Method for determining effective remaining oil production technical limit in high water cut period of continental thick oil reservoir

Abstract

The application provides a method for determining technical limits of effective utilization of remaining oil in a high water cut stage of a terrestrial heavy oil reservoir, and belongs to the technical field of oilfield development. The method is based on reservoir physical properties and fluid properties of a target oilfield, and uses a combination of centrifugal experiments, oil displacement experiments and core scanning technology by nuclear magnetic resonance to analyze the relationship between the flow capacity of crude oil corresponding to different pore throat radii and displacement pressure gradient, and to reveal the micro oil displacement characteristics of nonlinear seepage of the terrestrial heavy oil. A large-small pore throat parallel model is established, and the Busey flow equation is applied to study the retention mechanism of crude oil in the small pore throat and the technical limits of utilization under the main forces in the micro condition, to determine the technical limits of effective utilization of micro remaining oil, to propose a strategy for tapping the potential of remaining oil in the high water cut stage, to greatly improve the recovery rate, to greatly improve the development effect in the high water cut stage, to further develop and enrich the theory of efficient development of the terrestrial heavy oil reservoir, and to provide technical support and theoretical guidance for formulating the development technology policy of the terrestrial heavy oil reservoir in the high water cut stage.

Description

Technical Field

[0001] This invention belongs to the field of oilfield development technology, and in particular relates to a method for determining the effective utilization limit of remaining oil in high water-cut phases of continental heavy oil reservoirs. Background Technology

[0002] After long-term water injection development of continental heavy oil reservoirs enters the high water-cut period, they gradually exhibit development characteristics such as low single-well productivity, rapid production decline, and deteriorating water drive effect. However, there is still a lot of residual oil in the oil layer, and the oilfield recovery rate has the potential to be further improved. It is urgent to deepen the adjustment of oilfield development and flow field control to tap the residual oil and improve the development effect of the oilfield in the high water-cut period.

[0003] Currently, methods for tapping remaining oil potential in high water-cut reservoirs are mainly based on understanding reservoir detailing, development effect evaluation, and remaining oil distribution. Further refinement and deepening of development layers, injection-production well networks, well types, and well spacing are needed. While this method integrates research findings from multiple disciplines and possesses strong comprehensiveness, field application, and experiential basis, it lacks methodological and theoretical guidance. It fails to fully consider the distribution mechanism and flow patterns of remaining oil in high water-cut heavy oil reservoirs, and does not propose effective utilization technology limits or rationally optimize injection-production relationships. Therefore, it is difficult to achieve the goal of accurately and effectively tapping remaining oil potential and significantly improving oilfield recovery. There is an urgent need to conduct research on the mechanism of remaining oil in high water-cut reservoirs and the effective utilization technology limits to provide a theoretical basis for formulating oilfield development technology policies.

[0004] To address the shortcomings of current technology, a method for determining the effective utilization limit of remaining oil in continental heavy oil reservoirs during the high water-cut period is proposed. This method will guide the research and implementation of development adjustment in continental heavy oil fields during the high water-cut period, thereby maximizing the development effect and benefits during this period. Summary of the Invention

[0005] In view of this, the present invention aims to propose a method for determining the effective utilization technology limit of remaining oil in continental heavy oil reservoirs during the high water-cut period. This method starts from the reservoir physical properties and fluid properties of the target oilfield, and uses a combination of centrifugation experiments, oil displacement experiments, and nuclear magnetic resonance core scanning technology to analyze the relationship between crude oil flow capacity and displacement pressure gradient corresponding to different pore throat radii, revealing the microscopic oil displacement characteristics of nonlinear seepage in continental heavy oil. A parallel model of large and small pore throats is established, and the Bergeau flow equation is applied, considering the main forces under microscopic conditions, to study the retention mechanism and utilization technology limit of crude oil in small pore throats, determining the effective utilization technology limit of microscopic remaining oil. Finally, a strategy for tapping the remaining oil potential during the high water-cut period is proposed, further significantly improving the recovery rate and greatly enhancing the development effect during the high water-cut period, thus further developing and enriching the theory of efficient development of continental heavy oil reservoirs.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows: a method for determining the effective utilization limit of remaining oil in high water-cut phases of continental heavy oil reservoirs, comprising the following steps:

[0007] Step 1: Select representative core samples and crude oil viscosities based on the reservoir properties, fluid properties, and development characteristics of the target oilfield;

[0008] Step 2: Conduct identification of core casting thin sections and analyze pore structure, pore type, and pore throat radius distribution characteristics;

[0009] Step 3: Conduct centrifugation and displacement experiments, measure T2 spectrum, analyze the relationship between oil displacement efficiency and displacement pressure gradient, and determine the relationship between crude oil flow capacity and displacement pressure gradient corresponding to different pore throat radii.

[0010] Step 4: Based on the understanding in Step 3, establish a parallel displacement model of large and small orifice throats, apply the Bergeau flow equation to study the critical displacement pressure gradient for effective utilization of microscopic residual oil in small orifice throats, apply the time iteration method to study the relationship between microscopic oil displacement efficiency and displacement pressure gradient in large and small orifice throats, and determine the technical limit for utilization of microscopic residual oil in high water cut period.

[0011] Step 5: With the goal of improving displacement pressure gradient and oil displacement efficiency, optimize the design of injection-production well network, well type, well spacing and injection-production pressure difference, analyze the changes in displacement pressure gradient, oil displacement efficiency and water drive recovery rate before and after adjustment, and propose measures to tap the remaining oil potential in the high water cut period.

[0012] Step 6: Evaluate the development effect of the remaining oil tapping measures implemented in Step 5.

[0013] Furthermore, in step 1, selecting representative core samples and crude oil viscosity means selecting core samples whose porosity, permeability, and size, as well as crude oil viscosity, can represent the target oil field and meet the requirements for casting thin section identification, centrifugation experiments, and displacement experiments.

[0014] Furthermore, in step 3, the centrifugation experiment refers to using a centrifuge to analyze the degree of separation of crude oil in the core at different centrifugation speeds, and to simulate the range of crude oil mobilization and oil displacement efficiency in the core under different displacement pressure differentials.

[0015] Furthermore, in step 3, the displacement experiment refers to a one-dimensional waterflooding experiment using core samples to simulate the relationship between oil displacement efficiency and cumulative injection volume under different displacement pressure gradients.

[0016] Furthermore, in step 3, the T2 spectrum is obtained by using nuclear magnetic resonance scanning core technology to study the distribution of core pore throat radius and its oil content variation.

[0017] Furthermore, in step 4, a parallel displacement model of large and small orifice throats is established. Considering the orifice throat structure, fluid properties, and the main forces under microscopic conditions, the Bergeau flow equation is applied to study the critical displacement pressure gradient for the movement of microscopic residual oil in the small orifice throat. The calculation formula is shown below:

[0018]

[0019] In formula (6):

[0020] G is the displacement pressure gradient, MPa / m;

[0021] σ represents interfacial tension, in mN / m;

[0022] θ1 is the wetting angle of the macropore throat fluid, in °;

[0023] θ2 is the wetting angle of the fluid in the orifice throat, in °;

[0024] r1 is the radius of the macropore throat, in μm;

[0025] r2 is the radius of the throat of the orifice, in μm;

[0026] L is the throat length, in μm;

[0027] L1 is the length of the macrothroat, in μm;

[0028] L2 is the length of the orifice throat, in μm;

[0029] μ w The viscosity of the aqueous phase is mPa·s;

[0030] μ o Crude oil viscosity, mPa·s;

[0031] v2 is the fluid velocity, in m / s;

[0032] l o , where is the length of crude oil in the small throat, in μm.

[0033] Furthermore, in step 4, the relationship between the microscopic oil displacement efficiency and the displacement pressure gradient of large and small pore throats is studied using the time iteration method to determine the effective utilization limit of microscopic residual oil in the high water cut period. The calculation formula is shown in the following formula:

[0034]

[0035] In formula (10):

[0036] η represents the oil displacement efficiency, in percentages.

[0037] L is the throat length, in μm;

[0038] S1 is the cross-sectional area of ​​the macropore throat, in μm.2 ;

[0039] S2 is the cross-sectional area of ​​the throat at the small opening, in μm. 2 ;

[0040] x2 is the length of the aqueous phase in the throat of the small pore, in μm.

[0041] Furthermore, at each iteration, the displacement pressure gradient in the small orifice throat is compared with the starting pressure gradient until the starting pressure gradient in the small orifice throat is greater than the starting pressure gradient, and the iteration ends. The oil displacement efficiency of the large and small orifice throats is calculated using formula (10).

[0042] Furthermore, in step 5, the well network, well type, and well spacing are optimized so that the displacement pressure gradient between injection and production wells is greater than the critical displacement pressure gradient obtained in step 4, which greatly improves the oil displacement efficiency and forms a typical development model for offshore terrestrial heavy oil reservoirs.

[0043] Furthermore, in step 6, the measures for tapping the remaining oil potential during the high water cut period proposed in step 5 are applied to the actual oilfield, and the changes in the displacement pressure gradient, oil displacement efficiency, and water drive recovery rate of the oilfield before and after the adjustment are compared.

[0044] Compared with existing technologies, the method for determining the effective utilization limit of remaining oil in high water-cut continental heavy oil reservoirs described in this invention has the following advantages: This invention proposes a method for determining the effective utilization limit of remaining oil in high water-cut continental heavy oil reservoirs, providing technical support and theoretical guidance for formulating development technology policies for high water-cut continental heavy oil reservoirs. It can significantly improve the recovery rate and enhance the development effect of high water-cut continental heavy oil reservoirs, and is applicable to the development of heavy oil fields using water injection in continental areas. Attached Figure Description

[0045] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0046] Figure 1 This is a flowchart of a method according to an embodiment of the present invention;

[0047] Figure 2 This is a graph showing the T2 spectrum test results of an embodiment of the present invention;

[0048] Figure 3 This is a comparison chart of the cumulative oil content percentage of different pore throat radii after centrifugation compared to the initial state in an embodiment of the present invention;

[0049] Figure 4 This is a graph showing the relationship between oil displacement efficiency and displacement pressure difference for different pore throats in embodiments of the present invention.

[0050] Figure 5This is a graph showing the relationship between oil displacement efficiency and displacement ratio for different displacement pressure differentials in embodiments of the present invention.

[0051] Figure 6 This is a schematic diagram of the parallel connection of large and small orifice throats in an embodiment of the present invention (where a is the actual model and b is the simplified model);

[0052] Figure 7 This is a graph showing the relationship between the critical displacement pressure gradient and the pore throat radius in an embodiment of the present invention.

[0053] Figure 8 This is a graph showing the relationship between oil displacement efficiency and displacement pressure gradient in an embodiment of the present invention.

[0054] Figure 9 This is a diagram of the injection-production well network adjustment mode according to an embodiment of the present invention (where a is an injection-production well network with alternating rows of directional and horizontal wells; b is a five-point injection-production well network with directional and horizontal wells). Detailed Implementation

[0055] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0056] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0057] like Figure 1 As shown, this invention provides a method for determining the effective utilization limit of remaining oil in continental heavy oil reservoirs during the high water-cut period. This method can improve the development effect of continental heavy oil reservoirs during the high water-cut period and is applicable to the development of heavy oil fields using water injection in continental facies. The method includes the following steps:

[0058] Step 1: Select representative core samples and crude oil viscosities based on the reservoir properties, fluid properties, and development characteristics of the target oilfield. Selecting representative core samples and crude oil viscosities means that the porosity, permeability, and size of the selected core samples and the crude oil viscosity are representative of the target oilfield and meet the requirements for casting thin section identification, centrifugation experiments, and displacement experiments.

[0059] Step 2: Conduct identification of core casting thin sections and analyze pore structure, pore type, and pore throat radius distribution characteristics;

[0060] Step 3: Conduct centrifugation and displacement experiments, measure T2 spectrum, analyze the relationship between oil displacement efficiency and displacement pressure gradient, and determine the relationship between crude oil flow capacity and displacement pressure gradient corresponding to different pore throat radii.

[0061] The centrifugation experiment refers to using a centrifuge to analyze the degree of separation of crude oil in the core at different centrifugation speeds, and to simulate the range of crude oil movement and oil displacement efficiency in the core under different displacement pressure differentials.

[0062] The displacement experiment refers to a one-dimensional water-drive oil displacement experiment using core samples to simulate the relationship between oil displacement efficiency and cumulative injection volume under different displacement pressure gradients.

[0063] The T2 spectrum refers to the use of nuclear magnetic resonance scanning core technology to study the distribution of core pore throat radius and its oil content variation.

[0064] Step 4: Based on the understanding in Step 3, establish a parallel displacement model of large and small orifice throats. Consider the orifice throat structure, fluid properties and the main forces under microscopic conditions. Apply the Bergeau flow equation to study the critical displacement pressure gradient for effective utilization of microscopic residual oil in small orifice throats. Apply the time iteration method to study the relationship between microscopic oil displacement efficiency and displacement pressure gradient in large and small orifice throats, and determine the technical limit for effective utilization of microscopic residual oil in high water cut period.

[0065] Specifically, the critical displacement pressure gradient for the movement of microscopic residual oil in the orifice throat is studied using the Bergeau flow equation, and the calculation formula is shown below:

[0066]

[0067] In formula (6):

[0068] G is the displacement pressure gradient, MPa / m;

[0069] σ represents interfacial tension, in mN / m;

[0070] θ1 is the wetting angle of the macropore throat fluid, in °;

[0071] θ2 is the wetting angle of the fluid in the orifice throat, in °;

[0072] r1 is the radius of the macropore throat, in μm;

[0073] r2 is the radius of the throat of the orifice, in μm;

[0074] L is the throat length, in μm;

[0075] L1 is the length of the macrothroat, in μm;

[0076] L2 is the length of the orifice throat, in μm;

[0077] μ w The viscosity of the aqueous phase is mPa·s;

[0078] μ o Crude oil viscosity, mPa·s;

[0079] v2 is the fluid velocity, in m / s;

[0080] l o , where is the length of crude oil in the small throat, in μm.

[0081] The relationship between the microscopic oil displacement efficiency and displacement pressure gradient of large and small orifice throats was studied using the time iteration method. The effective technical limit for the mobilization of microscopic residual oil in the high water-cut period was determined. The calculation formula is shown in the following formula:

[0082]

[0083] In formula (10):

[0084] η represents the oil displacement efficiency, in percentages.

[0085] L is the throat length, in μm;

[0086] S1 is the cross-sectional area of ​​the macropore throat, in μm. 2 ;

[0087] S2 is the cross-sectional area of ​​the throat at the small opening, in μm. 2 ;

[0088] x2 is the length of the aqueous phase in the throat of the small pore, in μm.

[0089] At each iteration, the displacement pressure gradient in the small orifice throat is compared with the starting pressure gradient until the starting pressure gradient in the small orifice throat is greater than the starting pressure gradient, and the iteration ends. The oil displacement efficiency of the large and small orifice throats is calculated using formula (10).

[0090] Step 5: With the goal of improving the displacement pressure gradient and oil displacement efficiency, optimize the design of injection-production well network, well type, well spacing, and injection-production pressure difference, analyze the changes in displacement pressure gradient, oil displacement efficiency, and waterflood recovery rate before and after adjustment, and propose measures to tap the remaining oil potential during the high water-cut period; specifically, optimize the well network, well type, and well spacing so that the displacement pressure gradient between injection and production wells is greater than the critical displacement pressure gradient obtained in Step 4, significantly improve oil displacement efficiency, and form a typical development model for offshore-continental heavy oil reservoirs.

[0091] Step 6: Evaluate the development effect of the remaining oil potential tapping measures proposed in Step 5. Specifically, apply the remaining oil potential tapping measures proposed in Step 5 to the actual oilfield and compare the changes in displacement pressure gradient, oil displacement efficiency, and waterflood recovery rate before and after the adjustment.

[0092] The core of the method for determining the effective utilization technology limit of remaining oil in high water-cut continental heavy oil reservoirs is to start from the reservoir physical properties and fluid properties of the target oilfield, and use a combination of centrifugation experiments, oil displacement experiments and nuclear magnetic resonance core scanning technology to analyze the relationship between crude oil flow capacity and displacement pressure gradient corresponding to different pore throat radii, revealing the microscopic oil displacement characteristics of nonlinear seepage in continental heavy oil. A parallel model of large and small pore throats is established, and the Bergeau flow equation is applied, considering the main forces under microscopic conditions, to study the retention mechanism and utilization technology limit of crude oil in small pore throats, determine the effective utilization technology limit of microscopic remaining oil, and finally propose a strategy for tapping the potential of remaining oil in high water-cut stages, further significantly improving the recovery rate, greatly improving the development effect in high water-cut stages, further developing and enriching the theory of efficient development of continental heavy oil reservoirs, and providing technical support and theoretical guidance for formulating technical policies for the development of continental heavy oil reservoirs in high water-cut stages.

[0093] To better understand the invention's content, features, and effects, the following embodiments are provided, along with detailed descriptions in conjunction with the accompanying drawings:

[0094] Step 1: Briefly describe the reservoir, fluid, and development characteristics of the target oilfield, as well as the selection of core and crude oil viscosity.

[0095] The Bohai Sea continental sandstone heavy oil fields are dominated by deltaic and fluvial facies deposits. The Suizhong 36-1 oilfield is a typical deltaic facies oilfield, while the Qinhuangdao 32-6 oilfield is a typical fluvial facies oilfield. The reservoirs are characterized by high porosity, high permeability, and strong heterogeneity, with an average porosity of 30% and an average permeability of 3000 mD. The formation crude oil is characterized by high density, high viscosity, high asphalt content, low sulfur content, low wax content, and low pour point. The viscosity of the underground crude oil ranges from 50 to 350 mPa·s, with an average of 120 mPa·s. Vertically, multiple oil-bearing strata are developed, with significant differences in reservoir type, reservoir properties, and fluid properties among the various strata. Initially, a single strata were developed using a directional well and a reverse nine-spot well pattern. After nearly 20 years of development, the field has gradually entered a high water-cut phase, exhibiting a large decline in production and a rapid increase in water cut. In response to the above situation, during the high water-cut period, a closed-loop core drilling operation was conducted in the adjustment well. The core sample selected in this study had a length of 4.76 cm, a diameter of 2.45 cm, a porosity of 35.8%, and a pore volume of 8.03 cm³. 3 The gas permeability was 2915×10⁻⁶. -3 μm 2 Three crude oil viscosity values ​​of 60 mPa·s, 120 mPa·s, and 180 mPa·s were selected; the initial oil saturation was established as 0.81.

[0096] Step 2: Briefly describe the results of the thin section analysis of the core casting. The pore structure is a fine sandy structure, containing approximately 82% fine sand and 18% medium sand, with uniform distribution of particles of each size. The rock pores are relatively well-developed, uniformly distributed, and have good connectivity. The pore types are mainly primary intergranular pores, a small number of intergranular dissolution pores, and a small number of intragranular dissolution pores; the main pore size range is 0.1–35 μm, the average pore throat radius is 22.0 μm, the pore throat radius is 25.0 μm at 50% mercury saturation, and the maximum pore throat radius is 56.0 μm.

[0097] Step 3: Briefly describe the research process and understanding of the centrifugation and displacement experiments. Using Mn... 2+ Ionized water flooding was used in experiments at different centrifugation speeds (corresponding to displacement pressure differences of 0.03 MPa to 2.50 MPa), and the T2 spectra were measured. The experimental T2 spectra results are as follows: Figure 2 As shown, it can be seen that due to Mn 2+ Ion-shielded NMR signals indicate the crude oil content in the core. With increasing centrifugation speed (displacement pressure difference), the signal intensity decreases overall, indicating a decrease in crude oil content. The signal exhibits a bimodal distribution, with the right peak corresponding to the main flow channel containing large pores, where fluid flows first and is the primary contributor to the mobile fluid flow. The core is divided into three regions: the right side experiences continuous fluid outflow from large pores; the middle region shows lower fluid extraction at low speeds but outflows through large pores at high speeds; and the left side exhibits an interactive process due to dynamic changes in capillary force and displacement pressure. When the displacement pressure difference is less than 1.0 MPa, the crude oil content in the core changes significantly with increasing displacement pressure difference; however, beyond 1.0 MPa, the change in crude oil content becomes smaller.

[0098] The relationship between the orifice throat radius and the relaxation time is:

[0099]

[0100] Using equation (1) Figure 2 By performing the transformation, the cumulative oil content percentage for different pore throat radii under various displacement pressure differentials can be obtained, such as... Figure 3 As shown, the T2 relaxation time corresponding to the movable critical pore radius is called the movable fluid T2 cutoff value -1ms, and the critical movable pore radius is approximately 50nm. Further analysis yields the curve of oil displacement efficiency as the displacement pressure difference increases, as shown below. Figure 4 As shown, it can be seen that:

[0101] ①As the displacement pressure differential increases, the oil displacement efficiency increases and the residual oil decreases;

[0102] ② Macropores (throat radius greater than 80 nm) are the mainstream channels, with an oil displacement efficiency of 84.2%; mesopores (throat radius 60 nm to 80 nm) are the next best, with an oil displacement efficiency of 54.2%; micropores (throat radius less than 60 nm) have an oil displacement efficiency of only 12.4% when the displacement pressure difference is 1 MPa. As the displacement pressure difference gradually increases to 2.5 MPa, the oil in the micropores can gradually overcome capillary resistance and flow out through the macropores, thereby improving the mobility of the oil phase in the micropores and increasing the oil displacement efficiency to 26.9%.

[0103] Further displacement experiments were conducted on crude oil with viscosities of 60 mPa·s, 120 mPa·s, and 180 mPa·s under different displacement pressure gradients. Water viscosity was 1 mPa·s, salinity was 3000 mg / L, and displacement pressure gradients ranged from 0.001 MPa / m to 0.050 MPa / m. The relationship curves between oil displacement efficiency and displacement ratio for different displacement rates were obtained, as shown below. Figure 5 As shown, it can be seen that:

[0104] ① The oil displacement efficiency decreases with increasing crude oil viscosity. The maximum oil displacement efficiencies for low, medium, and high viscosity crude oils are 69.4%, 57.7%, and 53.8%, respectively.

[0105] ② The oil displacement efficiency increases with the increase of displacement velocity. Within a lower pressure gradient range, the increase is greater for low-viscosity oil than for medium- and high-viscosity oils, while within a higher pressure gradient range, the increase is less for low-viscosity oils than for medium- and high-viscosity oils. Increasing the displacement pressure gradient is beneficial for improving the mobilization of medium- and high-viscosity oils. This is because as the displacement pressure difference increases, the flow conditions of crude oil in the smaller radius orifice throat are gradually reached, thereby increasing the oil displacement efficiency.

[0106] ③ When the displacement pressure difference is constant, the oil displacement efficiency increases with the increase of the displacement ratio. After the displacement ratio exceeds 30 PV, the increase in oil displacement efficiency of low viscosity oil with the increase of the displacement ratio is relatively small, while the increase in oil displacement efficiency of medium and high viscosity oil with the increase of the displacement ratio is relatively large. Increasing the displacement ratio is beneficial to the mobilization effect of medium and high viscosity oil.

[0107] Step 4 briefly describes the calculation and understanding of the critical displacement pressure gradient for the mobilization of microscopic residual oil during high water cut periods. As shown in the experiment of Step 3, during waterflooding development, the differences in displacement pressure, capillary force, and viscous force at orifice throats of different radii lead to differences in fluid flow capacity within these throats. Orifice throats with larger radii exhibit stronger fluid flow capacity and higher oil displacement efficiency, while those with smaller radii have weaker fluid flow capacity and lower oil displacement efficiency. Therefore, when the displacement pressure differential is insufficient to overcome the capillary force and viscous resistance of the small orifice throat, crude oil will remain trapped within it, forming residual oil.

[0108] Assuming the fluid in the pore throat is incompressible, the wettability of the pore throat remains constant, and the size of the pore throat remains constant. Crude oil in the pore throat is subjected to the combined effects of displacement pressure, capillary force, and viscous force. The condition for crude oil to flow in the pore throat is that the displacement pressure difference is greater than the sum of the capillary force and other resistances. When the displacement pressure difference is insufficient to overcome the capillary force and viscous resistance, the crude oil will remain in the pore throat.

[0109] Figure 6 This model represents the flow of crude oil through large and small orifice throats, connected in parallel with equal displacement pressure differences at both ends. The radius of the large orifice throat is r1, and the radius of the small orifice throat is r2. The length of the large orifice throat is L1, and the length of the small orifice throat is L2. Based on the force equilibrium of the fluid within the orifice throats, the following equations can be derived:

[0110]

[0111] In equation (2), ΔP is the pressure difference across the throat, MPa; μ o Oil phase viscosity, mPa·s; μ w ρ is the viscosity of the aqueous phase, mPa·s; x is the length of the aqueous phase in the pore throat; v is the fluid velocity; r is the pore throat radius, μm; σ is the interfacial tension; θ is the wetting angle; μ o ρ is the crude oil viscosity, mPa·s; L is the pore throat length, μm.

[0112] Compared to small orifice throats, the fluid velocity in large orifice throats is faster. After a large orifice throat breaks through and forms a water channel, the pressure difference between the parallel orifice throats is ΔP. ​​Only when ΔP can overcome the resistance in the small orifice throat can the remaining oil be driven.

[0113] After the large pore throat breaks through and forms a water channel, the forces acting on the fluid are:

[0114]

[0115] In equation (3), v1 is the flow velocity of the fluid in the large pore throat.

[0116] When water breaks through the large pore throat, it amplifies the interference with fluid flow in the small pore throat. Fluid flow in the large pore throat becomes dominant, therefore the formation displacement pressure gradient is considered equal to the displacement pressure gradient in the large pore throat. The formula for calculating the displacement pressure gradient in the large pore throat is:

[0117]

[0118] In the formula, G is the driving pressure gradient, MPa / m.

[0119] The fluid in the small throat experiences the following forces:

[0120]

[0121] In the formula, v2 is the flow velocity of the fluid in the orifice throat; r2 is the radius of the orifice throat, μm; l o The length of the oil phase in the small pore throat.

[0122] Combining equations (4) and (5), we can obtain the relationship between the displacement pressure gradient and the orifice throat radius as follows:

[0123]

[0124] The critical displacement pressure gradient is defined as the displacement pressure gradient that allows fluid to just flow in the orifice throat. The critical displacement pressure gradient corresponding to different orifice throat radii can be calculated by equation (6).

[0125] The relationship curves between the critical displacement pressure gradient and the orifice throat radius for crude oil viscosities of 60 mPa·s, 120 mPa·s, and 180 mPa·s were calculated using equation (6), as follows: Figure 7 As shown, it can be seen that:

[0126] ① The critical driving pressure gradient increases with the increase of crude oil viscosity. Under the condition of average pore throat radius (35μm), the critical driving pressure gradients corresponding to low, medium and high viscosity crude oils are 0.029MPa / m, 0.058MPa / m and 0.088MPa / m, respectively.

[0127] ②With a fixed crude oil viscosity, the critical driving pressure gradient increases as the orifice throat radius decreases. Therefore, the utilization rate of crude oil in the orifice throat can be increased by increasing the driving pressure gradient, thereby increasing the oil displacement efficiency.

[0128] Next, the time iteration method was used to study the variation law of oil displacement efficiency and displacement pressure gradient between large and small orifice throats.

[0129] The seepage velocity in the macroporous throat is:

[0130]

[0131] The seepage velocity in the small orifice throat is:

[0132]

[0133] In equations (7) and (8), K1 is the macropore throat permeability, mD; K2 is the micropore throat permeability, mD; G1 is the macropore throat initiation pressure gradient, MPa / m; and G2 is the micropore throat initiation pressure gradient, MPa / m.

[0134] Since the displacement pressure gradient between the large and small pore throats remains constant, the sum of the seepage velocities between the large and small pore throats is a constant, which can be expressed as:

[0135] ν=ν1+ν2 (9)

[0136] As displacement proceeds, the water saturation of both large and small pore throats continuously increases, the seepage resistance gradually decreases, and the displacement pressure difference ΔP across the pore throats continuously decreases. When the displacement pressure difference decreases to the starting pressure difference of the small pore throat, residual oil will form in the small pore throat. The oil displacement efficiency of both large and small pore throats can be calculated as follows:

[0137]

[0138] In equation (10), η is the oil displacement efficiency; x2 is the length of the aqueous phase in the pore throat; and S2 is the cross-sectional area of ​​the pore throat (μm). 2 S1 is the cross-sectional area of ​​the macropore throat, in μm. 2 .

[0139] The calculation steps for oil displacement efficiency using large and small orifice throats are as follows:

[0140] ① Calculate the lengths of the water phases at time t for the large and small orifice throats:

[0141] x1(t)=x1(t-1)+ν1Δt (11)

[0142] x2(t)=x2(t-1)+ν2Δt (12)

[0143] ② Calculate the seepage velocity v1 of the large pore throat at time t using equation (7);

[0144] ③ Calculate the seepage velocity v2 of the small orifice throat at time t using equation (9);

[0145] ④ Calculate the pressure difference ΔP between the large and small orifice throats at time t using equation (8);

[0146] ⑤ Calculate the displacement pressure gradient of the small throat:

[0147]

[0148] ⑥ Compare the displacement pressure gradient of the small orifice throat with the starting pressure gradient. If the displacement pressure gradient is greater than the starting pressure gradient, repeat steps ①-⑤; otherwise, use equation (10) to calculate the oil displacement efficiency. Further calculations yield the oil displacement efficiency versus displacement pressure gradient curves corresponding to formation crude oil viscosities of 60 mPa·s, 120 mPa·s, and 180 mPa·s, as shown below. Figure 8 As shown, it can be seen that:

[0149] (1) When the driving pressure gradient is constant, the oil displacement efficiency increases as the crude oil viscosity decreases. The increase in oil displacement efficiency in the low viscosity range is greater than that in the high viscosity range. This is because when the viscosity is low, the critical driving pressure gradient is small, and the remaining oil in the small orifice throat is easy to reach the flow conditions.

[0150] (2) When the viscosity of crude oil is constant, the oil displacement efficiency increases with the increase of the driving pressure gradient. In the middle section, the oil displacement efficiency increases faster because the driving pressure gradient is greater than the critical driving pressure gradient, and the remaining oil in the small orifice throat is utilized.

[0151] Step 5: Briefly describe the remaining oil potential tapping strategy during the high water-cut stage. After years of development, the main heavy oil fields in the Bohai Sea continental facies have entered a high water-cut stage, posing a significant challenge to maintaining stable production. Based on the geological and reservoir characteristics of the Bohai Sea continental facies oil fields, the well network, well type, and well spacing are optimized to ensure that the displacement pressure gradient between injection and production wells is greater than the critical displacement pressure gradient, significantly improving oil displacement efficiency and forming two typical development models for offshore continental heavy oil reservoirs: a staggered injection-production well network of directional and horizontal wells. Figure 9 -a) Five-point injection-production well network of directional and horizontal wells ( Figure 9 -b).

[0152] Step 6: Briefly describe the effects of adjusting the injection and production well network.

[0153] Suizhong 36-1 oilfield is the first large-scale, integrated deltaic heavy oil field in the Bohai Sea, with an average permeability of 2000 mD and a formation crude oil viscosity of 30–450 mPa·s, averaging 185 mPa·s. Initially, it employed a single development system with a reverse nine-point well pattern and a well spacing of 360 m between injection and production wells, and commenced production in November 2000. During the high water-cut period, Suizhong 36-1 oilfield adopted a staggered well pattern with horizontal wells for separate production and directional wells for separate injection. After adjustment, the displacement pressure gradient increased from 0.04 MPa / m to 0.11 MPa / m, the oilfield production rate increased from 1.1% to 1.8%, and the recovery rate increased from 24.0% to 39.2%.

[0154] Qinhuangdao 32-6 oilfield is the first large fluvial heavy oilfield in the Bohai Sea. This oilfield has a complex oil-water relationship, low structural amplitude, an average permeability of 3000 mD, and a formation crude oil viscosity of 78–260 mPa·s, with an average of 220 mPa·s. Initially, a single development system with a reverse nine-point well pattern and a well spacing of 360 m was adopted, and it went into production in October 2001. During the high water-cut period, a five-point well pattern with horizontal wells for separate production and directional wells for separate injection was adopted. After adjustment, the displacement pressure gradient increased from 0.05 MPa / m to 0.10 MPa / m, the oilfield production rate increased from 0.8% to 1.8%, and the recovery rate increased from 21.5% to 35.6%.

[0155] Although preferred embodiments of the present invention have been described above in conjunction with the accompanying drawings, the present invention is not limited to the specific embodiments described above. The specific embodiments described above are merely illustrative and not restrictive. Those skilled in the art can make many other modifications under the guidance of the present invention without departing from the spirit and scope of the claims, and all of these modifications are within the scope of protection of the present invention.

Claims

1. A method for determining the effective recovery technology limit of remaining oil in high water-cut phases of continental heavy oil reservoirs, characterized in that, Includes the following steps: Step 1: Select representative core samples and crude oil viscosities based on the reservoir properties, fluid properties, and development characteristics of the target oilfield; Step 2: Conduct thin section identification of the core casting, and analyze the pore structure, pore type, and pore throat radius distribution characteristics; Step 3: Conduct centrifugation and displacement experiments, measure T2 spectrum, analyze the relationship between oil displacement efficiency and displacement pressure gradient, and determine the relationship between crude oil flow capacity and displacement pressure gradient corresponding to different pore throat radii. Step 4: Based on the understanding in Step 3, establish a parallel displacement model of large and small orifice throats, apply the Bergeau flow equation to study the critical displacement pressure gradient for effective utilization of microscopic residual oil in small orifice throats, apply the time iteration method to study the relationship between microscopic oil displacement efficiency and displacement pressure gradient in large and small orifice throats, and determine the technical limit for utilization of microscopic residual oil in high water cut period. In this study, a parallel displacement model of large and small orifice throats was established. Considering the orifice throat structure, fluid properties, and the main forces under microscopic conditions, the critical displacement pressure gradient for the movement of microscopic residual oil in the small orifice throat was studied using the Bergeau flow equation. The calculation formula is shown in the following equation: （6） In formula (6): G is the displacement pressure gradient, MPa / m; σ represents interfacial tension, in mN / m; θ1 is the wetting angle of the macropore throat fluid, in °; θ2 is the wetting angle of the fluid in the orifice throat, in °; r1 is the radius of the macropore throat, in μm; r2 is the radius of the throat of the orifice, in μm; L is the throat length, in μm; L1 is the length of the macrothroat, in μm; L2 is the length of the orifice throat, in μm; μ w The viscosity of the aqueous phase is mPa·s; μ o Crude oil viscosity, mPa·s; v2 is the fluid velocity, in m / s; l o The length of crude oil in the small-hole throat, in μm; The relationship between the microscopic oil displacement efficiency and displacement pressure gradient of large and small orifice throats was studied using the time iteration method. The effective technical limit for the mobilization of microscopic residual oil in the high water-cut period was determined. The calculation formula is shown in the following formula: （10） In formula (10): η is the oil displacement efficiency, % L is the throat length, in μm; S1 is the cross-sectional area of ​​the macropore throat, in μm. 2 ; S2 is the cross-sectional area of ​​the throat at the small opening, in μm. 2 ; x2 is the length of the aqueous phase in the throat of the orifice, in μm; At each iteration, the displacement pressure gradient in the small orifice throat is compared with the starting pressure gradient until the starting pressure gradient in the small orifice throat is greater than the starting pressure gradient, and the iteration ends. The oil displacement efficiency of the large and small orifice throats is calculated using formula (10). Step 5: With the goal of improving displacement pressure gradient and oil displacement efficiency, optimize the design of injection-production well network, well type, well spacing and injection-production pressure difference, analyze the changes in displacement pressure gradient, oil displacement efficiency and water drive recovery rate before and after adjustment, and propose measures to tap the remaining oil potential in the high water cut period. Step 6: Evaluate the development effect of the remaining oil tapping measures implemented in Step 5.

2. The method for determining the effective recovery limit of remaining oil in the high water-cut stage of continental heavy oil reservoirs according to claim 1, characterized in that: In step 1, selecting representative core samples and crude oil viscosity means that the porosity, permeability, and size of the selected core samples, as well as the crude oil viscosity, are representative of the target oil field and meet the requirements for casting thin section identification, centrifugation experiments, and displacement experiments.

3. The method for determining the effective recovery limit of remaining oil in the high water-cut stage of continental heavy oil reservoirs according to claim 1, characterized in that: In step 3, the centrifugation experiment refers to using a centrifuge to analyze the degree of separation of crude oil in the core at different centrifugation speeds, and to simulate the range of crude oil mobilization and oil displacement efficiency in the core under different displacement pressure differentials.

4. The method for determining the effective recovery limit of remaining oil in the high water-cut stage of continental heavy oil reservoirs according to claim 3, characterized in that: In step 3, the displacement experiment refers to a one-dimensional waterflooding and oil displacement experiment using core samples to simulate the relationship between oil displacement efficiency and cumulative injection volume under different displacement pressure gradients.

5. The method for determining the effective recovery limit of remaining oil in the high water-cut stage of continental heavy oil reservoirs according to claim 3, characterized in that: In step 3, the T2 spectrum is obtained by using nuclear magnetic resonance scanning core technology to study the distribution of core pore throat radius and its oil content variation.

6. The method for determining the effective recovery limit of remaining oil in the high water-cut stage of continental heavy oil reservoirs according to claim 1, characterized in that: In step 5, the well network, well type, and well spacing are optimized to make the displacement pressure gradient between injection and production wells greater than the critical displacement pressure gradient obtained in step 4, thereby significantly improving oil displacement efficiency and forming a typical development model for offshore terrestrial heavy oil reservoirs.

7. The method for determining the effective recovery limit of remaining oil in the high water-cut stage of continental heavy oil reservoirs according to claim 1, characterized in that: In step 6, the measures for tapping the remaining oil potential during the high water cut period proposed in step 5 are applied to the actual oilfield, and the changes in the displacement pressure gradient, oil displacement efficiency and water drive recovery rate of the oilfield before and after the adjustment are compared.

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