduct microthermometry analysis on the liquid-rich gas-liquid two-phase inclusions in the methane-rich fluid inclusion combination screened in S3, select the homogenization temperature of the first homogenized fluid inclusion as the capture temperature, and use the isochore extrapolation method of methane inclusions for PVT simulation. Calculate the fluid pressure during the capture process of this fluid inclusion combination using the above capture temperature, and reconstruct the fluid temperature-pressure field during the mineralization of massive crystals. In the implementation case, the lowest homogenization temperature of the early calcite crystallization obtained from the test of 5 methane-rich fluid inclusion combinations ranges from 89 to 106 °C (average 97 °C), which is overall higher than the lowest homogenization temperature of 78 to 104 °C (average 85 °C) of the late quartz precipitation obtained from the test of 6 methane-rich fluid inclusion combinations; the fluid pressure during the early calcite precipitation obtained from PVT simulation is 93 to 197 MPa (140 MPa), and the fluid pressure during the late quartz precipitation ranges from 59 to 133 MPa (average 88 MPa); S5. Analysis of the time coupling between the opening process of horizontal fractures and the cementation process of massive crystals The captured temperature data of the methane-rich fluid inclusion combination are 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 fractures, and then cause the natural horizontal hydraulic fracturing tensile fractures 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 is continuously opened. 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 P lithostatic , that is, the fluid pressure is the superstatic rock fluid pressure, indicating that the primary fluid inclusions formed during the growth of the massive calcite and quartz filling the fractures within this time range can record the fluid information when the fractures are opened; S6. Limitation of the duration of hydraulic fracturing tensile fracture opening The time when the super-static rock fluid pressure exists is the time when the horizontal hydraulic fracture tension cracks are opened. Through S5, the geological history period and time range corresponding to the existence of super-static rock fluid pressure in the horizontal fractures are clearly defined, so as to accurately define the duration of the opening of the natural horizontal hydraulic fracture tension cracks in the reservoir. Figure 2 and Figure 4 As shown in the figure, the super-static fluid pressure supports the opening of cracks and is accompanied by the cementation of early blocky calcite and the growth of late blocky quartz. The calcite cementation period of 41-28 Ma and the quartz cementation period of 40-14 Ma are both in the period of continuous opening of horizontal cracks under the conditions of super-static fluid pressure, that is, the duration of the opening of horizontal hydraulic fracturing tensile cracks is 27 Ma.

[0015] Through the experimental scheme provided by this illustrative example, it is first determined at the core scale that there are horizontal tensile fractures in the rock. Subsequently, the rock sample is collected and a thin section of fluid inclusions is made. Through techniques such as cathodoluminescence, the crystal size, morphology, sequence, etc. of the massive crystals filled in the fractures are determined. Combining the petrographic observation of fluid inclusions and the laser Raman spectroscopy analysis technique, a combination of methane-rich fluid inclusions is selected as the research object, and then microthermometry analysis is carried out on it. The minimum homogenization temperature among them is selected as the trapping temperature of this fluid inclusion combination. Based on the Raman shift of the previously obtained single-phase methane inclusions, the internal methane density is calculated. The isochore extrapolation method of methane inclusions is adopted for PVT simulation to obtain the trapping pressure of this inclusion combination, that is, the fluid pressure at the time of massive crystal cementation. By combining the PVT simulation results with the basin evolution history, the time range at the time of massive crystal cementation in the horizontal tensile fractures and the paleo-lithostatic load of the contemporaneous formation are obtained. By comparing the fluid pressure with the paleo-lithostatic load, the duration of the over-lithostatic fluid pressure is finally obtained, which is also the duration of the opening of the natural horizontal hydraulic fracturing tensile fractures.

[0016] The method of the present invention has low cost and strong operability, and can provide a new research direction and technical support for determining the duration of the opening of natural horizontal hydraulic fracturing tensile fractures in a defined reservoir.

[0017] Those of ordinary skill in the art should understand that the discussions of the above embodiments are only exemplary and are not intended to imply that the scope of the present invention is limited to these examples; under the idea of the present invention, the technical features in the above embodiments or different embodiments can also be combined, and the steps can be implemented in any order, and there are many other variations in different aspects of the present invention as described above, which are not provided in detail for the sake of brevity. Any omission, modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for determining the opening duration of natural horizontal hydraulic fracturing tensile fractures in a reservoir, characterized in that, It includes the following steps: S1. Core sample collection, thin section preparation, and identification of horizontal tensile fractures Determine that horizontal tensile fractures develop in the tight reservoir at the scale of rock hand specimens, and collect the rock samples; make fluid inclusion thin sections of the rock samples containing horizontal fractures, with both sides of the thin sections polished and without cover glasses; S2. Determination of the filling stage of massive crystal minerals in horizontal fractures Use polarized light microscopy and cathodoluminescence technology to identify the morphology of horizontal tensile fractures, identify the fracture opening direction, determine the size, morphology, and structure of the massive crystal filling minerals in horizontal tensile fractures, and identify the types of massive cementing minerals in horizontal fractures; determine the stages of massive crystal minerals; S3. Identification of methane-rich fluid inclusion assemblages in massive crystals of each stage Conduct petrographic observation and analysis on the primary fluid inclusion assemblages trapped in the massive crystals developed in horizontal tensile fractures, including the phase composition and volume ratio of the inclusions, and identify methane by combining laser Raman spectroscopy analysis technology, clarify the types of fluid inclusions developed in different fluid inclusion assemblages, obtain the Raman shift of single-phase methane inclusions, and screen out the primary methane-rich fluid inclusion assemblages, including single-phase inclusions, gas-liquid two-phase gas-rich inclusions, and gas-liquid two-phase liquid-rich inclusions; S4. Reconstruction of the fluid temperature and pressure conditions during the massive mineral cementation process Conduct microthermometric analysis on the gas-liquid two-phase fluid inclusions in the primary methane-rich fluid inclusion assemblages screened in S3, test the homogenization temperature range of the inclusions in the assemblage, select the minimum homogenization temperature in the methane-rich fluid inclusion assemblage as the trapping temperature of the fluid inclusion assemblage, calculate the methane density inside the single-phase methane inclusions based on the methane Raman peak shift in the result of Raman spectroscopy analysis, conduct PVT simulation on the gas-phase methane inclusions using the methane inclusion isochore extrapolation method, and calculate the trapping pressure of the methane-rich fluid inclusion assemblage in combination with the previously obtained trapping temperature, that is, restore the fluid temperature and pressure conditions during the massive crystal cementation process; S5. Analysis of the time coupling between the horizontal fracture opening process and the massive crystal cementation process Combine the above PVT simulation results with the basin thermal evolution history of the area to determine the geological time range and paleo-burial depth change corresponding to the massive crystal cementation process of natural horizontal tensile fractures in the reservoir, and analyze the time coupling relationship between the massive crystal cementation process and the fracture opening process in natural horizontal fractures; S6. Determination of the duration of the hydraulic fracturing tensile fracture opening Calculate the paleo-lithostatic load during the crystal crystallization process through paleo-burial depth data, and analyze the static rock load and the trapping pressure obtained from the PVT simulation of fluid inclusions to obtain the fracture continuous opening time.

2. A method for defining the opening duration of natural horizontal hydraulic fracturing tensile fractures in a reservoir according to claim 1, characterized in that In step S1, the thickness of the fluid inclusion thin section preparation is 90±10μm.

3. The method for defining the opening duration of natural horizontal hydraulic fracturing tensile fractures in a reservoir according to claim 1, characterized in that, In step S6, the specific steps are: correspond the PVT simulation results of multiple groups of methane-rich fluid inclusion assemblages to the basin thermal evolution history, compare the paleo-lithostatic pressure during the massive crystal crystallization process with the trapping pressure, and determine the fracture continuous opening time on the basis of clarifying the existence of over-lithostatic fluid pressure.

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