A method for determining the critical pressure gradient of gel-carbon dioxide alternating flooding in conglomerate reservoirs

Abstract

The invention belongs to the technical field of oilfield exploitation, and particularly relates to a method for determining a critical pressure gradient of gel-carbon dioxide alternating displacement in a conglomerate reservoir; the method solves the problem that the current unreasonable pressure gradient of gel-carbon dioxide alternating displacement in conglomerate reservoirs leads to low alternating displacement efficiency and sweep efficiency; the technical scheme is: establishing a large-scale physical model, preferably using gel-carbon dioxide as an alternating displacement medium, obtaining a change value of oil saturation in a wellbore plane under different displacement pressure gradients, calculating the alternating displacement efficiency and sweep efficiency under different displacement pressure gradients according to the change value of oil saturation in a wellbore plane, establishing a displacement pressure gradient, sweep efficiency and alternating displacement efficiency chart, and determining the alternating displacement critical pressure gradients of the conglomerate reservoir in the first 500 days and the last 500 days according to the two charts in different stages of actual production; the new method in the invention can provide a scientific basis for practical application, reduce operating costs, and improve the benefits of oil reservoir exploitation.

Description

Technical Field

[0001] The invention belongs to the technical field of oilfield exploitation, and in particular relates to a method for determining a critical pressure gradient of gel-carbon dioxide alternating displacement in a conglomerate reservoir. Background Art

[0002] In conglomerate reservoirs, the anisotropy of the reservoir and the complexity of the pore structure lead to high displacement pressure and low oil recovery efficiency during reservoir development. As an important means to improve oil recovery, the alternating displacement technology has been widely used in the development of conglomerate reservoirs. This technology optimizes reservoir development effects by alternately injecting fluids of different properties (water, gas, and polymer flooding, etc.) and utilizing the interaction between fluids and reservoir heterogeneity. However, the reasonable determination of the displacement pressure gradient during the alternating displacement process is the key to achieving efficient displacement.

[0003] After extensive research, the patent number CN202211429806.3, "An experimental system and method for determining the swept volume of gas-water alternating displacement", simulates the gas-water alternating displacement process and collects oil, gas and water saturation distribution data to determine the swept volume of gas-water alternating displacement, providing guidance for the gas-water alternating development of oil reservoirs; the patent number CN202011002770.1, "A method for enhancing the efficiency of lateral drive between wells in large bottom water oil reservoirs", calculates the amount of plugging agent used, the injection pressure value and injection displacement of the three-stage water plugging segment by establishing a three-dimensional physical model device, and injects the three-stage water plugging segment according to the calculated injection displacement and pressure value for displacement construction, thereby greatly improving the later recovery rate.

[0004] In general, there are many methods for determining the alternating displacement pressure gradient. However, these methods have their own limitations. Therefore, it is very important to study a scientific method for determining the critical pressure gradient of gel-CO2 alternating displacement in conglomerate reservoirs, so as to provide a scientific basis for practical application, reduce operating costs and improve the efficiency of reservoir exploitation. Summary of the invention

[0005] The purpose of the present invention is to solve the problem that the unreasonable pressure gradient of gel-carbon dioxide alternating displacement in conglomerate reservoirs leads to low alternating displacement efficiency and sweep efficiency. The present invention adopts a combination of experimental and theoretical means to establish a large-scale physical model, preferably gel-carbon dioxide is used as an alternating displacement medium, and the oil saturation change value of the wellbore plane under different displacement pressure gradients is obtained. According to the oil saturation change value of the wellbore plane, the alternating displacement efficiency and sweep efficiency under different displacement pressure gradients are calculated, and displacement pressure gradient, sweep efficiency and alternating displacement efficiency charts are established. In actual production, the alternating displacement critical pressure gradients of the conglomerate reservoir in the first 500 days and the last 500 days can be determined respectively according to the two charts, and the calculation is convenient and the reliability is strong.

[0006] To achieve the above object, the present invention provides a method for determining the critical pressure gradient of gel-carbon dioxide alternating displacement in conglomerate reservoirs, the method comprising the following steps:

[0007] The first step is to select the field study area, determine the test parameters and experimental conditions through similarity criteria, ensure that the established large-scale physical model is consistent with the various parameters of the actual formation, and establish a large-scale physical model;

[0008] In the second step, water-CO2 and gel-CO2 alternating displacement experiments were designed through large-scale physical models. The results showed that gel-CO2 alternating displacement of crude oil was better than water-CO2 alternating displacement. Gel-CO2 was selected as the alternating displacement medium and the optimal slug ratio of gel-CO2 was determined to be 2:1.

[0009] The third step is to use gel-CO2 as the alternating displacement medium in the large-scale physical model. Gel-CO2 is configured according to the optimal segment-plug ratio, and a one-injection-one-production method is set up to continuously increase the displacement pressure gradient of each group of experiments to obtain the change value of oil saturation in the wellbore plane under different displacement pressure gradients.

[0010] The fourth step is to calculate the alternating displacement efficiency and sweep efficiency under different displacement pressure gradients according to the change value of oil saturation in the plane around the well;

[0011] Step 5: Establish displacement pressure gradient, sweep efficiency and alternating displacement efficiency charts to determine the critical pressure gradient of alternating displacement in the first 500 days and the last 500 days;

[0012] The sixth step is to determine the critical pressure gradient of alternating displacement in the first 500 days and the last 500 days of the conglomerate reservoir in actual production based on the displacement pressure gradient, sweep efficiency and alternating displacement efficiency chart.

[0013] In the above method for determining the critical pressure gradient of gel-carbon dioxide alternating displacement in conglomerate reservoirs, the step of establishing a large-scale physical model is:

[0014] In the first step, two oil wells are arranged in a large-scale physical model. A fracture is designed between the two wells, and pressure and oil saturation detection probes are installed at various locations around the wells.

[0015] In the second step, a combination of rock cuttings, gravel and adhesive was used to fill and seal the large-scale physical model at a reservoir temperature of 86°C and a pressure of 44MPa;

[0016] In the third step, after the large-scale physical model is cemented and formed, it is dried and vacuumed, the formation water is quantitatively saturated, and the model is displaced with mine crude oil and displaced to a water saturation of 50% to obtain the initialization state of the large-scale physical model.

[0017] In the above method for determining the critical pressure gradient of gel-carbon dioxide alternating displacement in conglomerate reservoirs, the step of calculating the alternating displacement efficiency and the sweep efficiency under different displacement pressure gradients is as follows:

[0018] In the first step, after the alternating displacement, the oil saturation in the plane around the well decreases compared with the initial state. The closer to the oil well, the greater the decrease in the plane oil saturation. The effective swept area in the plane can be determined, and a sweep efficiency calculation model can be established.

[0019] ,

[0020] Among them, E s is the sweep efficiency, unit is %V s is the effective swept area in the plane, in m 2 ; V o is the total area of ​​the plane, in m 2 ;

[0021] The second step is to establish an alternating displacement efficiency calculation model based on the oil saturation at different locations around the well.

[0022] ,

[0023] Among them, E d is the alternating displacement efficiency, unit: %S o is the initial state plane oil saturation, unit is %; V o is the total area of ​​the plane, in m 2 ; S i is the oil saturation at different planes around the well, unit is %; V i is the plane area corresponding to the oil saturation at different planes around the well, in m 2 ; n is the final value of the summation, and its unit is dimensionless quantity.

[0024] In the above method for determining the critical pressure gradient of gel-carbon dioxide alternating flooding in a conglomerate reservoir, the steps of determining the critical pressure gradient of alternating flooding in the first 500 days and the last 500 days are as follows:

[0025] In the first step, the first 500 days of production in the conglomerate reservoir focus on sweep efficiency. A wider sweep area facilitates subsequent production and profile adjustment to enhance recovery, while the next 500 days focus on alternating displacement efficiency.

[0026] In the second step, as the displacement pressure gradient increases, the sweep efficiency first increases and then decreases to a constant value. The displacement pressure gradient P1 corresponding to the maximum sweep efficiency in the displacement pressure gradient and sweep efficiency chart is selected as the critical pressure gradient for alternating displacement in the first 500 days.

[0027] In the third step, the alternating displacement efficiency increases rapidly with the increase of the displacement pressure gradient and then does not change. While ensuring the maximum alternating displacement efficiency, the loss of the displacement pressure gradient needs to be reduced. The displacement pressure gradient P2 corresponding to the deflection point of the alternating displacement efficiency curve in the displacement pressure gradient and alternating displacement efficiency chart is selected as the critical pressure gradient for alternating displacement in the next 500 days.

[0028] In the above method for determining the critical pressure gradient of gel-carbon dioxide alternating displacement in conglomerate reservoirs, the method for determining the plane effective swept area is to determine the area where the plane oil saturation decreases by more than 3% as the plane effective swept area.

[0029] Compared with the prior art, the present invention has the following beneficial effects: (1) enhanced flexibility and customization; (2) convenient and effective method with high work efficiency; (3) high reliability and stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In the attached picture:

[0031] Figure 1 It is the overall technical roadmap of the method;

[0032] Figure 2 It is the displacement pressure gradient and sweep efficiency chart;

[0033] Figure 3 This is a graph showing displacement pressure gradient and alternating displacement efficiency. DETAILED DESCRIPTION

[0034] The present invention will be further described below in conjunction with the embodiments and accompanying drawings;

[0035] The present invention provides a method for determining the critical pressure gradient of gel-carbon dioxide alternating displacement in conglomerate reservoirs. Figure 1 It is the overall technical roadmap of the method, which includes the following steps:

[0036] The first step is to select the field study area, determine the test parameters and experimental conditions through similarity criteria, ensure that the established large-scale physical model is consistent with the various parameters of the actual formation, and establish a large-scale physical model;

[0037] In the second step, water-CO2 and gel-CO2 alternating displacement experiments were designed through large-scale physical models. The results showed that gel-CO2 alternating displacement of crude oil was better than water-CO2 alternating displacement. Gel-CO2 was selected as the alternating displacement medium and the optimal slug ratio of gel-CO2 was determined to be 2:1.

[0038] The third step is to use gel-CO2 as the alternating displacement medium in the large-scale physical model. Gel-CO2 is configured according to the optimal segment-plug ratio, and a one-injection-one-production method is set up to continuously increase the displacement pressure gradient of each group of experiments to obtain the change value of oil saturation in the wellbore plane under different displacement pressure gradients.

[0039] The fourth step is to calculate the alternating displacement efficiency and sweep efficiency under different displacement pressure gradients according to the change value of oil saturation in the plane around the well;

[0040] Step 5: Establish displacement pressure gradient, sweep efficiency and alternating displacement efficiency charts to determine the critical pressure gradient of alternating displacement in the first 500 days and the last 500 days;

[0041] The sixth step is to determine the critical pressure gradient of alternating displacement in the first 500 days and the last 500 days of the conglomerate reservoir in actual production based on the displacement pressure gradient, sweep efficiency and alternating displacement efficiency chart.

[0042] Furthermore, the steps of establishing a large-scale physical model are:

[0043] In the first step, two oil wells are arranged in a large-scale physical model. A fracture is designed between the two wells, and pressure and oil saturation detection probes are installed at various locations around the wells.

[0044] In the second step, a combination of rock cuttings, gravel and adhesive was used to fill and seal the large-scale physical model at a reservoir temperature of 86°C and a pressure of 44MPa;

[0045] In the third step, after the large-scale physical model is cemented and formed, it is dried and vacuumed, the formation water is quantitatively saturated, and the model is displaced with mine crude oil and displaced to a water saturation of 50% to obtain the initialization state of the large-scale physical model.

[0046] Further, the steps of calculating the alternating displacement efficiency and the sweep efficiency under different displacement pressure gradients are:

[0047] In the first step, after the alternating displacement, the oil saturation in the plane around the well decreases compared with the initial state. The closer to the oil well, the greater the decrease in the plane oil saturation. The effective swept area in the plane can be determined, and a sweep efficiency calculation model can be established.

[0048] ,

[0049] Among them, E s is the sweep efficiency, unit is %V s is the effective swept area in the plane, in m 2 ; V o is the total area of ​​the plane, in m 2 ;

[0050] The second step is to establish an alternating displacement efficiency calculation model based on the oil saturation at different locations around the well.

[0051] ,

[0052] Among them, E d is the alternating displacement efficiency, unit: %S o is the initial state plane oil saturation, unit is %; V o is the total area of ​​the plane, in m 2 ; S i is the oil saturation at different planes around the well, unit is %; V i is the plane area corresponding to the oil saturation at different planes around the well, in m 2 ; n is the final value of the summation, and its unit is dimensionless quantity.

[0053] Furthermore, the steps to determine the critical pressure gradient of alternating displacement in the first 500 days and the last 500 days are as follows:

[0054] In the first step, the first 500 days of production in the conglomerate reservoir focus on sweep efficiency. A wider sweep area facilitates subsequent production and profile adjustment to enhance recovery, while the next 500 days focus on alternating displacement efficiency.

[0055] In the second step, as the displacement pressure gradient increases, the sweep efficiency first increases and then decreases to a constant value. The displacement pressure gradient P1 corresponding to the maximum sweep efficiency in the displacement pressure gradient and sweep efficiency chart is selected as the critical pressure gradient for alternating displacement in the first 500 days.

[0056] In the third step, the alternating displacement efficiency increases rapidly with the increase of the displacement pressure gradient and then does not change. While ensuring the maximum alternating displacement efficiency, the loss of the displacement pressure gradient needs to be reduced. The displacement pressure gradient P2 corresponding to the deflection point of the alternating displacement efficiency curve in the displacement pressure gradient and alternating displacement efficiency chart is selected as the critical pressure gradient for alternating displacement in the next 500 days.

[0057] Furthermore, the method for determining the plane effective swept area is to determine the area where the plane oil saturation decreases by more than 3% as the plane effective swept area.

[0058] Taking a conglomerate reservoir as an example, the reservoir burial depth of the reservoir reaches 3700m, there are a large number of micro-fractures, the original reservoir pressure is 44MPa, the formation temperature is 86℃, and the initial oil saturation is 50.11%. A large-scale physical model with a diameter of 1m and a height of 0.4m is established by similarity criteria. Two wells with fracture communication are arranged for one injection and one production. Gel-carbon dioxide is injected alternately at a slug ratio of 2:1. The plane effective swept area of ​​the large-scale physical model is delineated according to the plane oil saturation decrease of more than 3%. The plane effective swept area and sweep efficiency under different displacement pressure gradients are shown in Table 1. The different oil saturations around the well and the corresponding plane areas under the displacement pressure gradient of 0.05MPa / m are shown in Table 2. The alternating displacement efficiency is calculated by the alternating displacement efficiency calculation model to be 2.08%. The other displacement pressure gradients are calculated in the same way. The results of calculating all alternating displacement efficiencies are shown in Table 3.

[0059] The displacement pressure gradient, sweep efficiency and alternating displacement efficiency charts are established. As the displacement pressure gradient increases, the sweep efficiency increases first and then decreases to a constant value. For example, Figure 2 As shown in the figure, the displacement pressure gradient of 0.25 MPa / m corresponding to the maximum sweep efficiency in the sweep efficiency chart is selected as the critical pressure gradient of alternating displacement in the first 500 days; the alternating displacement efficiency increases rapidly with the increase of the displacement pressure gradient and then does not change, as shown in Figure 3 As shown in the figure, while ensuring the maximum alternating displacement efficiency, it is necessary to reduce the loss of pressure gradient, and the displacement pressure gradient of 0.30MPa / m corresponding to the deflection of the alternating displacement efficiency curve in the alternating displacement efficiency plate is selected as the critical pressure gradient of alternating displacement in the next 500 days; it can be determined that the critical pressure gradients of alternating displacement in the conglomerate reservoir in the first 500 days and the last 500 days are 0.25MPa / m and 0.30MPa / m, respectively.

[0060] Table 1 Sweep efficiency under different displacement pressure gradients

[0061]

[0062] Table 2 Oil saturation corresponding to plane area

[0063]

[0064] Table 3 Alternating displacement efficiency under different displacement pressure gradients

[0065]

[0066] Compared with the prior art, the present invention has the following beneficial effects: (1) enhanced flexibility and customization; (2) convenient and effective method with high work efficiency; (3) high reliability and stability.

[0067] Finally, it should be noted that the above embodiments are only used to illustrate rather than limit the technical solutions of the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that the present invention can still be modified or replaced by equivalents. Any modification or partial replacement that does not depart from the spirit and scope of the present invention should be included in the scope of the claims of the present invention.

Claims

1. A method for determining the critical pressure gradient of gel-carbon dioxide alternating flooding in conglomerate reservoirs, characterized in that: The method comprises the following steps: S100, select the field research block, determine the test parameters and experimental conditions through similarity criteria, and ensure that the established large-scale physical model is consistent with the various parameters of the actual formation. The steps for establishing the large-scale physical model are as follows: S101, large-scale physical model with two oil wells arranged in plane, with fractures between the two wells, and pressure and oil saturation detection probes installed at various locations around the wells; S102, using a combination of rock cuttings, gravel and adhesive to fill, seal and compact the large-scale physical model at a reservoir temperature of 86°C and a pressure of 44 MPa; S103, after the large-scale physical model is cemented and formed, it is dried and vacuumed, the formation water is quantitatively saturated, and the model is injected with crude oil from the mine and driven to a water saturation of 50%, thereby obtaining an initialization state of the large-scale physical model; S200, through large-scale physical model, designed water-CO2 and gel-CO2 alternating displacement experiments, the results show that gel-CO2 alternating displacement of crude oil is better than water-CO2 alternating displacement, gel-CO2 is preferred as the alternating displacement medium and the optimal slug ratio of gel-CO2 is determined to be 2:1; S300, in the large-scale physical model, gel-CO2 is used as the alternating displacement medium. Gel-CO2 is configured according to the optimal slug ratio, and a one-injection-one-production mode is set. The displacement pressure gradient of each group of experiments is continuously increased to obtain the change value of oil saturation in the wellbore plane under different displacement pressure gradients; S400, the steps of calculating the alternating displacement efficiency and the sweep efficiency under different displacement pressure gradients according to the change value of the oil saturation in the plane around the well are: S401, after alternating displacement, the oil saturation in the plane around the well decreases compared with the initial state. The closer to the oil well, the greater the decrease in the plane oil saturation. The effective swept area in the plane can be determined, and a sweep efficiency calculation model can be established. ， Among them, E s is the sweep efficiency, unit is %V s is the effective swept area in the plane, in m 2 ; V o is the total area of ​​the plane, in m 2 ; S402, the oil saturation at different positions on the plane around the well is different. Extending outward from the well, the area is divided according to the oil saturation of different planes. Each plane oil saturation corresponds to a plane area, and an alternating displacement efficiency calculation model is established. ， Among them, E d is the alternating displacement efficiency, unit is %; S o is the initial state plane oil saturation, unit is %; V o is the total area of ​​the plane, in m 2 ; S i is the oil saturation at different planes around the well, unit is %; V i is the plane area corresponding to the oil saturation at different planes around the well, in m 2 ; n is the end value of the summation, and the unit is dimensionless quantity; S500, establish displacement pressure gradient and sweep efficiency and alternating displacement efficiency charts, and determine the alternating displacement critical pressure gradient for the first 500 days and the last 500 days as follows: S501, the first 500 days of production in conglomerate reservoirs focus on sweep efficiency, sweeping a wider area to facilitate subsequent production profile adjustment and enhanced recovery, while the next 500 days focus on alternate displacement efficiency; S502, as the displacement pressure gradient increases, the sweep efficiency first increases and then decreases to a constant value, and the displacement pressure gradient P1 corresponding to the maximum sweep efficiency in the displacement pressure gradient and sweep efficiency chart is selected as the critical pressure gradient of alternating displacement in the first 500 days; S503, the alternating displacement efficiency shows a trend of increasing rapidly and then not changing as the displacement pressure gradient increases. While ensuring the maximum alternating displacement efficiency, the loss of the displacement pressure gradient needs to be reduced. The displacement pressure gradient P2 corresponding to the deflection of the alternating displacement efficiency curve in the displacement pressure gradient and alternating displacement efficiency chart is selected as the alternating displacement critical pressure gradient for the next 500 days; S600, in actual production, according to the displacement pressure gradient and sweep efficiency and alternating displacement efficiency chart, the alternating displacement critical pressure gradient of the conglomerate reservoir in the first 500 days and the last 500 days can be determined.

2. The method for determining the critical pressure gradient of gel-carbon dioxide alternating flooding in conglomerate reservoirs according to claim 1, characterized in that: The method for determining the plane effective swept area is that the area where the plane oil saturation decreases by more than 3% is determined as the plane effective swept area.

Images