Method for enhanced oil recovery by post-fracturing co2 huff and puff

Abstract

The application provides a method for improving oil recovery by CO2 huff and puff after fracturing, which comprises the following steps: step 1, establishing a CO2 huff and puff model; step 2, determining a reasonable fracture length according to a geological condition; step 3, simulating a fracturing fracture parameter under the reasonable fracture length according to the geological condition and a construction condition; step 4, optimizing a CO2 huff and puff round and a CO2 huff and puff amount; and step 5, carrying out a field block application and implementing the CO2 huff and puff. The method for improving oil recovery by CO2 huff and puff after fracturing adopts a numerical simulation means to determine construction information such as a reasonable fracture length and a CO2 huff and puff amount, provides support for improving oil production of small fault blocks and small sand bodies, and improves the oil recovery effect of the small fault blocks and the small sand bodies by means of the combination of artificial fracturing and CO2 huff and puff.

Description

Technical Field

[0001] This invention relates to the field of simulation technology for CO2 huff and puff to enhance oil recovery in low-permeability reservoirs, and in particular to a method for enhancing oil recovery through CO2 huff and puff after fracturing. Background Technology

[0002] Since the industrial era, with the development of industrial levels, the massive emissions of greenhouse gases, primarily carbon dioxide, have caused a series of environmental and ecological problems. According to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), the concentration of carbon dioxide in the atmosphere has risen from 280 ppm before the Industrial Revolution to 379 ppm in 2005, exceeding the range of natural variations over the past 650,000 years. Recently, measurements have shown that the concentration of carbon dioxide in the atmosphere has exceeded 415 ppm, setting a new record high. CO2 emission reduction has become a global research hotspot, and CO2 enhanced oil recovery and CO2 storage are among the currently practical and effective methods for emission reduction.

[0003] CO2 flooding technology is a relatively ideal method. CO2 is a superior oil displacement agent, and CO2 flooding has been proven to be an effective method for enhancing oil recovery. CO2 has the characteristics of low viscosity and easy injection. Moreover, after being injected into the formation and coming into contact with crude oil, it can cause the crude oil to expand in volume, reduce the viscosity of crude oil, extract light components, reduce the oil-water interfacial tension, and improve the mobility ratio, which can effectively improve the oil recovery rate. After CO2 mixes with crude oil, it can eliminate interfacial tension and significantly improve the recovery rate. Carbon dioxide capture-and-storage enhanced oil recovery (CCS-EOR) technology, through CO2 capture, injection into the formation for oil displacement, and recovery and reuse, permanently seals CO2 underground, which can effectively reduce the content of greenhouse gases and achieve a "win-win" situation of increasing crude oil production and storage. It is currently the most realistic way to reduce carbon emissions and utilize resources. The application of CO2 flooding technology is an important tool for large oil companies to carry out carbon emission reduction work and implement the national green and low-carbon development concept. Therefore, the development of CO2 flooding technology is in line with the national low-carbon development strategy.

[0004] However, many small fault-block reservoirs or blocks with incomplete well networks exist in oilfields. These blocks contain considerable reserves, but are difficult to exploit. CO2 huff and puff is an effective method for utilizing the crude oil in these reservoirs. However, CO2 huff and puff in low-permeability reservoirs suffers from low production rates and limited well control areas.

[0005] Patent CN114109304A describes a method for oil recovery using temporary plugging agent-assisted carbon dioxide huff and puff. The method involves the following steps: injecting a temporary plugging agent slug, displacing it with water, and waiting for it to solidify; injecting a carbon dioxide slug, displacing it with water; shutting down the well; and then starting production. This invention provides a method for oil recovery using temporary plugging agent-assisted carbon dioxide huff and puff, which can effectively seal the dominant seepage channels in the reservoir, preventing carbon dioxide from channeling along these channels and thus expanding the carbon dioxide sweep volume, allowing it to enter and function in areas rich in residual oil. During well production, the temporary plugging agent degrades under reservoir conditions, avoiding permanent damage to the reservoir, significantly improving the sealing strength, and facilitating the flow of crude oil after huff and puff. Unlike this patent, the main invention of this patent is the use of a temporary plugging agent to seal dominant channels and improve CO2 oil recovery.

[0006] Patent CN107288594A describes an injection system and method for improving carbon dioxide huff and puff efficiency. Belonging to the field of tertiary oil recovery technology, this patent improves the single-well carbon dioxide huff and puff efficiency by modifying the injection system and rationally controlling injection parameters to achieve the mixed injection of liquid carbon dioxide and oilfield chemicals into the well. It ensures sufficient contact between the injected gas and liquid, working together to improve the remaining formation oil and thus increasing reservoir recovery. Unlike this patent, the main invention of this patent is the injection of CO2 and chemical agents to improve flow conditions and enhance CO2 huff and puff efficiency.

[0007] Patent CN104453812A: Acidification Method for Enhancing Production After Carbon Dioxide Huff and Puff. This method includes the following steps: After the ground equipment is securely connected, a pressure test of 30 MPa is conducted, requiring 25 minutes of pressure holding without any punctures or leaks. This pressure test is repeated three times, and the pressure is vented after each test until it returns to zero; 30-50 ml of composite foam acid solution is injected. 3 Large-volume injection is used, with a gradual increase in injection rate; displacement fluid is injected after foam acid injection. This method prevents the "gas blockage" effect in the near-wellbore zone, improves the success rate of the measure, and increases the oil production of a single well. Unlike this patent, the main invention of this patent is the injection of composite foam acid to alleviate the gas blockage effect and increase oil production.

[0008] The existing technologies described above are significantly different from the present invention and have failed to solve the technical problem we want to address. Therefore, we have invented a new method for enhancing oil recovery by CO2 huff and puff after fracturing. Summary of the Invention

[0009] The purpose of this invention is to provide a method for improving oil recovery rate by combining artificial fracturing with CO2 huff and puff, thereby enhancing the oil recovery effect in small fault blocks and small sand bodies.

[0010] The objective of this invention can be achieved through the following technical measures: a method for enhancing oil recovery through CO2 huff and puff after fracturing, comprising:

[0011] Step 1: Establish a CO2 throughput model;

[0012] Step 2: Determine the appropriate crack length based on geological conditions;

[0013] Step 3: Based on geological and construction conditions, simulate the fracturing parameters under a reasonable fracture length;

[0014] Step 4: Optimize CO2 throughput rounds and CO2 throughput;

[0015] Step 5: Implement the mining block application and carry out CO2 throughput.

[0016] The objective of this invention can also be achieved through the following technical measures:

[0017] In step 1, establish a numerical model for CO2 throughput:

[0018]

[0019] Where φ is porosity; ρ j Let be the molar density of the j-th phase; t be time; x be the molar density of the j-th phase i y is the molar density of the i-th component in the oil phase; i S is the molar density of the i-th component in the gas phase; j v represents the saturation of the j-th phase; j Let q be the velocity of the j-th phase; i For the source and sink terms of component i, q w For water phase source and sink items.

[0020] In step 1, based on formula 1, the discrete crack model DFN is used to represent the crack. DFN reduces the dimension of the crack and can be written as:

[0021]

[0022] Where FEQ represents the seepage equation, Ω is the overall flow region, and Ω f For the crack region, Ω m For the bedrock region, e represents the crack aperture.

[0023] In step 2, fractures of different lengths were set up and CO2 huff and puff numerical simulations were performed. The optimal fracture length was determined by comparing the two parameters of oil production intensity and oil exchange rate.

[0024] In step 3, the simulated fracturing parameters under a reasonable fracture length include the fracture aperture and the distribution pattern of the fracture trajectory.

[0025] Step 3 includes:

[0026] Step 31, establish a crack propagation model;

[0027] Step 32: Use the maximum circumferential stress criterion to simulate the crack propagation direction;

[0028] Step 33: Calculate the aperture distribution and fracture trajectory of the hydraulic fracturing fracture under a reasonable fracture length using a fracture propagation model.

[0029] In step 31, the rock mass deforms under the combined action of fluid pressure and geostress within the fracture. The rock deformation control equations are as follows:

[0030] σ=C:ε(u)

[0031] in σ represents the stress tensor, f represents the body force, C is the elastic tensor matrix, ε is the strain tensor, and u is the displacement;

[0032] Boundary conditions include outer boundary conditions and crack boundary conditions, i.e.

[0033]

[0034] Where g is the positional displacement; Γ g Γ represents the outer boundary of the displacement. c For the crack boundary; σ s Shear stress; For constant shear stress; σ n denoted as normal stress; p as fluid pressure; x as position; and t as time.

[0035] In step 32, the maximum circumferential stress criterion is applied. When the stress intensity factor at the crack tip is greater than the rock toughness, the crack will propagate along the direction of maximum circumferential stress. According to the maximum circumferential stress criterion, the following conditions must be met:

[0036]

[0037] Where θ M K is the angle of maximum circumferential stress direction, i.e., the angle between the crack initiation direction and the crack surface; I K is the first type of stress intensity factor; II K is the first type of stress intensity factor. IC The rock strength.

[0038] In step 33, the geometric parameters of the crack are determined:

[0039]

[0040] Where q is the flow rate per unit thickness section; w is the fracture width; μ is the viscosity of the fracturing fluid; x is the displacement along the fracture propagation direction; and p represents the pressure.

[0041] In step 4, the fracture parameters are input into the CO2 huff and puff model to perform CO2 huff and puff numerical simulation. Different huff and puff cycles and different CO2 injection volumes are designed. The cumulative oil production and oil exchange rate are compared to determine the optimal huff and puff scheme.

[0042] In step 5, based on the optimized fracturing scheme, a reasonable fracture length is first created, and then CO2 injection is implemented according to the optimized production scheme.

[0043] This invention presents a method for enhancing oil recovery via CO2 huff and puff after fracturing, combining fracture propagation simulation and CO2 huff and puff simulation technologies. Fracture propagation simulation can more accurately describe fracture orientation, length, and aperture, offering good realism. Based on this, CO2 huff and puff simulation is conducted to optimize parameters such as well shut-in time and huff and puff cycle. Using this patented method, CO2 can more fully contact the oil through the fractures during the huff and puff process, exerting its expansion and viscosity-reducing effects, thereby improving development efficiency. This provides support for increasing oil production in small fault blocks and small sand bodies. Attached Figure Description

[0044] Figure 1 This is a flowchart of a specific embodiment of the method for enhancing oil recovery after fracturing using CO2 huff and puff according to the present invention;

[0045] Figure 2 This is a schematic diagram of CO2 huff and puff in a fracturing reservoir according to a specific embodiment of the present invention;

[0046] Figure 3 This is a schematic diagram illustrating the optimization of CO2 huff and puff parameters in a fracturing reservoir according to a specific embodiment of the present invention. Detailed Implementation

[0047] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0048] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the exemplary embodiments of the present invention. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, and / or combinations thereof.

[0049] like Figure 1 As shown, Figure 1 This is a flowchart of the method for enhancing oil recovery through CO2 huff and puff after fracturing according to the present invention. The method for enhancing oil recovery through CO2 huff and puff after fracturing includes:

[0050] S1. First, determine the appropriate crack length based on the geological conditions;

[0051] S2. Based on geological and construction conditions, simulate the fracturing parameters under a reasonable fracture length, including fracture aperture and fracture trajectory distribution.

[0052] S3. Establish a CO2 throughput model;

[0053] S4. Optimize CO2 throughput rounds and CO2 throughput;

[0054] S5. Implement mining block applications and carry out CO2 throughput engineering.

[0055] The following are several specific embodiments of the application of the present invention.

[0056] Example 1

[0057] In a specific embodiment 1 of the present invention, the method for enhancing oil recovery by CO2 huff and puff after fracturing includes the following steps:

[0058] First, determine the appropriate fracture length based on the geological conditions;

[0059] Based on geological and construction conditions, simulate the parameters of hydraulic fracturing under a reasonable fracture length, including fracture aperture and fracture trajectory distribution.

[0060] Optimize CO2 throughput rounds and CO2 throughput;

[0061] The project implements CO2 throughput.

[0062] The specific steps for determining the appropriate fracture length based on geological conditions include:

[0063] Establish a numerical model for CO2 throughput

[0064]

[0065] Where φ is porosity; ρ j Let be the molar density of the j-th phase; t be time; x be the molar density of the j-th phase i y is the molar density of the i-th component in the oil phase; i S is the molar density of the i-th component in the gas phase; j v represents the saturation of the j-th phase; j Let be the velocity of the j-th phase; q be the source and sink terms. Based on this, a Discrete Crack Model (DFN) is used to represent the crack. DFN reduces the dimension of the crack and can be written as:

[0066]

[0067] Different fracture lengths were set up, and CO2 huff and puff numerical simulations were performed. The optimal fracture length was determined by comparing the two parameters of oil production intensity and oil exchange rate.

[0068] The specific steps for simulating hydraulic fracturing parameters, including fracture aperture and fracture trajectory distribution, under reasonable fracture length based on geological and construction conditions include:

[0069] A crack propagation model is established. The rock mass deforms under the combined effects of fluid pressure and geostress within the cracks. The governing equations for rock deformation are as follows:

[0070] σ=C:ε(u)

[0071] in Boundary conditions include outer boundary conditions and crack boundary conditions, i.e.

[0072] u|Γ g =g,

[0073] Where g is the positional displacement; Γ c Γ represents the outer boundary of the displacement. c For the crack boundary; σ s Shear stress; For constant shear stress; σ n ρ is the normal stress; p is the fluid pressure.

[0074] Using the maximum circumferential stress criterion, when the stress intensity factor at the crack tip exceeds the rock toughness, the crack will propagate along the direction of maximum circumferential stress. According to the maximum circumferential stress criterion, the following conditions must be met:

[0075]

[0076] Where θ M K is the angle of maximum circumferential stress direction, i.e., the angle between the crack initiation direction and the crack surface; I K is the first type of stress intensity factor; II It is the first type of stress intensity factor.

[0077] Using a fracture propagation model, the aperture distribution and fracture trajectory of hydraulic fracturing fractures under a reasonable fracture length are calculated. The geometric parameters of the fractures are then determined.

[0078]

[0079] Where q is the flow rate per unit thickness section; w is the fracture width; μ is the viscosity of the fracturing fluid; and x is the displacement along the fracture propagation direction.

[0080] The specific steps for optimizing CO2 throughput rounds and CO2 throughput include:

[0081] By incorporating fracture parameters into the CO2 huff and puff model, numerical simulations of CO2 huff and puff were conducted. Different huff and puff cycles and different CO2 injection volumes were designed, and the optimal huff and puff scheme was determined by comparing the two parameters of cumulative oil production and oil exchange rate.

[0082] The specific steps for implementing CO2 throughput in the project include:

[0083] Based on the optimized fracturing scheme, a reasonable fracture length is first created, and then CO2 injection is implemented according to the optimized production scheme.

[0084] Example 2

[0085] In a specific embodiment 2 of the present invention, a CO2 huff and puff model for a fracturing reservoir is established as follows: Figure 2 As shown, different artificial fracture lengths were set, and then CO2 huff and puff numerical simulations were performed to compare oil recovery intensity and oil exchange rate, thus obtaining the optimal fracture length. The reservoir scale was 765×765×15 meters, the porosity was 0.15, the matrix permeability was 10, the initial reservoir pressure was 34 MPa, and the initial water saturation was 0.35.

[0086] In the initial stage, as the fracture length increases, the contact area between CO2 and crude oil increases, leading to higher oil exchange rates and production rates. When the fracture length exceeds 180 meters, the increase in reservoir CO2 huff and puff efficiency slows down, and when the fracture length exceeds 240 meters, the CO2 huff and puff efficiency shows a downward trend. At this point, the fracture length causes rapid dissipation of bottomhole pressure after CO2 injection, affecting the CO2 huff and puff efficiency. By comparing the oil exchange rate and production intensity, it can be concluded that a fracture length of 240 meters yields the best oil production results.

[0087] Based on artificial fracturing to a depth of 240 meters, optimize parameters such as oil production rate, well shut-in time, and number of injection / pump cycles, for example... Figure 3 As shown. Figure 3 As shown in c, the increase in fuel consumption decreases with each CO2 huff and puff cycle. After 6 huff and puff cycles, the effect of CO2 huff and puff is not significant; therefore, the CO2 huff and puff cycle is 6 cycles. Figure 3 As shown in b, if the well shut-in time is too short, CO2 cannot fully contact the crude oil; if the well shut-in time is too long, the bottom hole pressure will decrease, affecting the throughput. Therefore, the well shut-in time is 10 days.

[0088] Example 3

[0089] In a specific embodiment 3 of the present invention, a CO2 huff and puff operation was conducted in a low-permeability block of the Shengli Oilfield. The reservoir is burial depth 2900 meters, oil layer thickness 4 meters, permeability 4.6 mD, and porosity 0.18. The well network in this block is imperfect, and since production began, a flexible development approach has been adopted. Initially, the daily oil production capacity per well was 7.8 t / d, and currently it is 2.1 t / d, with a water cut of 57%. As formation energy decreases, production declines rapidly.

[0090] A: Based on the method of this patent, a geological model is established using the geological parameters and well data of the block. Historical data fitting is then performed to obtain the current geological model.

[0091] B: Different artificial fracture lengths were designed for the wells, and the fracture length and direction were optimized by comparing the oil production intensity and oil exchange rate under different fracture lengths using the method of this patent. After optimization by this patent, the direction of the fracturing fractures in different wells is horizontal, and the fracture length is 160 meters.

[0092] C: With a fracture strength of 160 meters, after optimization by this patent, the injection rate of the fractured well is 105.2 t / d, and the well is shut in for 15 days.

[0093] D: Based on the optimization results, the well was artificially fracturing, with a fracture length of 160 meters, and CO2 was injected at a rate of 105.2 t / d. Production began 15 days after the well was shut down.

[0094] E: According to statistics, of the 14 low-permeability wells that underwent fracturing, 13 were effective, nearly 100%. A total of 2729m³ of CO2 was injected. 3 The oil production increased by 4781 tons, with an oil change rate of 1.75%. Practice shows that CO2 injection after artificial fracturing has a higher CO2 injection contact area and better results.

[0095] Finally, it should be noted that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

[0096] Except for the technical features described in the specification, all other technologies are known to those skilled in the art.

Claims

1. A method for enhancing oil recovery through CO2 huff and puff after fracturing, characterized in that, The method of enhancing oil recovery through CO2 huff and puff after fracturing includes: Step 1: Establish a CO2 throughput model; Step 2: Determine the appropriate crack length based on geological conditions; Step 3: Based on geological and construction conditions, simulate the fracturing parameters under a reasonable fracture length; Step 4: Optimize CO2 throughput rounds and CO2 throughput; Step 5: Implement mining block applications and carry out CO2 throughput in the project; In step 1, establish a CO2 throughput model: （1） in, Porosity; t represents the molar density of the oil, gas, or aqueous phase; t represents time. Let be the molar density of the i-th component in the oil phase; Let be the molar density of the i-th component in the gas phase; The saturation level of the oil phase, gas phase, or water phase; The velocity of the oil phase, gas phase, or water phase; q i For the source and sink terms of component i, q w For the source and sink of the aqueous phase; Based on Equation 1, the Discrete Crack Model (DFN) is used to represent the crack. DFN reduces the dimension of the crack and is written as follows: （2） Where FEQ represents the seepage equation, For the overall flow area, This is the crack area. For the bedrock region, e represents the fracture aperture; In step 2, fractures of different lengths were set up and CO2 huff and puff numerical simulations were performed. The optimal fracture length was determined by comparing the two parameters of oil production intensity and oil exchange rate. In step 3, the simulated fracturing parameters under a reasonable fracture length include the fracture aperture and the distribution pattern of the fracture trajectory. Step 3 includes: Step 31, establish a crack propagation model; Step 32: Use the maximum circumferential stress criterion to simulate the crack propagation direction; Step 33: Calculate the aperture distribution and fracture trajectory of the hydraulic fracturing fracture under a reasonable fracture length using a fracture propagation model. In step 4, the fracture parameters are input into the CO2 huff and puff model to perform CO2 huff and puff numerical simulation. Different huff and puff cycles and different CO2 injection volumes are designed. The two parameters of cumulative oil production and oil exchange rate are compared to determine the optimal huff and puff scheme. In step 5, based on the optimized fracturing scheme, a reasonable fracture length is first created, and then CO2 injection is implemented according to the optimized production scheme.

2. The method for enhancing oil recovery after fracturing with CO2 huff and puff as described in claim 1, characterized in that, In step 31, the rock mass deforms under the combined action of fluid pressure and geostress within the fracture. The rock deformation control equations are as follows: ， in ; Let f denote the stress tensor, f denote the volume force, and C be the elasticity tensor matrix. Let u be the strain tensor and u be the displacement. Boundary conditions include outer boundary conditions and crack boundary conditions, i.e. ， ， Where g is the position displacement; This is the outer boundary of the displacement; This is the boundary of the crack; Shear stress; The shear stress is constant. denoted as normal stress; p as fluid pressure; x as position; and t as time.

3. The method for enhancing oil recovery after fracturing with CO2 huff and puff as described in claim 1, characterized in that, In step 32, the maximum circumferential stress criterion is adopted. When the stress intensity factor at the crack tip is greater than the rock toughness, the crack will propagate along the direction of maximum circumferential stress. According to the maximum circumferential stress criterion, the following conditions are met: Where θ M The angle of maximum circumferential stress is the angle between the crack initiation direction and the crack surface. It is the first type of stress intensity factor; It is the first type of stress intensity factor. The rock strength.

4. The method for enhancing oil recovery after fracturing with CO2 huff and puff as described in claim 1, characterized in that, In step 33, determine the geometric parameters of the crack: Where q is the flow rate per unit thickness section; w is the fracture width; μ is the viscosity of the fracturing fluid; x is the displacement along the fracture propagation direction; and p represents the pressure.

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