Method for evaluating effective displacement distance of carbon dioxide injected into low-permeability-tight oil reservoir

Abstract

The invention belongs to the technical field of oil-gas field development tertiary oil recovery and unconventional oil-gas development, and discloses a low-permeability-tight oil reservoir carbon dioxide injection effective displacement distance evaluation method, which comprises the following steps: obtaining several groups of different permeability core starting pressure gradients, and obtaining a quasi-starting pressure gradient according to the starting pressure gradients; establishing a mathematical relationship between the real starting pressure gradient and the permeability and a mathematical relationship between the quasi-starting pressure gradient and the permeability; effective displacement distances of reservoirs with different permeability under different pressure gradient conditions are obtained through calculation, and an effective displacement distance and permeability relation chart is established; according to the oil field early-stage oil production well average well bottom pressure, oil production well bottom pressure is obtained, and then the pressure gradient between the gas injection well and the oil production well is obtained; and according to the actual permeability of the oil reservoir and the pressure gradient between the gas injection well and the oil producing well, the actual effective displacement distance of oil reservoir carbon dioxide flooding is obtained. According to the method, the limiting distance of fluid flowing is simply calculated by using a limiting well spacing method, the method is simple and rapid, and few basic parameters are needed.

Description

Technical Field

[0001] The invention relates to tertiary oil recovery in oil and gas fields and the technical field of unconventional oil and gas development, and in particular to a method for evaluating the effective displacement distance of carbon dioxide injection in low-permeability and tight oil reservoirs. Background Art

[0002] At present, the development practices of many low-permeability reservoirs show that some water injection wells have low water absorption capacity, high starting pressure and water injection pressure, and as the water injection time increases, the contradiction intensifies, and even no water can be injected. The bottom hole pressure of the water well is close to the formation fracture pressure, and it is difficult to establish an effective displacement pressure system in the reservoir. The reason is that there is a starting pressure gradient in low (ultra-low) permeability reservoirs, and this resistance must be overcome during water injection development. At the same time, due to the effect of the solid-liquid interface, there is an interface layer fluid on the pore surface, and the interface layer fluid needs to apply a certain external force to flow. For low-porosity and low-permeability cores, since the throat radius is very small, most or all of the seepage throat is occupied by the interface layer fluid, so the fluid seepage has different degrees of pseudo-starting pressure gradient and real starting pressure gradient. Therefore, the existing technology has not accurately simulated the nonlinear seepage state at reservoir temperature, and it is urgent to improve the experimental method and establish a test technology for the nonlinear seepage section of carbon dioxide drive under formation temperature and pressure.

[0003] The current nonlinear motion equations can be mainly divided into piecewise models and continuous models. The pseudo-start-up pressure gradient model is a typical piecewise model, which describes the nonlinear seepage problem by replacing the nonlinear segment with a straight line. It is essentially a start-up pressure gradient problem and is only applicable to conventional low-permeability reservoirs with inconspicuous nonlinear segments. Summary of the invention

[0004] In order to make up for the shortcomings of the prior art, the present invention establishes a method for evaluating the effective displacement distance of carbon dioxide injection in low-permeability-tight oil reservoirs for low-permeability-shale oil reservoir rock samples. On the basis of traditional starting pressure gradient testing technology, a mathematical relationship between starting pressure gradient and permeability is established, and the effective displacement distance under different permeability reservoirs and different pressure gradient conditions is calculated; according to the actual working system parameters of the oil reservoir, the actual carbon dioxide displacement distance of the oil reservoir is obtained.

[0005] The technical solution adopted by the present invention to solve its technical problem is:

[0006] A method for evaluating the effective displacement distance of carbon dioxide injection in a low permeability-tight oil reservoir comprises the following steps:

[0007] (1) Simulating the nonlinear seepage of carbon dioxide flooding under formation temperature and pressure, multiple cores were assembled into a long core, dried, vacuumed and saturated with simulated formation water, and then saturated with active oil. A nonlinear seepage experiment of carbon dioxide flooding in the core was carried out at formation temperature to analyze and obtain the real starting pressure gradient and simulated starting pressure gradient of carbon dioxide flooding in the core;

[0008] (2) Repeat step (1) to obtain several groups of core starting pressure gradients with different permeabilities, and obtain the pseudo starting pressure gradient based on the starting pressure gradient;

[0009] (3) According to the core starting pressure gradient corresponding to different permeabilities, the mathematical relationship between the real starting pressure gradient and the permeability and the mathematical relationship between the simulated starting pressure gradient and the permeability are established;

[0010] (4) Based on the mathematical relationship between the starting pressure gradient and permeability, the effective displacement distance under different permeability reservoirs and different pressure gradient conditions is calculated, and a relationship chart between the effective displacement distance and permeability is established;

[0011] (5) The bottom hole pressure of the gas injection well is calculated based on the depth of the carbon dioxide injection well and other parameters, and the bottom hole pressure of the oil production well is calculated based on the average bottom hole pressure of the oil production wells in the early stage of the oil field, and then the pressure gradient between the gas injection well and the oil production well is obtained;

[0012] (6) According to the actual permeability of the reservoir and the pressure gradient between the gas injection well and the oil production well, the effective displacement distance of carbon dioxide flooding in the actual reservoir is obtained by using the relationship chart between the effective displacement distance, permeability and pressure gradient.

[0013] Furthermore, the method for determining the real start-up pressure gradient and the simulated start-up pressure gradient in steps (1)-(2) is GBT 28912-2012 "Method for determining relative permeability of two-phase fluid in rock".

[0014] Furthermore, the simulated starting pressure gradient in steps (1)-(2) is 2.3 times the actual starting pressure gradient.

[0015] Furthermore, the mathematical relationship between the actual starting pressure gradient and the permeability in step (3) is:

[0016] y=0.1119x -1.523 , R 2＝ 0.9933.

[0017] Furthermore, the mathematical relationship between the proposed starting pressure gradient and the permeability in step (3) is:

[0018] y=0.0.1945x -1.022 , R 2＝ 0.99847.

[0019] Furthermore, the displacement distance and permeability relationship graph in step (4) is a relationship curve.

[0020] Furthermore, the calculation formula of the effective displacement distance in step (6) is:

[0021]

[0022] Where R is the distance between injection and production wells, m; r w is the wellbore radius, m; P H is the bottom flow pressure of the injection well, MPa; P w is the bottom hole flowing pressure of the oil well, MPa; λ is the starting pressure gradient.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] Nonlinear seepage experimental data are widely used in oil and gas development, and can be used to calculate the limit distance of fluid flow using the limit well spacing method, the effective utilization coefficient method and the numerical simulation method. The effective utilization coefficient method and the numerical simulation method can calculate the limit well spacing more accurately, but both require a large amount of data. Therefore, the present invention simply uses the limit well spacing method to calculate the limit distance of fluid flow, which has the advantages of simple and fast calculation and few basic parameters required. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a graph showing the relationship between the starting pressure and the permeability;

[0026] Figure 2 It is a curve diagram showing the relationship between displacement distance and permeability. DETAILED DESCRIPTION

[0027] The present invention is described in detail below in conjunction with specific drawings and examples. The examples described below are only preferred embodiments of the present invention. It should be noted that the following description is only for the purpose of explaining the present invention and is not intended to limit the present invention in any form. Any simple modification, equivalent changes and modifications made to the embodiments according to the technical essence of the present invention are within the scope of the technical solution of the present invention. In the following examples, the materials, reagents, etc. used are obtained from commercial sources unless otherwise specified.

[0028] Example 1

[0029] The basic data of the rock samples and the experimental results of the nonlinear seepage experiment are shown in Table 1. Figure 1 The relationship between starting pressure and permeability is given. Figure 2 The relationship between displacement distance and permeability is given.

[0030] Table 14 Results of CO2 flooding start-up pressure test on rock samples

[0031]

[0032] Figure 1 The relationship between the starting pressure gradient and permeability of the target reservoir is given. Figure 1It can be seen that there is a good negative correlation between the real start-up pressure gradient, the pseudo-start-up pressure gradient and the core permeability; the pseudo-start-up pressure gradient is 2.3 times the real start-up pressure gradient; the start-up pressure gradients corresponding to core permeabilities of 5.07mD, 3.13mD, 1.20mD and 0.53mD are 0.0087, 0.0210, 0.0970 and 0.2642MPa / m respectively.

[0033] Nonlinear seepage experimental data are widely used in oil and gas development. The limit well spacing method, effective utilization coefficient method and numerical simulation method can be used to calculate the limit distance of fluid flow. The effective utilization coefficient method and numerical simulation method can calculate the limit well spacing more accurately, but both require a large amount of data. Therefore, the present invention simply uses the limit well spacing method to calculate the limit distance of fluid flow. The advantage of this method is that the calculation is simple and fast, and few basic parameters are required. Its specific theoretical basis is:

[0034] The seepage theory of iso-yield-source-sink stable radial flow shows that among all streamlines, the seepage velocity on the mainstream line is the largest; on the same streamline, the seepage velocity at the same distance from the source and sink is the smallest.

[0035] In steady flow, Q = v·A = constant, and the seepage velocity can be expressed as:

[0036]

[0037] Plane radial flow production formula:

[0038]

[0039] Where: λ is the starting pressure gradient.

[0040] Substituting the yield formula into the seepage velocity formula, we can get:

[0041]

[0042] According to the flow rate formula, the pressure gradient at any point in the formation can be expressed as:

[0043]

[0044] Therefore, the pressure gradient at the midpoint of the iso-yield source and sink is:

[0045]

[0046] Where: P w is the bottom hole flowing pressure of the oil well, MPa; P H is the bottom flow pressure of the injection well, MPa; R is the distance between the injection well and the production well, m; r wis the wellbore radius, m; dp / dr: radial pressure gradient of the oil well, MPa / m; pe is the pressure at the oil supply radius, MPa; re is the supply edge radius, m; A is the radial seepage area, m 2 ; Q is flow rate: m 3 / s; K: reservoir permeability; h: reservoir thickness, m.

[0047] If the oil is to flow at the midpoint of the mainstream line, the driving pressure gradient at this point must be greater than the starting pressure gradient. Then the limit injection-production well spacing for different injection-production pressure differences under a certain permeability condition can be calculated, that is:

[0048]

[0049] Assuming that the effective injection-production pressure difference is 25 MPa, ignoring the wellbore radius, the average true starting pressure gradient under different permeability ranges and different states is used to calculate the limit distance of fluid flow. The calculation results are shown in the figure below.

[0050] Table 2 Relationship between displacement distance and permeability

[0051]

[0052] In the specific embodiment, the injection well head pressure is 18MPa, the well depth is 2400m, and the bottom pressure is 38MPa. However, the bottom pressure of the oil well in actual production is between 12 and 15MPa. Therefore, the pressure difference in actual production is close to 23 to 26MPa. From Table 2 and Figure 2 It can be seen that: calculated with a displacement pressure difference of 23MPa, the effective displacement distances corresponding to the 5mD, 2mD and 1mD reservoirs are 610m, 239m and 118m respectively; calculated with a displacement pressure difference of 26MPa, the effective displacement distances corresponding to the 5mD, 2mD and 1mD reservoirs are 690m, 271m and 133m respectively.

[0053] The experiment was carried out on typical rock samples, and the experimental results are as follows Figure 1-2 , as shown in Tables 1 and 2.

[0054] (1) Figure 1 The relationship between the starting pressure gradient and permeability of the target reservoir is given. Figure 1 It can be seen that there is a good negative correlation between the real start-up pressure gradient, the pseudo-start-up pressure gradient and the core permeability; the pseudo-start-up pressure gradient is 2.3 times the real start-up pressure gradient; the start-up pressure gradients corresponding to core permeabilities of 5.07mD, 3.13mD, 1.20mD and 0.53mD are 0.0087, 0.0210, 0.0970 and 0.2642MPa / m respectively.

[0055] (2) In the specific embodiment, the injection well head pressure is 18 MPa, the well depth is 2400m, and the bottom pressure is 38 MPa. However, the bottom pressure of the oil well in actual production is between 12 and 15 MPa. Therefore, the pressure difference in actual production is close to 23 to 26 MPa. From Table 2 and Figure 2 It can be seen that: calculated with a displacement pressure difference of 23MPa, the effective displacement distances corresponding to the 5mD, 2mD and 1mD reservoirs are 610m, 239m and 118m respectively; calculated with a displacement pressure difference of 26MPa, the effective displacement distances corresponding to the 5mD, 2mD and 1mD reservoirs are 690m, 271m and 133m respectively.

[0056] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not limitations on the implementation methods of the present invention. For ordinary technicians in the relevant field, other different forms of changes or modifications can be made based on the above description. It is impossible to list all the implementation methods here. All obvious changes or modifications derived from the technical solution of the present invention are still within the protection scope of the present invention.

Claims

1. A method for evaluating the effective displacement distance of carbon dioxide injection in low permeability and tight oil reservoirs, characterized in that: The following steps are involved: (1) Simulating the nonlinear seepage of carbon dioxide flooding under formation temperature and pressure, multiple cores were assembled into a long core, dried, vacuumed and saturated with simulated formation water, and then saturated with active oil. A nonlinear seepage experiment of carbon dioxide flooding in the core was carried out at formation temperature to analyze and obtain the real starting pressure gradient and simulated starting pressure gradient of carbon dioxide flooding in the core; (2) Repeat step (1) to obtain several groups of core starting pressure gradients with different permeabilities, and obtain the pseudo starting pressure gradient based on the starting pressure gradient; (3) According to the core starting pressure gradient corresponding to different permeabilities, the mathematical relationship between the real starting pressure gradient and the permeability and the mathematical relationship between the simulated starting pressure gradient and the permeability are established; (4) Based on the mathematical relationship between the starting pressure gradient and permeability, the effective displacement distance under different permeability reservoirs and different pressure gradient conditions is calculated, and a relationship chart between the effective displacement distance and permeability is established; (5) The bottom hole pressure of the gas injection well is calculated based on the depth of the carbon dioxide injection well and other parameters, and the bottom hole pressure of the oil production well is calculated based on the average bottom hole pressure of the oil production wells in the early stage of the oil field, and then the pressure gradient between the gas injection well and the oil production well is obtained; (6) According to the actual permeability of the reservoir and the pressure gradient between the gas injection well and the oil production well, the effective displacement distance of the actual reservoir carbon dioxide flooding is obtained by using the relationship chart between the effective displacement distance, permeability and pressure gradient.

2. The method for evaluating the effective displacement distance of carbon dioxide injection in low permeability-tight oil reservoirs according to claim 1, characterized in that: The method for determining the real start-up pressure gradient and the simulated start-up pressure gradient in steps (1)-(2) is GBT28912-2012 "Method for determining relative permeability of two-phase fluid in rock".

3. The method for evaluating the effective displacement distance of carbon dioxide injection in low permeability-tight oil reservoirs according to claim 1, characterized in that: The simulated starting pressure gradient in steps (1)-(2) is 2.3 times the actual starting pressure gradient.

4. The method for evaluating the effective displacement distance of carbon dioxide injection in low permeability-tight oil reservoirs according to claim 1, characterized in that: Furthermore, the mathematical relationship between the actual starting pressure gradient and the permeability in step (3) is: y=0.1119x -1.523 ,R 2＝ 0.9933。 5. The method for evaluating the effective displacement distance of carbon dioxide injection in low permeability-tight oil reservoirs according to claim 1, characterized in that: The mathematical relationship between the proposed starting pressure gradient and the permeability in step (3) is: y=0.0.1945x -1.022 ,R 2＝ 0.99847。 6. The method for evaluating the effective displacement distance of carbon dioxide injection in low permeability-tight oil reservoirs according to claim 1, characterized in that: Furthermore, the displacement distance and permeability relationship graph in step (4) is a relationship curve.

7. The method for evaluating the effective displacement distance of carbon dioxide injection in low permeability-tight oil reservoirs according to claim 1, characterized in that: The calculation formula for the effective displacement distance in step (6) is: Where R is the distance between injection and production wells, m; r w is the wellbore radius, m; P H is the bottom flow pressure of the injection well, MPa; P w is the bottom hole flowing pressure of the oil well, MPa; λ is the starting pressure gradient.

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