A method for screening polymer flooding systems suitable for use in medium-permeability oil reservoirs

Abstract

This invention relates to the field of oilfield development technology, specifically to a method for screening low-viscosity, high-molecular-weight polymer flooding systems suitable for medium-permeability oil reservoirs. The method includes: plotting viscosity-concentration curves for various polymers with different molecular weights; determining the concentration of each polymer solution under the viscosity requirements of the target reservoir based on the viscosity-concentration curves; then measuring the drag coefficient and residual drag coefficient of the corresponding concentration polymer solution in core samples with different permeability levels to obtain a permeability-drag coefficient curve; and, based on the permeability of the target reservoir, identifying one or more polymers that meet the permeability requirements, and determining the polymer solution with the highest residual drag coefficient as the preferred high-molecular-weight polymer. The method of this invention can rapidly screen polymers suitable for the target oil reservoir from a variety of polymers with different molecular weights, and the experimental method is simple and has good field applicability.

Description

Technical Field

[0001] This invention relates to the field of oilfield development technology, and specifically to a method for screening low-viscosity, high-molecular-weight polymer flooding systems suitable for medium-permeable oil-bearing reservoirs. Background Technology

[0002] Both laboratory experiments and oilfield practices demonstrate that selecting a polymer that matches the reservoir conditions is a prerequisite for successful polymer flooding. Different oilfields generally have varying formation conditions, necessitating the selection of different polymers based on these specific circumstances. When reservoir permeability is low, choosing a polymer with a larger molecular weight and higher concentration can effectively improve the oil-water mobility ratio, but it can also cause severe formation blockage, negatively impacting the polymer flooding effect. Therefore, selecting a suitable polymer based on the actual formation conditions is crucial for ensuring the smooth operation of polymer flooding.

[0003] Medium-permeability reservoirs have low permeability and porosity, are thin, and have poor connectivity. To ensure good polymer injectability, the polymer solution must penetrate as much of the reservoir pores as possible. This requires that the injected polymer molecular weight and concentration not be too high, and that the polymer molecules have a limited capacity to damage the medium-permeability reservoir, in order to meet the technical requirements for further improving recovery after polymer flooding in medium-permeability reservoirs. However, reducing the polymer molecular weight and concentration will correspondingly worsen the polymer flooding effect. Furthermore, the poor conditions of medium-permeability reservoirs increase the risks associated with polymer flooding.

[0004] Therefore, based on current research, it is an urgent problem to solve in order to conduct polymer flooding in medium-permeability reservoirs, clarify the compatibility between polymer molecular weight and medium-permeability cores, and then screen polymers.

[0005] In their study, "Screening and Application of Salt-Resistant Polymer Oil Displacement System", Han Peihui et al. selected five salt-resistant polymer varieties and screened out the LH2500 salt-resistant polymer variety with the best performance based on performance indicators such as viscosity, anti-adsorption, and oil displacement effect.

[0006] Chinese invention patent CN105651651B discloses a method for screening surfactants in polydimer-based binary flooding oil. The method includes the following steps: preparing different polydimer-based binary flooding solutions, wherein the different solutions are identical except for the type of surfactant; conducting comprehensive performance tests on the different prepared polydimer-based binary flooding solutions, wherein the comprehensive performance tests include interfacial tension, wettability, emulsifying properties, oil-washing ability, thermal stability, and adsorption stability; and selecting the surfactant that best meets the requirements of the comprehensive performance tests for use in the polydimer-based binary flooding oil.

[0007] The methods described above are labor-intensive and time-consuming, and are not suitable for screening multiple polymer flooding systems. Summary of the Invention

[0008] To address the aforementioned problems, this invention provides a method for screening polymer flooding systems suitable for medium-permeability oil reservoirs. The method described in this invention can rapidly screen polymers suitable for the target oil reservoir from a variety of polymers with different molecular weights. The experimental method is simple and has good field applicability.

[0009] To achieve the above objectives, the present invention adopts the following technical solution:

[0010] This invention provides a method for screening polymer flooding systems suitable for medium-permeability oil reservoirs, comprising the following steps:

[0011] S1. Prepare polymer solutions of different concentrations for the various polymers to be screened, and measure the viscosity of the polymer solutions; plot the viscosity-concentration relationship curves for each polymer.

[0012] S2. Based on the viscosity requirements of the target reservoir, and according to the viscosity-concentration relationship curves of each polymer obtained in step S1, determine the required concentration of each polymer solution and prepare polymer solutions of the corresponding concentrations.

[0013] S3. Determine the resistance coefficient and residual resistance coefficient of the various polymer solutions prepared in step S2 in cores with different permeability, and plot the core permeability-resistance coefficient relationship curve. Determine the core permeability corresponding to the abrupt change point of the slope of each curve as the minimum value of the applicable core permeability range for each polymer.

[0014] S4. Based on the target reservoir permeability and the minimum value of the applicable core permeability range for each polymer determined in step 3, select the polymer whose minimum applicable core permeability range is greater than the target reservoir permeability.

[0015] The method of the present invention also includes a screening step for the optimal polymer. Among the polymers selected from a variety of applicable core permeability ranges whose minimum value is greater than the target reservoir permeability, the polymer with the largest residual resistance coefficient is the optimal polymer.

[0016] Compared with the prior art, the present invention has the following advantages:

[0017] (1) By measuring the resistance coefficient and residual resistance coefficient of the polymer, this invention can improve the screening speed of the lowest core permeability that the polymer system can access and reduce the workload.

[0018] (2) The relationship curve established by this invention can provide a more convenient preferred method for the use of similar reservoirs and similar polymer systems;

[0019] (3) The water, core samples and experimental temperature used in this invention can simulate the target oil reservoir, which improves the specificity and field applicability of the obtained preferred polymer.

[0020] Other advantages, objectives and features of the present invention will become apparent in part from the following description, and in part from those skilled in the art through study and practice of the invention. Attached Figure Description

[0021] Figure 1 This is the viscosity-concentration curve of polymers with different molecular weights in Example 3 of the present invention.

[0022] Figure 2 This is a graph showing the permeability-resistance coefficient relationship of polymers with different molecular weights in Example 3 of the present invention. Detailed Implementation

[0023] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0024] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the exemplary embodiments of the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. Furthermore, it should be understood that when the terms "comprising" or "including" are used in this specification, they indicate the presence of features, steps, operations, and combinations thereof.

[0025] To address the shortcomings described in the background section, this invention provides a method for screening polymer flooding systems suitable for medium-permeability oil reservoirs, comprising the following steps:

[0026] S1. Prepare polymer solutions of different concentrations for the various polymers to be screened, and measure the viscosity of the polymer solutions; plot the viscosity-concentration relationship curves for each polymer.

[0027] S2. Based on the viscosity requirements of the target reservoir, and according to the viscosity-concentration relationship curves of each polymer obtained in step S1, determine the required concentration of each polymer solution and prepare polymer solutions of the corresponding concentrations.

[0028] S3. Determine the resistance coefficient and residual resistance coefficient of the various polymer solutions prepared in step S2 in cores with different permeability, and plot the core permeability-resistance coefficient relationship curve. Determine the core permeability corresponding to the abrupt change point of the slope of each curve as the minimum value of the applicable core permeability range for each polymer.

[0029] S4. Based on the target reservoir permeability and the minimum value of the applicable core permeability range for each polymer determined in step 3, select the polymer whose minimum applicable core permeability range is greater than the target reservoir permeability.

[0030] The method of the present invention also includes screening for the optimal polymer. Among the polymers selected from a variety of applicable core permeability ranges whose minimum value is greater than the target reservoir permeability, the polymer with the largest residual resistance coefficient is the optimal polymer.

[0031] As a preferred technical solution, the polymers to be screened in step S1 have a molecular weight range of 10 million to 25 million.

[0032] As a preferred technical solution, in step S1, polymer solutions with a concentration range of 500–2500 mg / L are prepared for each polymer.

[0033] As a preferred technical solution, the method for determining the drag coefficient and residual drag coefficient in step S3 is as follows:

[0034] Formation water was injected into a homogeneous core at a flow rate of 0.5–0.8 mL / min. The water-drive steady-state pressure in the core was measured, with a displacement of at least 2 PV. The flow rate and steady-state pressure were recorded during the test. The flow rate was then varied to inject formation water into the core at 1.0–1.2 mL / min and 1.5–2.0 mL / min, respectively. The water-drive steady-state pressure in the core was measured, with a displacement of at least 2 PV. The flow rate and steady-state pressure were recorded during the test.

[0035] The above steps were then repeated with each polymer solution and formation water, and the flow rate and steady-state pressure were recorded during the test.

[0036] The drag coefficient is the ratio of the stable injection pressure of the polymer solution to the stable water drive pressure, and the residual drag coefficient is the ratio of the subsequent stable water drive pressure to the stable water drive pressure. The result is the average value at three flow rates.

[0037] As a preferred technical solution, in step S3, the method for determining the abrupt change point of the curve's slope is as follows: take a point on the curve before and after the change in the curve's slope and draw a tangent line. The intersection of the two tangent lines is the abrupt change point of the curve.

[0038] To enable those skilled in the art to better understand the technical solution of the present invention, the technical solution of the present invention will be described in detail below with reference to specific embodiments.

[0039] Example 1

[0040] A method for screening polymer flooding systems suitable for medium-permeability oil reservoirs, comprising the following steps:

[0041] S1. Prepare polymer solutions of different concentrations for various polymers of different molecular weights to be screened, and measure the viscosity of the polymer solutions; plot the viscosity-concentration relationship curves for each polymer.

[0042] S2. Based on the viscosity requirements of the target reservoir, and according to the viscosity-concentration relationship curves of each polymer obtained in step S1, determine the required concentration of each polymer solution and prepare polymer solutions of the corresponding concentrations.

[0043] S3. Determine the resistance coefficient and residual resistance coefficient of the various polymer solutions prepared in step S2 in cores with different permeability, and plot the core permeability-resistance coefficient relationship curve. Determine the core permeability corresponding to the abscissa of the slope change point of each curve as the minimum value of the applicable core permeability range for each polymer.

[0044] The methods for determining the drag coefficient and residual drag coefficient are as follows:

[0045] Formation water was injected into a homogeneous core at a flow rate of 0.8 mL / min. The water drive stability pressure of the core was measured, with a displacement of at least 2 PV. The flow rate and stable pressure were recorded during the test. The flow rate was then varied to inject formation water into the core at 1.2 mL / min and 1.6 mL / min respectively. The water drive stability pressure of the core was measured, with a displacement of at least 2 PV. The flow rate and stable pressure were recorded during the test.

[0046] The above steps were then repeated with each polymer solution and formation water, and the flow rate and steady-state pressure were recorded during the test.

[0047] The drag coefficient is the ratio of the stable injection pressure of the polymer solution to the stable water drive pressure, and the residual drag coefficient is the ratio of the subsequent stable water drive pressure to the stable water drive pressure. The result is the average value at three flow rates.

[0048] The method for determining the abrupt change point of the slope of the curve is as follows: take a point on the curve before and after the change in slope and draw a tangent line. The intersection of the two tangent lines is the abrupt change point of the curve.

[0049] S4. Based on the target reservoir permeability and the minimum value of the applicable core permeability range for each polymer determined in step 3, select the polymer whose minimum applicable core permeability range is greater than the target reservoir permeability.

[0050] Example 2

[0051] A method for screening polymer flooding systems suitable for medium-permeability oil reservoirs, comprising the following steps:

[0052] S1. Prepare polymer solutions of different concentrations for various polymers of different molecular weights to be screened, and measure the viscosity of the polymer solutions; plot the viscosity-concentration relationship curves for each polymer.

[0053] S2. Based on the viscosity requirements of the target reservoir, and according to the viscosity-concentration relationship curves of each polymer obtained in step S1, determine the required concentration of each polymer solution and prepare polymer solutions of the corresponding concentrations.

[0054] S3. Determine the resistance coefficient and residual resistance coefficient of the various polymer solutions prepared in step S2 in cores with different permeability, and plot the core permeability-resistance coefficient relationship curve. Determine the core permeability corresponding to the abscissa of the slope change point of each curve as the minimum value of the applicable core permeability range for each polymer.

[0055] The methods for determining the drag coefficient and residual drag coefficient are as follows:

[0056] Formation water was injected into a homogeneous core at a flow rate of 0.5 mL / min. The water drive stability pressure of the core was measured, with a displacement of at least 2 PV. The flow rate and stable pressure were recorded during the test. The flow rate was then varied to inject formation water into the core at 1.0 mL / min and 1.5 mL / min respectively. The water drive stability pressure of the core was measured, with a displacement of at least 2 PV. The flow rate and stable pressure were recorded during the test.

[0057] The above steps were then repeated with each polymer solution and formation water, and the flow rate and steady-state pressure were recorded during the test.

[0058] The drag coefficient is the ratio of the stable injection pressure of the polymer solution to the stable water drive pressure, and the residual drag coefficient is the ratio of the subsequent stable water drive pressure to the stable water drive pressure. The result is the average value at three flow rates.

[0059] The method for determining the abrupt change point of the slope of the curve is as follows: take a point on the curve before and after the change in slope and draw a tangent line. The intersection of the two tangent lines is the abrupt change point of the curve.

[0060] S4. Based on the target reservoir permeability and the minimum value of the applicable core permeability range for each polymer determined in step 3, select the polymer whose minimum applicable core permeability range is greater than the target reservoir permeability; if multiple polymers are selected, the polymer solution with the largest residual resistance coefficient is the optimal polymer.

[0061] Example 3

[0062] In this embodiment, it is necessary to screen four polymers with molecular weights of 10 million, 15 million, 20 million, and 25 million to select the optimal molecular weight polymer suitable for polymer flooding of the target reservoir.

[0063] The specific steps of the screening method are as follows:

[0064] Step S1: First, using a polymer with a molecular weight of 10 million, a series of polymer solutions with concentrations of 500, 1000, 1500, 2000, and 2500 mg / L were prepared. The viscosity of the polymer solutions at different concentrations was measured using a Brookfield viscometer. Then, a viscosity-concentration curve for this polymer was obtained by plotting the polymer solution concentration on the x-axis and viscosity on the y-axis. Using polymers with molecular weights of 15 million, 20 million, and 25 million, the above method was followed to obtain viscosity-concentration curves for various high molecular weight polymers, as shown below. Figure 1 As shown.

[0065] Step S2: Since the target reservoir viscosity requirement is 10 mPa·s, determine the concentrations of each polymer solution corresponding to a viscosity of 10 mPa·s from the viscosity-concentration curve obtained in step S1. These concentrations are 2150 mg / L (molecular weight 10 million), 1350 mg / L (molecular weight 15 million), 1250 mg / L (molecular weight 20 million), and 1200 mg / L (molecular weight 25 million). See [link to relevant documentation]. Figure 1 Based on the determined four polymer concentrations, prepare new solutions of each polymer at these four concentrations.

[0066] Step S3: Measure the drag coefficient and residual drag coefficient of the four polymer solutions newly prepared in Step S2 in core samples of 100, 200, 300, and 500 mD respectively. Plot a permeability-drag coefficient curve with core permeability on the x-axis and drag coefficient on the y-axis. Determine the core permeability corresponding to the point of abrupt change in the slope of the curve; this is the minimum permeability range applicable to that polymer in the core sample. See [link to relevant documentation]. Figure 2 .

[0067] The methods for determining the drag coefficient and residual drag coefficient are as follows:

[0068] Formation water was injected into a homogeneous core at a flow rate of 0.5 mL / min. The water drive stability pressure of the core was measured, with a displacement of at least 2 PV. The flow rate and stable pressure were recorded during the test. The flow rate was then varied to inject formation water into the core at 1.0 mL / min and 1.5 mL / min respectively. The water drive stability pressure of the core was measured, with a displacement of at least 2 PV. The flow rate and stable pressure were recorded during the test.

[0069] The above steps were then repeated with polymer solution and formation water, and the flow rate and steady-state pressure were recorded during the test.

[0070] Based on the definitions of drag coefficient and residual drag coefficient and Darcy's formula, the drag coefficient can be obtained as the ratio of the injection steady pressure of the polymer system to the water drive steady pressure, and the residual drag coefficient is the ratio of the subsequent water drive steady pressure to the water drive steady pressure. The average value of the results at three flow rates is taken.

[0071] The method for determining the abrupt change point of the slope of the curve is as follows: take a point on the curve before and after the change in slope and draw a tangent line. The intersection of the two tangent lines is the abrupt change point of the curve.

[0072] Step S4: The target reservoir permeability is 300 mD. Select polymers whose minimum applicable core permeability range is greater than the target reservoir permeability of 300 mD. The polymers with molecular weights of 10 million, 15 million, and 20 million that meet the requirements of the applicable core permeability range are selected. From these three polymers, determine the polymer solution with the largest residual resistance coefficient. The corresponding polymer molecular weight is 20 million, which is the most preferred high molecular weight polymer.

[0073] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for screening polymer flooding systems suitable for medium-permeability oil reservoirs, characterized in that, Includes the following steps: S1. Prepare polymer solutions of different concentrations for various polymers of different molecular weights to be screened, and measure the viscosity of the polymer solutions. Plot the viscosity-concentration curves for each polymer; S2. Based on the viscosity requirements of the target reservoir, and according to the viscosity-concentration relationship curves of each polymer obtained in step S1, determine the required concentration of each polymer solution and prepare polymer solutions of the corresponding concentrations. S3. Determine the drag coefficient and residual drag coefficient of the various polymer solutions prepared in step S2 in core samples with different permeability, and plot the core permeability-drag coefficient relationship curve. The core permeability corresponding to the abrupt change point of the slope of each curve is determined as the minimum value of the applicable core permeability range for each polymer. The method for determining the abrupt change point of the slope of the curve is as follows: take a point on the curve before and after the change of the slope of the curve and draw a tangent line. The intersection of the two tangent lines is the abrupt change point of the curve. S4. Based on the target reservoir permeability and the minimum value of the applicable core permeability range for each polymer determined in step 3, select the polymer whose minimum applicable core permeability range is greater than the target reservoir permeability.

2. The method for screening polymer flooding systems suitable for medium-permeability oil reservoirs according to claim 1, characterized in that, The method also includes screening for the optimal polymer. Among the polymers selected from a variety of applicable core permeability ranges whose minimum value is greater than the target reservoir permeability, the polymer with the largest residual resistance coefficient is the optimal polymer.

3. The method for screening polymer flooding systems suitable for medium-permeability oil reservoirs according to claim 1, characterized in that, The polymers to be screened in step S1 have different molecular weights, ranging from 10 million to 25 million.

4. The method for screening polymer flooding systems suitable for medium-permeability oil reservoirs according to claim 1, characterized in that, In step S1, polymer solutions with a concentration range of 500~2500 mg / L are prepared for each polymer.

5. The method for screening polymer flooding systems suitable for medium-permeability oil reservoirs according to claim 1, characterized in that, In step S3, the methods for determining the drag coefficient and residual drag coefficient are as follows: Formation water was injected into a homogeneous core at a flow rate of 0.5–0.8 mL / min. The water-drive steady-state pressure in the core was measured, with a displacement of at least 2 PV. The flow rate and steady-state pressure were recorded during the test. The flow rate was then varied to inject formation water into the core at 1.0–1.2 mL / min and 1.5–2.0 mL / min, respectively. The water-drive steady-state pressure in the core was measured, with a displacement of at least 2 PV. The flow rate and steady-state pressure were recorded during the test. The above steps were then repeated with each polymer solution and formation water, and the flow rate and steady-state pressure were recorded during the test. The drag coefficient is the ratio of the stable injection pressure of the polymer solution to the stable water drive pressure, and the residual drag coefficient is the ratio of the subsequent stable water drive pressure to the stable water drive pressure. The result is the average value at three flow rates.

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