Method and system for determining shale oil reservoir matrix-fracture fluid channeling amount

Abstract

The invention provides a method and system for determining shale oil reservoir matrix-fracture fluid channeling capacity. The determining method comprises the steps that S1, the condition of a matrix-fracture interface is determined; s2, judging and calculating the relative size relationship between the fracture pressure and the pressure of each phase of the matrix, and determining different flow states; and S3, calculating the oil-water two-phase fluid channeling quantity according to different flow states. And the precision and reliability of shale oil reservoir numerical simulation are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of oil and gas field development, and particularly to a method and system for determining the matrix-fracture cross-flow rate in a shale oil reservoir. Background Art

[0002] With the gradual exploitation and consumption of high-quality oil and gas resources, the overall quality of remaining oil and gas resources has decreased, and most of them are low-quality and high-risk shale oil reservoirs. Shale oil reservoirs are widely distributed and have huge reserves in China, which is the key exploration and development direction of oil and gas resources in the next step in China. Shale oil reservoirs are characterized by ultra-low permeability and ultra-low porosity, and conventional production methods usually do not have economic productivity. The horizontal well volume fracturing technology is the main means for developing shale oil reservoirs. During the volume fracturing process, a large amount of fracturing fluid is injected into the formation to fracture the rock, forming a complex fracture network system. Different from conventional reservoirs, due to the very low permeability of shale oil reservoirs, the method of inter-well water flooding cannot be used. On the one hand, crude oil is discharged through the elastic energy of the fluid and the rock; on the other hand, due to the small pore throats and large capillary pressure in the shale reservoir, the crude oil is discharged by relying on capillary pressure.

[0003] In numerical simulation of fractured reservoirs, a dual-porosity medium model is generally used for modeling. For shale oil reservoirs, the capillary force in the matrix is very large, while the capillary force in the fractures is very small. The huge capillary force difference will lead to the interface effect between the matrix and the fractures. In the calculation of the cross-flow rate in the traditional dual-porosity medium model, the interface effect between the matrix and the fractures caused by the capillary force difference is not considered, resulting in inaccurate calculation results. Summary of the Invention

[0004] In view of the above problems, the present invention is proposed to provide a method and system for determining the matrix-fracture cross-flow rate in a shale oil reservoir that can overcome or at least partially solve the above problems.

[0005] According to one aspect of the present invention, a method for determining the matrix-fracture cross-flow rate in a shale oil reservoir is provided. The determination method includes:

[0006] Step S1: Determine the conditions at the matrix-fracture interface;

[0007] Step S2: Judge the relative magnitude relationship between the calculated fracture pressure and the phase pressures of the matrix, and determine different flow states;

[0008] Step S3: Calculate the oil-water two-phase cross-flow rate according to different flow states.

[0009] Optionally, the step S3: calculating the oil-water two-phase cross-flow rate according to different flow states specifically includes:

[0010] Calculating the oil-water two-phase cross-flow rate under the condition; the fracture pressure is P (f), the average pressures of the aqueous phase and the oil phase in the matrix are respectively and

[0011] Under the conditions of

[0012] calculate the cross - flow rate of the oil - water two - phase;

[0013] Optionally, the step S1: determining the conditions at the matrix - fracture interface specifically includes:

[0014] The capillary force in the fracture is zero;

[0015] The water saturations in the matrix and the fracture are both non - zero. When crossing the matrix - fracture interface, since the pressures of the aqueous phase and the oil phase are continuous, the capillary force should be continuous;

[0016] The capillary force at the matrix interface needs to match the capillary force in the fracture;

[0017] The capillary force in the fracture is zero, and the capillary force in the matrix at the interface should also be zero, resulting in the water saturation in the matrix at the interface being 1.

[0018] Optionally, the step S2: judging the relative magnitude relationship between the calculated fracture pressure and the pressures of each phase in the matrix, and determining different flow patterns specifically includes:

[0019] The first one, The flow pattern is that both the oil - water two - phase flow from the matrix into the fracture;

[0020] The second one, The flow pattern is that the aqueous phase flows from the fracture into the matrix, and the oil phase flows from the matrix into the fracture;

[0021] The third one, The flow pattern is that the aqueous phase flows from the fracture into the matrix, and the cross - flow rate of the oil phase is 0.

[0022] Optionally, the calculation of the cross - flow rate of the oil - water two - phase under the conditions specifically includes:

[0023] Both the oil - water two - phase flow from the matrix into the fracture, corresponding to the release of elastic energy;

[0024] The control equations for the flow of each phase in the matrix are expressed as:

[0025]

[0026] When considering the condition of capillary force continuity, the water saturation at x = 0 at the matrix interface is 1, that is, S| x＝0 = 1, and at the same time, the pressure P (f) is given at the matrix - fracture interface;

[0027] At the center of the matrix, x = L / 2, and the oil-phase pressure is given and the water saturation S x＝L / 2 ;

[0028] Ignoring the unsteady terms and the change in density in the above equation, we have:

[0029]

[0030] It is easy to know that in the above equation, dP / dx > dP c / dx > 0. Denote the water-oil cross-flow rate ratio θ = V w / V o , then we can obtain

[0031]

[0032] Integrating the above equation from the matrix interface to the matrix center, we have

[0033]

[0034] Furthermore, from

[0035]

[0036] Integrating the V o in equation (3) gives

[0037]

[0038] Using equation (5), rewrite the above equation as:

[0039]

[0040] where the corresponding water-phase saturation is in the interval Obviously In the traditional model is used to calculate the oil-phase cross-flow rate, and the traditional model equation (1) overestimates the oil-phase cross-flow rate;

[0041] Integrating the V w in equation (3) gives

[0042]

[0043] Using equation (5), rewrite the above equation as:

[0044]

[0045] where the corresponding water-phase saturation is in the interval Obviously

[0046] Define the matrix apparent saturation S app such that in equations (8) and (10), there is approximately

[0047]

[0048] Next, estimate the apparent saturation S app value;

[0049] Assume that in equation (5), is large enough, then there is

[0050]

[0051] Divide equation (10) by equation (8) to get

[0052]

[0053] Combined with equation (12), we get S app should satisfy

[0054]

[0055] It is easy to know that S app > S x＝L / 2 ;

[0056] After obtaining the matrix apparent saturation, calculate the cross-flow rate of the aqueous phase and the oil phase according to the following formula:

[0057]

[0058] Optionally, under the described conditions, calculating the cross-flow rate of the oil-water two-phase specifically includes:

[0059] The aqueous phase flows from the fracture into the matrix, and the oil phase flows from the matrix into the fracture. This situation corresponds to imbibition;

[0060] Since the matrix saturation is 1 at the matrix-fracture interface, write the cross-flow rate calculation formula as

[0061]

[0062] The meanings of each symbol are the same as before, represents the value of the aqueous phase mobility when the matrix water saturation is 1, that is

[0063] In this case, the oil phase cross-flow rate calculated by equation (16) is the same as that of the traditional dual-porosity W-R model, but the aqueous phase cross-flow rate is different from that of the traditional dual-porosity W-R model;

[0064] In the traditional dual-porosity W-R model, the aqueous phase mobility takes the saturation value in the upstream fracture in this case, that is

[0065] When the water saturation value in the fracture is small, the W-R model will greatly underestimate the water-phase crossflow rate.

[0066] In fact, the interfacial effect caused by the capillary force difference will result in a saturation of 1 at the interface of the matrix. The water-phase crossflow rate depends on the saturation distribution in the matrix and is independent of the water saturation value in the upstream fracture.

[0067] Optionally, the Under the conditions, calculating the oil-water two-phase crossflow rate specifically includes:

[0068] The water phase flows from the fracture into the matrix, and the oil-phase crossflow rate is 0;

[0069] The flow velocities of the water phase and the oil phase at the matrix interface are respectively and

[0070] Since Therefore, at the matrix interface, there is Also, the matrix saturation is 1 at the matrix-fracture interface, that is

[0071] is a finite value, while

[0072] Therefore, the flow velocities of the water phase and the oil phase at the matrix interface satisfy ||(V w ) x→0 || >> ||(V o ) x→0 ||, that is (V o ) x→0 = 0;

[0073] Using the shape factor, the crossflow rate calculation formula can be written as

[0074]

[0075] The water-phase mobility does not take the value in the upstream fracture but takes the value when the matrix saturation is 1

[0076] Optionally, the flow velocities of the water phase and the oil phase at the matrix interface satisfy ||(V w ) x→0 || >> ||(V o ) x→0 ||, that is (V o ) x→0 = 0 specifically includes:

[0077] When the pressures of both the oil phase and the water phase in the matrix are less than the fracture pressure, only the water phase flows from the fracture into the matrix, while the oil phase cannot;

[0078] When the fracture is full of oil, the conclusion that the water saturation in the fracture is 0 and the water saturation at the matrix interface is 1 does not hold, and the oil phase enters the matrix from the fracture;

[0079] After all the water in the fracture enters the matrix, the oil in the fracture will start to enter the matrix.

[0080] Optionally, the calculation formula for the crossflow rate of the traditional model is as follows

[0081]

[0082] where q w and q o are the crossflow rates of the water phase and the oil phase per unit volume respectively; σ is the shape factor; k (m) is the matrix permeability; the water phase mobility and the oil phase mobility take the matrix average value, that is is the average water saturation of the matrix; P (f) is the fracture pressure; and are the average pressures of the water phase and the oil phase in the matrix respectively;

[0083] Since the water saturation is 1 at the matrix - fracture interface, the traditional model will underestimate the water phase flow rate and overestimate the oil phase flow rate.

[0084] The present invention also provides a determination system for the matrix - fracture crossflow rate of a shale oil reservoir, applying the determination method for the matrix - fracture crossflow rate of a shale oil reservoir described above. The determination system includes:

[0085] A condition determination module for determining the conditions at the matrix - fracture interface;

[0086] A flow state determination module for judging and calculating the relative magnitude relationship between the fracture pressure and the pressures of each phase in the matrix, and determining different flow states;

[0087] A flow rate calculation module for calculating the crossflow rates of the oil - water two - phase according to different flow states.

[0088] The determination method and system for the matrix - fracture crossflow rate of a shale oil reservoir provided by the present invention. The determination method includes: Step S1: determining the conditions at the matrix - fracture interface; Step S2: judging and calculating the relative magnitude relationship between the fracture pressure and the pressures of each phase in the matrix, and determining different flow states; Step S3: calculating the crossflow rates of the oil - water two - phase according to different flow states. It improves the accuracy and reliability of numerical simulation of shale oil reservoirs.

[0089] The above description is only an overview of the technical solution of the present invention. In order to better understand the technical means of the present invention, it can be implemented according to the content of the specification. And in order to make the above and other purposes, features and advantages of the present invention more obvious and understandable, the following specific embodiments of the present invention are given. BRIEF DESCRIPTION OF THE DRAWINGS

[0090] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0091] Figure 1 It is a flow chart of a method for determining the matrix-fracture crossflow rate of a shale oil reservoir provided by an embodiment of the present invention;

[0092] Figure 2 Provided by an embodiment of the present invention Under the condition, calculation and analysis of the oil-phase crossflow rate;

[0093] Figure 3 Provided by an embodiment of the present invention Under the condition, the relationship between the matrix apparent saturation and the average saturation;

[0094] Figure 4 Provided by an embodiment of the present invention Under the condition, comparison of the crossflow rate between the model proposed by the present invention and the traditional dual-porosity medium model;

[0095] Figure 5 Provided by an embodiment of the present invention Under the condition, comparison of the crossflow rate between the model proposed by the present invention and the traditional dual-porosity medium model;

[0096] Figure 6 Provided by an embodiment of the present invention Under the condition, when the fracture water saturation is small, comparison of the water-phase and oil-phase crossflow rates calculated by the traditional dual-porosity medium model. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0097] The exemplary embodiments of the present disclosure will be described in more detail below with reference to the drawings. Although the exemplary embodiments of the present disclosure are shown in the drawings, it should be understood that the present disclosure can be implemented in various forms and should not be limited by the embodiments set forth herein. On the contrary, these embodiments are provided so that the present disclosure can be more thoroughly understood and the scope of the present disclosure can be fully conveyed to those skilled in the art.

[0098] The terms "comprising", "having" and any variations thereof in the description of the embodiments, claims and drawings of the present invention are intended to cover non-exclusive inclusion. For example, a series of steps or units are included.

[0099] The technical solutions of the present invention will be further described in detail below with reference to the drawings and embodiments.

[0100] Embodiment 1

[0101] As Figure 1 shown, an improved method for calculating the cross-flow rate of a shale reservoir matrix-fracture dual-porosity medium model includes the following steps:

[0102] Step 1: Determine the conditions at the matrix-fracture interface. The capillary force in the matrix of the shale reservoir is very large, while the capillary force in the fracture is very small. It can be considered that the capillary force in the fracture is zero. The water saturations in the matrix and the fracture are generally not zero. When crossing the matrix-fracture interface, since the pressures of the water phase and the oil phase are both continuous, the capillary force should be continuous. That is to say, the capillary force at the matrix interface needs to match the capillary force of the fracture. Therefore, since the capillary force in the fracture is zero, it can be known that the capillary force in the matrix at the interface should also be zero, and thus the matrix water saturation (after normalization) is 1 at the interface.

[0103] Step 2: Judge the relative magnitude relationship between the calculated fracture pressure and the pressures of each phase in the matrix, and determine different flow states accordingly. Denote the fracture pressure as P (f) , and the average pressures of the water phase and the oil phase in the matrix are respectively and The relative magnitude relationship can be divided into three types. The first type is At this time, the flow state is that both the oil and water phases flow from the matrix into the fracture (the calculation of the cross-flow rate of the water-oil two-phase under this flow state is shown in Step 3); the second type is At this time, the flow state is that the water phase flows from the fracture into the matrix, and the oil phase flows from the matrix into the fracture (the calculation of the cross-flow rate of the water-oil two-phase under this flow state is shown in Step 4); the third type is At this time, the flow state is that the water phase flows from the fracture into the matrix, and the cross-flow rate of the oil phase is 0 (the calculation of the cross-flow rate of the water-oil two-phase under this flow state is shown in Step 5).

[0104] Step 3: Calculation of the cross-flow rate of the oil-water two-phase under the condition of. Under this condition, both the oil and water phases flow from the matrix into the fracture, and this situation corresponds to the release of elastic energy. The cross-flow rate calculation formula of the traditional model is as follows

[0105]

[0106] Among them, q w and q oare the cross - flow rates of the aqueous and oil phases per unit volume, respectively; σ is the shape factor; k (m) is the matrix permeability; the aqueous phase mobility and the oil phase mobility take the matrix average value, that is is the average water saturation of the matrix; P (f) is the fracture pressure; and are the average pressures of the aqueous and oil phases in the matrix, respectively. Since the water saturation is 1 at the matrix - fracture interface, this traditional model will underestimate the aqueous phase flow rate and overestimate the oil phase flow rate. The following presents the correction scheme.

[0107] The governing equations for the flow of each phase in the matrix can be expressed as:

[0108]

[0109] If the capillary force continuity condition is not considered, nor the variation of density and porosity with pressure, and it is assumed that the saturation distribution in the matrix is constant, that is, the average saturation of the matrix then a linear distribution of the matrix pressure is obtained, corresponding to the cross - flow rate calculation formula of the traditional model.

[0110] When the capillary force continuity condition is considered, the water saturation at the interface of the matrix (x = 0) is 1, that is, S| x＝0 = 1, and at the same time, the pressure P (f) is given at the matrix - fracture interface; at the center of the matrix (x = L / 2), the oil phase pressure and the water saturation Sx ＝L / 2 (in practical applications, due to the scale of the interface effect being much smaller than the matrix scale, S x＝L / 2 can take the average saturation of the matrix ). Ignoring the unsteady terms and the variation of density in the above formula (the traditional model is also based on this quasi - steady - state assumption when deriving the cross - flow rate), then there is:

[0111]

[0112] It is easy to know that in the above formula, dP / dx > dP c / dx > 0. Denote the water - oil cross - flow rate ratio θ = V w / V o , then we can get

[0113]

[0114] Integrating the above formula from the matrix interface to the center of the matrix, we have

[0115]

[0116] Then from

[0117]

[0118] For V in equation (3) o Integrating equation

[0119]

[0120] Using equation (5), the above equation can be rewritten as (by the mean value theorem for integrals):

[0121]

[0122] where The corresponding aqueous phase saturation is in the interval Obviously In the traditional model is used to calculate the oil phase crossflow rate. That is to say, equation (1) of the traditional model overestimates the oil phase crossflow rate.

[0123] Similarly, for V in equation (3) w Integrating equation

[0124]

[0125] Using equation (5), the above equation can be rewritten as (by the mean value theorem for integrals):

[0126]

[0127] where The corresponding aqueous phase saturation is in the interval Obviously In the traditional model is used to calculate the aqueous phase crossflow rate. That is to say, equation (1) of the traditional model underestimates the aqueous phase crossflow rate.

[0128] Define the matrix apparent saturation S app such that approximately in equations (8) and (10)

[0129]

[0130] Next, estimate the value of the apparent saturation S app value.

[0131] Assume that in equation (5) is large enough (when the saturation is small, the capillary force is large, and this condition holds), then there is

[0132]

[0133] Dividing equation (10) by equation (8), we get

[0134]

[0135] Combining with Equation (12), we get S app should satisfy

[0136]

[0137] It is easy to know that S app > S x＝L / 2 .

[0138] After obtaining the matrix apparent saturation, calculate the cross-flow rate of the aqueous phase and the oil phase according to the following formula:

[0139]

[0140] Step 4: Calculation of the cross-flow rate of the oil-water two-phase under the condition of. Under this condition, the aqueous phase flows from the fracture into the matrix, and the oil phase flows from the matrix into the fracture. This situation corresponds to imbibition. Since the matrix saturation is 1 at the matrix-fracture interface, the cross-flow rate calculation formula can be written as

[0141]

[0142] The meanings of each symbol are the same as before, represents the value of the aqueous phase mobility when the matrix water saturation is 1, that is In this case, the oil phase cross-flow rate calculated by Equation (16) is the same as that of the traditional dual-porosity W-R model, but the aqueous phase cross-flow rate is different from that of the traditional dual-porosity W-R model. In the traditional dual-porosity W-R model, the aqueous phase mobility takes the saturation value in the upstream fracture under this condition, that is If the water saturation value in the fracture is small, the W-R model will greatly underestimate the aqueous phase cross-flow rate. In fact, due to the interfacial effect caused by the capillary force difference, the saturation at the interface of the matrix is 1, and the aqueous phase cross-flow rate depends on the saturation distribution in the matrix and has nothing to do with the water saturation value in the upstream fracture.

[0143] Step 5: Calculation of the cross-flow rate of the oil-water two-phase under the condition of. Under this condition, the aqueous phase flows from the fracture into the matrix, and the oil phase cross-flow rate is 0. This is because, under this condition, the flow velocities of the aqueous phase and the oil phase at the matrix interface are respectively and Since Therefore, at the matrix interface, there is (see Figure 1 ), and since the matrix saturation is 1 at the matrix-fracture interface, that is It can be seen that is a finite value, while Therefore, the flow rates of the aqueous and oil phases at the matrix interface satisfy ||(V w ) x→0 || >> ||(V o ) x→0 ||, that is, (V o ) x→0 = 0. In other words, when the pressures of the oil and aqueous phases in the matrix are both less than the fracture pressure, only the aqueous phase can flow from the fracture into the matrix, while the oil phase cannot. (When the fracture is full of oil, that is, the water saturation in the fracture is 0, the conclusion that the water saturation at the matrix interface is 1 does not hold, and the oil phase can enter the matrix from the fracture. That is to say, after all the water in the fracture enters the matrix, the oil in the fracture will start to enter the matrix. This situation does not occur in development practice, so it is not considered.) Using the shape factor, the crossflow rate calculation formula can be written as

[0144]

[0145] Here, for the same reason, the mobility of the aqueous phase does not take the value in the upstream fracture but takes the value when the matrix saturation is 1

[0146] Example 2: As Figure 2 shown, under the conditions, the calculation and analysis process of the oil-phase crossflow rate includes:

[0147] Take k rw = S 2 , k ro = (1 - S) 2 , μ o / μ w = 2, P c = -P e lnS, Denote and denote η = ΔP / P e . At this time, it obviously meets the condition Both the aqueous and oil phases flow from the matrix into the fracture, corresponding to the release of elastic energy. Under these conditions, equation (14) is specifically

[0148]

[0149] Solving this equation gives

[0150]

[0151] where

[0152]

[0153] Figure 3 gives S effRelationship with the relationship between

[0154] Example 3: Given the characteristic length of the matrix block \(L = 10m\), the absolute permeability of the matrix \(k = 10\) -15 m 2 , the viscosity of the aqueous phase \(\mu\) w = 0.2mPa·s, the viscosity of the oil phase \(\mu\) o = 1.0mPa·s; the relative permeability of the matrix and the fracture is the same, which is \(k\) rw (S)=S 4 , \(k\) ro (A)=(1 - S) 2 ; the capillary pressure curve in the matrix is the capillary pressure in the fracture is 0. The average water saturation of the matrix is denoted as The average aqueous and oil phase pressures of the matrix are respectively denoted as and Then there is Take the fracture pressure Obviously, the conditions are met at this time Figure 4 The comparison of the aqueous and oil phase cross - flow rates calculated by the model proposed in the present invention and the traditional dual - porosity medium model is given respectively when the fracture water saturation takes 1.0, 0.9, 0.8, 0.7, 0.5, 0.1. It can be seen from the figure that under conditions, the aqueous phase flows from the fracture into the matrix (\(q\) w > 0), and the oil phase flows from the matrix into the fracture (\(q\) o < 0). The oil phase cross - flow rate calculated by the new model proposed in the present invention is consistent with that of the traditional dual - porosity medium model, but the aqueous phase cross - flow rate is greater than the calculated value of the traditional dual - porosity medium model. The aqueous phase cross - flow rate calculated by the traditional dual - porosity medium model is related to the fracture water saturation. When the fracture water saturation is 1, the aqueous phase cross - flow rate calculated by the traditional model is the same as that of the new model; when the fracture water saturation is less than 1, the aqueous phase cross - flow rate calculated by the traditional model is significantly less than that of the new model. This means that the traditional model often underestimates the aqueous phase cross - flow rate, which is consistent with the relevant imbibition research conclusions.

[0155] Example 4: Given the relevant parameters (the same as in Example 1), take the fracture pressure Obviously, the conditions are met at this time Figure 5 The comparison of the aqueous and oil phase cross - flow rates calculated by the model proposed in the present invention and the traditional dual - porosity medium model is given respectively when the fracture water saturation takes 1.0, 0.9, 0.8, 0.7, 0.5, 0.1. It can be seen from the figure that under Under such conditions, the new model gives an oil-phase flow rate of 0. Although the oil-phase crossflow rate calculated by the traditional dual-porosity medium model is not 0, its value is very small, and the difference from the new model is not obvious. However, for the calculation of the water-phase crossflow rate, there are significant differences between the traditional dual-porosity medium model and the new model. The reason is also that the traditional dual-porosity medium model does not take into account the interfacial effect brought about by the capillary force difference between the matrix and the fracture. When calculating the water-phase crossflow rate, the water-phase mobility in the fracture is used. When the water saturation in the fracture is small, the water-phase crossflow rate value will be underestimated. Another point worthy of note is that when the water saturation in the fracture is small and the oil saturation is large, the oil-phase crossflow rate calculated by the traditional dual-porosity medium model will be greater than the water-phase crossflow rate, which is not in line with physics. As Figure 5 shown, when the water saturation in the fracture is 0.1, the oil-phase crossflow rate per unit volume calculated by the traditional dual-porosity medium model is about 0.003 day -1 , while the water-phase crossflow rate is about 0. In fact, the water-phase crossflow rate will not be less than the oil-phase crossflow rate (see the analysis in Figure 1 ), which indicates that in this case, the traditional dual-porosity medium model may bring serious errors.

[0156] Beneficial effects: In the calculation of the crossflow rate of the traditional dual-porosity medium model, both the water-phase mobility and the oil-phase mobility take the upstream values. That is to say, when the fracture is upstream, the mobility in the fracture is taken; when the matrix is upstream, the mobility in the matrix is taken. In fact, the huge capillary force difference between the matrix and the fracture will cause an interfacial effect, that is, at the matrix-fracture interface, to ensure the continuity of the capillary force, the water saturation on the matrix side should be 1. The water saturation at the matrix interface always remains 1, which makes the crossflow rate actually independent of the water saturation in the fracture. In the traditional model, the interfacial effect brought about by the capillary force difference is not taken into account, and the saturation value in the fracture is used in the calculation of the crossflow rate, which will bring serious errors in some cases.

[0157] To make up for the defects of the traditional model in the calculation of the crossflow rate, the present invention proposes a new method for calculating the crossflow rate. Compared with the crossflow rate of the traditional dual-porosity medium model, the new algorithm has two prominent features. One is that the crossflow rate is no longer related to the saturation in the fracture, which ensures that the calculation of the crossflow rate will not produce results that violate physics. The other is that according to the relative magnitudes of the fracture pressure and the water-phase and oil-phase pressures in the matrix, the crossflow patterns are divided into three categories, and the crossflow rate calculation formulas are given respectively. It should be noted that the crossflow rate calculation formula given by the present invention can be degraded to the case of incompressible simple imbibition, and the accuracy is basically consistent with the corresponding analytical solution.

[0158] By comparing with the traditional model, it is found that the improvements of the new model over the traditional model are mainly reflected in two aspects. First, the water-phase cross-flow value is often significantly higher than that of the traditional model, which is related to the imbibition effect; second, when the fracture pressure is greater than the oil-phase and water-phase pressures in the matrix, the physical result that the oil-phase cross-flow is greater than the water-phase cross-flow will not occur. Compared with the calculation method of the traditional dual-porosity model, the present invention can avoid the wrong results that violate physics, can significantly improve the numerical simulation accuracy of shale oil reservoirs, can provide a more reasonable calculation model for the history matching of shale oil reservoirs, can more accurately predict the development effect of shale oil reservoirs, and guide the development and production practice of shale oil reservoirs.

[0159] The above specific implementation manners further elaborate on the purpose, technical solution and beneficial effects of the present invention. It should be understood that the above are only the specific implementation manners of the present invention and are not used to limit the protection scope of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for determining matrix-fracture crossflow in shale reservoirs, characterized in that: The determination method comprises: Step S1: determining the conditions at the matrix-crack interface; Step S2: determining the relative magnitude relationship between the calculated fracture pressure and the pressure of each phase of the matrix, and determining different flow patterns; Step S3: Calculate the oil-water two-phase crosstalk flow rate according to different flow patterns.

2. The method for determining matrix-fracture crossflow rate in shale reservoirs according to claim 1, characterized in that: The step S3: calculating the oil-water two-phase cross-flow rate according to different flow states specifically includes: Calculate the oil-water two-phase crossflow rate under the condition; the fracture pressure is P (f) The average pressures of the water phase and the oil phase in the matrix are and Under these conditions, calculate the oil-water two-phase crosstalk flow rate; Under these conditions, calculate the oil-water two-phase crosstalk flow rate.

3. The method for determining matrix-fracture crossflow rate in shale reservoirs according to claim 1, characterized in that: The step S1: determining the conditions at the matrix-crack interface specifically includes: The capillary force in the crack is zero; The water saturation in the matrix and the fracture is not zero. When crossing the matrix-fracture interface, the capillary force should be continuous because the pressures of the water phase and the oil phase are continuous. The capillary force at the matrix interface needs to match the capillary force in the fracture; The capillary force in the fracture is zero, and the matrix capillary force at the interface should also be zero, so the matrix water saturation at the interface is 1.

4. The method for determining matrix-fracture crosstalk in a shale reservoir according to claim 1, characterized in that: The step S2: determining the relative magnitude relationship between the calculated fracture pressure and the pressure of each phase of the matrix, and determining different flow states specifically includes: The first one, The flow pattern is that both oil and water phases flow from the matrix into the fracture; The second type, The flow pattern is that the water phase flows from the fracture into the matrix, and the oil phase flows from the matrix into the fracture; The third type, The flow pattern is that the water phase flows from the fracture into the matrix, and the oil phase crossflow is 0.

5. The method for determining matrix-fracture crosstalk in shale reservoirs according to claim 1, characterized in that: Said The calculation of oil-water two-phase cross-flow rate under the following conditions specifically includes: Both oil and water phases flow from the matrix into the fracture, corresponding to the release of elastic energy; The governing equations for the flow of each phase in the matrix are expressed as: When the capillary force continuity condition is considered, the water saturation of the matrix at the interface x = 0 is 1, that is, S| x＝0 = 1, and at the same time, the pressure P is given at the matrix fracture interface (f) ; At the center of the matrix, x = L / 2, given the oil phase pressure and water saturation S x＝L / 2 ; Ignoring the non-steady-state terms and density changes in the above formula, we have: It is easy to see that in the above formula, dP / dx>dP c / dx>0. Let the water-oil cross-flow ratio θ=V w / V o , then we can get The above equation is integrated from the matrix interface to the matrix center, Then by For V in formula (3), o The integral of the formula is Using formula (5), the above formula can be rewritten as: in The corresponding water phase saturation is in the interval Inside, obviously In the traditional model Used to calculate the oil phase cross-flow rate, the traditional model (1) overestimates the oil phase cross-flow rate; For V in formula (3), w The integral of the formula is Using formula (5), the above formula can be rewritten as: in The corresponding water phase saturation is in the interval Inside, obviously Definition of matrix apparent saturation S app , so that in equations (8) and (10) we have approximately The apparent saturation S is estimated as follows: app The value of Assume that in formula (5), If it is large enough, then Dividing equation (10) by equation (8), we get Combining (12), we get S app Should meet Yi Zhi, S app >S x＝L / 2 ; After obtaining the apparent saturation of the matrix, the cross-flow rate of the water phase and the oil phase is calculated as follows:

6. The method for determining matrix-fracture crossflow rate in shale reservoirs according to claim 2, characterized in that: Said Under these conditions, the calculation of oil-water two-phase cross-flow rate specifically includes: The water phase flows from the fractures into the matrix, and the oil phase flows from the matrix into the fractures, which corresponds to imbibition; Since the matrix saturation is 1 at the matrix-fracture interface, the crosstalk calculation formula is written as The meanings of the symbols are the same as before. It means that the water phase mobility is the value when the matrix water saturation is 1, that is, In this case, the oil phase cross-flow rate calculated by equation (16) is the same as that of the traditional dual-porosity media WR model, but the water phase cross-flow rate is different from that of the traditional dual-porosity media WR model; In the traditional dual-porosity WR model, the water phase mobility in this case is taken as the saturation value in the upstream fracture, that is, If the water saturation value in the fracture is small, the WR model will greatly underestimate the water phase flow rate; In fact, due to the interfacial effect caused by the difference in capillary forces, the saturation of the matrix at its interface is 1, and the water phase crossflow rate depends on the saturation distribution in the matrix and has nothing to do with the water saturation value in the upstream fracture.

7. The method for determining matrix-fracture crosstalk in shale reservoirs according to claim 2, characterized in that: Said Under these conditions, the calculation of oil-water two-phase cross-flow rate specifically includes: The water phase flows from the fracture into the matrix, and the oil phase crossflow is 0; The flow rates of water and oil phases at the matrix interface are and because Therefore, at the matrix interface, The matrix saturation is 1 at the matrix-fracture interface, that is, is a finite value, and Therefore, the flow rates of water and oil phases at the matrix interface satisfy ||(V w ) x→0 ||>>||(V o ) x→0 ||, that is (V o ) x→0 =0; The cross-flow calculation formula can be written as The water phase mobility does not take the value in the upstream fracture The value when the matrix saturation is 1 is taken 8. The method for determining matrix-fracture crossflow rate in shale reservoirs according to claim 1, characterized in that: The flow rates of the water phase and the oil phase at the matrix interface satisfy ||(V w ) x→0 ||>>||(V o ) x→0 ||, that is (V o ) x→0 =0 specifically includes: When the pressures of the oil phase and water phase in the matrix are both lower than the fracture pressure, only the water phase flows from the fracture into the matrix, but the oil phase cannot; When the fracture is full of oil, the water saturation in the fracture is 0, and the conclusion that the water saturation at the matrix interface is 1 is not valid, and the oil phase enters the matrix from the fracture; After all the water in the cracks enters the matrix, the oil in the cracks will begin to enter the matrix.

9. The method for determining matrix-fracture crosstalk in shale reservoirs according to claim 5, characterized in that: The cross-flow calculation formula of the traditional model is as follows Among them, q w and q o are the cross-flow rates of water phase and oil phase per unit volume, respectively; σ is the shape factor; k (m) is the matrix permeability; water phase mobility and oil phase fluidity Take the matrix average value, that is is the average water saturation of the matrix; P (f) is the crack pressure; and are the average pressures of water and oil phases in the matrix, respectively; Since the water saturation is 1 at the matrix-fracture interface, the conventional model will underestimate the water phase flow rate and overestimate the oil phase flow rate.

10. A system for determining the matrix-fracture crosstalk of a shale reservoir, using the method for determining the matrix-fracture crosstalk of a shale reservoir as described in claims 1 to 9, characterized in that: The determination method comprises: a condition determination module for determining conditions at a matrix-crack interface; The flow pattern determination module is used to determine the relative magnitude relationship between the calculated fracture pressure and the pressure of each phase of the matrix, and to determine different flow patterns; The flow calculation module is used to calculate the oil-water two-phase crosstalk flow according to different flow states.

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