An experimental method for improving oil recovery by stepwise polymer injection in heterogeneous reservoirs

Abstract

The present disclosure relates to an experimental method for improving oil recovery by stepwise polymer injection in heterogeneous reservoirs, which includes the steps of: preparing separate injection and production and homogeneous in-layer heterogeneous cores according to the obtained target reservoir characteristics, and forming parallel cores; performing primary water flooding on the parallel cores to determine their oil recovery; performing the first-stage polymer flooding on the homogeneous in-layer heterogeneous cores in a constant-rate and constant-pressure manner to determine their oil recovery; performing the second-stage polymer flooding on the parallel cores in a constant-pressure manner to determine their oil recovery; and performing secondary water flooding on the parallel cores to determine their oil recovery. The embodiments of the present disclosure can simulate the stepwise polymer injection process of the real heterogeneous reservoir of the target well through an experimental method, determine the oil recovery by measuring the produced fluid collected during the stepwise polymer injection experiment, compare and analyze the oil recovery obtained by stepwise polymer injection with the oil recovery obtained by the original polymer flooding method, and determine whether there is an increase in the oil recovery obtained by using this stepwise polymer flooding method in the target well.

Description

Technical Field

[0001] The present disclosure relates to the technical field of reservoir development, and particularly to an experimental method for improving oil recovery by stepwise polymer injection in heterogeneous reservoirs. Background Art

[0002] The polymer (HPAM) flooding technology has been highly regarded by oilfield developers due to its relatively simple injection process, low cost, and strong reservoir adaptability, and has become one of the effective means to significantly improve the oil recovery of water-flooded reservoirs. Although obvious oil production increase and water cut reduction effects have been achieved in polymer flooding, due to the limitation of the reservoir fracture pressure, the actual injection pressure increase in the field is difficult to reach the same amplitude as in the laboratory experiment, and even causes injection difficulties. In addition, the limitation of the injection pressure increase in the field also accelerates the "reversal of the liquid absorption profile" and restricts the oil production increase and water cut reduction effects of polymer flooding to a certain extent. The "reversal of the liquid absorption profile" is caused by different seepage resistances generated after displacement agents with the same retention ability enter different permeability parts in the thick oil layer.

[0003] In recent years, petroleum scientists and technicians have proposed injection methods such as alternating injection of "high molecular weight" and "low molecular weight" polymer solutions, "high concentration" and "low concentration" polymer solutions, alternating injection of weak gels and water, and "stepwise viscosity reduction and speed increase" to improve the development effect of polymer flooding. The oil displacement mechanism is to avoid the entry of "high concentration" or "high molecular weight" into low-permeability layers, reduce the rising speed of their liquid absorption starting pressure, and thus achieve the purpose of slowing down the "reversal of the liquid absorption profile" speed and improving the development effect of polymer flooding.

[0004] The patent "Core displacement experimental method for different displacement modes in heterogeneous reservoirs" proposed a core displacement experimental method for polymer flooding with alternating injection of "high molecular weight" and "low molecular weight" polymer solutions and "gradient viscosity reduction and speed increase" using heterogeneous cores, and analyzed the variation law of oil displacement efficiency under different displacement modes. However, during the alternating injection of different polymer solutions or the injection of "gradient viscosity reduction and speed increase", due to the differences in the average permeability and heterogeneity of the reservoir, it is very difficult to control the alternating timing of "high concentration or high molecular weight" polymer solutions and "low concentration or low molecular weight" polymer solutions. Therefore, some "high concentration or high molecular weight" polymer solutions will enter the medium and low permeability layers and be retained during the alternating injection process, resulting in an increase in their seepage resistance and liquid absorption starting pressure, and a decrease in the liquid absorption pressure difference, liquid absorption volume, and relative liquid absorption volume, thereby reducing the actual effect of alternating injection in delaying the "reversal of the liquid absorption profile".

[0005] The literature "The Law of Enhanced Oil Recovery by Variable Mobility Polymer Flooding and Its Application Effect" proposed the injection method (way) of "stepwise viscosity reduction and speed increase". This method was tested in the 86 well injection area in the eastern part of the central area of the Daqing Oilfield. The test was carried out by first injecting a "high-concentration" polymer solution at a "low speed" and then injecting a "low-concentration" polymer solution at a "high speed". Compared with the conventional injection method, the injection method of "stepwise viscosity reduction and speed increase" achieved better oil production increase and water cut reduction effects. However, practice has shown that the injection method of "stepwise viscosity reduction and speed increase" also has deficiencies and problems. First, the "low speed" and "high concentration" injection stages will cause injection-production imbalance, that is, the injection volume of the water well is less and the liquid production volume of the oil well is more (which is determined by the development plan and is also a necessary condition for completing the oil production), the formation shows a deficit, resulting in a pressure reduction and a decrease in the liquid supply capacity, and ultimately reducing the oilfield development effect; second, it is difficult to control the best timing of implementing "stepwise viscosity reduction and speed increase", and it is impossible to completely avoid the "high-concentration" polymer solution from entering the medium and low permeability layers (parts), thereby weakening the effect of this method in slowing down the "inversion of the liquid absorption profile". Summary of the Invention

[0006] The present disclosure provides an experimental method for enhancing oil recovery by stepwise polymer injection in heterogeneous reservoirs. By simulating the stepwise polymer injection process of the target well's real heterogeneous reservoir through the experimental method, determining its oil recovery rate by measuring the produced fluid collected during the stepwise polymer injection experiment, and comparing and analyzing the oil recovery rate obtained by stepwise polymer injection with the oil recovery rate obtained by the original polymer flooding method, it is determined whether there is an increase in the oil recovery rate of the target well using this stepwise polymer flooding method.

[0007] According to one aspect of the present disclosure, there is provided an experimental method for enhancing oil recovery by stepwise polymer injection in heterogeneous reservoirs, including the steps of:

[0008] Obtain the core porosity, permeability, and heterogeneity of the target reservoir, and prepare heterogeneous cores according to the core porosity, permeability, and heterogeneity. Among them, the heterogeneous cores include: in-layer heterogeneous cores for separate injection and separate production and in-layer heterogeneous cores of the blank plate;

[0009] Pretreat the heterogeneous cores, form parallel cores with the pretreated heterogeneous cores, perform a primary water drive on the parallel cores, and measure and determine the water drive oil recovery rate of the heterogeneous cores;

[0010] After the first water flooding is completed, the heterogeneous cores in the separate injection and production layers of the parallel cores are subjected to the first-stage polymer flooding. The first polymer solution is injected at a constant rate into the injection end of the heterogeneous cores in the separate injection and production layers at a predetermined initial pressure and a predetermined second flow rate. When it is determined that the real-time injection pressure when injecting the first polymer is equal to the predetermined injection pressure, the injection is maintained at a constant pressure of the predetermined injection pressure until the injection volume of the first polymer solution is equal to the predetermined first injection volume. During the injection process, the produced fluids from each small layer at the production end of the heterogeneous cores in the separate injection and production layers are collected regularly, and the recovery factor of the first-stage polymer flooding of the heterogeneous cores in the separate injection and production layers is measured and determined;

[0011] After the first-stage polymer flooding is completed, the parallel cores are subjected to the second-stage polymer flooding. The second polymer solution is injected at a constant pressure into the injection end of the parallel cores until the injection volume of the second polymer solution is equal to the predetermined second injection volume. During the injection of the second polymer solution, the produced fluids from each small layer at the production end of the parallel cores are collected regularly, and the recovery factor of the second-stage polymer flooding of the parallel cores is measured and determined;

[0012] After the second-stage polymer flooding is completed, the parallel cores are subjected to secondary water flooding, and the subsequent water flooding recovery factor of the parallel cores is measured and determined.

[0013] Preferably, the method for pre-treating the heterogeneous cores includes:

[0014] The heterogeneous cores are evacuated for 6 to 12 hours, saturated with formation water for 6 to 12 hours, and simulated oil is injected from the injection end of the heterogeneous cores under the reservoir temperature conditions of the target reservoir until no water comes out from the production end of the heterogeneous cores. After the injection of the simulated oil is completed, the heterogeneous cores are left to age statically for 12 to 24 hours.

[0015] Preferably, the method for the first water flooding of the parallel cores includes:

[0016] Water is injected into the parallel cores at a constant rate of a predetermined first flow rate from the injection end of the parallel cores. During the water injection process, the water cut of the produced fluids from each small layer at the production end of the parallel cores is collected and measured regularly. When the water cut of the produced fluids reaches the predetermined water cut, the water injection is stopped, and the water flooding recovery factor of each small layer of the parallel cores is measured and determined.

[0017] Preferably, the first polymer solution is partially hydrolyzed polyacrylamide, and its relative molecular mass is: M = 1900 - 2500×10 4 ;

[0018] The polymer concentration of the first polymer solution is: Cp = 1000 - 1500 mg / L;

[0019] The predetermined first injection volume is: 0.1 PV - 0.3 PV.

[0020] Preferably, the second polymer solution is partially hydrolyzed polyacrylamide, and its relative molecular mass is: M < 500×10 4 ;

[0021] The polymer concentration of the second polymer solution is: Cp = 1000 - 1500 mg / L;

[0022] The predetermined second injection volume is: 0.3PV - 0.5PV.

[0023] Preferably, the method for secondary water flooding of parallel cores includes:

[0024] Inject water into the parallel cores at a constant pressure with a predetermined injection pressure from the injection end of the parallel cores. During the water injection process, regularly collect the produced fluids from each small layer at the production end of the parallel cores, measure and stop water injection when the water cut of the produced fluid reaches the predetermined water cut, and measure and determine the subsequent water flooding recovery rate of each small layer of the parallel cores.

[0025] It should be understood that the above general description and the following detailed description are only exemplary and explanatory, and do not limit the present disclosure.

[0026] According to the following detailed description of exemplary embodiments with reference to the accompanying drawings, other features and aspects of the present disclosure will become clear. Description of the Drawings

[0027] The accompanying drawings here are incorporated into the specification and constitute a part of this specification. These drawings show embodiments consistent with the present disclosure and, together with the specification, are used to explain the technical solutions of the present disclosure.

[0028] Figure 1 A schematic structural diagram showing the connection of the experimental device during the first-stage polymer flooding and the second-stage polymer flooding in the embodiments of the present disclosure.

[0029] Figure 2 A curve showing the relationship between the injection pressure and the PV number of the injection volume during the implementation of the general injection method and the step-by-step polymer injection method in the embodiments of the present disclosure;

[0030] Figure 3 A curve showing the relationship between the water cut and the PV number of the injection volume during the implementation of the general injection method and the step-by-step polymer injection method in the embodiments of the present disclosure;

[0031] Figure 4 A curve showing the relationship between the recovery rate and the PV number of the injection volume during the implementation of the general injection method and the step-by-step polymer injection method in the embodiments of the present disclosure;

[0032] Figure 5Shows the relationship curve between the shunt ratio at the injection end of a heterogeneous core and the number of PVs of the injection volume during the implementation processes of the general injection method and the step-by-step polymer injection method according to the embodiments of the present disclosure;

[0033] Figure 6 Shows the relationship curve between the shunt ratio at the production end of a heterogeneous core and the number of PVs of the injection volume during the implementation processes of the general injection method and the step-by-step polymer injection method according to the embodiments of the present disclosure;

[0034] In the figure, 1 - constant pressure pump, 2 - first intermediate container, 3 - second intermediate container, 4 - heterogeneous core in the layer with separate injection and production, 5 - heterogeneous core in the bare plate layer, 6 - pressure sensor, 7 - displacement fluid, 10 - first polymer solution, 11 - second polymer solution. Detailed implementation manners

[0035] Various exemplary embodiments, features, and aspects of the present disclosure will be described in detail below with reference to the accompanying drawings. The same reference numerals in the drawings denote elements having the same or similar functions. Although various aspects of the embodiments are shown in the drawings, the drawings do not have to be drawn to scale unless otherwise specified.

[0036] The special term "exemplary" herein means "serving as an example, embodiment, or illustration". Any embodiment described as "exemplary" herein does not have to be construed as superior to or better than other embodiments.

[0037] The term "and / or" in this article is merely a description of the association relationship of associated objects, indicating that there can be three relationships. For example, A and / or B can represent: A exists alone, A and B exist simultaneously, and B exists alone. In addition, the term "at least one" in this article means any one of multiple or any combination of at least two of multiple. For example, including at least one of A, B, and C can represent including any one or more elements selected from the set composed of A, B, and C.

[0038] In addition, for a better illustration of the present disclosure, numerous specific details are given in the following detailed implementation manners. Those skilled in the art should understand that the present disclosure can also be implemented without some specific details. In some instances, methods, means, elements, and circuits well-known to those skilled in the art are not described in detail so as to highlight the gist of the present disclosure.

[0039] The present disclosure provides an experimental method for improving oil recovery by stepwise polymer injection in heterogeneous reservoirs, including the following steps: Step 1: Obtain the core porosity, permeability, and heterogeneity of the target reservoir, and prepare heterogeneous cores according to the core porosity, permeability, and heterogeneity. The heterogeneous cores include: the heterogeneous core 4 within the separate injection and production layer and the heterogeneous core 5 within the blank layer. Step 2: Pretreat the heterogeneous cores, form parallel cores with the pretreated heterogeneous cores, perform a primary water drive on the parallel cores, and measure and determine the water drive oil recovery rate of the heterogeneous cores. Step 3: After the primary water drive, perform a first-stage polymer drive on the heterogeneous core 4 within the separate injection and production layer in the parallel cores. Inject the first polymer solution 10 at a predetermined initial pressure and a predetermined second flow rate into the injection end of the heterogeneous core 4 within the separate injection and production layer at a constant speed. When it is determined that the real-time injection pressure when injecting the first polymer is equal to the predetermined injection pressure, maintain a constant pressure injection at the predetermined injection pressure until the injection volume of the first polymer solution 10 is equal to the predetermined first injection volume. During the injection process, regularly collect the produced fluids from each sub-layer at the production end of the heterogeneous core 4 within the separate injection and production layer, and measure and determine the first-stage polymer drive oil recovery rate of the heterogeneous core 4 within the separate injection and production layer. Step 4: After the first-stage polymer drive, perform a second-stage polymer drive on the parallel cores. Inject the second polymer solution 11 at a constant pressure into the injection end of the parallel cores until the injection volume of the second polymer solution 11 is equal to the predetermined second injection volume. During the injection of the second polymer solution 11, regularly collect the produced fluids from each sub-layer at the production end of the parallel cores, and measure and determine the second-stage polymer drive oil recovery rate of the parallel cores. Step 5: After the second-stage polymer drive, perform a secondary water drive on the parallel cores, and measure and determine the subsequent water drive oil recovery rate of the parallel cores.

[0040] In the experimental method of the present invention, the high-permeability layer and the medium-low permeability layer of the target reservoir are opened by stepwise polymer injection, avoiding the entry of "high-concentration or high-molecular-weight" polymer solutions into the medium-low permeability layer, greatly reducing the "inversion rate of the liquid absorption profile" and improving the polymer flooding development effect. The experimental results confirm that the distribution polymer injection method further improves the polymer flooding development effect of heterogeneous oil reservoirs, slows down the "inversion rate of the liquid absorption profile", increases the liquid absorption volume of the medium-low permeability layer (area), and expands the swept volume.

[0041] Step 1: Obtain the core porosity, permeability, and heterogeneity of the target reservoir, and prepare heterogeneous cores according to the core porosity, permeability, and heterogeneity. The heterogeneous cores include: the heterogeneous core 4 within the separate injection and production layer and the heterogeneous core 5 within the blank layer.

[0042] In the embodiments of the present invention, the target reservoir is selected as a heterogeneous reservoir with an interlayer inside. According to the core porosity, permeability and heterogeneity of the target reservoir, a simulated heterogeneous core required in the experiment is made. Among them, the method for making the heterogeneous core can refer to the patents: Method for making an in-layer heterogeneous core for separate injection and production and its experimental method, CN201710487705.4; Method for making a quartz sand epoxy resin cemented heterogeneous model, CN200510063665.8, for production.

[0043] Among them, the appearance geometric dimensions of the in-layer heterogeneous core 4 for separate injection and production are: width × height × length = 4.5 cm × 9.0 cm × 30 cm, including two (or more) permeable layers of high and low permeability. The thickness of each small layer is 4.5 cm. The gas-measured permeability Kg of the high-permeability layer is 1500×10 -3 μm 2 ~15000×10 -3 μm 2 ; the gas-measured permeability Kg of the low-permeability layer is 150×10 -3 μm 2 ~1500×10 -3 μm 2 .

[0044] Among them, the appearance geometric dimensions of the in-layer heterogeneous core 5 of the bare plate are: width × height × length = 4.5 cm × 4.5 cm × 30 cm, including two permeable layers of high and low permeability. The thickness of each small layer is 2.25 cm. The gas-measured permeability Kg of the high-permeability layer is 150×10 -3 μm 2 ~1500×10 -3 μm 2 ; the gas-measured permeability Kg of the low-permeability layer is 15×10 -3 μm 2 ~150×10 -3 μm 2 .

[0045] The in-layer heterogeneous core 4 for separate injection and production and the in-layer heterogeneous core 5 of the bare plate simulate the core characteristics of the target reservoir. Among them, the average permeability of the in-layer heterogeneous core 4 for separate injection and production is relatively high, and the injection end of the core realizes the separation of high and low permeability layers, but the seepage characteristics of the in-layer heterogeneous core are still retained between the permeable layers inside the core; the average permeability of the in-layer heterogeneous core 5 of the bare plate is relatively low.

[0046] Step 2: Pretreat the heterogeneous core, form a parallel core with the pretreated heterogeneous cores, and perform a primary water flooding on the parallel core to measure and determine the water flooding recovery rate of the heterogeneous core.

[0047] In the present invention, a method for pretreating heterogeneous cores includes: evacuating the heterogeneous cores for 6 - 12 hours, saturating them with formation water for 6 - 12 hours, injecting simulated oil from the injection end of the heterogeneous cores at the reservoir temperature of the target reservoir until no water comes out from the production end of the heterogeneous cores, and after the injection of the simulated oil is completed, allowing the heterogeneous cores to stand and age for 12 - 24 hours.

[0048] In the present invention, a method for performing primary water flooding on parallel cores includes: injecting water into the parallel cores at a constant rate of a predetermined first flow rate from the injection end of the parallel cores. During the water injection process, regularly collect and measure the water cut of the produced fluids from each small layer at the production end of the parallel cores. Stop water injection when the water cut of the produced fluids reaches the predetermined water cut, and measure and determine the water flooding recovery factor of each small layer of the parallel cores.

[0049] In an embodiment of the present invention, the pretreatment of the heterogeneous cores is as follows: casting the heterogeneous core 4 within the separate injection and production layer with epoxy resin, placing the heterogeneous core 5 within the blank layer into a core holder, and then evacuating the heterogeneous core 4 within the separate injection and production layer and the heterogeneous core 5 within the blank layer for 6 hours respectively, and then saturating them with formation water for 6 hours. After the saturation with water is completed, measure the pore volume of the heterogeneous cores and calculate the porosity of the heterogeneous cores. After the saturation with water is completed, saturate the heterogeneous cores with simulated oil. The experimental oil is composed of a mixture of produced oil from the oilfield and light hydrocarbons, with a viscosity of 9.8 mPa·s at 45°C. When saturating with the simulated oil, load the simulated oil into an intermediate container for simulated oil, connect the injection end of the core holder of the heterogeneous core 4 within the separate injection and production layer and the heterogeneous core 5 within the blank layer to the intermediate container for simulated oil, place the intermediate container for simulated oil and the core holders of the heterogeneous core 4 within the separate injection and production layer and the heterogeneous core 5 within the blank layer into an incubator, keep the temperature in the incubator at 45°C, connect the intermediate container for simulated oil to a peristaltic pump through a pipeline, start the peristaltic pump to inject the simulated oil in the intermediate container for simulated oil from the injection end of the heterogeneous cores until no water comes out from the production end of the heterogeneous cores, collect the oil - water mixture at the production end, and calculate the oil saturation. The above process simulates the formation process of crude oil in the reservoir. Among them, the porosity, pore volume, and oil saturation of the heterogeneous cores are used to calculate the recovery factor during subsequent water flooding and polymer flooding.

[0050] In an embodiment of the present invention, during primary water flooding, raise the temperature of the incubator to the simulated reservoir temperature, and connect the input end of the peristaltic pump to the experimental water. Among them, the experimental water is the injection water and formation water of the oilfield, and the ionic composition of the solvent water is shown in Table 1 below:

[0051] Table 1 Ionic composition of the solvent water

[0052]

[0053] The output end of the advection pump is respectively connected to the core holders of the heterogeneous core 4 in the separate injection and production layer and the heterogeneous core 5 in the blank plate layer through a four-way pipeline, so that the heterogeneous core 4 in the separate injection and production layer and the heterogeneous core 5 in the blank plate layer form a parallel core; a pressure sensor 6 is installed on one pipeline near the output end of the advection pump, and the pressure sensor 6 is used to detect the pressure of the fluid output at the output end of the advection pump.

[0054] During water injection, start the advection pump, and the advection pump injects water into the injection end of the parallel core through the pipeline. During injection, control the advection pump to inject water at a constant speed at a predetermined first flow rate. Among them, the value range of the predetermined first flow rate is 0.1 mL / min to 1.5 mL / min. During the injection process, regularly collect the produced fluids (oil-water mixture) of each small layer at the production end of the heterogeneous core 4 in the separate injection and production layer and the heterogeneous core 5 in the blank plate layer, and measure the water cut at the same time until the water cut reaches the predetermined water cut and then end the water injection. Among them, the predetermined water cut is: 98%. Measure and calculate the water flooding recovery rate and split flow rate of the produced fluids of each small layer of the heterogeneous core 4 in the separate injection and production layer and the heterogeneous core 5 in the blank plate layer. At the same time, detect and record the injection pressure P at the end of water flooding through the pressure sensor 6.

[0055] Step 3: After the first water flooding ends, perform the first-stage polymer flooding on the heterogeneous core 4 in the separate injection and production layer in the parallel core. Inject the first polymer solution 10 at a predetermined initial pressure and a predetermined second flow rate into the injection end of the heterogeneous core 4 in the separate injection and production layer at a constant speed. Judge that when the real-time injection pressure when injecting the first polymer is equal to the predetermined injection pressure, maintain constant pressure injection at the predetermined injection pressure until the injection volume of the first polymer solution 10 is equal to the predetermined first injection volume. During the injection process, regularly collect the produced fluids of each small layer at the production end of the heterogeneous core 4 in the separate injection and production layer, and measure and determine the first-stage polymer flooding recovery rate of the heterogeneous core 4 in the separate injection and production layer.

[0056] In the present invention, the first polymer solution 10 is partially hydrolyzed polyacrylamide, and its relative molecular mass is: M = 1900 - 2500×10 4 ; the polymer concentration of the first polymer solution 10 is: Cp = 1000 - 1500 mg / L; the predetermined first injection volume is: 0.1 PV - 0.3 PV.

[0057] In the embodiment of the present invention, after the water flooding ends, connect the output end of the constant pressure pump 1 to the first intermediate container 2 through a pipeline. The first intermediate container 2 is filled with the first polymer solution 10. Among them, the first polymer solution 10 is a "super high" and "medium concentration" polymer solution, and the molecular mass is M = 2500×10 4, with an effective content of 89.7%, and the polymer concentration is Cp = 1000 mg / L. The pump outlet of the constant-pressure pump 1 is connected to the inlet of the first intermediate container 2 through a pipeline. The outlet of the first intermediate container 2 is respectively connected to the heterogeneous core 4 in the layer with separate injection and production through the first valve. The pump inlet of the constant-pressure pump 1 is connected to the displacement fluid 7. A pressure sensor 6 is installed on the pipeline between the pump outlet of the constant-pressure pump 1 and the first intermediate container 2. Among them, the first intermediate container 2 is placed in a constant-temperature box, and the temperature in the constant-temperature box is maintained at 45 °C.

[0058] When performing the first-stage polymer flooding, first close the first valve and the second valve, start the constant-pressure pump 1, the pump outlet of the constant-pressure pump 1 outputs the displacement fluid 7, so that the pressure in the pipeline rises. The pressure sensor 6 detects the real-time pressure in the pipeline. When the real-time pressure is equal to the predetermined initial pressure, control the first valve to open, and the displacement fluid 7 enters the first intermediate container 2, and pushes the first polymer solution 10 in the first intermediate container 2 into the injection end of the heterogeneous core 4 in the layer with separate injection and production. During the injection, control the constant-pressure pump 1 to inject the first polymer solution 10 at a constant speed of a predetermined second flow rate of 0.6 mL / min. During the injection process, detect the real-time injection pressure through the pressure sensor 6, and judge that when the real-time injection pressure rises to the predetermined injection pressure, control the flow rate and flow rate output by the constant-pressure pump 1, so that the pressure in the pipeline, that is, the injection pressure remains unchanged, and inject the first polymer solution 10 at a constant pressure of the predetermined injection pressure. Among them, the predetermined initial pressure is the injection pressure P at the end of the first water flooding; the predetermined injection pressure is 2P (P is the injection pressure at the end of the first water flooding). During the injection process, regularly collect the produced fluids from each small layer at the production end of the heterogeneous core 4 in the layer with separate injection and production, and measure and calculate the first-stage polymer flooding recovery factor and split flow rate of the produced fluids from the heterogeneous core 4 in the layer with separate injection and production when injecting the first polymer solution 10. When the injection volume of the injected first polymer solution 10 is equal to 0.3PV, end the injection.

[0059] Step 4: After the first-stage polymer flooding ends, perform the second-stage polymer flooding on the parallel cores, inject the second polymer solution 11 into the injection end of the parallel cores at a constant pressure of the predetermined injection pressure until the injection volume of the second polymer solution 11 is equal to the predetermined second injection volume. During the injection of the second polymer solution 11, regularly collect the produced fluids from each small layer at the production end of the parallel cores, and measure and determine the second-stage polymer flooding recovery factor of the parallel cores.

[0060] In the present invention, the second polymer solution 11 is partially hydrolyzed polyacrylamide, and its relative molecular mass is: M < 500×10 4 ; the polymer concentration of the second polymer solution 11 is: Cp = 1000 - 1500 mg / L; the predetermined second injection volume is: 0.3PV - 0.5PV.

[0061] In the embodiment of the present invention, after the first-stage polymer flooding ends, the output end of the constant-pressure pump 1 is connected to the second intermediate container 3 through a pipeline. The second intermediate container 3 is filled with a second polymer solution 11. Among them, the relative molecular mass of the second polymer solution 11 is: M = 1200×10 4 , and the polymer concentration is Cp = 1000 mg / L. The pump outlet of the constant-pressure pump 1 is connected to the inlet of the second intermediate container 3 through a pipeline. The outlet of the second intermediate container 3 is connected to the core holders of the heterogeneous core 4 in the injection-production layer with separate injection and production and the heterogeneous core 5 in the blank layer through a second valve and a third valve respectively. The pump inlet of the constant-pressure pump 1 is connected to the displacement fluid 7. A pressure sensor 6 is installed on the pipeline between the pump outlet of the constant-pressure pump 1 and the second intermediate container 3. Among them, the second intermediate container 3 is placed in a constant-temperature box, and the temperature in the constant-temperature box is maintained at 45°C.

[0062] When performing the second-stage polymer flooding, first close the first valve, the second valve and the third valve, start the constant-pressure pump 1, and the pump outlet of the constant-pressure pump 1 outputs the displacement fluid 7 to increase the pressure in the pipeline. The pressure sensor 6 detects the real-time pressure in the pipeline. When the real-time pressure is equal to the predetermined injection pressure (2P), control the second valve and the third valve to open, and the displacement fluid 7 enters the second intermediate container 3, and the second polymer solution 11 in the second intermediate container 3 is pushed into the injection end of the heterogeneous core 4 in the injection-production layer with separate injection and production.

[0063] During the injection process, control the flow rate and flow volume output by the constant-pressure pump 1 to keep the pressure in the pipeline, that is, the injection pressure, unchanged, and inject the first polymer solution 10 at a constant pressure with the predetermined injection pressure. The predetermined injection pressure is 2P. During the injection process, regularly collect the produced fluids of each small layer at the production ends of the heterogeneous core 4 in the injection-production layer with separate injection and production and the heterogeneous core 5 in the blank layer, and measure and calculate the second-stage polymer flooding recovery rate and the split flow rate of the produced fluid of the heterogeneous core when injecting the second polymer solution 11. When the injection volume of the injected second polymer solution 11 is equal to 0.3PV, end the injection.

[0064] In the embodiment of the present invention, the sum of the predetermined first injection volume and the predetermined second injection volume should be 0.6PV. The second polymer solution 11 used in the second-stage polymer flooding is a "low molecular weight", "medium concentration" polymer solution. During the second-stage polymer flooding, the second polymer solution 11 can be injected alone, or alternately injected with other polymer solutions ("medium molecular weight", "low concentration" polymer solution, "high molecular weight", "low concentration" polymer solution) or with water. Among them, the relative molecular mass of the "ultra-high molecular weight" polymer is M = 1900 - 2500×10 4 ; the relative molecular mass of the "high molecular weight" polymer is M = 1200 - 1900×10 4 ; the relative molecular mass of the "medium molecular weight" polymer is M = 500 - 1200×10 4 ; the relative molecular mass of the "low molecular weight" polymer is <500×104 For the "medium concentration" polymer solution, Cp = 1000 - 1500 mg / L, and for the "low concentration" polymer solution, Cp = 500 - 1000 mg / L. Among them, during the alternating injection of the second polymer solution 11 and water, the "water / polymer" volume ratio is 0.05 - 0.15.

[0065] Step 5: After the second-stage polymer flooding is completed, perform secondary water flooding on the parallel cores, and measure and determine the water flooding recovery rate of the parallel cores.

[0066] In the present invention, the method for performing secondary water flooding on the parallel cores includes: injecting water into the parallel cores at a constant pressure with a predetermined injection pressure from the injection end of the parallel cores. During the water injection process, regularly collect the produced fluids from each small layer at the production end of the parallel cores, measure and stop water injection when the water cut of the produced fluid reaches a predetermined water cut, and measure and determine the subsequent water flooding recovery rate of each small layer of the parallel cores.

[0067] In an embodiment of the present invention, after the second-stage polymer flooding is completed, connect the inlet of the constant pressure pump 1 to a water source, and the outlet of the pump is respectively connected to the core holders of the heterogeneous core 4 in the layer with separate injection and production and the heterogeneous core 5 in the blank layer through the fourth valve and the fifth valve. A pressure sensor 6 is connected to the outlet pipeline of the constant pressure pump 1.

[0068] During the secondary water flooding, close the fourth valve and the fifth valve, start the constant pressure pump 1, detect the real-time pressure in the pipeline through the pressure sensor 6. When the real-time pressure rises to the predetermined injection pressure (2P), control the fourth valve and the fifth valve to open, and inject water into the injection ends of the heterogeneous core 4 in the layer with separate injection and production and the heterogeneous core 5 in the blank layer. During the injection process, keep the injection pressure of the water in the pipeline unchanged at the predetermined injection pressure and inject at a constant pressure. During the injection process, regularly collect the produced fluids from each small layer at the production ends of the heterogeneous core 4 in the layer with separate injection and production and the heterogeneous core 5 in the blank layer, measure and end the secondary water flooding when the water cut reaches 98%, and measure and calculate the subsequent water flooding recovery rate and the split ratio of the produced fluids of the heterogeneous cores during the secondary water injection process.

[0069] In an embodiment of the present invention, during the primary water injection, the first-stage polymer flooding, the second-stage polymer flooding, and the secondary water flooding processes, the time for regularly collecting the produced fluids is once every 30 minutes.

[0070] Remake heterogeneous cores according to the core porosity, permeability and heterogeneity of the target well, and conduct the original general injection oil displacement experiment on the heterogeneous cores. During the experiment, regularly collect the produced fluid and calculate the recovery rate. The recovery rates obtained from the general injection experiment and the step-by-step polymer injection experiment of the present invention are shown in Table 2 below. In Table 2, the primary water flooding recovery rate is the water flooding recovery rate, the polymer flooding recovery rates in the first and second stages are the sum of the polymer flooding recovery rates in the first stage and the second stage, and the secondary water flooding recovery rate is the subsequent water flooding recovery rate.

[0071] Table 2 Recovery Rates of General Polymer Injection Experiment and Step-by-Step Polymer Injection Experiment

[0072]

[0073]

[0074] The experimental data on the influence of the injection method, i.e., the opening time of the low-permeability part, on the recovery rates in each stage of polymer flooding are shown in Table 2. It can be seen from Table 2 that compared with the "general injection" method, after adopting the "step-by-step polymer injection" method, as the opening time of the low-permeability part is postponed, the increase in polymer flooding recovery rate shows a trend of "increasing first and then decreasing". Compared with the "general injection" method, the total increase in recovery rate of the "step-by-step polymer injection" method is higher. Thus, it can be seen that under the condition of the same polymer solution and slug size, the oil-increasing effect of the "step-by-step polymer injection" method in polymer flooding is better than that of the "general injection" method.

[0075] During the primary water flooding, the first-stage polymer flooding, the second-stage polymer flooding, and the secondary water flooding processes, regularly collect and record the injection pressure, the injection and produced fluid volumes of each small layer of the heterogeneous core, calculate the recovery rate, water cut, and small layer diversion rate of the heterogeneous core, and plot the linear relationships between the injection pressure, water cut, and recovery rate of the heterogeneous core and the injected volume PV number, as well as the relationship curve between the small layer diversion rate and the injected volume PV number.

[0076] The relationship between the injection pressure and the injected volume PV number during the implementation of the general injection method (Scheme 1-1) and the step-by-step polymer injection method (Scheme 1-2) of the present invention is shown in Figure 2 , Figure 2 It can be shown that the injection pressure was limited during the experiment of the present invention.

[0077] The relationship between the water cut and the injected volume PV number is shown in Figure 3 , Figure 3 It can be shown that the water cut can be reduced during the polymer flooding stage.

[0078] The relationship between the recovery rate and the injected volume PV number is shown in Figure 4 , Figure 4 It can be shown that the recovery rate of the target reservoir is improved through the step-by-step polymer injection method.

[0079] FromFigures 2 to 4 It can be seen that during the water flooding stage, the injection pressures of both schemes show a trend of "decreasing - tending to be stable", the water cut gradually increases, and the recovery factor shows a trend of "rapidly increasing first - then flattening out". At the initial stage of polymer flooding in the first stage, during the "constant rate" injection process, the injection pressure shows a rapid upward trend. When the injection pressure rises to 2P (the injection pressure at the end of water flooding), and then switches to "constant pressure (2P)" injection, the water cut shows a trend of "decreasing - increasing" and the recovery factor shows an obvious increasing trend. During the subsequent water flooding stage, both the water cut and the recovery factor show a trend of "increasing - stabilizing".

[0080] The relationship between the split - flow rate at the injection end of heterogeneous cores and the number of PVs of injected volume for the general injection method (Scheme 1 - 1) and the step - by - step polymer injection method of the present invention (Scheme 1 - 2) is shown in Figure 5 ;

[0081] The relationship between the split - flow rate at the production end of heterogeneous cores and the number of PVs of injected volume for the general injection method (Scheme 1 - 1) and the step - by - step polymer injection method of the present invention (Scheme 1 - 2) is shown in Figure 6 .

[0082] From Figure 5 and Figure 6 it can be seen that during the water flooding stage, the split - flow rate at the injection end of the high - permeability layer is less than that at the production end, while the split - flow rate at the injection end of the medium - low permeability layer is greater than that at the production end, indicating that there is an "inter - permeability" phenomenon among the various permeable layers of the core during water flooding, that is, part of the water entering the medium - low permeability layer from the injection end will turn inside the core and enter the high - permeability layer, resulting in an increase in the split - flow rate of the high - permeability layer at the production end and a decrease in the split - flow rate of the medium - low permeability layer. Compared with the "general injection" method, the split - flow rate of the high - permeability layer is higher during the polymer flooding stage after polymer injection by the "step - by - step polymer injection" method, and the split - flow rate of the medium - low permeability layer is higher during the subsequent water flooding stage, indicating that this injection method has a better effect on fluid diversion.

[0083] During the experiment of the present invention, constant - pressure or constant - rate injection was carried out during the injection process, truly simulating the step - by - step polymer injection process of the target well. The experimental results show that the step - by - step polymer injection method for heterogeneous reservoirs helps to improve the polymer flooding development effect of heterogeneous oil reservoirs. Before polymer injection, first close the medium - low permeability layer, and only inject the first polymer solution 10 with high retention ability (high concentration or high molecular weight) into the high - permeability layer. After completing the predetermined first injection volume, then open the medium - low permeability layer and inject the polymer solution with low retention ability (low concentration or low molecular weight) to achieve "step - by - step polymer injection". Adopting the "step - by - step polymer injection" method can ensure that the polymer solution with high retention ability in the first stage only enters the high - permeability layer, and in the second stage, most of the polymer solution with low retention ability only enters the medium - low permeability layer (because the high - permeability layer has inhaled a large amount of polymer solution with high retention ability in the previous stage, its seepage resistance increases greatly, and the suction start - up pressure is higher, so the suction pressure difference and suction volume are smaller), ultimately achieving the effect of delaying the "reversal of the fluid absorption profile" speed and expanding the swept volume.

[0084] It can be understood that, without violating the principle logic, the above-mentioned various method embodiments mentioned in the present disclosure can be combined with each other to form an embodiment after combination. Due to space limitations, the present disclosure will not elaborate further.

[0085] The embodiments of the present disclosure have been described above. The above description is exemplary and not exhaustive, and is also not limited to the disclosed embodiments. Many modifications and variations are obvious to those of ordinary skill in the art in the technical field without departing from the scope and spirit of the described embodiments. The choice of terms used herein is intended to best explain the principles of the embodiments, the practical application, or the technical improvement of the technology in the market, or to enable other ordinary technical personnel in the technical field to understand the embodiments disclosed herein.

Claims

1. An experimental method for improving oil recovery by stepwise polymer injection in heterogeneous reservoirs, characterized in that, it includes the steps: Obtain the core porosity, permeability and heterogeneity of the target reservoir, and prepare heterogeneous cores according to the core porosity, permeability and heterogeneity. Among them, the heterogeneous cores include: heterogeneous cores within the separate injection and production layers and heterogeneous cores within the blank layer; the average permeability of the heterogeneous cores within the separate injection and production layers is relatively high, and the average permeability of the heterogeneous cores within the blank layer is relatively low; Pretreat the heterogeneous cores, form parallel cores with the pretreated heterogeneous cores, perform a primary water drive on the parallel cores, and measure and determine the water drive oil recovery rate of the heterogeneous cores; After the primary water drive is completed, perform a first-stage polymer drive on the heterogeneous cores within the separate injection and production layers in the parallel cores. Inject the first polymer solution at a predetermined initial pressure and a predetermined second flow rate into the injection end of the heterogeneous cores within the separate injection and production layers at a constant speed. When it is judged that the real-time injection pressure when injecting the first polymer is equal to the predetermined injection pressure, maintain a constant pressure injection at the predetermined injection pressure until the injection volume of the first polymer solution is equal to the predetermined first injection volume. During the injection process, regularly collect the produced fluids from each small layer at the production end of the heterogeneous cores within the separate injection and production layers, and measure and determine the first-stage polymer drive oil recovery rate of the heterogeneous cores within the separate injection and production layers; the first polymer solution has a high retention capacity; After the first-stage polymer drive is completed, perform a second-stage polymer drive on the parallel cores. Inject the second polymer solution into the injection end of the parallel cores at a constant pressure with the predetermined injection pressure until the injection volume of the second polymer solution is equal to the predetermined second injection volume. During the injection of the second polymer solution, regularly collect the produced fluids from each small layer at the production end of the parallel cores, and measure and determine the second-stage polymer drive oil recovery rate of the parallel cores; the second polymer solution has a low retention capacity; After the second-stage polymer drive is completed, perform a secondary water drive on the parallel cores, and measure and determine the subsequent water drive oil recovery rate of the parallel cores; Remake heterogeneous cores according to the core porosity, permeability and heterogeneity of the target reservoir, perform a general injection displacement experiment on the heterogeneous cores, regularly collect the produced fluids during the experiment and calculate the oil recovery rate; compare and analyze the oil recovery rate obtained by stepwise polymer injection with the oil recovery rate obtained by general injection.

2. The experimental method for improving oil recovery by stepwise polymer injection in heterogeneous reservoirs according to claim 1, characterized in that, The method for pretreating the heterogeneous cores includes: Vacuum the heterogeneous cores for 6 - 12 hours, saturate them with formation water for 6 - 12 hours, inject simulated oil from the injection end of the heterogeneous cores under the reservoir temperature conditions of the target reservoir until no water comes out from the production end of the heterogeneous cores. After the injection of the simulated oil is completed, let the heterogeneous cores stand and age for 12 - 24 hours.

3. The experimental method for improving oil recovery by stepwise polymer injection in heterogeneous reservoirs according to claim 1, characterized in that, The method for performing a primary water drive on the parallel cores includes: Water is injected into the parallel cores at a constant speed at a predetermined first flow rate from the injection end of the parallel cores. During the water injection process, the water cut of the produced fluid from each small layer at the production end of the parallel cores is regularly collected and measured. When the water cut of the produced fluid reaches the predetermined water cut, the water injection is stopped, and the water flooding recovery factor of each small layer of the parallel cores is measured and determined.

4. The method for the experiment of improving the recovery factor by stepwise polymer injection in heterogeneous reservoirs according to claim 1, characterized in that: The first polymer solution is partially hydrolyzed polyacrylamide, and its relative molecular mass is: M = 1900 - 2500×10 4 ; the polymer concentration of the first polymer solution is: Cp = 1000 - 1500 mg / L; the predetermined first injection volume is: 0.1 PV - 0.3 PV.

5. The method for the experiment of improving the recovery factor by stepwise polymer injection in heterogeneous reservoirs according to claim 1, characterized in that: The second polymer solution is partially hydrolyzed polyacrylamide, and its relative molecular mass is: M < 500×10 4 ; the polymer concentration of the second polymer solution is: Cp = 1000 - 1500 mg / L; the predetermined second injection volume is: 0.3 PV - 0.5 PV.

6. The method for the experiment of improving the recovery factor by stepwise polymer injection in heterogeneous reservoirs according to any one of claims 1 - 5, characterized in that the method for secondary water flooding of the parallel cores includes: injecting water into the parallel cores at a constant pressure at a predetermined injection pressure from the injection end of the parallel cores. During the water injection process, the produced fluid from each small layer at the production end of the parallel cores is regularly collected, and the water injection is stopped when the water cut of the produced fluid reaches the predetermined water cut, and the subsequent water flooding recovery factor of each small layer of the parallel cores is measured and determined.

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