Preposed carbon dioxide composite fracturing parameter optimization design method and system

Abstract

The invention provides a preposed carbon dioxide composite fracturing parameter optimization design method and system. The method comprises the following steps: establishing a composite fracturing design geometric model by combining natural fracture development conditions based on physical property parameters and rock mechanical properties of a reservoir; a composite fracturing crack propagation model considering the change condition of the carbon dioxide physical property parameters is constructed based on the model; crack propagation numerical simulation operation is carried out according to different geological factors and engineering factors; and according to a simulation operation result obtained by solving, analyzing a relationship between different factors under various geological factors and a fracture expansion form, and optimizing and designing engineering factor parameters. By the adoption of the scheme, the problem that current on-site preposed carbon dioxide composite fracturing construction parameter design lacks practical construction parameter guidance can be effectively solved, and reliable parameter configuration guidance is provided for composite fracturing construction operation.

Description

Technical Field

[0001] The present invention relates to the technical field of shale oil reservoir fracturing and reconstruction, and in particular to a method and system for optimizing design of pre-carbon dioxide composite fracturing parameters. Background Art

[0002] Continental shale oil resources are an important successor energy source in the oil field. Shale oil reservoirs are characterized by dense reservoirs, high crude oil viscosity and poor fluidity. They need to be transformed through horizontal well volume fracturing to form a large-scale "high conductivity" fracture network to achieve long-term and effective utilization of reservoir crude oil.

[0003] Pre-placement of carbon dioxide can utilize the ultra-low viscosity and easy diffusion characteristics of carbon dioxide to enter micro-cracks and natural fracture tips, increase pore pressure, and reduce fracture pressure; the pre-placement of carbon dioxide composite fracturing process combines the advantages of carbon dioxide fracturing and hydraulic fracturing; carbon dioxide dissolved in formation crude oil can reduce crude oil viscosity, increase fluidity, increase pore fluid pressure, and replenish formation energy. In the later stage, large-scale sand addition, main fracture creation, and improved fracture network conductivity can be achieved by injecting water-based fracturing fluid. Therefore, pre-placement of carbon dioxide composite fracturing technology has become one of the key technologies for achieving efficient development of shale oil resources.

[0004] There are some operating methods for the pre-placed carbon dioxide composite fracturing process in the prior art. For example, the invention patent with application number 201610216886.2 provides a carbon dioxide-drag-reducing water composite fracturing method. The method includes: (1) injecting liquid carbon dioxide fracturing fluid into the formation every day; (2) after a period of time, injecting drag-reducing water fracturing fluid without adding proppant; (3) after a period of time, injecting drag-reducing water fracturing fluid carrying proppant. Through the performance differences of different fracturing fluid systems, the effects of expanding the scale of the fracture network and increasing oil and gas production capacity can be achieved. The invention patent with application number 202011014962.4 provides a composite fracturing transformation system and method based on multi-stage liquid CO2 phase change. CO2 gas is filled into each stage of the liquid CO2 phase change generating device, and then the liquid CO2 phase change generating device is assembled into a multi-stage liquid CO2 phase change blasting device. The safety of liquid CO2 phase change blasting is utilized to achieve the step-by-step release of blasting energy, effectively reducing the rock fracture pressure. At the same time, CO2 pre-energization is achieved through a one-pass process. The existing technology mainly involves the injection sequence design and on-site construction process of different types of fracturing fluid systems, and does not involve the optimization design method of pre-carbon dioxide composite fracturing parameters; and the amount of pre-carbon dioxide has an important influence on the final fracturing crack morphology and the efficiency of fracturing fluid fracture creation.

[0005] It can be seen that the current pre-fracturing carbon dioxide composite fracturing technology is only based on a feasible theoretical operation level, lacking specific construction parameter design guidance to effectively achieve the expected fracturing effect, resulting in great blindness in actual operation applications. Therefore, providing a pre-fracturing carbon dioxide composite fracturing optimization design method suitable for shale oil reservoirs is an urgent problem to be solved in the field.

[0006] The information disclosed in the background technology section of the present invention is only intended to deepen the understanding of the general background technology of the present invention, and should not be regarded as acknowledging or suggesting in any form that the information constitutes the prior art known to those skilled in the art. Summary of the invention

[0007] To solve the above problems, the present invention provides a method for optimizing the design of pre-placed carbon dioxide composite fracturing parameters. The method establishes a composite fracturing design geometric model based on information such as reservoir physical properties, rock mechanical properties, and natural fracture development conditions; establishes a composite fracturing crack extension model that takes into account changes in carbon dioxide physical property parameters; conducts numerical simulations of crack extension under different geological and engineering factors, obtains crack morphology, and draws a curve showing the relationship between different factors and fracture length, and optimizes pre-placed carbon dioxide composite fracturing parameters; the scheme can effectively overcome the problem that the current on-site pre-placed carbon dioxide composite fracturing construction parameter design lacks a theoretical basis; preferably, in one embodiment, the method comprises: a geometric model establishment step: establishing a composite fracturing design geometric model based on reservoir physical properties, rock mechanical properties, and natural fracture development conditions;

[0008] The fracturing extension model establishment step: based on the composite fracturing design geometric model, constructing a composite fracturing crack extension model that takes into account the changes in carbon dioxide physical property parameters;

[0009] Extension simulation operation step: using the composite fracturing fracture extension model to carry out fracture extension numerical simulation operation for different geological factors and engineering factors, and solving the model based on the shifting discontinuous boundary element method;

[0010] Fracturing parameter optimization design steps: Based on the simulation results, analyze the relationship between different factors and crack extension morphology under various geological factors, and optimize the design of engineering factor parameters.

[0011] Furthermore, in one embodiment, in the geometric model establishment step, the natural fracture development of the current reservoir is determined based on the core observation, logging interpretation and seismic interpretation results as constraints, a reservoir geometric model with a scale that meets the requirements is formed based on the physical parameters of the reservoir and the mechanical properties of the rock, and the fracture distribution is set in combination with the natural fracture development, as a composite fracturing design geometric model.

[0012] Optionally, in one embodiment, in the fracturing extension model establishment step, the composite fracturing crack extension model established includes a stress equation of a crack unit, a flow equation of a fluid in a crack, a crack extension criterion, and a carbon dioxide property calculation equation.

[0013] In a preferred embodiment, in the step of establishing the fracturing extension model, the total stress at a certain point is set as the sum of the induced stresses generated by all fracture units at the point, and the stress equation of a single fracture unit at a certain point is established based on the displacement discontinuity method.

[0014] Further, in an optional embodiment, in the step of establishing the fracturing extension model, the stress equation of a single fracture unit at a certain point is established as follows:

[0015]

[0016] Where G is the shear modulus; ν is the Poisson's ratio; a is the half length of the crack unit; D x Tangential displacement discontinuity; D y is the normal displacement discontinuity; f ,xy 、f ,xyy 、f ,yy 、f ,yyy is the stress influence coefficient of each crack unit.

[0017] Preferably, in one embodiment, in the step of establishing the fracturing extension model, a crack extension criterion is established based on the principle of fracture mechanics, and the maximum hoop stress criterion is used as the extension judgment condition criterion of the crack tip.

[0018] Specifically, in an optional embodiment, in the step of establishing the fracturing extension model, the physical property parameters of carbon dioxide are calculated using the Peng-Robinson model.

[0019] Furthermore, in one embodiment, in the fracturing parameter optimization design step, for a certain combination of geological factors, curves of the changing relationship between different factors and fracturing crack length are drawn respectively according to the simulation calculation results, and combined with the balance principle of carbon dioxide injection rate and filtration loss rate in fracturing engineering, the curve trend is analyzed to determine the optimal value of the geological factor under the current combination of geological factors.

[0020] Based on other aspects of the method described in any one or more of the above embodiments, the present invention further provides a storage medium storing program codes that can implement the method described in any one or more of the above embodiments.

[0021] Based on the application aspects of the method described in any one or more of the above embodiments, the present invention also provides a pre-carbon dioxide composite fracturing parameter optimization design system, which executes the method described in any one or more of the above embodiments.

[0022] Compared with the closest prior art, the present invention also has the following beneficial effects:

[0023] The present invention provides a method and system for optimizing the design of parameters of pre-treatment carbon dioxide composite fracturing. The method establishes a composite fracturing design geometric model based on the physical parameters of the reservoir, the mechanical properties of the rock and the development of natural fractures. The geometric model presets the distribution of natural fractures based on the reservoir conditions, and can provide a reservoir model foundation that is closer to the actual situation for subsequent numerical model establishment and solution operations.

[0024] Construct a composite fracturing fracture propagation model that takes into account the changes in carbon dioxide physical parameters; take into account the impact of changes in carbon dioxide physical parameters in the pre-carbon dioxide composite fracturing project, and further control the error between the calculation results and the actual situation to the minimum;

[0025] Then, numerical simulation calculations of crack extension are carried out for different geological factors and engineering factors. According to the simulation calculation results obtained, the relationship between different factors and crack extension morphology under various geological factors is analyzed, and the design parameters of engineering factors are optimized. Based on this scheme, practical construction guidance on the design of construction parameters of pre-carbon dioxide composite fracturing is obtained, which is helpful to optimize the fracturing crack morphology of the operation and the fracturing efficiency of the fracturing fluid.

[0026] Other features and advantages of the present invention will be described in the following description, and partly become apparent from the description, or understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained by the structures particularly pointed out in the description, claims and drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The accompanying drawings are used to provide a further understanding of the present invention and constitute a part of the specification. Together with the embodiments of the present invention, they are used to explain the present invention and do not constitute a limitation of the present invention. In the accompanying drawings:

[0028] Figure 1 It is a schematic flow chart of a method for optimizing design of parameters of pre-carbon dioxide composite fracturing provided by one embodiment of the present invention;

[0029] Figure 2 This is an example diagram of a geometric model for composite fracturing optimization design of a method for optimizing design of parameters of pre-carbon dioxide composite fracturing provided in an embodiment of the present invention;

[0030] Figure 3 This is an example diagram of the relationship curve between the liquid injection amount and the fracture length in the composite fracturing of the method for optimizing the design of the parameters of the pre-carbon dioxide composite fracturing provided in the embodiment of the present invention;

[0031] Figure 4This is an example graph of total injection volume and fracture length curves at different displacements in composite fracturing in a method for optimizing design of pre-carbon dioxide composite fracturing parameters provided in an embodiment of the present invention;

[0032] Figure 5 This is an example graph of the curve between the liquid injection volume and the fracture length under different pre-carbon dioxide ratio conditions of the pre-carbon dioxide composite fracturing parameter optimization design method provided in an embodiment of the present invention;

[0033] Figure 6 It is a schematic structural diagram of a pre-carbon dioxide composite fracturing parameter optimization design system provided in another embodiment of the present invention. DETAILED DESCRIPTION

[0034] The following will describe the implementation methods of the present invention in detail in conjunction with the accompanying drawings and embodiments, so that the implementers of the present invention can fully understand how the present invention applies technical means to solve technical problems and achieve the implementation process of technical effects and implement the present invention specifically according to the above implementation process. It should be noted that as long as there is no conflict, the various embodiments and various features of the embodiments in the present invention can be combined with each other, and the technical solutions formed are all within the protection scope of the present invention.

[0035] Although the flowcharts describe the operations as sequential processes, many of the operations may be performed in parallel, concurrently, or simultaneously. The order of the operations may be rearranged. A process may be terminated when its operations are completed, but may also have additional steps not included in the accompanying drawings. A process may correspond to a method, function, procedure, subroutine, subprogram, etc.

[0036] Computer devices include user devices and network devices. Among them, user devices or clients include but are not limited to computers, smart phones, PDAs, etc.; network devices include but are not limited to a single network server, a server group consisting of multiple network servers, or a cloud composed of a large number of computers or network servers based on cloud computing. Computer devices can be operated alone to implement the present invention, or they can be connected to the network and implement the present invention through interactive operations with other computer devices in the network. The network where the computer device is located includes but is not limited to the Internet, wide area network, metropolitan area network, local area network, VPN network, etc.

[0037] The terms "first", "second", etc. may be used herein to describe various units, but these units should not be limited by these terms, and these terms are used only to distinguish one unit from another unit. The term "and / or" used herein includes any and all combinations of one or more of the listed associated items. When a unit is referred to as being "connected" or "coupled" to another unit, it can be directly connected or coupled to the other unit, or there can be intermediate units.

[0038] The terms used herein are only for describing specific embodiments and are not intended to limit exemplary embodiments. Unless the context clearly indicates otherwise, the singular forms "one", "one" and "item" used herein are also intended to include plural numbers. It should also be understood that the terms "include" and / or "comprise" used herein specify the existence of stated features, integers, steps, operations, units and / or components, without excluding the existence or addition of one or more other features, integers, steps, operations, units, components and / or combinations thereof.

[0039] Due to the wide distribution of continental shale oil resources and huge development potential, it is an important alternative energy source in the oil field. Shale oil reservoirs are characterized by dense reservoirs, high crude oil viscosity, and poor fluidity. They need to be transformed through horizontal well volume fracturing to form a large-scale "high conductivity" fracture network to achieve long-term effective use of reservoir crude oil.

[0040] Pre-placement of carbon dioxide can utilize the ultra-low viscosity and easy diffusion characteristics of carbon dioxide to enter micro-cracks and natural fracture tips, increase pore pressure, and reduce fracture pressure; the pre-placement of carbon dioxide composite fracturing process combines the advantages of carbon dioxide fracturing and hydraulic fracturing; carbon dioxide dissolved in formation crude oil can reduce crude oil viscosity, increase fluidity, increase pore fluid pressure, and replenish formation energy. In the later stage, large-scale sand addition, main fracture creation, and improved fracture network conductivity can be achieved by injecting water-based fracturing fluid. It can be seen that pre-placement of carbon dioxide composite fracturing technology has become one of the key technologies for achieving efficient development of shale oil resources.

[0041] There are some operating methods for the pre-placement carbon dioxide composite fracturing process in the prior art. For example, the invention patent with application number 201610216886.2 provides a carbon dioxide-drag-reducing water composite fracturing method. The method includes: (1) injecting liquid carbon dioxide fracturing fluid into the formation daily; (2) after a period of time, injecting drag-reducing water fracturing fluid without adding proppant; (3) after a period of time, injecting drag-reducing water fracturing fluid carrying proppant instead. Through the performance differences of different fracturing fluid systems, the effects of expanding the scale of the fracture network and increasing oil and gas production capacity can be achieved.

[0042] The invention patent with application number 201810497419.0 provides a carbon dioxide-slick water intermittent mixed fracturing design method. The method includes: (1) injecting a first slick water fracturing fluid into the target reservoir; (2) injecting a liquid carbon dioxide fracturing fluid into the target reservoir; (3) injecting a second slick water fracturing fluid into the target reservoir; (4) injecting a liquid carbon dioxide fracturing fluid into the target reservoir; (5) soaking the well; (6) injecting a second slick water fracturing fluid into the target reservoir; (7) injecting a first slick water fracturing fluid into the target reservoir. By injecting fracturing fluids of different properties at different stages, the physical and chemical effects of carbon dioxide are brought into play, thereby improving the fracturing transformation effect.

[0043] The invention patent with application number 202011014962.4 provides a composite fracturing transformation system and method based on multi-stage liquid CO2 phase change. CO2 gas is filled into each stage of the liquid CO2 phase change generator, and then the liquid CO2 phase change generator is assembled into a multi-stage liquid CO2 phase change blasting device. The safety of liquid CO2 phase change blasting is utilized to achieve the step-by-step release of blasting energy, effectively reduce the rock fracture pressure, and at the same time, achieve CO2 pre-energization through a single process.

[0044] The invention patent with application number 202110216556.4 discloses a composite fracturing process based on supercritical carbon dioxide pre-fracturing. The process mainly involves injecting supercritical carbon dioxide into the formation for pre-fracturing to form a complex fracture network, and then injecting different types of sand-carrying fracturing fluids into the fracture network in turn to expand, support, and polish the fracture network, while mixing with liquid nitrogen, and then injecting slick water for displacement operations, so as to achieve complex fracture network volume transformation of the reservoir, effectively support the fracture network, and enhance the formation energy.

[0045] The above-mentioned technologies mainly involve the injection sequence design and on-site construction process of different types of fracturing fluid systems, and do not involve the optimization design method of the pre-fracturing parameters of carbon dioxide composite fracturing; and the amount of pre-fracturing carbon dioxide has an important influence on the final fracturing crack morphology and the fracturing efficiency of the fracturing fluid.

[0046] It can be seen that the current pre-fracturing technology of carbon dioxide composite fracturing is only based on feasible theoretical operation level, lacking specific construction parameter design guidance to effectively achieve the expected fracturing effect, resulting in great blindness in actual operation application. Therefore, providing a pre-fracturing optimization design method of carbon dioxide composite fracturing suitable for shale oil reservoirs is an urgent problem to be solved.

[0047] In order to solve the above problems, the present invention provides a method and system for optimizing the design of pre-placed carbon dioxide composite fracturing parameters, which mainly includes the following operations: establishing a composite fracturing design geometric model based on information such as reservoir physical properties, rock mechanical properties, and natural fracture development; establishing a composite fracturing crack expansion model that takes into account changes in carbon dioxide physical property parameters; carrying out numerical simulations of crack expansion under different geological and engineering factors, obtaining crack morphology, and drawing curves of the relationship between different factors and the length of the fracturing crack, and optimizing pre-placed carbon dioxide composite fracturing parameters. Based on this scheme, the influence of horizontal stress difference, reservoir matrix permeability, degree of development of natural fractures in the reservoir, total amount of fracturing fluid, viscosity of fracturing fluid, displacement, pre-placed carbon dioxide amount, pre-placed carbon dioxide ratio, etc. on the expansion morphology of composite fracturing cracks can be effectively studied, effectively overcoming the problem that the current on-site pre-placed carbon dioxide composite fracturing construction parameter design lacks a theoretical basis, and providing reliable parameter configuration guidance for composite fracturing construction operations.

[0048] Next, the detailed process of the method of the embodiment of the present invention is described in detail based on the accompanying drawings. The steps shown in the flowchart of the accompanying drawings can be executed in a computer system including a set of computer executable instructions. Although the logical order of each step is shown in the flowchart, in some cases, the steps shown or described can be executed in a different order than here.

[0049] Embodiment 1:

[0050] Figure 1 The schematic diagram of the process of the method for optimizing the design of the parameters of the pre-embedded carbon dioxide composite fracturing provided in the first embodiment of the present invention is shown. Figure 1 It can be seen that the method includes the following steps.

[0051] Geometric model establishment steps: Establish a composite fracturing design geometric model based on reservoir physical parameters, rock mechanical properties and natural fracture development;

[0052] The fracturing extension model establishment step: based on the composite fracturing design geometric model, constructing a composite fracturing crack extension model that takes into account the changes in carbon dioxide physical property parameters;

[0053] Extension simulation calculation step: using the composite fracturing fracture extension model to carry out fracture extension numerical simulation calculation for different geological factors and engineering factors;

[0054] Fracturing parameter optimization design steps: Based on the simulation results, analyze the relationship between different factors and crack extension morphology under various geological factors, and optimize the design of engineering factor parameters.

[0055] By adopting the technical solution provided by the embodiment of the present invention, a composite fracturing fracture expansion model consisting of stress equations, flow equations, fracture expansion criteria, and carbon dioxide physical property calculation equations is established. The model is solved based on the shifting discontinuous boundary element method, and the Peng-Robinson equation is introduced to realize the simulation of the carbon dioxide injection process, so that the influence of horizontal stress difference, reservoir matrix permeability, degree of development of natural fractures in the reservoir, total amount of fracturing fluid, viscosity of fracturing fluid, displacement, amount of pre-placed carbon dioxide, and proportion of pre-placed carbon dioxide on the composite fracturing fracture expansion morphology can be studied.

[0056] In a preferred embodiment, in the geometric model establishment step, the natural fracture development of the current reservoir is determined based on the core observation, logging interpretation and seismic interpretation results as constraints, a reservoir geometric model with a scale that meets the requirements is formed based on the physical parameters and rock mechanical properties of the reservoir, and the fracture distribution is set in combination with the natural fracture development, as a composite fracturing design geometric model, such as Figure 2 The geometric model of composite fracturing optimization design is shown.

[0057] In actual application, a large number of randomly distributed natural fractures set in the composite fracturing design geometric model are analyzed and set in advance with core observation, well logging interpretation and seismic interpretation results as constraints.

[0058] Furthermore, a fracturing extension model establishment step is performed to construct a composite fracturing crack extension model that takes into account changes in carbon dioxide physical property parameters based on the composite fracturing design geometric model.

[0059] Optionally, in one embodiment, in the fracturing extension model establishment step, the composite fracturing crack extension model established is arranged to include a stress equation of a crack unit, a flow equation of a fluid in a crack, a crack extension criterion, and a carbon dioxide property calculation equation.

[0060] Specifically, in a preferred embodiment, in the step of establishing the fracturing extension model, the total stress at a certain point is set to be the sum of the induced stresses generated by all fracture units at that point; wherein the stress equation of a single fracture unit at a certain point is established based on the displacement discontinuity method.

[0061] In a preferred embodiment, further, the stress equation is established based on the displacement discontinuity method, and the stress equation of a certain crack unit for any point is:

[0062]

[0063] Where: G is the shear modulus; ν is the Poisson's ratio; a is the half length of the crack unit; D x Tangential displacement discontinuity; D y is the normal displacement discontinuity; f ,xy 、f ,xyy 、f ,yy 、f ,yyy is the stress influence coefficient of each fracture unit; the total stress at a point is the sum of the induced stresses generated by all fracture units at that point; σ xx is the induced normal stress in the x-direction, σ yy is the induced normal stress in the y direction, σ xy To induce tangential stress.

[0064] f represents the analytical model formula of the Kelvin problem in elasticity mechanics, as shown in the following formula (2):

[0065]

[0066] Where v is Poisson's ratio, a is the half length of the fracture unit, and f ,xy 、f ,xyy 、f ,yy 、f ,yyy represents the partial derivative of f.

[0067] Furthermore, the flow equation is established by considering that the flow in the fracture satisfies Poiseuille's law. Therefore, in a preferred embodiment, in the step of establishing the fracturing extension model, the flow equation in the fracture is established according to Poiseuille's law as follows:

[0068]

[0069] Where: q is the volume flow rate; μ is the viscosity; d is the opening of the crack, Represents the pressure gradient, which is calculated based on pressure and distance data.

[0070] Usually, the original permeability of natural fractures is significantly higher than that of shale matrix. Therefore, it is assumed that natural fractures have an initial fracture width d0, that is, the minimum opening of the fracture unit. Natural fractures are fractures that originally exist in the formation and have a certain permeability. Permeability is related to width, so the initial fracture width d0 can be used to characterize the permeability of natural fractures; optionally, the initial fracture width d0 of natural fractures is determined by the permeability of the original fractures in the reservoir;

[0071] Specifically, in practical applications, the original permeability of natural fractures in the reservoir is used as the model input parameter (generally obtained through indoor experimental tests) to calculate the initial width of the natural fractures (the two satisfy a certain relationship). The width of the natural fractures will affect the fluid flow in the natural fractures, as shown in formula (3), where d is the fracture width. After affecting the fluid flow, it will affect the fluid pressure p in the fracture, which will further affect the natural fracture expansion and failure criterion.

[0072] Further, in an optional embodiment, in the step of establishing the fracturing extension model, a crack extension criterion is established based on fracture mechanics; the maximum hoop stress criterion is used as the extension judgment condition of the crack tip (crack extension criterion):

[0073]

[0074] In the formula, K I is the type I stress intensity factor; K II is the type II stress intensity factor; θ is the extension direction of the crack tip, K IC Indicates fracture toughness. Among them, θ represents the angle between the current crack extension direction and the next crack extension direction. Therefore, according to the current crack direction and the calculated θ, the next crack extension direction can be known.

[0075] The propagation direction of the crack tip satisfies the requirements shown in the following equation:

[0076] K I sinθ+K II (3cosθ-1)=0 (5)

[0077] When the crack opens, the crack extension failure criterion, that is, the condition for shear failure, is:

[0078] |τ|≤μ f (σ n -p) (6)

[0079] Where τ is the shear stress on the closed crack surface; μ f is the friction coefficient of the crack surface; σ n is the normal stress on the closed fracture surface; p is the fluid pressure in the fracture. In practical applications, the above formula is used to determine whether the conditions for opening a natural fracture are met. If the left side of the formula is smaller than the right side, it indicates that the shear stress is smaller than the friction stress on the fracture surface, and the fracture surface is not opened, otherwise it is opened. The present invention establishes a composite fracturing fracture extension model that takes into account the changes in the physical properties of carbon dioxide. For the carbon dioxide fracturing fluid system, the carbon dioxide physical property characterization model is the Peng-Robinson model; therefore, in the step of establishing the fracturing extension model, the carbon dioxide physical property parameters are calculated using the Peng-Robinson model.

[0080] The calculation equation for the density of carbon dioxide is:

[0081]

[0082] Where: T and V are the absolute temperature and molar volume of carbon dioxide respectively; T r is the comparison temperature, that is, the absolute temperature T and the critical temperature T c ratio; M is the molar mass of carbon dioxide.

[0083] The calculation equation for the viscosity of carbon dioxide is:

[0084]

[0085] Where: M is the molar mass; V c is the critical volume of carbon dioxide; Ω u is the collision integral; μ r is the reduced dipole moment; κ is the correlation factor of highly polar substances; n is the number of moles per unit volume; η * , η ** , T * , G1, G2, F c All are intermediate variables; E i is a function of the eccentricity factor and the correlation factor of highly polar substances; Y is an intermediate variable; T r is the comparison temperature; ω represents the eccentricity factor; V c is the critical volume.

[0086] Next, the expansion simulation operation step is performed, and the composite fracturing crack expansion model is used to carry out numerical simulation operations of crack expansion for different geological factors and engineering factors, and the model is solved based on the shifting discontinuous boundary element method to obtain the corresponding simulated crack shape parameters.

[0087] Further, in an optional embodiment, the geological factors include but are not limited to horizontal stress difference, reservoir matrix permeability, degree of development of natural fractures in the reservoir, etc., and the numerical information of the geological factors can be obtained based on experimental test data related to the target reservoir;

[0088] The engineering factors include but are not limited to the total amount of fracturing fluid, fracturing fluid viscosity, displacement, pre-carbon dioxide dosage, pre-carbon dioxide ratio, etc. These engineering factor data and effective ranges can be obtained based on field engineering data and criteria. In actual application, numerical simulation calculations of different engineering factors are carried out for different numerical combinations of geological factors, and simulation calculation results of different engineering factors corresponding to each geological factor combination are obtained. Specifically for a certain geological factor combination, one of the engineering factors can be further controlled to remain unchanged, and the fracture expansion simulation plan is calculated for the values ​​of other engineering factors that change; until all available geological factors and engineering factors are calculated, the numerical simulation calculation of fracture expansion is completed.

[0089] Furthermore, through the fracturing parameter optimization design step, the relationship between different factors and crack extension morphology under various geological factors is analyzed based on the simulation calculation results, and the engineering factor parameters are optimized.

[0090] Preferably, in one embodiment, in the fracturing parameter optimization design step, for a certain combination of geological factors, curves of the changing relationship between different factors and fracturing crack length are drawn respectively according to the simulation calculation results, and combined with the balance principle of carbon dioxide injection rate and filtration loss rate in fracturing engineering, the curve trend is analyzed to determine the optimal value of the geological factor under the current combination of geological factors.

[0091] This embodiment of the present invention optimizes the pre-carbon dioxide composite fracturing parameters by carrying out numerical simulation of fracture extension under different geological factors and engineering factors, obtaining fracture morphology and drawing a curve of the relationship between different factors and fracturing fracture length; and optimizes reasonable pre-carbon dioxide composite fracturing engineering parameters according to different reservoir conditions.

[0092] Furthermore, the method for determining the optimal pre-placement carbon dioxide dosage is the first inflection point position on the pre-placement carbon dioxide dosage and fracture length curve, that is, the carbon dioxide injection rate and the filtration rate are balanced.

[0093] Optional, fixed front CO2 with 50% displacement and 6m 3 / min, and the total simulated injection volume was 300m3 、500m 3 、800m 3 、1500m 3 According to the simulation results, the curve of fracture length changing with liquid injection volume is drawn to optimize the optimal total liquid volume and pre-placed carbon dioxide dosage.

[0094] like Figure 3 The curve of the relationship between the liquid injection volume and the fracture length in composite fracturing shows that the fracture length-total injection volume curve shows a similar trend under different fluid injection scales. In the early stage of carbon dioxide injection, the curve slope is large, and after a period of injection, it will turn into near-horizontal development, and the so-called "inflection point" will appear; the main reason for the near-horizontal development is that after the fracture expands to a certain extent, the carbon dioxide filtration loss is basically the same as the injection. Therefore, although the injection volume continues to increase, no new cracks appear, and the fracture length is almost unchanged. Therefore, the carbon dioxide injection volume corresponding to this inflection point is the optimal pre-carbon dioxide dosage.

[0095] Optional, fixed injection volume constant 800m 3 , the front carbon dioxide accounts for 50%, and the simulated displacement is set to 3m 3 / min、6m 3 / min、10m 3 / min. Based on the simulation results, the curve of fracture length changing with liquid injection volume under different displacement conditions was drawn to clarify the influence of displacement on the fracturing effect. Figure 4 The curves of total injection volume and fracture length at different displacements in composite fracturing show that the higher the displacement, the longer the fracture length when the "inflection point" appears for the first time after a certain period of carbon dioxide injection, and the higher the fracture creation efficiency. Therefore, combined with the construction pressure prediction results of the on-site fracturing string, the displacement should be increased as much as possible.

[0096] Optional, fixed injection volume constant 800m 3 , the displacement is constant at 6m 3 / min, simulating the front carbon dioxide consumption of 100m 3 、200m 3 、400m 3 , that is, the proportion of pre-placed carbon dioxide is 12.5%, 25%, and 50% respectively. According to the simulation results, the curve of the change of fracture length with liquid injection volume under different pre-placed carbon dioxide proportions is drawn to optimize the optimal pre-placed carbon dioxide proportion. Figure 5The curves of liquid injection volume and fracture length under different pre-CO2 ratios show that with the increase of pre-CO2 ratio, the fracture length gradually shortens. The main reason is that after a certain period of pre-CO2 injection, the injection displacement and the filtration rate gradually tend to balance, and the fracture efficiency in the pre-CO2 fracturing stage is low. Therefore, the optimization principle of the pre-CO2 ratio is to see an inflection point on the curve of the relationship between the total injection volume and the fracture length. After the inflection point, the water-based fracturing fluid is injected in time to achieve the expected fracture length.

[0097] The present invention can be used to study the influence of horizontal stress difference, reservoir matrix permeability, reservoir natural fracture development degree, total fracturing fluid dosage, fracturing fluid viscosity, displacement, pre-placed carbon dioxide dosage, pre-placed carbon dioxide ratio, etc. on the composite fracturing fracture expansion morphology, effectively overcoming the problem of lack of theoretical basis for the current on-site pre-placed carbon dioxide composite fracturing construction parameter design.

[0098] For the above-mentioned method embodiments, for the sake of simplicity, they are all described as a series of action combinations, but those skilled in the art should know that the present invention is not limited by the order of the actions described, because according to the present invention, some steps can be performed in other orders or simultaneously. Secondly, those skilled in the art should also know that the embodiments described in the specification are all preferred embodiments, and the actions and modules involved are not necessarily required by the present invention.

[0099] It should be pointed out that in other embodiments of the present invention, the method can also obtain a new pre-carbon dioxide composite fracturing parameter optimization design method by combining one or several of the above embodiments to achieve fine optimization of the pre-carbon dioxide composite fracturing process.

[0100] It should be noted that, based on the method in any one or more of the above embodiments of the present invention, the present invention also provides a storage medium, on which is stored a program code that can implement the method as described in any one or more of the above embodiments, and when the code is executed by the operating system, it can implement the above-mentioned pre-carbon dioxide composite fracturing parameter optimization design method.

[0101] Embodiment 2:

[0102] The method is described in detail in the embodiments disclosed in the present invention. The method of the present invention can be implemented by various forms of devices or systems. Therefore, based on other aspects of the method described in any one or more embodiments, the present invention also provides a pre-carbon dioxide composite fracturing parameter optimization design system, which is used to execute the pre-carbon dioxide composite fracturing parameter optimization design method described in any one or more embodiments. Specific embodiments are given below for detailed description.

[0103] Specifically, Figure 6 FIG. 4 is a schematic diagram showing the structure of the pre-carbon dioxide composite fracturing parameter optimization design system provided in an embodiment of the present invention. Figure 6 As shown, the system includes:

[0104] A geometric model building module, which is configured to build a composite fracturing design geometric model based on reservoir physical parameters, rock mechanical properties and natural fracture development conditions;

[0105] A fracturing extension model building module, configured to build a composite fracturing crack extension model taking into account the changes in carbon dioxide physical property parameters based on the composite fracturing design geometric model;

[0106] An extended simulation operation module is configured to use the composite fracturing fracture extension model to carry out fracture extension numerical simulation operations for different geological factors and engineering factors, and solve the model based on the shifting discontinuous boundary element method;

[0107] The fracturing parameter optimization design module is configured to analyze the relationship between different factors and crack extension morphology under various geological factors based on simulation calculation results, and optimize the design engineering factor parameters.

[0108] Furthermore, in one embodiment, the geometric model building module is configured as follows: based on core observations, logging interpretations and seismic interpretation results, natural fracture development conditions of the current reservoir are determined as constraints; based on the physical parameters of the reservoir and rock mechanical properties, a reservoir geometric model with a scale that meets the requirements is formed; and in combination with the natural fracture development conditions, fracture distribution is set as a composite fracturing design geometric model.

[0109] Optionally, in one embodiment, the composite fracturing crack extension model established by the fracturing extension model establishment module includes a stress equation of a crack unit, a flow equation of a fluid in a crack, a crack extension criterion, and a carbon dioxide property calculation equation.

[0110] In a preferred embodiment, the fracturing extension model building module sets the total stress at a certain point to be the sum of the induced stresses generated by all fracture units at the point, and establishes the stress equation of a single fracture unit at a certain point based on the displacement discontinuity method.

[0111] Furthermore, in an optional embodiment, the fracturing extension model building module is configured to: establish a stress equation of a single fracture unit at a certain point according to the following formula:

[0112]

[0113] Where G is the shear modulus; ν is the Poisson's ratio; a is the half length of the crack unit; D x Tangential displacement discontinuity; D yis the normal displacement discontinuity; f ,xy 、f ,xyy 、f ,yy 、f ,yyy is the stress influence coefficient of each crack unit.

[0114] Preferably, in one embodiment, the fracturing extension model building module is configured to establish a crack extension criterion based on the principle of fracture mechanics, and adopt the maximum hoop stress criterion as the extension judgment condition criterion of the crack tip.

[0115] Specifically, in an optional embodiment, the fracturing extension model building module is configured to use the Peng-Robinson model to calculate the physical property parameters of carbon dioxide.

[0116] Furthermore, in one embodiment, the fracturing parameter optimization design module is configured as follows: for a certain combination of geological factors, curves of the changing relationship between different factors and fracturing crack length are drawn respectively according to the simulation results, and the balance principle between the injection rate and the filtration rate of carbon dioxide in the fracturing project is combined to analyze the curve trend and determine the optimal value of the geological factor under the current combination of geological factors.

[0117] In the pre-carbon dioxide composite fracturing parameter optimization design system provided by the embodiment of the present invention, each module or unit structure can be independently operated or combined according to the actual influencing factor setting requirements and numerical simulation calculation requirements to achieve corresponding technical effects.

[0118] It should be understood that the embodiments disclosed in the present invention are not limited to the specific structures, processing steps or materials disclosed herein, but should be extended to equivalent substitutions of these features understood by ordinary technicians in the relevant field. It should also be understood that the terms used herein are only used for the purpose of describing specific embodiments and are not meant to be limiting.

[0119] The "one embodiment" mentioned in the specification means that a particular feature, structure or characteristic described in conjunction with the embodiment is included in at least one embodiment of the present invention. Therefore, the phrase "one embodiment" appearing in various places throughout the specification does not necessarily refer to the same embodiment.

[0120] It should be noted that the simulation parameters involved in this embodiment do not limit the present invention in any form. Although the present invention has been disclosed through the above embodiments, any technician familiar with the profession can make some changes or modifications to equivalent embodiments of equivalent changes using the above disclosed technical contents without departing from the scope of the technical method of the present invention. However, any simple modification, equivalent change and modification made to the above embodiments according to the technology of the present invention, which does not depart from the content of the technical solution of the present invention, still falls within the scope of the technical solution of the present invention.

Claims

1. A method for optimizing the design of pre-carbon dioxide composite fracturing parameters, characterized in that: The method comprises: Geometric model establishment steps: Establish a composite fracturing design geometric model based on reservoir physical parameters, rock mechanical properties and natural fracture development; The fracturing extension model establishment step: based on the composite fracturing design geometric model, constructing a composite fracturing crack extension model that takes into account the changes in carbon dioxide physical property parameters; Extension simulation operation step: using the composite fracturing fracture extension model to carry out fracture extension numerical simulation operation for different geological factors and engineering factors, and solving the model based on the shifting discontinuous boundary element method; Fracturing parameter optimization design steps: Based on the simulation results, analyze the relationship between different factors and crack extension morphology under various geological factors, and optimize the design of engineering factor parameters.

2. The method according to claim 1, characterized in that In the geometric model establishment step, the natural fracture development of the current reservoir is determined based on the core observation, logging interpretation and seismic interpretation results as constraints, a reservoir geometric model with a scale that meets the requirements is formed based on the physical parameters of the reservoir and the mechanical properties of the rock, and the fracture distribution is set in combination with the natural fracture development, as a composite fracturing design geometric model.

3. The method according to claim 1, characterized in that In the fracturing extension model establishment step, the composite fracturing crack extension model established includes a stress equation of a crack unit, a flow equation of a fluid in a crack, a crack extension criterion, and a carbon dioxide property calculation equation.

4. The method according to claim 1, characterized in that: In the step of establishing the fracturing extension model, the total stress at a certain point is set as the sum of the induced stresses generated by all fracture units at that point, and the stress equation of a single fracture unit at a certain point is established based on the displacement discontinuity method.

5. The method according to claim 3, characterized in that: In the fracturing extension model establishment step, the stress equation of a single fracture unit at a certain point is established as follows: Where: G is the shear modulus; ν is the Poisson's ratio; a is the half length of the crack unit; D x Tangential displacement discontinuity; D y is the normal displacement discontinuity; f ,xy 、f ,xyy 、f ,yy 、f ,yyy is the stress influence coefficient of each crack unit.

6. The method according to claim 1, characterized in that In the step of establishing the fracturing extension model, the crack extension criterion is established based on the principle of fracture mechanics, and the maximum hoop stress criterion is used as the extension judgment condition criterion of the crack tip.

7. The method according to claim 1, characterized in that In the step of establishing the fracturing extension model, the physical properties of carbon dioxide are calculated using the Peng-Robinson model.

8. The method according to claim 1, characterized in that In the fracturing parameter optimization design step, for a certain combination of geological factors, the relationship curves between different factors and fracturing crack length are drawn according to the simulation results. Combined with the balance principle between carbon dioxide injection rate and filtration rate in fracturing engineering, the curve trend is analyzed to determine the optimal value of the geological factor under the current combination of geological factors.

9. A storage medium, characterized in that: The storage medium stores program codes that can implement the method as claimed in any one of claims 1 to 8.

10. A pre-carbon dioxide composite fracturing parameter optimization design system, characterized in that: The system executes the method according to any one of claims 1 to 8.

Images