Method for evaluating pollution radius and blockage removal effect of oilfield reinjection sewage

Abstract

The invention relates to the technical field of oil and gas field development, and provides an oil field reinjection sewage pollution radius and plug removal effect evaluation method which comprises the following steps: S1, preparing a rock core and forming a plurality of injection holes, S2, testing the water permeability before pollution by clear water flooding, S3, testing the water permeability and resistance coefficient of each area after pollution by sewage flooding to evaluate the pollution degree, and S4, carrying out plug removal by a plug removal agent. And S5, sequentially testing the water permeability and the residual resistance coefficient of each region after blockage removal by clear water drive region by region and section to obtain the pollution blockage removal effect of each region, and making an evaluation, so as to simulate the blockage flow-back process of the wellhead after the blockage removal of the oil field. The method can be used for evaluating the pollution degree of the reinjection well and the effect of the blockage removal measure, guiding the site to take related measures in time, and reducing the problem of water injection capacity reduction caused by pollution, so that the service life of the reinjection well is prolonged, and the oilfield development benefit is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of oil and gas field development, and in particular, to a method for evaluating the pollution radius and plugging removal effect of reinjected sewage in an oilfield. Background Art

[0002] In the initial stage of oilfield development, the reserves of petroleum are quite abundant, the reservoir pressure is high, and most of the oil production wells are flowing wells. Therefore, the production efficiency is high and the cost is low. As the crude oil in the formation is continuously exploited, the formation pressure gradually decreases, and the crude oil cannot rise from the formation to the surface by itself. The original flowing oil production method can no longer be continuously carried out. In order to enable the oilfield to continue production operation, it is necessary to inject corresponding media into the formation to supplement the formation energy, so as to achieve the purpose of efficient oil production. Among them, the method of water injection for oil production is the most common and widespread, and is the method commonly used in major oilfields at home and abroad.

[0003] With the continuous increase in the intensity of oil and gas exploration and development in China and the fact that most oil and gas fields have entered the middle and late stages of development, the produced water volume in the oil and gas production process has increased sharply. Due to the complex characteristics of produced water in oil and gas fields, such as high salinity, high content of petroleum-bacteria-suspended solids, many types and high content of organic matter-toxic heavy metals, and containing natural radioactive substances, it is difficult and costly to treat and dispose of. Under the existing technical conditions, in order to maintain normal production, after treatment by processes such as coagulation sedimentation and filtration, and taking safe and effective measures, reinjecting the produced water into active oil reservoirs, depleted and abandoned oil and gas reservoirs or geologically closed formations has become a commonly used disposal method in the oil and gas industry at home and abroad. Its advantages are: on the one hand, it is used to maintain the formation pressure, improve the crude oil recovery rate, and solve the water source problem of water flooding for oil production; on the other hand, it reduces the cost of treating and discharging up to standard and reduces the impact of produced water in oil and gas fields on the surface environment.

[0004] However, with the reinjection of sewage, affected by factors such as reservoir sensitivity and water quality incompatibility, the reservoir is prone to plugging of injection wells, resulting in an increase in the pressure of injection wells, a decrease in the injection volume, and a reduction in the productivity of corresponding oil wells, affecting the development level of the oilfield. For evaluating the pollution distance of reinjected sewage in the reservoir and the degree of plugging of the reservoir, generally, artificial cores or natural cores are used to simulate the pollution situation of the water injection interval in this block by plugging substances, and the plugging radius is calculated according to the core plugging law. At present, all relevant studies reported in the literature calculate the reduction of permeability using the pressure value at the injection end of a single pressure measurement point core, which fails to reflect the along - path distribution of suspended solids in the reinjected sewage in the core and cannot establish an evaluation standard for the degree of along - path pollution. Therefore, the existing physical simulation technical means lack an experimental process and standard for grading and evaluating the pollution radius and plugging removal effect of reinjected sewage in oilfields. Summary of the Invention

[0005] The object of the present invention is to provide a method for evaluating the pollution radius and plugging removal effect of reinjected sewage in oilfields, which can be used to evaluate the pollution degree of injection wells and the effect of plugging removal measures, guide the on-site timely adoption of relevant measures, reduce the problem of the decline in water injection capacity caused by pollution, thereby extending the service life of injection wells and improving the oilfield development benefit.

[0006] To solve the above technical problems, the technical solution adopted in this application is as follows:

[0007] The embodiments of this application provide a method for evaluating the pollution radius and plugging removal effect of reinjected sewage in oilfields, including the following steps: S1. Prepare an artificial homogeneous cast core, evenly open n injection holes along the length direction of the artificial homogeneous cast core and divide it into n - 1 regions equally. S2. Vacuumize and saturate the artificial homogeneous cast core with water to obtain the pore volume of the artificial homogeneous cast core, put it into a high-pressure container chamber for pressurization, set one injection hole as injection hole 1, number the remaining injection holes in sequence, and use clear water to displace from injection hole 1. The produced liquid is collected at injection hole n, and the remaining injection holes 1 to n - 1 are used as monitoring points until the pressure values at each monitoring point are stable, and record the pressure P 0i , calculate the water permeability value K before pollution for each region 0i , where i = 1 to n - 1. During the displacement process, set the pressure in the high-pressure container chamber to be always higher than the injection pressure of the injection hole. S3. Use reinjected sewage for displacement, displace from injection hole 1, collect the produced liquid at injection hole n, and the remaining injection holes 1 to n - 1 are used as monitoring points to simulate the process of reinjected sewage in oilfields polluting the reservoir. Record the pressure P of each monitoring point region throughout the process 1i , calculate the water permeability value K after pollution for each region 1i , and the resistance coefficient F Ri , evaluate the pollution degree and pollution radius according to the size of the resistance coefficient, where i = 1 to n - 1. S4. Inject a plugging removal agent for displacement to simulate the process of plugging removal from the pollution of reinjected sewage in oilfields. Close all injection holes for reaction, open injection hole 1 to conduct backflow using the remaining pressure after the reaction, and simulate the process of the wellhead backflowing and blocking substances after plugging removal in oilfields. S5. Set any injection hole as injection hole i, open injection hole i and injection hole i + 1, close the remaining injection holes, and use clear water to conduct segmented displacement on n - 1 regions in sequence from injection hole i. The produced liquid is collected at injection hole i + 1. Displace until the pressure value at injection hole i is stable, and record its pressure value P 2i , obtain different pressure values for each of the n - 1 regions, calculate the water permeability value K after pollution and plugging removal for each region 2i , and the residual resistance coefficient F RRi , evaluate the pollution and plugging removal degree according to the size of the residual resistance coefficient, where i = 1 to n - 1.

[0008] Further, in the above step S1, the artificial homogeneous cast core is prepared by the quartz sand-epoxy resin cementing method, and three-way valves and pressure sensors are connected to all n injection holes, which are used as the liquid injection end and the pressure monitoring point.

[0009] Further, the size of the above artificial homogeneous cast core is width × height × length = 4.5 cm × 4.5 cm × 60 - 3000 cm.

[0010] Further, in the above step S2, the water permeability value K before contamination 0i The calculation formula is as shown in Equation 1:

[0011]

[0012] In the formula, i - the i-th region of the artificial homogeneous cast core; K 0i The water permeability of the i-th region before contamination, mD; q0 - the volume flow rate of the injected clear water, cm 3 / s; μ0 - the viscosity during the displacement of clear water, mPa·s; L - the distance between two injection holes, cm; P 0i - the pressure at the i-th injection hole during the displacement of clear water, MPa; P 0(i+1) - the pressure at the (i + 1)-th injection hole during the displacement of clear water, MPa; A - the cross-sectional area of the clear water flow, cm 2 .

[0013] Further, in the above step S3, the water permeability value K after contamination 1i and the resistance coefficient F Ri The calculation formulas are as shown in Equation 3 and Equation 4:

[0014]

[0015] In the formula, i - the i-th region of the artificial homogeneous cast core; K 1i The water permeability of the i-th region after contamination, mD; q1 - the volume flow rate of the injected sewage, cm 3 / s; μ1 - the viscosity during the displacement of sewage, mPa·s; L - the distance between two injection holes, cm; P 1i - the pressure at the i-th injection hole during the displacement of sewage, MPa; P 1(i+1) - the pressure at the (i + 1)-th injection hole during the displacement of sewage, MPa; F Ri - the resistance coefficient of the i-th region; P 0i - the pressure at the i-th injection hole during the displacement of clear water, MPa; P 0(i+1) - the pressure at the (i + 1)-th injection hole during the displacement of clear water, MPa; A - the cross-sectional area of the clear water flow, cm 2 .

[0016] Further, the specific method for evaluating the pollution degree and pollution radius according to the drag coefficient in step S3 above is: F Ri > 15, then it is extremely polluted, and the length of this area from the injection hole is the extremely polluted radius. 15 ≥ F Ri > 10, then it is severely polluted, and the length of this area from the injection hole is the severely polluted radius. 10 ≥ F Ri > 5, then it is moderately polluted, and the length of this area from the injection hole is the moderately polluted radius. 5 ≥ F Ri > 1, then it is slightly polluted, and the length of this area from the injection hole is the slightly polluted radius.

[0017] Further, the plugging removal agent in step S4 above includes one or more of acid plugging removal agent, oxidation plugging removal agent, surfactant plugging removal agent and bioenzyme plugging removal agent.

[0018] Further, the reaction time for closing all injection holes for reaction in step S4 above is 0.1 - 120 h.

[0019] Further, the injection volume of the plugging removal agent in step S4 above is 0.01 PV - 0.3 PV.

[0020] Further, the water - measured permeability value K 2i and the residual resistance coefficient F RRi after pollution plugging removal are calculated as shown in Equations 5 and 6:

[0021]

[0022] In the formula, i - the i - th area of the artificial homogeneous cast core; K 2i is the water - measured permeability of the i - th area after pollution plugging removal, mD; q2 - the volume flow rate of the injected clear water after plugging removal, cm 3 / s; μ2 - the viscosity during the displacement of clear water after plugging removal, mPa·s; L - the distance between two injection holes, cm; P 2i - the pressure of the i - th injection hole during the displacement of clear water after plugging removal, MPa; P 2(i+1) - the pressure of the (i + 1) - th injection hole during the displacement of clear water after plugging removal, MPa; F RRi - the residual resistance coefficient of the i - th area; P 0i - the pressure of the i - th injection hole during the displacement of clear water; P 0(i+1) - the pressure of the (i + 1) - th injection hole during the displacement of clear water; A - the cross - sectional area of the clear water flow, cm 2 .

[0023] Further, the specific method for evaluating the pollution plugging removal degree according to the residual resistance coefficient in step S5 above is: 3 ≥ F RRi > 1, then the pollution plugging removal effect is good. 5 ≥ FRRi > 3, it indicates that the pollution removal effect is the second best, 10 ≥ F RRi > 5, it indicates that the pollution removal effect is better, 15 ≥ F RRi > 10, it indicates that the pollution removal effect is relatively poor, F RRi > 15, it indicates that the pollution removal effect is poor.

[0024] The method of the present application can be used to evaluate the pollution degree of injection wells and the effect of plugging removal measures. First, using artificial homogeneous multi-pressure-point cast cores can reflect the along-path distribution of suspended solids in the reinjected sewage within the core, thereby providing accurate values for establishing evaluation indicators of the along-path pollution degree of reinjected sewage. Secondly, calculating in stages during the sewage flooding can accurately measure the pollution degree of each region of the core. Furthermore, the backflow operation after plugging removal can prevent the tiny suspended solids generated by plugging removal from migrating deep into the core along with the subsequent fresh water flooding. At the same time, the staged displacement of the subsequent fresh water flooding can also prevent the suspended solids in the reinjected sewage from migrating backward under the drive of fresh water, thus avoiding affecting the evaluation of the plugging removal effect of each region. By accurately evaluating the pollution degree and plugging removal effect of injection wells, it is possible to guide the on-site timely adoption of relevant measures, reduce the problem of reduced water injection capacity caused by pollution, thereby extending the life of injection wells and enhancing the oilfield development benefit.

[0025] Compared with the prior art, the embodiments of the present invention have at least the following advantages or beneficial effects:

[0026] 1. Using multi-pressure points can reflect the along-path distribution of suspended solids in the reinjected sewage within the core, and at the same time conduct dynamic monitoring of multiple regions to achieve sub-regional evaluation of reinjected sewage and determine its pollution radius;

[0027] 2. Calculating in stages for sewage flooding can accurately measure the pollution degree of each region of the core;

[0028] 3. The backflow operation after plugging removal can prevent the tiny suspended solids generated by plugging removal from migrating deep into the core along with the subsequent fresh water flooding, thus avoiding affecting the evaluation of the pollution removal effect of each region;

[0029] 4. The staged displacement of the subsequent fresh water flooding can also prevent the suspended solids in the reinjected sewage from migrating backward under the drive of fresh water, thus avoiding affecting the evaluation of the plugging removal effect of each region. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required for the embodiments. It should be understood that the following drawings only show some embodiments of the present invention, and therefore should not be regarded as limiting the scope. For those of ordinary skill in the art, without creative efforts, other related drawings can also be obtained based on these drawings.

[0031] Figure 1 Schematic flow chart of the method provided by the embodiment of the present invention;

[0032] Figure 2 Schematic diagram of the artificial homogeneous cast core structure provided by the embodiment of the present invention;

[0033] Figure 3 Physical diagram of the artificial homogeneous cast core provided by the embodiment of the present invention;

[0034] Figure 4 Schematic flow chart of the experimental device provided by the embodiment of the present invention.

[0035] Icons: 1 - High-temperature and high-pressure vessel; 2 - Intermediate container for formation sewage; 3 - Intermediate container for clear water; 4 - Intermediate container for plugging removal agent; 5 - Pressure sensor; 6 - Displacement pump; 7 - Temperature sensor; 8 - Hand pump; 9 - Pressure relief valve; 10 - Beaker; 11 - Produced fluid collector; 12 - Valve; 13 - Four-way joint; 14 - Confining pressure manometer; 15 - Artificial homogeneous cast core. Specific embodiments

[0036] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. For those not specified in the embodiments, they are carried out according to conventional conditions or conditions recommended by the manufacturer. For reagents or instruments not specified by the manufacturer, they are all conventional products that can be obtained through commercial purchase.

[0037] It should be noted that, without conflict, the embodiments in this application and the features in the embodiments can be combined with each other. The present invention will be described in detail below with reference to specific embodiments.

[0038] As Figure 1 shown, based on the basic theoretical route concept of the present invention, a method for evaluating the pollution radius and plugging removal effect of reinjected sewage in oil fields is proposed, including the following steps:

[0039] S1. Prepare an artificial homogeneous cast core 15 by the quartz sand epoxy resin cementing method. n injection holes are evenly opened along the length direction of the artificial homogeneous cast core 15 and divided into n - 1 regions. Each of the n injection holes is connected with a three-way valve 12 and a pressure sensor 5, which are used as the liquid injection end and the pressure monitoring point;

[0040] S2. Vacuumize and saturate the artificial homogeneous cast core 15 with water to obtain the pore volume of the artificial homogeneous cast core 15. Place it in the high-temperature and high-pressure vessel 1 to apply pressure. Designate one injection hole as injection hole 1, number the remaining injection holes in sequence, and displace with clear water from injection hole 1. Collect the produced fluid from injection hole n, and use the remaining injection holes 1 to n - 1 as monitoring points until the pressure values at each monitoring point are stable, and record the pressure P. 0i , and calculate the water permeability value K before pollution in each area. 0i , where i = 1 to n - 1. During the displacement process, set the pressure in the high-temperature and high-pressure vessel 1 to be always higher than the injection pressure of the injection hole. The water permeability value K before pollution. 0i The calculation formula is shown in Equation 1:

[0041]

[0042] In the formula, i - the i-th area of the artificial homogeneous cast core 15; K 0i The water permeability of the i-th area before pollution, mD; q0 - the volume flow rate of the injected clear water, cm 3 / s; μ0 - the viscosity during the displacement of clear water, mPa·s; L - the distance between two injection holes, cm; P 0i - the pressure of the i-th injection hole during the displacement of clear water, MPa; P 0(i+1) - the pressure of the (i + 1)-th injection hole during the displacement of clear water, MPa; A - the cross-sectional area of the clear water flow, cm 2 .

[0043] S3. Use reinjected sewage for displacement. Displace from injection hole 1, collect the produced fluid from injection hole n, and use the remaining injection holes 1 to n - 1 as monitoring points to simulate the process of oilfield reinjected sewage polluting the reservoir. Record the pressure P in each monitoring point area throughout the process. 1i , and calculate the water permeability value K after pollution in each area. 1i , and the resistance coefficient F Ri , evaluate the pollution degree and pollution radius according to the size of the resistance coefficient, where i = 1 to n - 1. The water permeability value K after pollution. 1i and the resistance coefficient F Ri The calculation formulas are shown in Equation 3 and Equation 4:

[0044]

[0045] In the formula, i - the i-th area of the artificial homogeneous cast core 15; K 1i The water permeability of the i-th area after pollution, mD; q1 - the volume flow rate of the injected sewage, cm 3 / s; μ1 - the viscosity during the displacement of sewage, mPa·s; L - the distance between two injection holes, cm; P 1i - the pressure of the i-th injection hole during the displacement of sewage, MPa; P1(i+1) - The pressure of the (i + 1)-th injection hole during sewage displacement, MPa; F Ri - The resistance coefficient of the i-th region; P 0i - The pressure of the i-th injection hole during clear water displacement, MPa; P 0(i+1) - The pressure of the (i + 1)-th injection hole during clear water displacement, MPa; A - The cross-sectional area of clear water flow, cm 2 ;

[0046] The method for evaluating the pollution degree and pollution radius according to the resistance coefficient is as follows:

[0047] F Ri > 15, then it is extremely polluted, and the length of this area from the injection hole is the extremely polluted radius;

[0048] 15 ≥ F Ri > 10, then it is severely polluted, and the length of this area from the injection hole is the severely polluted radius;

[0049] 10 ≥ F Ri > 5, then it is moderately polluted, and the length of this area from the injection hole is the moderately polluted radius;

[0050] 5 ≥ F Ri > 1, then it is slightly polluted, and the length of this area from the injection hole is the slightly polluted radius;

[0051] S4. Inject 0.01PV - 0.3PV of the plugging removal agent for displacement, simulate the plugging removal process of oilfield reinjected sewage pollution. The plugging removal agent includes acid plugging removal agent, oxidation plugging removal agent, surfactant plugging removal agent, and bioenzyme plugging removal agent, etc. Close all injection holes and react for 0.1 - 120 h, open injection hole 1, and use the remaining pressure after the reaction of the plugging removal agent for backflow, simulate the process of wellhead backflow of plugged substances after oilfield plugging removal;

[0052] S5. Continue to use clear water to conduct displacement in sub-regions and in segments in turn. Open injection hole i (i = 1 ~ n) and injection hole i + 1 in turn, close the remaining injection holes, use clear water to conduct displacement from injection hole i, and collect the produced fluid at injection hole i + 1. Displace until the pressure value at monitoring point i (injection hole i) is stable, record its pressure value P 2i , calculate the water permeability value K after pollution plugging removal of each region 2i , the residual resistance coefficient F RRi , evaluate the degree of pollution plugging removal according to the size of the residual resistance coefficient, where i = 1 ~ n - 1, the water permeability value K after pollution plugging removal 2i and the residual resistance coefficient F RRi The calculation formulas are shown in Formulas 5 and 6:

[0053]

[0054] Where, i is the ith region of artificial homogeneous cast core 15; K 2i The water permeability of the i-th area after the pollution is unblocked, mD; q2-the volume flow rate of clean water injected after unblocking, cm 3 / s; μ2-viscosity during water displacement after unblocking, mPa·s; L-distance between two injection holes, cm; P 2i - Pressure of the i-th injection hole during water displacement after unblocking, MPa; P 2(i+1) - Pressure of the i+1th injection hole during water displacement after unblocking, MPa; F RRi - residual drag coefficient of the ith region; P 0i - Pressure of the ith injection hole during water displacement, MPa; P 0(i+1) - Pressure of the i+1th injection hole during water displacement, MPa; A- Cross-sectional area of ​​water flow, cm 2 ;

[0055] The specific evaluation method for evaluating the degree of pollution removal based on the residual resistance coefficient is as follows:

[0056] 3≥F RRi >1, the pollution removal effect is good;

[0057] 5≥F RRi >3, the pollution removal effect is second best;

[0058] 10≥F RRi >5, the pollution removal effect is good;

[0059] 15≥F RRi >10, the pollution removal effect is poor;

[0060] F RRi >15, the pollution relief effect is poor.

[0061] The features and performance of the present invention are further described in detail below in conjunction with the embodiments.

[0062] Example 1

[0063] Please refer to Figure 4 In order to better understand the technical solution of this embodiment, this embodiment provides an oilfield reinjection wastewater pollution radius and a method for evaluating the blocking effect, and an experimental device is as follows:

[0064] It includes a high-temperature and high-pressure container 1, in which an artificial homogeneous cast core 15 is placed. The upper end of the high-temperature and high-pressure container 1 is connected to a confining pressure gauge 14 and a temperature sensor 7 for monitoring the confining pressure and temperature data. The upper end of the high-temperature and high-pressure container 1 is also connected to a pressure relief valve 9.

[0065] A beaker 10 is connected to a hand pump 8, and the hand pump 8 is connected to the high-temperature and high-pressure vessel 1 for pressurizing the high-temperature and high-pressure vessel 1.

[0066] A displacement pump 6 is successively connected to a plugging agent intermediate container 4, a clear water intermediate container 3, and a formation sewage intermediate container 2 through a tee. The plugging agent intermediate container 4, the clear water intermediate container 3, and the formation sewage intermediate container 2 are connected to an artificial homogeneous cast core 15 through a four-way 13 passing through the high-temperature and high-pressure vessel 1. Valves 12 are provided at the tops of the plugging agent intermediate container 4, the clear water intermediate container 3, and the formation sewage intermediate container 2.

[0067] A produced fluid collector 11 passes through the high-temperature and high-pressure vessel 1 for pressurization and is communicated with the injection hole of the artificial homogeneous cast core 15.

[0068] A number of pressure sensors 5 are also provided on the artificial homogeneous cast core 15.

[0069] Example 2

[0070] Please refer to Figures 1 - 3 , Figure 1 shown in the schematic flow chart of the method of this example; Figure 2 shown in the schematic structural diagram of the artificial homogeneous cast core 15 of this example; Figure 3 shown in the physical diagram of the artificial homogeneous cast core 15 of this example.

[0071] This example provides a method for evaluating the pollution radius and plugging removal effect of oilfield reinjected sewage. The oilfield reinjected sewage used in this example is taken from the Fourth Oil Production Plant of Daqing Oilfield and includes the following steps:

[0072] S1. An artificial homogeneous cast core 15 is made by the quartz sand epoxy resin cementing method. The designed size of the artificial homogeneous cast core 15 is width × height × length = 4.5 cm × 4.5 cm × 270 cm. Seven injection holes are uniformly arranged along the core length direction and numbered 1-7 in sequence. The core is evenly divided into 6 regions. After each injection hole is connected to a three-way valve 12, a pressure sensor 5 is connected, which is used as the liquid injection end and the pressure monitoring point;

[0073] S2. The standby core is evacuated by a vacuum pump for 0.5 h, and then the core is saturated with water. The pore volume of the core is obtained as 623 ml through the amount of saturated water;

[0074] S3. The saturated core is placed in the high-temperature and high-pressure vessel 1. The pressure in the high-temperature and high-pressure vessel 1 is increased to 6 MPa by using the hand pump 8, and the temperature in the high-temperature and high-pressure vessel 1 is increased to 45 °C by using an electric heater;

[0075] S4. Start the displacement pump 6, set the flow rate to 0.6 ml / min, inject clear water from the core injection hole 1, extract it from the injection hole 7, use other injection holes as monitoring points, displace with clear water until the pressure values at each monitoring point are stable, and record the pressure P between each pressure measurement point 0i (i = 1 - 6), calculate the water permeability value before contamination in each area of the core as K 0i (i = 1 - 6), the pressure of each injection hole and the permeability results of each area at the end of the clear water injection are shown in Table 1:

[0076] Table 1 Pressure of each pressure measurement point and permeability results of each area at the end of the clear water injection

[0077]

[0078] S5. Start the displacement pump 6, adjust the injection speed to 0.6 ml / min, inject reinjected sewage with a viscosity of 1.3 mPa·s from the core injection hole 1, extract it from the injection hole 7, use other injection holes as monitoring points, and end until 623 ml of experimental produced water is collected in the collector 11 at the injection hole 7. Record the pressure value P of each monitoring point throughout the process 1i (i = 1 - 6), calculate the water permeability value after contamination in each area of the core as K 1i (i = 1 - 6), the resistance coefficient is F Ri (i = 1 - 6), evaluate the degree of contamination and the contamination radius according to the size of the resistance coefficient. The pressure of each injection hole and the situation of each area after the sewage displacement are shown in Table 2:

[0079] Table 2 Pressure of each injection hole and situation of each area after the sewage displacement

[0080]

[0081]

[0082] From the results in Table 2, it can be seen that the maximum extreme contamination radius of the reinjected sewage in the artificial homogeneous extra-long cast core is at a distance of 45 cm from the core injection hole 1, the maximum severe contamination radius is at a distance of 90 cm from the core injection hole 1, the maximum moderate contamination radius is at a distance of 135 cm from the core injection hole 1, and the maximum mild contamination radius is at a distance of 270 cm from the core injection hole 1;

[0083] S6. Displace with the composite plugging removal agent, which is configured as mud acid: 1% potassium persulfate: light hydrocarbon = 1:1:1, simulate the plugging removal process of the oilfield reinjected sewage contamination, inject 0.1 PV and then close all injection holes for reaction for 12 h;

[0084] S7. Open the injection hole 1, use the remaining pressure after the reaction of the plugging removal agent for backflow, and simulate the process of backflow of the plugged substances at the wellhead after the oilfield plugging removal;

[0085] S8. Continue the zonal displacement with clear water. Open injection hole 1 and injection hole 2, close the other injection holes, and displace with clear water until the pressure value at monitoring point 1 (injection hole 1) is stable, and record its pressure value P 2i (i = 1), calculate the water permeability value of area 1 after core pollution removal and plugging removal as K 21 and the residual resistance coefficient as F RR1 , repeat the above operations for areas 2 - 6 to obtain the permeability K 2i (i = 2 - 6) and the residual resistance coefficient F RRi (i = 2 - 6), evaluate the degree of pollution removal according to the magnitude of the residual resistance coefficient. The situation of each area at the end of the subsequent clear water displacement is shown in Table 3 as follows:

[0086] Table 3 Situation of Each Area at the End of the Subsequent Clear Water Displacement

[0087]

[0088]

[0089] It can be seen from the results in Table 3 that the pollution removal effect of the composite plugging removal agent in each area of the artificial homogeneous extra-long cast core is good.

[0090] The embodiments described above are some, but not all, of the embodiments of the present invention. The detailed description of the embodiments of the present invention is not intended to limit the scope of the claimed present invention, but merely represents selected embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without making creative efforts fall within the scope of protection of the present invention.

Claims

1. A method for evaluating the pollution radius and plugging removal effect of reinjected sewage in oilfields, characterized in that, Including the following steps: S1. Prepare an artificial homogeneous cast core, on which n injection holes are evenly drilled and divided into n - 1 regions equally; S2. Place the artificial homogeneous cast core into the high-pressure vessel chamber, apply pressure and displace it with clear water from one of the injection holes, collect the produced fluid from the other injection hole, and monitor the pressure P at the remaining injection holes 0i , and calculate the water permeability value K before contamination in each of the regions 0i , where i = 1 to n - 1; S3. Use reinjected sewage for displacement and monitor the pressure P of each of the said areas 1i , calculate the water permeability value K after contamination of each of the said areas 1i , the resistance coefficient F Ri , evaluate the degree of contamination and the contamination radius according to the magnitude of the resistance coefficient, where i = 1 to n - 1; S4. Inject a plugging removal agent for displacement, close all injection holes for reaction, and open injection hole 1 to conduct backflow using the remaining pressure after the reaction; S5. Set any one of the injection holes as injection hole i, open injection hole i and injection hole i + 1, close the remaining injection holes, and use clear water to perform segmented displacement on the n - 1 regions in sequence from injection hole i. Injection hole i + 1 collects the produced fluid, and record its pressure value P when the pressure value is stable 2i , obtain different pressure values of the n - 1 regions, and calculate the water - measured permeability values K after pollution removal and plugging of the n - 1 regions 2i , and the residual resistance coefficient F RRi . Evaluate the degree of pollution removal and plugging according to the magnitude of the residual resistance coefficient, where i = 1 to n - 1 2. The method according to claim 1, characterized in that, In step S1, the artificial homogeneous cast core is prepared by the quartz sand epoxy resin cementing method. Three-way valves and pressure sensors are connected to all n injection holes, which are used as the liquid injection end and the pressure monitoring point.

3. The method according to claim 1, wherein In step S1, the size of the artificial homogeneous cast core is width × height × length = 4.5 cm × 4.5 cm × 60 - 3000 cm.

4. The method according to claim 1, wherein The pre - pollution water permeability value K in step S2 0i The calculation formula is as shown in Equation 1: Where, i - the i-th region of the artificial homogeneous cast core; K 0i The water permeability of the i-th region before contamination, mD; q0 - the volume flow rate of the injected fresh water, cm 3 / s; μ0 - the viscosity during fresh water displacement, mPa·s; L - the distance between the two injection holes, cm; P 0i - the pressure at the i-th injection hole during fresh water displacement, MPa; P 0(i+1) - the pressure at the (i + 1)-th injection hole during fresh water displacement, MPa; A - the cross-sectional area of fresh water flow, cm 2 .

5. The method according to claim 1, wherein The water permeability value K after contamination in step S3 1i and the resistance coefficient F Ri The calculation formulas are shown in Formulas 3 and 4 as follows: Where, i - the i-th region of the artificial homogeneous cast core; K 1i The water permeability of the i-th region after contamination, mD; q1 - the volume flow rate of the injected sewage, cm 3 / s; μ1 - the viscosity during sewage displacement, mPa·s; L - the distance between two injection holes, cm; P 1i - the pressure at the i-th injection hole during sewage displacement, MPa; P 1(i+1) - the pressure at the (i + 1)-th injection hole during sewage displacement, MPa; F Ri - Resistance coefficient of the i-th region; P 0i - Pressure at the i-th injection hole during water flooding, MPa; P 0(i+1) - Pressure at the (i + 1)-th injection hole during water flooding, MPa; A - Cross-sectional area of water flow, cm 2 。 6. The method according to claim 1, characterized in that In step S3, the specific method for evaluating the pollution degree according to the resistance coefficient and the pollution radius evaluation method is as follows: F Ri > 15, it is extremely polluted, and the length of this area from the injection hole is the extremely polluted radius; 15≥F Ri If it is > 10, it is severe pollution, and the length of this area from the injection hole is the severe pollution radius; 10≥F Ri >5, it is moderately polluted, and the length of this area from the injection hole is the moderately polluted radius; 5≥F Ri If it is > 1, it is lightly polluted, and the length of this area from the injection hole is the lightly polluted radius.

7. The method according to claim 1, characterized in that In step S4, the plugging removal agent includes one or more of an acid plugging removal agent, an oxidation plugging removal agent, a surfactant plugging removal agent, and a bioenzyme plugging removal agent.

8. The method according to claim 1, wherein In step S4, the reaction time for closing all injection holes for reaction is 0.1 - 120 h; the injection volume of the plugging removal agent is 0.01 PV - 0.3 PV.

9. The method according to claim 1, wherein The water permeability value K after pollution removal and plugging in the step S5 2i and the residual resistance coefficient F RRi The calculation formulas are shown in Formulas 5 and 6 as follows: Where, i - the i-th region of the artificial homogeneous cast core; K 2i The water permeability of the i-th region after pollution removal and plugging removal, mD; q2 - the volume flow rate of the injected clear water after plugging removal, cm 3 / s; μ2 - the viscosity during the displacement of clear water after plugging removal, mPa·s; L - the distance between two injection holes, cm; P 2i - the pressure of the i-th injection hole during the displacement of clear water after plugging removal, MPa; P 2(i+1) - the pressure of the (i + 1)-th injection hole during the displacement of clear water after plugging removal, MPa; F RRi - Residual resistance coefficient of the i-th region; P 0i - Injection hole pressure at the i-th during clear water displacement, MPa; P 0(i+1) - Injection hole pressure at the (i + 1)-th during clear water displacement, MPa; A - Cross-sectional area of clear water flow, cm 2 。 10. The method according to claim 1, wherein In step S5, the specific method for evaluating the plugging removal degree of pollution according to the residual resistance coefficient is as follows: 3≥F RRi > 1, it indicates good pollution removal and plugging removal effect; 5≥F RRi > 3, then the pollution removal and plugging effect is the second best; 10≥F RRi > 5, it indicates a better pollution removal and plugging effect; 15≥F RRi > 10, the pollution removal and plugging removal effect is poor; F RRi > 15 indicates poor pollution removal and plugging removal effect.

Images