Carbon dioxide energy storage fracturing process method for tight oil vertical well

Abstract

The invention relates to the technical field of tight oil reservoir fracturing, in particular to a tight oil vertical well carbon dioxide energy storage fracturing process method. The method comprises the following steps: acquiring a crack image of a tight oil reservoir; determining the type of a crack node based on the position of the crack node in a crack region in the crack image, constructing a topological structure diagram, and determining the connectivity rate of the topological structure diagram; according to the number of the crack nodes of each type, the length distribution of the cracks and the connectivity rate, the crack density is obtained; constructing an equivalent permeability matrix of the tight oil reservoir by combining the matrix permeability of the tight oil reservoir, the porosity of the fracture, the fracture density and the extension direction of the fracture; and the effective thickness of the reservoir, the equivalent permeability matrix, the difference between the formation fracture pressure and the original formation pressure and the critical carbon dioxide viscosity are integrated, and the type of the fracture node is combined, so that the target injection amount of carbon dioxide is determined. According to the method, the sufficiency of hydraulic fracturing energy supplementation is guaranteed, and the yield of a tight oil reservoir is increased.

Description

Technical Field

[0001] This invention relates to the field of tight oil reservoir fracturing technology, specifically to a carbon dioxide energy storage fracturing process for tight oil vertical wells. Background Technology

[0002] With the continuous development of my country's oil and gas resources, tight oil reservoirs have become an important replacement resource. However, tight oil reservoirs are characterized by low porosity, low permeability, and strong heterogeneity, making their development difficult, and overall development is still near the economic limit. Currently, the main technologies for increasing the production and recovery of tight oil are volumetric fracturing and huff-and-puff energy replenishment technologies, but the following problems still exist: First, the development method of volumetric fracturing and elastic energy development of tight oil, due to the lack of external energy replenishment during the production stage, generally exhibits a high-production period with a short period of high production and a rapid decline in production, with a stage recovery rate of only 6-8%, and the utilization and recovery of reserves are relatively low. Second, based on the current development methods of tight oil reservoirs and the integrated geological engineering design concept, development is mainly carried out in the form of independent horizontal wells and vertical well groups with large well spacing and row spacing. Due to the influence of the microstructure and physical properties of tight oil reservoirs, it is difficult to establish effective displacement under the existing well network, and conventional injection and production modes cannot further improve the development effect. Thirdly, single-well injection technologies such as carbon dioxide and activated water are mainly applied to low-yield wells with depleted energy. These technologies achieve short-term production increases with small injection doses, but the increase in oil production per well is limited, resulting in an enhanced recovery rate of less than three percentage points. Therefore, innovative production-boosting technologies and approaches are still needed to achieve efficient, long-term, and economical development of tight oil.

[0003] Existing tight reservoir methods cannot achieve efficient production and recovery throughout the entire life cycle of tight reservoirs. They can only achieve short-term oil increase and do not fully utilize the oil-enhancing mechanism of the displacement medium and the optimal effect of the process technology. Furthermore, the design of carbon dioxide fracturing related processes and key parameters all adopt empirical parameters and lack theoretical guidance. As a result, there are problems such as low porosity, strong heterogeneity, and insufficient energy replenishment from conventional hydraulic fracturing in tight reservoirs. Summary of the Invention

[0004] To address the problems of low porosity, strong heterogeneity, and insufficient energy replenishment in conventional hydraulic fracturing in existing tight oil reservoir methods, this invention aims to provide a carbon dioxide energy storage fracturing process for vertical tight oil wells. The specific technical solution adopted is as follows: This invention provides a carbon dioxide energy storage fracturing process for tight oil vertical wells, the method comprising the following steps: Obtain fracture images of tight oil reservoir samples; Based on the location distribution characteristics of crack nodes in the crack region of the crack image, the type of crack node is determined, a topology graph is constructed, and the connectivity of the topology graph is determined. The fracture density is obtained based on the number distribution of fracture nodes, the length distribution of fractures, and the connectivity of each type of fracture. The equivalent permeability matrix of the tight oil reservoir is constructed by combining the matrix permeability of the tight oil reservoir, the porosity of the fractures, the fracture density, and the direction of fracture extension. The initial injection amount of carbon dioxide into the tight oil reservoir is determined by considering the effective thickness of the reservoir, the equivalent permeability matrix, the difference between the formation fracture pressure and the original formation pressure, and the critical carbon dioxide viscosity. The target injection amount is determined based on the type of crack node and the initial injection amount.

[0005] Preferably, determining the type of crack node based on the positional distribution characteristics of crack nodes in the crack region of the crack image includes: The node at the end of the crack segment is denoted as an isolated node; The node at the intersection of two crack line segments is denoted as the first connected node; The point where the branches of three or more crack segments intersect is denoted as the second connecting node.

[0006] Preferably, the construction of the topology diagram includes: constructing a topology diagram based on the type of crack nodes in the crack image; the crack image is a binarized image.

[0007] Preferably, determining the connectivity of the topology graph includes: The ratio between the number of first connected nodes in the topology graph and the total number of all crack nodes is taken as the connectivity of the topology graph.

[0008] Preferably, the step of obtaining the crack density based on the number distribution of crack nodes of each type, the length distribution of cracks, and the connectivity includes: The total number of crack traces is determined based on the number of isolated nodes and the number of second connected nodes; The total number of crack branches is determined based on the number of isolated nodes, the number of first connected nodes, and the number of second connected nodes; Based on the values ​​of the average length of all crack traces relative to the standard length, and the values ​​of the average length of all crack branches relative to the standard length, the crack trace weights and crack branch weights are obtained respectively. The crack density is obtained by combining the total number of crack traces, the total number of crack branches, the crack trace weight, the crack branch weight, and the connectivity.

[0009] Preferably, determining the total number of crack traces based on the number of isolated nodes and the number of second connected nodes includes: taking the average of the number of isolated nodes and the number of second connected nodes as the total number of crack traces.

[0010] Preferably, determining the total number of crack branches based on the number of isolated nodes, the number of first connected nodes, and the number of second connected nodes includes: Calculate the sum of the products of the number of nodes of all types and their corresponding quantity coefficients; Half of the sum is taken as the total number of crack branches.

[0011] Preferably, the coefficient for the number of isolated nodes is 1, the coefficient for the number of first connected nodes is 4, and the coefficient for the number of second connected nodes is 3.

[0012] Preferably, obtaining the crack trace weight and crack branch weight based on the average length of all crack traces relative to the standard length and the average length of all crack branches relative to the standard length, respectively, includes: The ratio between the average length of all crack traces and the standard length is used as the crack trace weight. The ratio between the average length of all crack branches and the standard length is used as the crack branch weight.

[0013] Preferably, the step of obtaining the crack density by combining the total number of crack traces, the total number of crack branches, the crack trace weight, the crack branch weight, and the connectivity includes: The first eigenvalue is calculated using the total number of crack traces, the total number of crack branches, the crack trace weight, and the crack branch weight. The product of the first feature value and the connectivity rate is taken as the crack density.

[0014] Preferably, the construction of the equivalent permeability matrix of the tight oil reservoir by combining the matrix permeability, fracture porosity, fracture density, and fracture extension direction includes: Based on the direction of crack extension, construct a directional feature matrix; Based on the directional feature matrix, the matrix permeability of the tight oil reservoir, the porosity of the fractures, and the fracture density, the equivalent permeability matrix of the tight oil reservoir is obtained.

[0015] Preferably, the construction of the directional feature matrix based on the crack extension direction includes: The directional feature matrix is A 3D matrix; The element in the first row and first column of the directional feature matrix is ​​the square of the cosine of the fracture composite direction angle. The elements in the first row and second column of the directional feature matrix and the elements in the second row and first column of the directional feature matrix are the product of the sine and cosine of the fracture composite direction angle. The element in the second row and second column of the directional feature matrix is ​​the square of the sine of the fracture composite direction angle. The comprehensive direction angle of the crack is the average of the angles between all cracks in the topology diagram and the preset direction.

[0016] Preferably, obtaining the equivalent permeability matrix of the tight oil reservoir based on the directional feature matrix, the matrix permeability of the tight oil reservoir, the porosity of the fractures, and the fracture density includes: Calculate the first product of the matrix permeability and the identity matrix of tight oil reservoirs; Calculate the second product between the matrix permeability, fracture porosity, fracture density, and directional feature matrix of the tight oil reservoir; The sum of the first product and the second product is used as the equivalent permeability matrix of the tight oil reservoir.

[0017] Preferably, the initial carbon dioxide injection amount for the tight oil reservoir is determined by using the effective thickness of the integrated reservoir, the equivalent permeability matrix, the difference between the formation fracture pressure and the original formation pressure, and the critical carbon dioxide viscosity, including: The equivalent permeability matrix is ​​decomposed into eigenvalues ​​to obtain the maximum and minimum eigenvalues. The initial injection rate of carbon dioxide into the tight oil reservoir is obtained based on the effective thickness of the reservoir, the maximum characteristic value, the minimum characteristic value, the difference between the formation fracture pressure and the original formation pressure, the supercritical carbon dioxide viscosity, and the wellbore radius.

[0018] Preferably, the step of obtaining the initial carbon dioxide injection amount of the tight oil reservoir based on the effective thickness of the reservoir, the maximum characteristic value, the minimum characteristic value, the difference between the formation fracture pressure and the original formation pressure, the supercritical carbon dioxide viscosity, and the wellbore radius includes: Calculate the first ratio between the preset radius and the wellbore radius; The initial injection volume of carbon dioxide into the tight oil reservoir is obtained based on the effective thickness of the reservoir, the maximum characteristic value, the minimum characteristic value, the difference between the formation fracture pressure and the original formation pressure, the supercritical carbon dioxide viscosity, and the first ratio. The effective thickness, the maximum characteristic value, the minimum characteristic value, and the difference between the formation fracture pressure and the original formation pressure are all positively correlated with the initial injection volume, while the supercritical carbon dioxide viscosity and the wellbore radius are both negatively correlated with the initial injection volume.

[0019] Preferably, determining the target injection amount based on the type of crack node and the initial injection amount includes: Calculate the sum of the number of the first connected nodes and the number of the second connected nodes; determine the ratio between the number of isolated nodes and the sum as the relative proportion of isolated nodes; The target injection amount is determined by comparing the relative proportion of isolated nodes with a preset proportion threshold.

[0020] Preferably, the step of comparing the relative proportion of isolated nodes with a preset proportion threshold to determine the target injection amount includes: If the relative proportion of isolated nodes is greater than a preset proportion threshold, then a preset multiple of the initial injection amount is determined as the target injection amount, wherein the preset multiple is less than 1; If the relative proportion of isolated nodes is less than or equal to a preset proportion threshold, then the initial injection amount is determined as the target injection amount.

[0021] The present invention has at least the following beneficial effects: This invention constructs a topological map based on the types of fracture nodes in fracture regions of fracture images from tight oil reservoir samples, evaluates their connectivity, and comprehensively assesses reservoir connectivity by combining matrix permeability, fracture porosity, and fracture extension direction. Based on the strength of reservoir connectivity, the target carbon dioxide injection amount is precisely determined, maximizing formation energy replenishment without compromising wellbore integrity or creating early gas channeling pathways. This ensures that carbon dioxide truly enters the matrix micropores, rather than simply escaping along large fractures. Compared to existing technologies, the method provided by this invention guarantees sufficient energy replenishment for hydraulic fracturing, thereby increasing the production of tight oil reservoirs. Attached Figure Description

[0022] To more clearly illustrate the technical solutions and advantages in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0023] Figure 1 This is a flowchart of a carbon dioxide energy storage fracturing process for tight oil vertical wells provided in an embodiment of the present invention. Detailed Implementation

[0024] To further illustrate the technical means and effects adopted by the present invention to achieve the intended purpose, the following detailed description of a tight oil vertical well carbon dioxide energy storage fracturing process method proposed according to the present invention is provided in conjunction with the accompanying drawings and preferred embodiments.

[0025] Unless otherwise defined, 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.

[0026] The following describes in detail, with reference to the accompanying drawings, a specific scheme for a carbon dioxide energy storage fracturing process for tight oil vertical wells provided by the present invention.

[0027] An example of a carbon dioxide energy storage fracturing process for tight oil vertical wells: This embodiment proposes a carbon dioxide energy storage fracturing process for tight oil vertical wells, such as... Figure 1 As shown, a carbon dioxide energy storage fracturing process for tight oil vertical wells in this embodiment includes the following steps: Step S1: Obtain fracture images of tight oil reservoir samples.

[0028] This embodiment applies carbon dioxide to multiple production-enhancing factors in tight oil reservoirs. By deeply integrating energy replenishment and reservoir stimulation, it proposes a tight oil carbon dioxide energy storage fracturing technology with "pre-fracturing gas injection + well-sealing seepage + fracturing stimulation" as its core. Through the deep integration of micro-fracture topology characterization and macro-fracturing process, a collaborative mechanism for energy field construction and volume transformation throughout the entire life cycle is established.

[0029] The purpose of pre-pressure gas injection is to inject a certain amount of liquid or supercritical carbon dioxide into the target reservoir to establish a formation energy field. The specific operation involves pumping cryogenic liquid carbon dioxide from a carbon dioxide storage tank into the target formation downhole using a high-pressure pump unit. The injection pressure is typically controlled below the reservoir fracture pressure to avoid macroscopic fractures. The key is to utilize the high permeability of carbon dioxide to allow it to penetrate the reservoir matrix and natural microfractures.

[0030] Replenishing formation energy: Increases reservoir pore pressure, providing driving force for subsequent production; Improving tight oil properties: Carbon dioxide reacts with minerals and fluids, which may change the mechanical properties of the oil layer, creating favorable conditions for subsequent fracturing; Pre-treating crude oil: Carbon dioxide begins to dissolve in crude oil, initially reducing viscosity and interfacial tension.

[0031] First, a tight oil reservoir sample is selected. To obtain clear images of fractures in the tight oil reservoir, this embodiment uses a micron-level CT scanner, such as the Phoenix V | TOME | XS180 or Bruker Skyscan series, without specific limitations. The tight oil reservoir sample is placed on a rotating stage, and the oil layer slice image is acquired by the CT scanner. This is recorded as the original CT slice image of the tight oil reservoir, with a window size of 2500 pixels. Then, the original CT slice image is binarized. In the processed image, the black areas are the background areas, and the white areas are the fracture areas. The image of the fracture areas in the binarized image is recorded as the fracture image of the tight oil reservoir sample, i.e., the fracture image is a binarized image. Image binarization is a prior art technique and will not be elaborated further here.

[0032] Step S2: Based on the location distribution characteristics of crack nodes in the crack region of the crack image, determine the type of crack nodes, construct a topology graph, and determine the connectivity of the topology graph.

[0033] Tight oil reservoirs contain numerous natural fractures with varying orientations. When the number and number of fractures develop to a certain extent, the fractures in tight oil reservoirs exhibit complex network characteristics. When the fracture network is complex, characterizing the developmental features of fractures in tight oil reservoirs becomes difficult, making accurate characterization of fractures challenging using conventional methods. This embodiment utilizes binarized images of oil reservoir fractures to analyze and characterize fractures in tight oil reservoirs.

[0034] Because the fracture network in tight oil reservoirs exhibits complex and disordered characteristics, the construction of the topology can significantly affect the calculation of the fracture network connectivity and permeability, thus impacting the subsequent calculation of CO2 fracturing parameters. Therefore, to improve the accuracy of CO2 fracturing parameter analysis, it is necessary to calculate the fracture topology of tight oil reservoirs.

[0035] Typically, in binarized images of tight oil reservoirs, natural fracture nodes exhibit three different appearance types. Therefore, they are categorized based on their location distribution characteristics. Specifically, nodes at the ends of fracture segments are designated as isolated nodes; nodes at the intersection of two fracture segments are designated as first connected nodes, equivalent to X nodes; and nodes at the intersection of branches of three or more fracture segments are designated as second connected nodes, equivalent to Y nodes. An isolated branch is a line connecting two isolated nodes; a connected branch is a line connecting two isolated nodes or an isolated node with a second connected node; and a multi-connected branch is a line connecting two second connected nodes or a first connected node with a second connected node.

[0036] Furthermore, a topology graph is constructed based on the types of crack nodes in the crack image. The method for constructing the topology graph is existing technology and will not be elaborated further here. The number of nodes of each type is counted, and the ratio between the number of the first connected node in the topology graph and the total number of all crack nodes is taken as the connectivity of the topology graph. The larger the proportion of X nodes in the topology graph, the more complex crack connection points exist in the topology, the higher the connectivity of the topology graph will be, and the more tightly connected the crack network will be.

[0037] Step S3: Based on the distribution of the number of fracture nodes of each type, the length distribution of fractures, and the connectivity, the fracture density is obtained; combined with the matrix permeability of the tight oil reservoir, the porosity of the fractures, the fracture density, and the direction of fracture extension, the equivalent permeability matrix of the tight oil reservoir is constructed.

[0038] The X-nodes formed by the intersection of two cracks represent intermediate stages for both cracks, not their termination. Therefore, each X-node contributes 0 crack terminations. However, in a binarized image, cracks are continuous traces, and each crack should have 2 termination nodes.

[0039] Since fractures are the primary channels for fluid (oil, gas, water, and fracturing fluid, etc.) migration, higher trace density means more possible flow paths for the fluid, typically leading to a significant increase in the overall permeability of the fracture network. During fracture development, multiple intersecting fractures often form a network, further enhancing permeability. In this embodiment, the line segment between two nodes is defined as a fracture branch, and the total number of fracture branches can be calculated using the graph theory handshake theorem.

[0040] Based on the above characteristics, the total number of crack traces is determined according to the number of isolated nodes and the number of second connected nodes; the total number of crack branches is determined according to the number of isolated nodes, the number of first connected nodes, and the number of second connected nodes.

[0041] As a concrete example, the average of the number of isolated nodes and the number of second connected nodes is taken as the total number of crack traces. The sum of the products of the number of all types of nodes and their corresponding quantity coefficients is calculated, and half of this sum is taken as the total number of crack branches, where the quantity coefficient corresponding to the number of isolated nodes is 1, the quantity coefficient corresponding to the number of first connected nodes is 4, and the quantity coefficient corresponding to the number of second connected nodes is 3.

[0042] In this embodiment, the total number of crack branches can be expressed as:

[0043] in, This indicates the total number of crack branches. Indicates the number of isolated nodes. Indicates the number of second connected nodes. This indicates the number of nodes in the first connected node.

[0044] Furthermore, based on the values ​​of the average length of all crack traces relative to the standard length and the average length of all crack branches relative to the standard length, crack trace weights and crack branch weights are obtained respectively.

[0045] As a specific example, the ratio between the average length of all crack traces and the standard length is used as the crack trace weight; the ratio between the average length of all crack branches and the standard length is used as the crack branch weight. The standard length refers to the original window size, which is 2500 in this embodiment.

[0046] The first characteristic value is calculated using the total number of crack traces, the total number of crack branches, the crack trace weight, and the crack branch weight; the product of the first characteristic value and the connectivity is taken as the crack density.

[0047] In this embodiment, a specific formula for calculating crack density is given, which can be expressed as:

[0048] in, Indicates crack density. Indicates connectivity. Indicates the crack trace weight. Indicates the crack branch weight. This indicates the total number of crack traces. This indicates the total number of crack branches. This represents the total number of nodes in the topology graph. This represents the first eigenvalue.

[0049] This embodiment utilizes graph theory and statistical methods to analyze fracture characteristics and obtain fracture density. In the topological structure diagram, a higher number of fracture traces and fracture branches indicates that the tight oil reservoir possesses a highly connected and easily flowable reservoir state. By introducing a weighting coefficient based on average length, the interference of short, non-connected fractures is effectively suppressed, and the degree of fracture interweaving can be more quantified, establishing a strong correlation between fracture geometry and reservoir permeability.

[0050] Specifically, if the total number of nodes in the topology graph is 0 or 1, then the crack density is set to 0.

[0051] Carbon dioxide fracturing is a complex, nonlinear, and time-varying dynamic process. The injected carbon dioxide fluid interacts with and causes fracture initiation, propagation, and reversal, and interacts with natural fractures. The final fracture network morphology directly determines the reservoir stimulation effect. Therefore, it is necessary to analyze and calculate the changing characteristics of the natural fractures to further analyze and determine the carbon dioxide injection parameters.

[0052] Based on the extension direction of the cracks, a direction feature matrix is ​​constructed. Specifically, the average angle between all cracks in the topology diagram and a preset direction is taken as the comprehensive crack direction angle, where the preset direction can be horizontal to the right. The direction feature matrix is ​​as follows: The directional feature matrix is ​​a dimensional matrix. The elements in the first row and first column are the squares of the cosine of the fracture direction angle. The elements in the first row and second column, and the elements in the second row and first column, are the products of the sine and cosine of the fracture direction angle. The elements in the second row and second column are the squares of the sine of the fracture direction angle. The directional feature matrix can be represented as: ,in, Represents the directional feature matrix. Indicates the angle of the crack's composite direction.

[0053] The fracture density obtained through the above steps reflects the density distribution of fractures on the surface of the tight oil reservoir during carbon dioxide fracturing. Next, the density distribution needs to be converted into permeability to characterize the subsequent diffusion rate of carbon dioxide within the reservoir.

[0054] In the process of carbon dioxide fracturing, liquid carbon dioxide enters the reservoir and becomes a supercritical fluid, thus having a flow direction. The direction of the fractures during the carbon dioxide fracturing process can be simulated by adding directional carbon dioxide.

[0055] As a specific implementation method, the equivalent permeability matrix of a tight oil reservoir can be determined in the following way: Specifically, the product of the matrix permeability of the tight oil reservoir and the identity matrix is ​​calculated, and this product is recorded as the first product; the product of the matrix permeability, fracture porosity, fracture density, and directional characteristic matrix of the tight oil reservoir is calculated, and this product is recorded as the second product; the sum of the first product and the second product is used as the equivalent permeability matrix of the tight oil reservoir.

[0056] In this embodiment, a specific formula for calculating the equivalent permeability matrix of a tight oil reservoir is given. The equivalent permeability matrix of a tight oil reservoir can be expressed as:

[0057] in, This represents the equivalent permeability matrix of a tight oil reservoir. Indicates the matrix permeability of tight oil reservoirs. Represents the identity matrix. Indicates the porosity of the crack. Indicates crack density. This represents the directional feature matrix.

[0058] Represents the first product. This represents the second product. It should be noted that the identity matrix... Too A 3D matrix. The porosity of the cracks is obtained through CT scanning. In this embodiment, The value is 0.087; the matrix permeability of the tight oil reservoir was obtained through experimental measurement. In this embodiment, The value is 0.48 millidarcy (mD).

[0059] When the fracture density is greater, the element values ​​in the equivalent permeability matrix of the tight oil reservoir are also larger, and under the same pumping pressure, the tight oil reservoir in the current area can be injected with more carbon dioxide.

[0060] Step S4: Based on the effective thickness of the reservoir, the equivalent permeability matrix, the difference between the formation fracture pressure and the original formation pressure, and the critical carbon dioxide viscosity, determine the initial injection amount of carbon dioxide into the tight oil reservoir.

[0061] The elements in the equivalent permeability matrix can represent the fluid flow direction in tight oil reservoirs, and are characterized using a second-order tensor. However, in practical engineering, since the input to the pumping equipment is a scalar value, further processing of the second-order tensor is required.

[0062] In tight oil reservoirs, the flow velocity of supercritical carbon dioxide varies in different directions. Therefore, we first perform eigenvalue decomposition on the equivalent permeability matrix of the second-order tensor tight oil reservoir to obtain the maximum and minimum eigenvalues ​​of the equivalent permeability matrix. The direction with the largest eigenvalue represents the main direction of natural fracture development, while the direction with the smaller eigenvalue represents the permeability perpendicular to the fracture direction.

[0063] The initial injection rate of carbon dioxide into the tight oil reservoir is obtained based on the effective thickness of the reservoir, the maximum and minimum eigenvalues ​​of the equivalent permeability matrix, the difference between the formation fracture pressure and the original formation pressure, the supercritical carbon dioxide viscosity, and the wellbore radius.

[0064] As a specific example, the ratio between the preset radius and the wellbore radius is calculated and recorded as the first ratio. Based on the effective thickness of the reservoir, the maximum characteristic value, the minimum characteristic value, the difference between the formation fracture pressure and the original formation pressure, the supercritical carbon dioxide viscosity, and the first ratio, the initial injection volume of carbon dioxide in the tight oil reservoir is obtained. The effective thickness, the maximum characteristic value, the minimum characteristic value, the difference between the formation fracture pressure and the original formation pressure are all positively correlated with the initial injection volume, while the supercritical carbon dioxide viscosity and the wellbore radius are both negatively correlated with the initial injection volume.

[0065] In this embodiment, a specific formula for calculating the initial injection amount of carbon dioxide into a tight oil reservoir is given. The initial injection amount can be expressed as:

[0066] in, The initial injection rate of carbon dioxide into the tight oil reservoir is expressed in cubic meters per second; h represents the effective thickness of the reservoir in meters. This represents the largest eigenvalue of the equivalent permeability matrix. This represents the smallest eigenvalue of the equivalent permeability matrix; Represents pi (π). It represents the formation fracture pressure, that is, the critical pressure at which the oil reservoir undergoes macroscopic tensile failure; Indicates the original formation pressure; This indicates the viscosity of supercritical carbon dioxide. Indicates the preset radius. Represents the wellbore radius.

[0067] In this embodiment, the effective thickness of the reservoir is taken as 10, in meters; formation fracture pressure The value is 34 MPa; the original formation pressure The value is 20.2 MPa; supercritical carbon dioxide viscosity The value is 0.06 centipoise (cP); preset radius The value is 150m, and the wellbore radius is... The value is 0.1m.

[0068] The initial carbon dioxide injection rate of tight oil reservoirs can be calculated using the above formula. Since formation permeability varies in different directions and pressure wave diffusion is elliptical, to better reflect actual permeability conditions, the permeability in two different directions after tensor decomposition is analyzed, thus better reflecting formation permeability. This represents the difference between the fractured formation pressure and the original formation pressure, reflecting the net pressure differential. Higher injection pressure allows tight oil reservoirs to achieve greater injection pressure, but it needs to be kept below the fracture pressure to ensure matrix absorption and microfracture opening, rather than prematurely forming large fractures that could lead to gas channeling. During carbon dioxide fracturing injection, limited by the wellbore diameter, most of the pressure drop is concentrated near the wellbore. This can be calculated... This reflects the pressure injection process.

[0069] The initial carbon dioxide injection rate of the tight oil reservoir was obtained using the above methods.

[0070] Step S5: Determine the target injection amount based on the type of crack node and the initial injection amount.

[0071] During the pre-pressure gas injection stage, carbon dioxide needs to be injected into the formation microfractures as quickly as possible without opening the macroscopic main fractures.

[0072] Considering the complex micro-network inside the oil layer in natural fractures, when there are many intersections and branch nodes in the oil layer, the oil layer exhibits a highly interconnected permeable body. After supercritical carbon dioxide enters, it will quickly divert and can accommodate high-speed injection without pressure buildup. However, when there are many isolated nodes inside the oil layer, supercritical carbon dioxide will quickly encounter a flow dead zone after entering, and at this time, the carbon dioxide injection pressure will rise sharply.

[0073] Calculate the sum of the number of the first connected nodes and the number of the second connected nodes; determine the ratio between the number of isolated nodes and this sum as the relative proportion of isolated nodes; specifically, if the number of the first connected nodes and the number of the second connected nodes are both 0, then set the relative proportion of isolated nodes to 1. If the relative proportion of isolated nodes is greater than a preset proportion threshold, it indicates that the natural connectivity of the current tight oil reservoir is poor. In this case, a low-volume injection method should be adopted to avoid the bottom hole pressure exceeding the fracturing pressure, which would cause the oil layer near the wellbore to break and the distal end to be unenergized. Therefore, a preset multiple of the initial injection volume is determined as the target injection volume, where the preset multiple is less than 1, and the value of the preset multiple can be in the range of 0.2~0.5. In this embodiment, the preset multiple is 0.25, and the preset proportion threshold is 0.08. If the relative proportion of isolated nodes is less than or equal to the preset proportion threshold, it indicates that the natural connectivity of the current tight oil reservoir formation is poor. The initial injection volume is determined as the target injection volume, and a large-volume rapid injection is used to shorten the construction cycle.

[0074] When pre-charging energy or directly initiating fracturing, the purpose of this stage is to create artificial fractures with high conductivity. The specific operational process includes: using carbon dioxide as the main fracturing fluid, or mixing it with a small amount of water and chemical additives to form a mixed fracturing fluid. Carbon dioxide and proppant (such as ceramic aggregate or quartz sand) are pumped downhole using a fracturing pump truck. When the bottomhole pressure exceeds the formation fracturing pressure, the oil layer fractures, forming the main fracture. The fracture is extended and effectively supported by continuously pumping carbon dioxide fracturing fluid carrying proppant.

[0075] By establishing high-speed channels for oil and gas to flow from the matrix to the wellbore, and by utilizing the low viscosity and high filtration characteristics of carbon dioxide to create natural fractures and form a complex fracture network system, the reservoir stimulation volume (SRV) is maximized.

[0076] Furthermore, well shut-in energy storage (shut-in well) is a key step in fracturing operations where the well is shut down for a period of time after the fracturing is completed. This is a crucial difference between carbon dioxide energy storage fracturing and conventional fracturing. The specific procedure is as follows: after stopping injection, the wellhead is closed, allowing the injected carbon dioxide to further diffuse, dissolve, and interact with the crude oil in the reservoir. The shut-in time is one of the key parameters that needs optimization, typically lasting several days.

[0077] By allowing high-pressure carbon dioxide to diffuse further from fractures into the matrix, formation energy is replenished more evenly; time is provided for carbon dioxide to fully dissolve into crude oil, achieving viscosity reduction, expansion, and extraction, thus improving fluid flowability; and fractures are allowed to close slowly under the action of proppant, forming stable flow channels. In terms of mechanism, the integrated effect of carbon dioxide fracturing, energy enhancement, and permeation is utilized. Regarding construction steps, carbon dioxide is injected at super-fracture pressure after perforation in new wells, followed by water-based fracturing after well shut-in, changing the previous pre-fracturing injection method. Through pre-injection, a supercritical state of carbon dioxide throughout the reservoir and the formation of a high-pressure field before fracturing are ensured; permeation and replacement effects are enhanced and the affected area is expanded through well shut-in diffusion; and subsequent fracturing and modification increase the complexity of fractures, significantly improving the fracture modification body and increasing the drainage area.

[0078] Field data from production wells show that well A, which adopted this method, produced 1.9 times more oil per day in the initial stage than adjacent wells, and maintained a formation pressure level significantly higher than that of conventional wells (1.7 times the original formation pressure), confirming the effectiveness of energy storage.

[0079] Table 1 shows the comparison results of the formation pressure increase factor in tight oil CO2-storage fracturing wells, and Table 2 shows the comparison results of the initial production increase in tight oil CO2-storage fracturing wells. Table 1. Comparison of Formation Pressure Increase Factors in Tight Oil Carbon Dioxide Storage Fracturing Wells

[0080] Table 2 Comparison of Initial Production Increase in Tight Oil Carbon Dioxide Storage Fracturing Wells

[0081] Taking the tight oil reservoir A well as an example, we explored carbon dioxide energy storage fracturing technology based on the idea of ​​integrating carbon dioxide-assisted fracturing with huff and puff energy replenishment.

[0082] Well A is a Class I tight oil well with an oil layer thickness of 12.0 m and an effective thickness of 10.8 m. The reservoir temperature is 100.2℃, the pressure coefficient is 1.0, and the original formation pressure is 20.2 MPa.

[0083] (1) Gas injection parameter design With production targets as the core and high injection volume as the goal for significant energy increase, a multi-method approach combining numerical simulations and theoretical formulas was employed for optimization. The designed injection volume is 5600t, and the injection rate is 4... 600 tons / day, well shut-in time 10-15 days.

[0084] (2) Process design After perforation of the new well, carbon dioxide was injected at the super-fracture pressure, followed by water-based fracturing and stimulation. The gas injection process used was casing injection, with a P110 casing short-connection installed at the wellhead and a 70MPa fracturing pressure applied.

[0085] (3) Fracturing parameter design Casing fracturing 8-12 The liquid addition intensity is 425.4. The strength of the sand added is 20.2. The suture was temporarily plugged 5 times.

[0086] Implementation status: 4 The well had a daily gas injection capacity of 600 tons, an operational pressure of approximately 31 MPa, a shutdown pressure of approximately 23 MPa, and a cumulative gas injection of 5600 tons. The planned injection volume was executed at 100% of the target, and the final injection pressure reached twice the original formation pressure. The well was shut in for 15 days, and fracturing was completed according to the plan. The shutdown pressure was 32.3 MPa, approximately 6 MPa higher than that of the adjacent well.

[0087] Application results: Well A initially produced 16.8 tons of oil per day. After 128 days of production, it now produces 10.3 tons of oil per day, with a cumulative production of 1,571 tons. This has significantly increased the production of tight oil wells. Compared with neighboring platform wells, Well A's initial production increased by 1.9 times.

[0088] This embodiment constructs a topological map based on the types of fracture nodes in the fracture regions of fracture images from tight oil reservoir samples, evaluates their connectivity, and comprehensively assesses reservoir connectivity by combining matrix permeability, fracture porosity, and fracture extension direction. Based on the strength of reservoir connectivity, the target carbon dioxide injection amount is precisely determined, maximizing formation energy replenishment without compromising wellbore integrity or creating early gas channeling pathways. This ensures that carbon dioxide truly enters the matrix micropores, rather than simply escaping along large fractures. Compared to existing technologies, the method provided in this embodiment improves the sufficiency of hydraulic fracturing energy replenishment and increases tight oil reservoir production.

[0089] For more details about the above modules, please refer to other parts of this manual; they will not be repeated here.

[0090] It should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for carbon dioxide energy storage fracturing in tight oil vertical wells, characterized in that, The method includes the following steps: Obtain fracture images of tight oil reservoir samples; Based on the location distribution characteristics of crack nodes in the crack region of the crack image, the type of crack node is determined, a topology graph is constructed, and the connectivity of the topology graph is determined. The fracture density is obtained based on the number distribution of fracture nodes, the length distribution of fractures, and the connectivity of each type of fracture. The equivalent permeability matrix of the tight oil reservoir is constructed by combining the matrix permeability of the tight oil reservoir, the porosity of the fractures, the fracture density, and the direction of fracture extension. The initial injection amount of carbon dioxide into the tight oil reservoir is determined by considering the effective thickness of the reservoir, the equivalent permeability matrix, the difference between the formation fracture pressure and the original formation pressure, and the critical carbon dioxide viscosity. The target injection amount is determined based on the type of crack node and the initial injection amount.

2. The tight oil vertical well carbon dioxide energy storage fracturing process method according to claim 1, characterized in that, The determination of the type of crack node based on the positional distribution characteristics of crack nodes in the crack region of the crack image includes: The node at the end of the crack segment is denoted as an isolated node; The node at the intersection of two crack line segments is denoted as the first connected node; The point where the branches of three or more crack segments intersect is denoted as the second connecting node.

3. The tight oil vertical well carbon dioxide energy storage fracturing process method according to claim 2, characterized in that, The construction of the topology diagram includes: constructing a topology diagram based on the type of crack nodes in the crack image; the crack image is a binarized image.

4. The tight oil vertical well carbon dioxide energy storage fracturing process method according to claim 2, characterized in that, Determining the connectivity of the topology graph includes: The ratio between the number of first connected nodes in the topology graph and the total number of all crack nodes is taken as the connectivity of the topology graph.

5. The tight oil vertical well carbon dioxide energy storage fracturing process method according to claim 2, characterized in that, The method of obtaining crack density based on the number distribution of crack nodes of each type, crack length distribution, and connectivity includes: The total number of crack traces is determined based on the number of isolated nodes and the number of second connected nodes; The total number of crack branches is determined based on the number of isolated nodes, the number of first connected nodes, and the number of second connected nodes; Based on the values ​​of the average length of all crack traces relative to the standard length, and the values ​​of the average length of all crack branches relative to the standard length, the crack trace weights and crack branch weights are obtained respectively. The crack density is obtained by combining the total number of crack traces, the total number of crack branches, the crack trace weight, the crack branch weight, and the connectivity.

6. The tight oil vertical well carbon dioxide energy storage fracturing process method according to claim 5, characterized in that, The step of determining the total number of crack traces based on the number of isolated nodes and the number of second connected nodes includes: taking the average of the number of isolated nodes and the number of second connected nodes as the total number of crack traces.

7. The tight oil vertical well carbon dioxide energy storage fracturing process method according to claim 5, characterized in that, The determination of the total number of crack branches based on the number of isolated nodes, the number of first connected nodes, and the number of second connected nodes includes: Calculate the sum of the products of the number of nodes of all types and their corresponding quantity coefficients; Half of the sum is taken as the total number of crack branches.

8. The tight oil vertical well carbon dioxide energy storage fracturing process method according to claim 7, characterized in that, The coefficient for the number of isolated nodes is 1, the coefficient for the number of the first connected nodes is 4, and the coefficient for the number of the second connected nodes is 3.

9. The tight oil vertical well carbon dioxide energy storage fracturing process method according to claim 5, characterized in that, The step of obtaining the crack trace weight and crack branch weight based on the average length of all crack traces relative to the standard length and the average length of all crack branches relative to the standard length, respectively, includes: The ratio between the average length of all crack traces and the standard length is used as the crack trace weight. The ratio between the average length of all crack branches and the standard length is used as the crack branch weight.

10. The tight oil vertical well carbon dioxide energy storage fracturing process method according to claim 5, characterized in that, The process of obtaining the crack density by combining the total number of crack traces, the total number of crack branches, the crack trace weight, the crack branch weight, and the connectivity includes: The first eigenvalue is calculated using the total number of crack traces, the total number of crack branches, the crack trace weight, and the crack branch weight. The product of the first feature value and the connectivity rate is taken as the crack density.

11. The tight oil vertical well carbon dioxide energy storage fracturing process method according to claim 1, characterized in that, The equivalent permeability matrix of the tight oil reservoir is constructed by combining the matrix permeability, fracture porosity, fracture density, and fracture extension direction, including: Based on the direction of crack extension, construct a directional feature matrix; Based on the directional feature matrix, the matrix permeability of the tight oil reservoir, the porosity of the fractures, and the fracture density, the equivalent permeability matrix of the tight oil reservoir is obtained.

12. The tight oil vertical well carbon dioxide energy storage fracturing process method according to claim 11, characterized in that, The construction of the directional feature matrix based on the extension direction of the crack includes: The directional feature matrix is A 3D matrix; The element in the first row and first column of the directional feature matrix is ​​the square of the cosine of the fracture composite direction angle. The elements in the first row and second column of the directional feature matrix and the elements in the second row and first column of the directional feature matrix are the product of the sine and cosine of the fracture composite direction angle. The element in the second row and second column of the directional feature matrix is ​​the square of the sine of the fracture composite direction angle. The comprehensive direction angle of the crack is the average of the angles between all cracks in the topology diagram and the preset direction.

13. The tight oil vertical well carbon dioxide energy storage fracturing process method according to claim 11, characterized in that, The process of obtaining the equivalent permeability matrix of the tight oil reservoir based on the directional feature matrix, the matrix permeability of the tight oil reservoir, the porosity of the fractures, and the fracture density includes: Calculate the first product of the matrix permeability and the identity matrix of tight oil reservoirs; Calculate the second product between the matrix permeability, fracture porosity, fracture density, and directional feature matrix of the tight oil reservoir; The sum of the first product and the second product is used as the equivalent permeability matrix of the tight oil reservoir.

14. The tight oil vertical well carbon dioxide energy storage fracturing process method according to claim 1, characterized in that, The effective thickness of the integrated reservoir, the equivalent permeability matrix, the difference between the formation fracture pressure and the original formation pressure, and the critical carbon dioxide viscosity are used to determine the initial injection amount of carbon dioxide into the tight oil reservoir, including: The equivalent permeability matrix is ​​decomposed into eigenvalues ​​to obtain the maximum and minimum eigenvalues. The initial injection rate of carbon dioxide into the tight oil reservoir is obtained based on the effective thickness of the reservoir, the maximum characteristic value, the minimum characteristic value, the difference between the formation fracture pressure and the original formation pressure, the supercritical carbon dioxide viscosity, and the wellbore radius.

15. The tight oil vertical well carbon dioxide energy storage fracturing process method according to claim 14, characterized in that, The initial injection rate of carbon dioxide into the tight oil reservoir is determined based on the effective thickness of the reservoir, the maximum characteristic value, the minimum characteristic value, the difference between the formation fracture pressure and the original formation pressure, the supercritical carbon dioxide viscosity, and the wellbore radius, including: Calculate the first ratio between the preset radius and the wellbore radius; The initial injection volume of carbon dioxide into the tight oil reservoir is obtained based on the effective thickness of the reservoir, the maximum characteristic value, the minimum characteristic value, the difference between the formation fracture pressure and the original formation pressure, the supercritical carbon dioxide viscosity, and the first ratio. The effective thickness, the maximum characteristic value, the minimum characteristic value, and the difference between the formation fracture pressure and the original formation pressure are all positively correlated with the initial injection volume, while the supercritical carbon dioxide viscosity and the wellbore radius are both negatively correlated with the initial injection volume.

16. The tight oil vertical well carbon dioxide energy storage fracturing process method according to claim 2, characterized in that, The determination of the target injection amount based on the type of crack node and the initial injection amount includes: Calculate the sum of the number of the first connected nodes and the number of the second connected nodes; determine the ratio between the number of isolated nodes and the sum as the relative proportion of isolated nodes; The target injection amount is determined by comparing the relative proportion of isolated nodes with a preset proportion threshold.

17. The tight oil vertical well carbon dioxide energy storage fracturing process method according to claim 16, characterized in that, The step of comparing the relative proportion of isolated nodes with a preset proportion threshold to determine the target injection amount includes: If the relative proportion of isolated nodes is greater than a preset proportion threshold, then a preset multiple of the initial injection amount is determined as the target injection amount, wherein the preset multiple is less than 1; If the relative proportion of isolated nodes is less than or equal to a preset proportion threshold, then the initial injection amount is determined as the target injection amount.

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