Method for reducing fracture pressure of shale oil super-long horizontal well during volume fracturing

Abstract

The invention belongs to the technical field of petroleum and natural gas development, and particularly provides a shale oil ultra-long horizontal well volume fracturing fracture pressure reduction method which comprises the following steps: taking rock samples underground at different positions of a horizontal section of an ultra-long horizontal well, and testing initial physical property parameters, mineral component contents and rock mechanical parameters of the rock samples; a rock sample and liquid CO2 soaking experiment is carried out under the reservoir temperature and pressure condition, rock mechanical parameters of the rock sample after the experiment are tested, rock damage factors are defined, the fracture reduction capacity is evaluated according to the rock damage factors, and a fracturing section is selected; and establishing a shale oil reservoir geologic model based on the reservoir physical property parameters of the selected fracturing section, performing CO2 injection productivity numerical simulation, and determining the optimal CO2 injection rate. The problems that an existing method for reducing the fracture pressure of the shale oil ultra-long horizontal well is small in application object range, large in construction risk, low in operation efficiency and high in cost are solved, the CO2 injection amount is optimized, the dual effects of reducing the fracture pressure and increasing the yield through CO2 are achieved, and the construction risk and the operation cost are reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of petroleum and natural gas development, and in particular relates to a method for reducing the fracture pressure of an ultra-long horizontal well of shale oil by volume fracturing. Background Art

[0002] The commercial development of shale oil has further accelerated the energy independence of the United States and pushed the wave of energy revolution to the world. China's shale oil resources are extremely rich. In 2020, PetroChina evaluated that the technically recoverable shale oil resources in China were 145×10 8 t, mainly distributed in large basins such as the Ordos Basin, Junggar Basin, Bohai Bay Basin, and Songliao Basin, with huge development potential. China has made breakthrough progress in continuous research on shale oil exploration and development technology. In 2019, Changqing Oilfield discovered the 1 billion-ton unconventional shale oil Qingcheng Oilfield in the Ordos Basin. Among them, long horizontal wells + volume fracturing are the technical direction for shale oil and gas to increase single-well production. At the same time, the reserves in sensitive areas such as water sources and forest edges reach 177 million tons and are difficult to mobilize. For this part of the reserves, ultra-long horizontal wells (more than 3000m) are currently the engineering and technical means to effectively mobilize reserves in sensitive areas. A large number of mine practices have confirmed that ultra-long horizontal well volume fracturing faces technical difficulties in high wellbore pressure. Long-term construction brings huge challenges to fracturing tools and wellbore pressure bearing. Among them, the fracture pressure is the core factor affecting high-pressure construction, and it is urgent to explore the method of breaking pressure. At present, there are mainly the following methods to reduce the fracture pressure of horizontal wells:

[0003] (1) Guo Jianchun et al. (Guo Jianchun, Li Xinyang, Wang Kun et al. A method and application for reducing reservoir fracture pressure based on discharge shock wave technology, patent number: CN202210961319.5). This method first obtains the initial fracture pressure of the reservoir rock; secondly, calculates the single discharge energy of the discharge device under different capacitances, different initial voltages and different conversion efficiencies, and obtains the rock fracture pressure of the rock under different discharge energies and after different discharge times, calculates the influence parameters of the discharge energy on the rock fracture pressure, and thus determines the discharge energy attenuation coefficient. Finally, by determining the degree to which the shock wave energy affects the reformed layer section at different perforation densities, the value of the discharge shock wave reducing the rock fracture pressure under different discharge energies, different discharge times and different perforation densities of the reformed layer section is calculated.

[0004] (2) Li Ning et al. (Li Ning, Wang Haibo, Li Fengxia et al. A fracturing method for reducing the fracture pressure of hot dry rock formations, patent number: CN202110330008.4). The method firstly drills to the target layer in the hot dry rock development area, completes the well with an open hole, and lowers the fracturing tubing into the well to a predetermined depth. The fracturing tubing is provided with a packer and a temperature and pressure sensor; secondly, cold water is continuously injected into the fracturing tubing to continuously cool the rock on the wall of the open hole section, forming microcracks on the wall of the well until the formation temperature drops to the preset temperature, and cold water is repeatedly injected back and forth to perform a cyclic low-temperature shock on the rock on the wall of the well, thereby achieving the purpose of reducing the fracture pressure.

[0005] (3) Gou Bo et al. (Gou Bo, Guo Jianchun, Yu Ting. A new method for calculating the fracture pressure of rocks reduced by acid damage [J]. Journal of Central South University (Acta Scientiarum Naturalium Universitatis Sinica, 2015, 46(01): 271-281). This method aims at the bottleneck problem of fracture in deep and tight oil and gas reservoirs. It uses the fact that the pore space increases after the rock is damaged by strong acid. The change of porosity is defined as the damage variable, and the damage variable is introduced into the fracture pressure prediction model. The damage variable is calculated using the sandstone acidizing model, and the fracture pressure after acid damage is predicted based on the damage variable, forming a fracture pressure change chart.

[0006] The representative method (1) mentioned above uses a discharge pulse wave to reduce the fracture pressure, but the method requires a discharge device to be installed at the wellhead, and a long-term discharge physical method is required to stimulate the reservoir rock. This method takes a considerable amount of time and is very inefficient. At the same time, the reduction in fracture pressure is very small. It is an ideal method, which is very different from reality and difficult to promote and apply. Method (2) is a method for hot dry rock to form microcracks in the rock by alternately injecting cold and hot water. It is a method for special rock bodies of hot dry rock. It also has low operating efficiency and a small range of applicable objects. Method (3) uses acid to reduce the fracture pressure. It is a commonly used method, but it has high acid corrosion and risk factors, and is difficult to promote and apply in unconventional volume fracturing. Therefore, a new method for reducing fracture pressure that can be promoted and applied is needed to provide strong support for the efficient development of shale oil. Summary of the invention

[0007] The method for reducing the fracture pressure of an ultra-long horizontal well of shale oil by volume fracturing provided by the present invention aims to overcome the problems in the prior art of reducing the fracture pressure of an ultra-long horizontal well of shale oil, such as a small scope of application, high construction risk, low operating efficiency and high cost.

[0008] To this end, the present invention provides a method for reducing the fracture pressure of an ultra-long horizontal well of shale oil by volume fracturing, comprising the following steps:

[0009] S1. Take rock samples from different positions in the horizontal section of the ultra-long horizontal well, and test the initial physical properties, mineral content, and rock mechanics parameters of the rock samples;

[0010] S2. The rock samples were subjected to liquid CO under reservoir temperature and pressure conditions. 2 Immersion experiment, and test the rock mechanical parameters of the rock samples after the experiment, define the rock damage factor, evaluate the fracture reduction capacity and select the fracturing stage according to the rock damage factor;

[0011] S3. Establish a shale oil reservoir geological model based on the reservoir physical property parameters of the selected fracturing stage, and conduct CO 2 Numerical simulation of injection capacity to determine CO 2 Optimal injection volume.

[0012] Preferably, the method for preparing the rock sample in step S1 is: preparing the rock of the shale reservoir section into a standard rock sample of target diameter and length, and drying the standard rock sample to a constant weight to obtain the rock sample.

[0013] Preferably, the initial physical property parameters in step S1 include porosity and permeability, the mineral component content includes the whole rock mineral content and clay mineral content, and the rock mechanics parameters include the rock fracture strength value of the rock sample before the experiment.

[0014] Preferably, the rock mechanics parameter testing method is to classify shales with initial physical parameters and mineral composition content within the target difference range into one category, and mark the shales in the same category as target rock samples and parallel rock samples, respectively; wherein the target rock samples are used for rock mechanics parameter testing before the experiment to obtain the rock fracture strength value of the rock samples before the experiment.

[0015] Preferably, step S2 comprises the following steps:

[0016] S2.1. Determine the relationship between rock sample and liquid CO based on formation stress and formation temperature 2 Immersion test loading test confining pressure and test temperature;

[0017] S2.2, based on the determined experimental confining pressure and experimental temperature, using CO 2 The fracture pressure reduction potential evaluation experimental device conducts experiments on parallel rock samples to obtain the rock failure strength value of the rock samples after the experiment;

[0018] S2.3. Define the rock damage factor according to the rock failure strength value of the rock sample before the experiment and the rock failure strength value of the rock sample after the experiment. The larger the rock damage factor value, the stronger the fracture reduction ability;

[0019] S2.4. Sort the rock damage factors and define that when the damage factor value is greater than the target value, the location of the rock sample is the selected fracturing section.

[0020] Preferably, the CO 2The experimental device for evaluating the potential for reducing fracture pressure includes a constant speed and constant pressure pump, an intermediate container, a vacuum pump, a reactor, a heater, a core holder, a cushion block and a confining pressure pump. The constant speed and constant pressure pump is connected to the inlet of the reactor through the intermediate container, the vacuum pump's exhaust port is connected to pipeline two between the intermediate container and the reactor through pipeline one, the liquid outlet of the reactor is connected to one end of the core holder, the core holder is connected to the confining pressure pump, the core is connected to the inside of the core holder, one end of the core contacts the medium in the reactor, the other end of the core is connected to one end of the cushion block, the other end of the cushion block is connected to the drainage pipeline, the outsides of the reactor and the core holder are connected to the heater, and the reactor is provided with an outlet pipeline.

[0021] Preferably, the step S2.2 comprises the following steps:

[0022] S2.2.1 Load the parallel rock samples into the core holder, apply the determined experimental confining pressure using a confining pressure pump, and heat the reactor, core and core holder to the determined experimental temperature using a heater;

[0023] S2.2.2. Use a constant speed and constant pressure pump to transfer liquid CO 2 Inject the liquid CO into the reactor through the intermediate container. 2 Target time of contact and immersion with the core end face;

[0024] S2.2.3. After the soaking is completed, the parallel rock samples after the experiment are taken out and subjected to triaxial rock mechanics testing to obtain the rock failure strength value of the rock samples after the experiment.

[0025] Preferably, the calculation formula for defining the damage factor value is:

[0026] Where: F r is the rock damage factor, dimensionless; R 0 is the rock failure strength value of the rock sample after the experiment, MPa; R 1 is the rock fracture strength value of the rock sample after the experiment, MPa.

[0027] Preferably, the defined damage factor value is greater than the target value of 0.5.

[0028] Preferably, step S3 comprises the following steps:

[0029] S3.1. Collect the basic reservoir parameters of the selected fracturing stage: including target reservoir burial depth, reservoir thickness, permeability, porosity, oil saturation, reservoir temperature, reservoir pressure and crude oil viscosity;

[0030] S3.2, establish geological model: according to the basic reservoir parameters of step S3.1, use reservoir numerical simulation software to establish the reservoir geological model where the horizontal well is located, and obtain the distribution of permeability field, porosity field, oil saturation field and formation pressure field of the reservoir where the horizontal well is located;

[0031] S3.3. Optimize fracture parameters: Carry out production capacity numerical simulation based on geological model, obtain the correlation between different fracture half-lengths and the first cumulative oil production of a single well, and obtain the optimal fracture half-length; on this basis, simulate the correlation between different conductivity and the first cumulative oil production of a single well under the optimal fracture half-length, and obtain the optimal fracture conductivity;

[0032] S3.4. Optimizing CO 2 Injection volume: Input the optimal fracture parameters in step S3.3 into the geological model in step S3.2 and perform different CO 2 Numerical simulation of injection capacity and production capacity, obtaining different CO 2 The correlation between injection volume and pressure sweep area of ​​the selected fracturing stage is used to determine the CO 2 Optimal injection volume.

[0033] Beneficial effects of the present invention:

[0034] 1. The method for reducing the fracture pressure of shale oil ultra-long horizontal well volume fracturing provided by the present invention comprises the following steps: S1, taking rock samples at different positions of the horizontal section of the ultra-long horizontal well, and testing the initial physical property parameters, mineral component content, and rock mechanical parameters of the rock samples; S2, mixing the rock samples with liquid CO under reservoir temperature and pressure conditions; 2 Soaking experiment, and testing the rock mechanical parameters of the rock samples after the experiment, defining the rock damage factor, evaluating the fracture reduction capacity and selecting the fracturing section according to the rock damage factor; S3, establishing the shale oil reservoir geological model based on the reservoir physical property parameters of the selected fracturing section, and conducting CO 2 Numerical simulation of injection capacity to determine CO 2 The core of this method is to optimize the potential section for reducing the fracture pressure, and optimize the CO 2 Injection volume, achieving CO 2 The dual effects of reducing the fracture pressure and increasing production greatly reduce the construction risk and operating cost, and can be extended to unconventional similar reservoirs to reduce the fracture pressure and efficiently utilize CO 2 Advantages.

[0035] 2. The method for reducing the fracture pressure by volume fracturing of ultra-long horizontal wells of shale oil provided by the present invention quantitatively evaluates the CO 2 The potential for reducing fracture pressure is achieved by optimizing the fracturing potential section. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The present invention will be further described in detail below with reference to the accompanying drawings.

[0037] Figure 1 It is a flow chart of a method for reducing the fracture pressure by volume fracturing of ultra-long horizontal wells of shale oil;

[0038] Figure 2 It is CO 2 Schematic diagram of the structure of the experimental device for evaluating the potential of reducing the burst pressure;

[0039] Figure 3 is the correlation diagram between fracture half-length and cumulative oil production in the first year;

[0040] Figure 4 is the correlation diagram between fracture conductivity and cumulative oil production in the first year;

[0041] Figure 5 It is different CO 2 Correlation diagram between injection volume and pressure sweep surface.

[0042] Explanation of the accompanying drawings: 1. constant speed and constant pressure pump; 2. constant speed and constant pressure pump outlet valve; 3. intermediate container; 4. intermediate container outlet valve; 5. vacuum pump; 6. vacuum pump inlet valve; 7. reactor; 8. heater; 9. reactor outlet valve; 10. core; 11. core clamp; 12. pad; 13. confining pressure pump; 14. core clamp outlet valve. DETAILED DESCRIPTION

[0043] The principles and features of the present invention are described below in conjunction with the accompanying drawings.

[0044] Embodiment 1:

[0045] like Figure 1 As shown, a method for reducing the fracture pressure of an ultra-long horizontal well of shale oil by volume fracturing comprises the following steps:

[0046] S1. Take rock samples from different positions in the horizontal section of the ultra-long horizontal well, and test the initial physical properties, mineral content, and rock mechanics parameters of the rock samples;

[0047] Preferably, the method for preparing the rock sample in step S1 is: preparing the rock of the shale reservoir section into a standard rock sample of target diameter and length, and drying the standard rock sample to a constant weight to obtain the rock sample.

[0048] Specifically, the rocks of the shale reservoir section are made into standard rock samples with a diameter of 2.5 cm and a length of 5 cm, and the standard rock samples are placed in an oven at 100° C. and dried to constant weight.

[0049] Preferably, the initial physical property parameters in step S1 include porosity and permeability, the mineral component content includes the whole rock mineral content and clay mineral content, and the rock mechanics parameters include the rock fracture strength value of the rock sample before the experiment.

[0050] Specifically, the porosity and permeability of the rock sample after drying in step S1 are tested using a helium porosity automatic tester and an ultra-low permeability tester, respectively. The mineral content of the rock sample in step S1 is tested using an X-ray diffractometer, including the whole rock mineral content and the clay mineral content.

[0051] Preferably, the rock mechanics parameter testing method is to classify shales with initial physical parameters and mineral composition content within the target difference range into one category, and mark the shales in the same category as target rock samples and parallel rock samples, respectively; wherein the target rock samples are used for rock mechanics parameter testing before the experiment to obtain the rock fracture strength value of the rock samples before the experiment.

[0052] S2. The rock samples were subjected to liquid CO under reservoir temperature and pressure conditions. 2 Immersion experiment, and test the rock mechanical parameters of the rock samples after the experiment, define the rock damage factor, evaluate the fracture reduction capacity and select the fracturing stage according to the rock damage factor;

[0053] Preferably, step S2 comprises the following steps:

[0054] S2.1. Determine the relationship between rock sample and liquid CO based on formation stress and formation temperature 2 Immersion test loading test confining pressure and test temperature;

[0055] Specifically, the determination method is as follows: the experimental loading confining pressure is determined by equations (1) to (4), and the reservoir temperature is the experimental temperature

[0056] σ' z =σ z -αP p (1)

[0057] σ' H =σ H -αP p (2)

[0058] σ' h =σ h -αP p (3)

[0059] σ c =(σ' z +σ' H +σ' h ) / 3 (4)

[0060] Where: σ' z is the vertical effective stress, MPa; σ' H is the maximum horizontal effective principal stress, MPa; σ' h is the minimum horizontal effective principal stress, MPa; σ zis the vertical stress, MPa; σ H is the maximum horizontal principal stress, MPa; σ h is the minimum horizontal principal stress, MPa; α is the effective stress coefficient, decimal; σ c is the experimental confining pressure, MPa; P P is the formation pressure, MPa.

[0061] S2.2, based on the determined experimental confining pressure and experimental temperature, using CO 2 The fracture pressure reduction potential evaluation experimental device conducts experiments on parallel rock samples to obtain the rock failure strength value of the rock samples after the experiment;

[0062] Preferably, Figure 2 As shown, the CO 2 The experimental device for evaluating the potential for reducing fracture pressure includes a constant speed and constant pressure pump 1, an intermediate container 3, a vacuum pump 5, a reactor 7, a heater 8, a core holder 11, a cushion block 12 and a confining pressure pump 13. The constant speed and constant pressure pump 1 is connected to the inlet of the reactor 7 through the intermediate container 3, the exhaust port of the vacuum pump 5 is connected to the pipeline 2 between the intermediate container 3 and the reactor 7 through the pipeline 1, the liquid outlet of the reactor 7 is connected to one end of the core holder 11, the core holder 11 is connected to the confining pressure pump 13, the core is connected inside the core holder 11, one end of the core contacts the medium in the reactor 7, the other end of the core is connected to one end of the cushion block 12, the other end of the cushion block 12 is connected to the drainage pipeline, the outside of the reactor 7 and the core holder 11 are both connected to the heater 8, and the reactor 7 is provided with an outlet pipeline.

[0063] Specifically, the outlet of the constant speed and constant pressure pump 1 is connected to the constant speed and constant pressure pump outlet valve 2, the outlet of the intermediate container 3 is connected to the intermediate container outlet valve 4, the exhaust port of the vacuum pump 5 is connected to the vacuum pump inlet valve 6, the outlet of the reactor 7 is connected to the reactor outlet valve 9, the pad 12 is a cylindrical pad, and the contact surface between the cylindrical pad and the core is provided with a guide groove, and the center of the pad 12 is provided with a hole 12-1 for fluid flow, and the fluid can flow to the core clamp outlet valve 14 through the hole 12-1.

[0064] The core 10 is placed in a lithology holder 11, one end surface of the core 10 contacts the experimental liquid, and the other end surface contacts the pad 12. During the experiment, the confining pressure is applied to the core 10 by the confining pressure pump 13; the experimental liquid and the core 10 in the reactor 9 are heated by the heating jacket 8. The structure is simple and the operation is convenient.

[0065] Preferably, the step S2.2 comprises the following steps:

[0066] S2.2.1 Load the parallel rock samples into the core holder 11, apply the determined experimental confining pressure by using the confining pressure pump 13, and heat the reactor 7, the core and the core holder 11 to the determined experimental temperature by using the heater 8;

[0067] S2.2.2. Use constant speed and constant pressure pump 1 to pump liquid CO 2 Injected into the reactor 7 through the intermediate container 3, the liquid CO 2 Contacting and soaking with the core end face for a target time; specifically, the target soaking time is 5 days;

[0068] S2.2.3. After the soaking is completed, the parallel rock samples after the experiment are taken out and subjected to triaxial rock mechanics testing to obtain the rock failure strength value of the rock samples after the experiment.

[0069] Preferably, the calculation formula for defining the damage factor value is:

[0070] In formula (5): F r is the rock damage factor, dimensionless; R 0 is the rock failure strength value of the rock sample after the experiment, MPa; R 1 is the rock fracture strength value of the rock sample after the experiment, MPa.

[0071] S2.3. Define the rock damage factor according to the rock failure strength value of the rock sample before the experiment and the rock failure strength value of the rock sample after the experiment. The larger the rock damage factor value, the stronger the fracture reduction ability;

[0072] S2.4. Sort the rock damage factors and define that when the damage factor value is greater than the target value, the location of the rock sample is the selected fracturing section.

[0073] Preferably, the defined damage factor value is greater than the target value of 0.5.

[0074] S3. Establish a shale oil reservoir geological model based on the reservoir physical property parameters of the selected fracturing stage, and conduct CO 2 Numerical simulation of injection capacity to determine CO 2 Optimal injection volume.

[0075] Preferably, step S3 comprises the following steps:

[0076] S3.1. Collect the basic reservoir parameters of the selected fracturing stage: including target reservoir burial depth, reservoir thickness, permeability, porosity, oil saturation, reservoir temperature, reservoir pressure and crude oil viscosity;

[0077] S3.2, establish geological model: according to the basic reservoir parameters of step S3.1, use the reservoir numerical simulation software Eclipse to establish the reservoir geological model where the horizontal well is located, and obtain the distribution of permeability field, porosity field, oil saturation field and formation pressure field of the reservoir where the horizontal well is located;

[0078] S3.3. Optimize fracture parameters: Carry out production capacity numerical simulation based on geological model, obtain the correlation between different fracture half-lengths and the first cumulative oil production of a single well, and obtain the optimal fracture half-length; on this basis, simulate the correlation between different conductivity and the first cumulative oil production of a single well under the optimal fracture half-length, and obtain the optimal fracture conductivity;

[0079] S3.4. Optimizing CO 2 Injection volume: Input the optimal fracture parameters in step S3.3 into the geological model in step S3.2 and perform different CO 2 Numerical simulation of injection capacity and production capacity, obtaining different CO 2 The correlation between injection volume and pressure sweep area of ​​the selected fracturing stage is used to determine the CO 2 Optimal injection volume.

[0080] Compared with the prior art, the present invention has the following beneficial effects:

[0081] The present invention proposes a method for reducing the fracture pressure of ultra-long horizontal wells in shale oil reservoirs by volume fracturing. This method quantitatively evaluates the CO 2 The potential to reduce the fracture pressure is achieved to optimize the potential fracturing section; on this basis, the reservoir numerical simulation method quantitatively optimizes the CO 2 injection volume; the core of this method is to optimize the potential section for reducing the fracture pressure, and optimize the CO 2 Injection volume, achieving CO 2 The dual effects of reducing the fracture pressure and increasing production greatly reduce the construction risk and operating costs, and can be extended to unconventional similar reservoirs to reduce the fracture pressure. 2 Advantages.

[0082] Embodiment 2:

[0083] According to the attached Figure 3-5 A method for reducing the fracture pressure by volume fracturing of an ultra-long horizontal well in a shale oil reservoir in a certain block is taken as an example to describe the specific implementation mode of the present invention in detail and illustrate the practicability of the method.

[0084] A certain test well QH1 well adopts volume fracturing transformation, with a horizontal section length of 3050m, an oil layer drilling rate of 73.2%, a reservoir temperature of 58.9℃, a reservoir burial depth of 2005m, a formation pore pressure of 15.8MPa, a maximum horizontal well principal stress of 33.0MPa, a minimum horizontal principal stress of 26.0MPa, and a vertical stress of 20MPa, which illustrates the specific implementation process of the present invention.

[0085] A method for reducing the fracture pressure by volume fracturing of an ultra-long horizontal well of shale oil, comprising the following steps:

[0086] S1. Take rock samples from different locations in the horizontal section of the ultra-long horizontal well, and test the initial physical properties, mineral content, and rock mechanics parameters of the rock samples; specifically:

[0087] The preparation method of the rock sample is as follows: the rock of the shale reservoir section is made into a standard rock sample with a diameter of 2.5 cm and a length of 5 cm, and the standard rock sample is placed in an oven at 100°C and dried to a constant weight. Some standard rock samples are shown in Table 1. The present invention uses the typical standard rock samples in Table 1 to illustrate the CO 2 Specific implementation process of the method for reducing burst pressure.

[0088] Initial physical property parameter test: The porosity and permeability of the rock sample after drying described in step S1 were tested using a helium porosity automatic tester and an ultra-low permeability tester, respectively, as shown in Table 1.

[0089] Table 1 Porosity and permeability test table of shale oil samples

[0090]

[0091]

[0092] Mineral content test: The mineral content of the rock sample in step S1 was tested using an X-ray diffractometer, including the whole rock mineral content and clay mineral content, see Table 2.

[0093] Table 2 Whole rock mineral and clay mineral composition of shale samples

[0094]

[0095] Testing of rock mechanical parameters: Shale samples with similar physical parameters and mineral content were grouped into one category and marked as target rock samples S1, S3, and S5, and their corresponding parallel rock samples were S2, S4, and S6. The rock failure strength values ​​Ro of the target rock samples S1, S3, and S5 before the experiment were 95.6 MPa, 85.7 MPa, and 70.6 MPa, respectively.

[0096] S2. The rock samples were subjected to liquid CO under reservoir temperature and pressure conditions. 2 Immersion experiment, and test the rock mechanical parameters of the rock sample after the experiment, define the rock damage factor, evaluate the fracture reduction capacity and select the fracturing section according to the rock damage factor; specifically include the following steps:

[0097] S2.1. Determine the relationship between rock sample and liquid CO based on formation stress and formation temperature 2The immersion test was loaded with experimental confining pressure and experimental temperature. The experimental temperature can be determined to be 58.9°C from the formation temperature. According to the formation pore pressure of QH1 well of 15.8 MPa, the maximum horizontal well principal stress of 33.0 MPa, the minimum horizontal principal stress of 26.0 MPa, the vertical stress of 20 MPa, and the reservoir effective stress coefficient of 0.5, the experimental loading confining pressure can be determined to be 15.0 MPa using formulas (1) to (4).

[0098] S2.2, based on the determined experimental confining pressure and experimental temperature, using CO 2 The fracture pressure reduction potential evaluation experimental device conducts experiments on parallel rock samples to obtain the rock failure strength values ​​of the rock samples after the experiments; specifically:

[0099] S2.2.1 Load the parallel rock sample S2 of the target rock sample S1 into the core holder 11, apply the determined experimental confining pressure by using the confining pressure pump 13, and heat the reactor 7, the core and the core holder 11 to the determined experimental temperature by using the heater 8;

[0100] S2.2.2. Use constant speed and constant pressure pump 1 to pump liquid CO 2 Injected into the reactor 7 through the intermediate container 3, the liquid CO 2 Soak in contact with the core end for 5 days;

[0101] S2.2.3. After the immersion is completed, the parallel rock sample S2 after the experiment is taken out and subjected to triaxial rock mechanics test to obtain the rock failure strength value R of the rock sample after the experiment. 1 The rock failure strength value R1 after the parallel rock sample S4 experiment is 40.5 MPa, and the rock failure strength value R2 after the parallel rock sample S5 experiment is 70.8 MPa. 1 It is 59.6MPa.

[0102] S2.3, according to the rock failure strength value R of the target rock sample before the test 0 The rock failure strength value R of the rock sample after the parallel rock sample test 1 The rock damage factor is defined by using formula (5), where the rock damage factor of target rock sample S1 (parallel rock sample S2) after the experiment is 0.26, the rock damage factor of target rock sample S3 (parallel rock sample S4) after the experiment is 0.53, and the rock damage factor of target rock sample S5 (parallel rock sample S6) after the experiment is 0.16.

[0103] S2.4. Sort the rock damage factors and define that when the damage factor value is greater than the target value, the location of the rock sample is the selected fracturing section. Sort the rock damage factors after the experiment as S3>S1>S5, where the S3 damage factor value is greater than 0.5, which is the ultra-long horizontal well CO 2 The fracturing section that can reduce the breaking pressure is preferred.

[0104] S3. Establish a shale oil reservoir geological model based on the reservoir physical property parameters of the selected fracturing stage, and conduct CO 2 Numerical simulation of injection capacity to determine CO 2 Optimal injection volume.

[0105] The specific steps include:

[0106] S3.1. Collect the basic reservoir parameters of the selected fracturing stage: including target reservoir depth, reservoir thickness, permeability, porosity, oil saturation, reservoir temperature, reservoir pressure and crude oil viscosity; see Table 3

[0107] Table 3 Basic parameters of the reservoir where the QH1 well is located

[0108] parameter Numeric parameter Numeric Reservoir depth (m) 2005 Oil saturation (%) 70.8 Reservoir thickness (m) 10.0 Volume coefficient of formation crude oil ( / ) 1.25 Reservoir pressure (MPa) 15.8 Crude oil viscosity (mPa.s) 1.25 Reservoir temperature (℃) 58.9 Horizontal section length (m) 3050 Porosity(%) 4.2 Permeability (mD) 0.25

[0109] S3.2, establish geological model: according to the basic reservoir parameters of step S3.1, use the reservoir numerical simulation software Eclipse to establish the reservoir geological model where the horizontal well is located, and obtain the distribution of permeability field, porosity field, oil saturation field and formation pressure field of the reservoir where the horizontal well is located;

[0110] S3.3. Optimize fracture parameters: Carry out productivity numerical simulation based on geological model to obtain the correlation between different fracture half-lengths and the first cumulative oil production of a single well. Figure 3 The optimal fracture half-length is 140 m. Based on this, the correlation between different conductivity and the first cumulative oil production of a single well under the optimal fracture half-length is simulated. See the attached figure. Figure 4 , the optimal fracture conductivity is 20D.cm;

[0111] S3.4. Optimizing CO 2 Injection volume: Input the optimal fracture parameters in step S3.3 into the geological model in step S3.2 and perform different CO 2 Numerical simulation of injection capacity and production capacity, obtaining different CO 2 The correlation between injection volume and pressure sweep area of ​​the selected fracturing stage is shown in the attached Figure 5 , when the pressure wave area increases with CO 2 When the injection volume increases slightly, the production increase effect is the best. 2 The best injection volume is 300m 3 .

[0112] In the description of the present invention, it should be understood that if there is an orientation or positional relationship indicated by terms such as "inside", it is based on the orientation or positional relationship shown in the drawings, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation. Therefore, the terms describing the positional relationship in the drawings are only used for illustrative purposes and cannot be understood as a limitation on the present invention.

[0113] The above examples are merely illustrative of the present invention and do not constitute a limitation on the protection scope of the present invention. All designs that are the same or similar to the present invention fall within the protection scope of the present invention.

Claims

1. A method for reducing the fracture pressure by volume fracturing of ultra-long horizontal wells of shale oil. Features: The steps include: S1. Take rock samples from different positions in the horizontal section of the ultra-long horizontal well, and test the initial physical properties, mineral content, and rock mechanics parameters of the rock samples; S2. The rock samples were subjected to liquid CO under reservoir temperature and pressure conditions. 2 Immersion experiment, and test the rock mechanical parameters of the rock samples after the experiment, define the rock damage factor, evaluate the fracture reduction capacity and select the fracturing stage according to the rock damage factor; S3. Establish a shale oil reservoir geological model based on the reservoir physical property parameters of the selected fracturing stage, and conduct CO 2 Numerical simulation of injection capacity to determine CO 2 Optimal injection volume.

2. The method for reducing the fracture pressure by volume fracturing of ultra-long horizontal wells of shale oil according to claim 1, Features: The method for preparing the rock sample in step S1 is as follows: the rock of the shale reservoir section is made into a standard rock sample with a target diameter and length, and the standard rock sample is dried to a constant weight to obtain a rock sample.

3. The method for reducing the fracture pressure by volume fracturing of ultra-long horizontal wells of shale oil according to claim 1, Features: In step S1, the initial physical property parameters include porosity and permeability, the mineral component content includes the whole rock mineral content and clay mineral content, and the rock mechanical parameters include the rock fracture strength value of the rock sample before the experiment.

4. The method for reducing the fracture pressure by volume fracturing of ultra-long horizontal wells of shale oil according to claim 3, Features: The rock mechanics parameter testing method is to classify shales with initial physical parameters and mineral composition content within the target difference range into one category, and mark the shales in the same category as target rock samples and parallel rock samples respectively; the target rock samples are used for rock mechanics parameter testing before the experiment to obtain the rock fracture strength value of the rock samples before the experiment.

5. The method for reducing the fracture pressure by volume fracturing of ultra-long horizontal wells of shale oil according to claim 4, Features: The step S2 comprises the following steps: S2.

1. Determine the relationship between rock sample and liquid CO based on formation stress and formation temperature 2 Immersion test loading test confining pressure and test temperature; S2.2, based on the determined experimental confining pressure and experimental temperature, using CO 2 The fracture pressure reduction potential evaluation experimental device conducts experiments on parallel rock samples to obtain the rock failure strength value of the rock samples after the experiment; S2.

3. Define the rock damage factor according to the rock failure strength value of the rock sample before the experiment and the rock failure strength value of the rock sample after the experiment. The larger the rock damage factor value, the stronger the fracture reduction ability; S2.

4. Sort the rock damage factors and define that when the damage factor value is greater than the target value, the location of the rock sample is the selected fracturing section.

6. The method for reducing the fracture pressure by volume fracturing of ultra-long horizontal wells of shale oil according to claim 5, Features: The CO 2 The experimental device for evaluating the potential of reducing fracture pressure comprises a constant speed and constant pressure pump (1), an intermediate container (3), a vacuum pump (5), a reactor (7), a heater (8), a core holder (11), a cushion block (12) and a confining pressure pump (13). The constant speed and constant pressure pump (1) is connected to the inlet of the reactor (7) through the intermediate container (3). The suction port of the vacuum pump (5) is connected to the pipeline 2 between the intermediate container (3) and the reactor (7) through the pipeline 1. The liquid outlet of the reactor (7) is connected to one end of the core holder (11). The core holder (11) is connected to the confining pressure pump (13). The core is connected inside the core holder (11). One end of the core contacts the medium in the reactor (7). The other end of the core is connected to one end of the cushion block (12). The other end of the cushion block (12) is connected to the drainage pipeline. The outsides of the reactor (7) and the core holder (11) are both connected to the heater (8). The reactor (7) is provided with an outlet pipeline.

7. The method for reducing the fracture pressure by volume fracturing of ultra-long horizontal wells of shale oil according to claim 6, Features: The step S2.2 comprises the following steps: S2.2.1 Load the parallel rock samples into the core holder (11), apply a determined experimental confining pressure using a confining pressure pump (13), and heat the reactor (7), the core and the core holder (11) to a determined experimental temperature using a heater (8); S2.2.

2. Use a constant speed and constant pressure pump (1) to transfer liquid CO 2 The liquid CO is injected into the reactor (7) through the intermediate container (3) to 2 Target time of contact and immersion with the core end face; S2.2.

3. After the soaking is completed, the parallel rock samples after the experiment are taken out and subjected to triaxial rock mechanics testing to obtain the rock failure strength value of the rock samples after the experiment.

8. The method for reducing the fracture pressure by volume fracturing of ultra-long horizontal wells of shale oil as claimed in claim 5, Features: The calculation formula for defining the damage factor value is: Where: F r is the rock damage factor, dimensionless; R 0 is the rock failure strength value of the rock sample after the experiment, MPa; R 1 is the rock fracture strength value of the rock sample after the experiment, MPa.

9. The method for reducing the fracture pressure by volume fracturing of ultra-long horizontal wells of shale oil as claimed in claim 5, Features: The defined damage factor value is greater than the target value, in which the target value is 0.

5.

10. The method for reducing the fracture pressure by volume fracturing of ultra-long horizontal wells of shale oil according to claim 1, Features: The step S3 comprises the following steps: S3.

1. Collect the basic reservoir parameters of the selected fracturing stage: including target reservoir burial depth, reservoir thickness, permeability, porosity, oil saturation, reservoir temperature, reservoir pressure and crude oil viscosity; S3.2, establish geological model: according to the basic reservoir parameters of step S3.1, use reservoir numerical simulation software to establish the reservoir geological model where the horizontal well is located, and obtain the distribution of permeability field, porosity field, oil saturation field and formation pressure field of the reservoir where the horizontal well is located; S3.

3. Optimize fracture parameters: Carry out productivity numerical simulation based on geological model, obtain the correlation between different fracture half-lengths and the first cumulative oil production of a single well, and obtain the optimal fracture half-length; on this basis, simulate the correlation between different conductivity and the first cumulative oil production of a single well under the optimal fracture half-length, and obtain the optimal fracture conductivity; 、 S3.

4. Optimizing CO 2 Injection volume: Input the optimal fracture parameters in step S3.3 into the geological model in step S3.2 and perform different CO 2 Numerical simulation of injection capacity and production capacity, obtaining different CO 2 The correlation between injection volume and pressure sweep area of ​​the selected fracturing stage is used to determine the CO 2 Optimal injection volume.

Images