A method of hydraulic fracturing of offshore low permeability reservoirs

Abstract

The present application relates to a kind of offshore low-permeability reservoir hydraulic fracturing method, comprising: obtaining the basic parameter of target reservoir;According to the basic parameter of the target reservoir, development interval division is carried out, and multiple development intervals are divided from the target reservoir;According to the basic parameter of each development interval, preferred perforation position;Balanced displacement evaluation index suitable for offshore fracturing well water drive condition is constructed;According to the balanced displacement evaluation index, fracture morphology parameter is optimized;According to the fracture morphology parameter obtained by optimization, hydraulic fracturing construction parameter is optimized;According to the fracture morphology parameter obtained by optimization, the laying form of proppant in fracture is optimized.The scheme can form complex fracture network in the process of hydraulic fracturing, and proppant can be effectively laid in the fracture, so that the longitudinal position and plane trend of fracture can realize interlayer and plane balanced displacement, thereby improving the effect of hydraulic fracturing, and the economic benefit of offshore reservoir exploitation can be improved.

Description

Technical Field

[0001] This invention relates to the field of marine oil and gas development technology, specifically to a hydraulic fracturing method for low-permeability offshore oil reservoirs. Background Technology

[0002] Fracturing technology is an important means to increase oil and gas production; however, currently, offshore fracturing is generally limited to single wells and small-scale operations (average sand content 50 m³). 3 , liquid volume 500m 3 This is the fracturing test stage. Therefore, the research and development of offshore hydraulic fracturing technology has important guiding significance for the development of low-permeability offshore reservoirs.

[0003] The inventors of this application discovered in their research that existing offshore fracturing technology only focuses on single-well fracturing without optimizing the design of the fractures. This makes it difficult to meet the multi-well water drive development conditions of low-permeability offshore reservoirs, and thus difficult to achieve balanced displacement in offshore production, resulting in suboptimal economic benefits. Summary of the Invention

[0004] To address the aforementioned problems, the present invention aims to provide a hydraulic fracturing method for low-permeability offshore oil reservoirs. This method can form a complex fracture network during the hydraulic fracturing process and effectively place proppant within the fractures, enabling balanced displacement between layers and along the plane in terms of the longitudinal position and planar orientation of the fractures. This improves the hydraulic fracturing effect. The method is theoretically reliable, highly operable in the field, and can enhance the economic benefits of offshore oil reservoir development.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: In a first aspect, this application provides a hydraulic fracturing method for offshore low-permeability oil reservoirs, the method comprising: Obtain the basic parameters of the target reservoir; Based on the basic parameters of the target reservoir, development zones are divided, and multiple development zones are divided from the target reservoir. Based on the basic parameters of each development layer, the perforation location is selected; Construct an evaluation index for balanced displacement suitable for water drive conditions in offshore fracturing wells; Based on the aforementioned equilibrium displacement evaluation index, optimize the crack morphology parameters; Based on the optimized fracture morphology parameters, optimize the hydraulic fracturing construction parameters; Based on the optimized crack morphology parameters, the proppant placement pattern in the crack is optimized.

[0006] In one implementation of this application, obtaining the basic parameters of the target reservoir includes obtaining the rock mechanics parameters, permeability parameters, natural fracture parameters, geostress parameters, artificial fracture closure pressure parameters, temperature data, and interlayer conditions of the target reservoir.

[0007] In one implementation of this application, the step of dividing the target reservoir into multiple development segments based on the basic parameters of the target reservoir includes: Based on the permeability parameters and interlayer conditions of the target reservoir, multiple development layers of preset thickness are divided from the target reservoir.

[0008] In one implementation of this application, the step of selecting the preferred perforation location based on the basic parameters of each development layer includes: Based on the rock mechanics parameters, natural fracture parameters, and geostress parameters of each development layer, the compressibility index is calculated, and the perforation location is selected based on the calculation results.

[0009] In one implementation of this application, the construction of an equilibrium displacement evaluation index suitable for water drive conditions in offshore fracturing wells includes: From multiple preset indicators, equilibrium displacement evaluation indicators suitable for water drive conditions in offshore fracturing wells were selected; and numerical simulation methods were used to verify the selected equilibrium displacement evaluation indicators suitable for water drive conditions in offshore fracturing wells.

[0010] In one implementation of this application, the step of using numerical simulation to verify the selected equilibrium displacement evaluation index suitable for offshore fracturing well water drive conditions includes: Using reservoir numerical simulation, the relationship curve between the selected equilibrium displacement evaluation index and cumulative oil production is simulated and calculated. If the curve satisfies a linear relationship, the reliability of the equilibrium displacement evaluation index is verified.

[0011] In one implementation of this application, optimizing the crack morphology parameters based on the equilibrium displacement evaluation index includes: Using the selected equilibrium displacement evaluation index suitable for water drive conditions of offshore fracturing wells as the optimization objective, the fracture azimuth angle and fracture length of the fracture morphology as optimization variables, and the geostress direction and perforation location of the geostress parameters as constraints, a mathematical model for optimizing fracture morphology parameters is established. Using reservoir numerical simulation methods, we simulate and predict the equilibrium displacement evaluation index under any combination of fracture azimuth and fracture length parameters. Based on any combination of crack azimuth angle and crack length parameters and its corresponding equilibrium displacement evaluation index, the mathematical model for optimizing crack morphology parameters is solved using an intelligent optimization algorithm to obtain the optimal crack morphology parameters.

[0012] In one implementation of this application, optimizing the hydraulic fracturing construction parameters based on the optimized fracture morphology parameters includes: A geomechanical model is constructed based on the direction of geostress and rock mechanical parameters. With the goal of minimizing the difference between the optimized fracture morphology parameters and the simulated fracture propagation parameters, the perforation direction and fracturing fluid pumping rate are the optimization variables, and the on-site fracturing equipment construction capacity is the constraint condition, a mathematical model for optimizing fracturing construction parameters is constructed. Numerical simulation of fracture propagation is used to predict the fracture propagation morphology obtained under different fracturing operation parameters; By utilizing the fracture propagation morphology corresponding to the different fracturing construction parameters, an intelligent optimization algorithm is used to solve the mathematical model for optimizing the fracturing construction parameters, thereby obtaining reasonable fracturing construction parameters for achieving the fracture morphology, including the perforation direction and the fracturing fluid pumping rate.

[0013] In one implementation of this application, optimizing the proppant placement in the crack based on the optimized crack morphology parameters includes: Based on the artificial fracture closure pressure parameters and temperature data, the relevant parameters of the high-viscosity fracturing fluid and low-density proppant in the first stage and the relevant parameters of the low-viscosity fracturing fluid and high-density proppant in the second stage were determined respectively.

[0014] In one implementation of this application, the relevant parameters of the high-viscosity fracturing fluid and low-density proppant in the first stage are specifically as follows: When T≤90℃ =20 mPa·s; When 90℃ < T < 120℃a =30 mPa·s; When T≥120℃ =40 mPa·s; In the formula: T represents the temperature data; The viscosity of the fracturing fluid in the first stage; When P≤50MPa =13 kg / m 3 ; When 50MPa < P < 70MPa =15 kg / m 3 ; When P ≥ 70 MPa =17 kg / m 3 ; In the formula: P is the artificial crack closure pressure parameter; The density of the first-stage proppant; The relevant parameters for the low-viscosity fracturing fluid and high-density proppant in the second stage are as follows: When T≤90℃ =5 mPa·s; When 90℃ < T < 120℃a =10 mPa·s; When T≥120℃ =20 mPa·s; In the formula: T represents the temperature data; The viscosity of the fracturing fluid in the second stage; When P≤50MPa =15 kg / m 3 ; When 50MPa < P < 70MPa =17 kg / m 3 ; When P ≥ 70 MPa =18.3 kg / m 3 ; In the formula: P is the artificial crack closure pressure parameter; This refers to the density of the proppant in the second stage.

[0015] This invention, by adopting the above technical solutions and considering the division of development layers, facilitates balanced displacement between vertical layers during the development phase; it optimizes perforation locations based on compressibility calculations, effectively increasing the complexity of the fracture network; it optimizes fracture morphology based on balanced displacement evaluation indicators, enabling planar balanced displacement; it ensures the effective realization of the target fracture morphology by optimizing fracturing construction parameters; and it controls the proppant delivery distance by finely adjusting the fracturing fluid viscosity and proppant density based on reservoir parameters, improving the proppant placement pattern in near-wellbore zones and distal fractures, effectively enhancing reservoir permeability and improving hydraulic fracturing performance. Attached Figure Description

[0016] Figure 1 A schematic flowchart of the hydraulic fracturing method for low-permeability oil reservoirs at sea provided in an embodiment of the present invention; Figure 2 This is a diagram showing the vertical permeability distribution of the reservoir within a single well area in practical applications. Figure 3 This is a typical five-point well pattern model for a low-permeability offshore oil reservoir in practical applications; Figure 4 This is a graph showing the relationship between the standard deviation of saturation and cumulative oil production in a specific embodiment of the present invention; Figure 5 This is a model of different fracture azimuth angles in a five-point well network according to a specific embodiment of the present invention; Figure 6 This is a comparison of the construction curves of two fracturing wells in two well groups in a specific embodiment of the present invention; Figure 7 This is a comparison of the production curves of two fractured wells in two well groups in a specific embodiment of the present invention. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the described embodiments of the present invention are within the scope of protection of the present invention.

[0018] like Figure 1 As shown in the figure, this application provides a hydraulic fracturing method for offshore low-permeability oil reservoirs, including: 101. Obtain the basic parameters of the target reservoir; 102. Based on the basic parameters of the target reservoir, the development layers are divided into multiple development layers from the target reservoir. 103. Based on the basic parameters of each development layer, the perforation location is selected. 104. Construct an evaluation index for balanced displacement suitable for water drive conditions in offshore fracturing wells; 105. Optimize crack morphology parameters based on the aforementioned balanced displacement evaluation index; 106. Based on the optimized fracture morphology parameters, optimize the hydraulic fracturing construction parameters; 107. Based on the optimized crack morphology parameters, optimize the proppant placement pattern in the crack.

[0019] The embodiments of this application, by adopting the above technical solutions and considering the division of development layers, are conducive to achieving balanced displacement between vertical layers during the development phase; the perforation location is optimized based on the compressibility calculation results, which can effectively improve the complexity of the fracture network; the fracture morphology is optimized based on the balanced displacement evaluation index, which can achieve planar balanced displacement; the target fracture morphology is effectively achieved by optimizing the fracturing construction parameters; and finally, the proppant delivery distance is controlled by finely adjusting the fracturing fluid viscosity and proppant density based on reservoir parameters, which improves the proppant placement pattern in the near-wellbore zone and in the far-end fractures, effectively enhancing the reservoir permeability and improving the hydraulic fracturing effect.

[0020] The following is combined with Figures 2 to 7 The process and advantages of the above method are illustrated in a specific application example of this application.

[0021] Taking a five-point well group in a block of a Bohai oilfield as an example, there are oil wells P1, P2, P3, P4, and water injection well I1. It is a low-permeability offshore reservoir with a formation fracture closure pressure of 52 MPa, a reservoir temperature of 136℃, well-developed natural fractures, and a large difference in geostress coefficient. The permeability distribution of each sub-layer (which can serve as a buffer or development zone) is shown in Table 1. The fourth sub-layer is a buffer. The vertical permeability distribution of each sub-layer within the single well area is shown in the figure below. Figure 2 As shown.

[0022]

[0023] Table 1 The hydraulic fracturing method for low-permeability offshore reservoirs in this example includes the following steps: (1) Collect the basic parameters of the target reservoir to be fracturing, including rock mechanical parameters (e.g., Young's modulus, dilatation angle, peak strain, etc.), permeability parameters (e.g., porosity, permeability, viscosity, formation pressure, effective thickness, etc.), natural fracture parameters (e.g., natural fracture approach angle, which refers to the angle between the natural fracture and the maximum horizontal principal stress of the formation), geostress parameters (e.g., geostress difference coefficient), artificial fracture closure pressure parameters, temperature data, and interlayer conditions.

[0024] (2) The target reservoir was divided into 7 sub-layers, each with a thickness of 1m. The 4th sub-layer was a partition layer, and the rest were development layers. The physical properties of each sub-layer were further normalized, as shown in Table 2.

[0025]

[0026] Table 2 Calculate the average value and sum of squares of the seepage properties within any combination of adjacent sublayers. When sublayers 1-3 are divided into one segment and sublayers 5-7 are divided into another segment, the total sum of squares of the overall combination scheme is minimized. Therefore, sublayers 1-3 are designated as the first development segment, and sublayers 5-7 are designated as the second development segment.

[0027] (3) Calculate the average value of the basic rock mechanics parameters of each sublayer, and then normalize the Young's modulus, dilatation angle and peak strain.

[0028] (4) Based on the compressibility evaluation model, the compressibility index of the target layer is calculated, and the calculation results are shown in Table 3.

[0029]

[0030] Table 3 (5) Optimize perforation location based on compressibility index. Since the overall compressibility index of the reservoir in this well is between 0.1 and 0.6, and considering the perforation length requirement, determine the compressibility index threshold. If the compressibility index is 0.4, then the perforation location is selected as a small layer with a compressibility index greater than 0.4. The preferred results are as follows:

[0031] Table 4 (6) Investigate existing equilibrium displacement evaluation indicators and analyze their applicable scope.

[0032]

[0033] Table 5 Given that offshore low-permeability oil reservoirs are in the early stages of exploration and development and require life-cycle guidance for fracture parameter optimization, the initial selected index is the minimum standard deviation of saturation.

[0034] (7) Construct a typical five-point well pattern model for low-permeability offshore reservoirs, such as Figure 3 As shown, by setting four regions with different porosity and permeability parameters, the porosity is 0.08, 0.09, 0.1, and 0.11 respectively; the permeability is 50 × 10⁻⁶. -3 μm 2 100×10 -3 μm 2 150×10 -3 μm 2 200×10 -3 μm 2 To account for reservoir heterogeneity, and considering the effects of bottom hole flowing pressure and water injection volume under different conditions, an orthogonal experimental table was designed, as shown in Table 6, with 24 sets of numerical simulation schemes designed.

[0035]

[0036] Table 6 Using reservoir numerical simulation methods, the above 24 scenarios were simulated and calculated, obtaining the saturation standard deviation and cumulative oil production for each scenario. These were then plotted in a single figure, as shown below. Figure 4 As shown, the equilibrium displacement evaluation index based on the minimum standard deviation of saturation has a good linear relationship with the cumulative oil production. Therefore, the equilibrium displacement evaluation index based on the minimum standard deviation of saturation is suitable for evaluating the development effect of fracturing wells in offshore low-permeability reservoirs.

[0037] (8) Taking the minimum of the equilibrium displacement evaluation index as the optimization objective, the fracture azimuth angle and fracture length as optimization variables, and the geostress direction and perforation location as constraints, a mathematical model for optimizing fracture morphology parameters is established as follows:

[0038] in, u This represents the optimal combination of crack azimuth and crack length parameters; u This represents a combination of optimization variables; U This represents the space of available optimization variables; SD ( u This indicates the use of a combination of crack azimuth and crack length parameters. u The obtained standard deviation of saturation.

[0039] (9) Using the reservoir numerical simulation method based on the embedded discrete fracture model, establish a multi-fracturing well model of the target reservoir, such as... Figure 5 As shown, the standard deviation of saturation is calculated under different combinations of crack azimuth angle and crack length parameters.

[0040] Using an intelligent optimization algorithm, the optimal mathematical model was solved to obtain the optimal crack azimuth angle of 11° and crack length parameter of 55m.

[0041] (10) Taking the minimum difference between the optimized fracture morphology parameters and the fracture morphology parameters simulated by the fracture propagation as the optimization objective, the perforation direction and the fracturing fluid pumping volume as optimization variables, and the on-site fracturing equipment construction capacity (fracturing fluid pumping volume and pump pressure) as constraints, a mathematical model for optimizing fracturing construction parameters is constructed.

[0042] in, u This represents the optimal combination of fracturing operation parameters (perforation direction, fracturing fluid injection rate); u This represents a combination of fracturing operation parameters; U This represents the space of available optimization variables; D ( u This indicates the use of a specific combination of fracturing construction parameters. u The difference between the obtained crack coordinates and the target crack coordinates.

[0043] (11) Using numerical simulation of fracture propagation, predict the fracture propagation morphology obtained by different fracturing operation parameters (perforation direction, fracturing fluid injection rate); using the fracture propagation morphology corresponding to the different fracturing operation parameters, solve the mathematical model of the fracturing operation parameters using an intelligent optimization algorithm to obtain reasonable fracturing operation parameters to achieve the fracture morphology, with a perforation angle of 10° and a fracturing fluid injection rate of 475m³. 3 .

[0044] (12) The proppant placement pattern in the fracture is optimized by finely controlling the fracturing fluid viscosity and proppant density. The optimization process of construction parameters is as follows: ① The formation fracture closure pressure is 52 MPa, and the formation temperature T is 136℃. Therefore, the viscosity of the fracturing fluid injected in the first stage is determined to be 40 mPa·s, and the proppant density is 15 kg / m³. 3 .

[0045] ②The viscosity of the fracturing fluid injected in the second stage is 20 mPa·s, and the density of the proppant is 17 kg / m³. 3 .

[0046] Well group W2 and well group W1 are adjacent well groups located in the same block and the same reservoir. Well group W2 selected the perforation location, fracturing fluid viscosity, and proppant density using conventional methods. Both well groups underwent fracturing using the same scale of operations. Then, the pressure changes during the fracturing process and the oil well production results after fracturing were compared. Figure 6 As shown, due to the higher brittleness and well-developed natural fractures in the stratigraphic intervals with higher compressibility indices, the W1 well group, with its optimized perforation locations based on the compressibility index, experienced lower fracture pressure. Throughout the entire construction process, optimizing the perforation locations significantly reduced the construction pressure by 5-10 MPa. For example... Figure 7 As shown, the W1 well group initially produced 17.6 tons of oil per day, which was 5.6 tons more than the W2 well group, and the well maintained a stable production of 15 tons for a long time.

[0047] In the several embodiments provided by this invention, it should be understood that the disclosed methods can be implemented in other ways. For example, the device embodiments described above are merely illustrative; for instance, the division of the units described above is only a logical functional division, and in actual implementation, there may be other division methods. For example, multiple units or components may be combined or integrated into another system, or some features may be ignored or not executed. Furthermore, the coupling or direct coupling or communication connection shown or discussed may be through some interfaces; the indirect coupling or communication connection between devices or units may be electrical, mechanical, or other forms.

[0048] The integrated units implemented as software functional units described above can be stored in a computer-readable storage medium. These software functional units, stored in a storage medium, include several instructions to cause a computer device (which may be a personal computer, server, or network device, etc.) or processor to execute some steps of the methods described in the various embodiments of the present invention. The aforementioned storage medium includes various media capable of storing program code, such as USB flash drives, portable hard drives, read-only memory (ROM), random access memory (RAM), magnetic disks, or optical disks.

[0049] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A hydraulic fracturing method for low-permeability offshore oil reservoirs, characterized in that, The method includes: Obtain the basic parameters of the target reservoir; Based on the basic parameters of the target reservoir, development zones are divided into multiple development zones, including: obtaining the rock mechanics parameters, permeability parameters, natural fracture parameters, geostress parameters, artificial fracture closure pressure parameters, temperature data, and interlayer conditions of the target reservoir; and dividing the target reservoir into multiple development zones of predetermined thickness based on the permeability parameters and interlayer conditions of the target reservoir. The perforation location is determined based on the basic parameters of each development layer. Constructing an equilibrium displacement evaluation index suitable for water drive conditions in offshore fracturing wells includes: selecting an equilibrium displacement evaluation index suitable for water drive conditions in offshore fracturing wells from multiple preset indexes; and verifying the selected equilibrium displacement evaluation index suitable for water drive conditions in offshore fracturing wells using numerical simulation methods. Based on the equilibrium displacement evaluation index, the fracture morphology parameters are optimized, including: using the selected equilibrium displacement evaluation index suitable for offshore fracturing well water drive conditions as the optimization objective, the fracture azimuth angle and fracture length as optimization variables, and the geostress direction and perforation location as constraints, to establish a mathematical model for optimizing fracture morphology parameters; using reservoir numerical simulation methods to simulate and predict the equilibrium displacement evaluation index under any combination of fracture azimuth angle and fracture length parameters; and using an intelligent optimization algorithm to solve the mathematical model for optimizing fracture morphology parameters based on any combination of fracture azimuth angle and fracture length parameters and its corresponding equilibrium displacement evaluation index, to obtain the optimal fracture morphology parameters. Based on the optimized fracture morphology parameters, the hydraulic fracturing construction parameters are optimized, including: constructing a geomechanical model based on the geostress direction and rock mechanics parameters; constructing a mathematical model for optimizing fracturing construction parameters with the optimization objective of minimizing the difference between the optimized fracture morphology parameters and the simulated fracture propagation parameters, using the perforation direction and fracturing fluid injection rate as optimization variables, and the on-site fracturing equipment construction capacity as constraints; predicting the fracture propagation morphology obtained by different fracturing construction parameters using numerical simulation methods for fracture propagation; and solving the mathematical model for optimizing fracturing construction parameters using intelligent optimization algorithms based on the fracture propagation morphologies corresponding to the different fracturing construction parameters to obtain reasonable fracturing construction parameters for achieving the fracture morphology, including the perforation direction and fracturing fluid injection rate. Based on the optimized crack morphology parameters, the proppant placement pattern in the crack is optimized.

2. The hydraulic fracturing method for low-permeability offshore reservoirs according to claim 1, characterized in that, Determining the perforation location based on the basic parameters of each development layer includes: Based on the rock mechanics parameters, natural fracture parameters, and geostress parameters of each development layer, the compressibility index is calculated, and the perforation location is determined based on the calculation results.

3. The hydraulic fracturing method for low-permeability offshore reservoirs according to claim 1, characterized in that, The numerical simulation method is used to verify the selected equilibrium displacement evaluation indexes suitable for offshore fracturing well water drive conditions, including: Using reservoir numerical simulation, the relationship curve between the selected equilibrium displacement evaluation index and cumulative oil production is simulated and calculated. If the curve satisfies a linear relationship, the reliability of the equilibrium displacement evaluation index is verified.

4. The hydraulic fracturing method for low-permeability offshore reservoirs according to claim 1, characterized in that, The step of optimizing the proppant placement in the crack based on the optimized crack morphology parameters includes: Based on the artificial fracture closure pressure parameters and temperature data, the relevant parameters of the high-viscosity fracturing fluid and low-density proppant in the first stage and the relevant parameters of the low-viscosity fracturing fluid and high-density proppant in the second stage were determined respectively.

5. The hydraulic fracturing method for low-permeability offshore reservoirs according to claim 4, characterized in that, The relevant parameters for the high-viscosity fracturing fluid and low-density proppant in the first stage are as follows: When T≤90℃ =20 mPa·s; When 90℃ < T < 120℃a =30 mPa·s; When T≥120℃ =40 mPa·s; In the formula: T represents the temperature data; The viscosity of the fracturing fluid in the first stage; When P≤50MPa =13 kg / m 3 ; When 50MPa < P < 70MPa =15 kg / m 3 ; When P ≥ 70 MPa =17 kg / m 3 ; In the formula: P is the artificial crack closure pressure parameter; The density of the first-stage proppant; The relevant parameters for the low-viscosity fracturing fluid and high-density proppant in the second stage are as follows: When T≤90℃ =5 mPa·s; When 90℃ < T < 120℃a =10 mPa·s; When T≥120℃ =20 mPa·s; In the formula: T represents the temperature data; The viscosity of the fracturing fluid in the second stage; When P≤50MPa =15 kg / m 3 ; When 50MPa < P < 70MPa =17 kg / m 3 ; When P ≥ 70 MPa =18.3 kg / m 3 ; In the formula: P is the artificial crack closure pressure parameter; This refers to the density of the proppant in the second stage.

Images