CN111272630B - Method for calculating artificial fracture parameters of compact rock core - Google Patents

Abstract

The invention belongs to the technical field of oil and gas field development, and discloses a method for calculating artificial fracture parameters in tight rock cores, which can comprehensively, accurately and real-time acquire various parameter changes of tight rock core fractures during experiments. The invention establishes a method for calculating the parameters of the dense matrix-fracture dual medium model, which is simple and convenient, has strong operability and accurate results, and solves the problem that the existing technical scheme cannot accurately and real-time obtain the variation law of the core fracture parameters during the experiment. The calculation method of the present invention is suitable for core fractures with dip angles, and these parameters have practical significance for indoor research on the law of material diffusion, mass transfer and seepage in the dual medium system of dense matrix-fracture, and the process of liquid injection and gas injection for enhanced oil recovery.

Description

Calculation Method of Artificial Fracture Parameters in Tight Cores

technical field

The invention belongs to the technical field of oil and gas field development, and relates to the calculation of indoor artificial fracture parameters, in particular to a calculation method of tight rock core artificial fracture parameters.

Background technique

In order to study the multi-phase and multi-scale seepage characteristics of tight reservoirs after multi-stage fracturing, the model of artificially cutting cores to create fractures is usually used in laboratory simulations to construct artificial fractures with different properties, and then explore the diffusion of substances in the tight matrix-fracture dual medium. , the law of mass transfer and seepage, and the matrix-fracture mass exchange mechanism in the process of liquid injection and gas injection for enhanced oil recovery. Therefore, the accurate acquisition of fracture parameters (fracture opening, permeability, fracture volume, conductivity, etc.) during the experiment is the key to describe the fluid seepage law and reveal the mechanism of enhanced oil recovery.

Kun Ma et al. disclosed crack opening, permeability, permeability and other cracks in "Low-IFT Foaming System for Enhanced Oil Recovery in Highly Heterogeneous/Fractured Oil-Wet Carbonate Reservoirs" (SPE Journal, 2018, 12, 2243-2259) However, the volume of fractures is not considered in the calculation process, and the parameters of fractures with dip angle cannot be calculated.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a method for calculating artificial fracture parameters of tight rock cores, so as to solve the problem of inability to obtain accurate, comprehensive and real-time changes in core fracture parameters in the experimental process in the prior art solutions.

In order to achieve the above object, the technical scheme adopted in the present invention is:

直径D、基质渗透率k_m、平均渗透率

所述致密岩心长度方向上的压力梯度

及通过所述致密岩心的流体的流体粘度μ；所述致密岩心人工裂缝参数的计算方法如下：A method for calculating parameters of artificial fractures in tight cores is to divide the calculated tight cores with artificial fractures into three layers from top to bottom: upper layer of fractures, fractures, and lower layers of fractures and marked as i=1-3, On the premise that the fluid flows in one direction and passes through the tight core horizontally, the fracture angle θ and matrix porosity of the tight core are

Diameter D, matrix _permeability km, average permeability

pressure gradient along the length of the tight core

and the fluid viscosity μ of the fluid passing through the tight core; the method for calculating the artificial fracture parameters of the tight core is as follows:

其中，h为裂缝开度；而采用达西公式表达时，

其中，k_f为裂缝渗透率；average velocity of fluid in the fracture

Among them, h is the crack opening; and when expressed by Darcy's formula,

where k _f is the fracture permeability;

Combining Equation (1) and Equation (2), we get

其中，V_i为任一层所述致密岩心的体积，k_i为任一层所述致密岩心的渗透率；The average permeability of the tight core when expressed by the method of core volume weighting

Wherein, _Vi is the volume of the tight core in any layer, and _ki is the permeability of the tight core in any layer;

Substituting equation (3) into equation (4), we get:

According to the fracture opening h obtained by the formula (5), it is substituted into the formula (3) to obtain the fracture permeability k _f .

As a limitation, the crack angle θ is 0-15°.

As a second limitation, the ratio of fracture permeability to matrix permeability is calculated as follows:

As a third limitation,

The total flow q is the sum of the flow q _i of the tight cores in each layer, that is, q=∑q _i ------ Equation (7);

From formula (7), υA=∑υ _i A _i ------ formula (8), where υ is the total flux, υ _i is the flux of any layer of the tight core, and A is the The total cross-sectional area of the tight core, A _i is the cross-sectional area of the tight core in any layer;

Combining Darcy's formula with formula (8), we get

其中，k_i为任一层岩心的渗透率；

Among them, _ki is the permeability of any layer of core;

The flow rate of the upper layer of the fracture is equal to the flow rate of the lower layer of the fracture,

According to formula (7), formula (9) and Darcy's formula, the matrix flow in the upper layer of the fracture and the matrix flow in the lower layer of the fracture are:

According to formula (7), formula (9) and Darcy formula, the flow rate of fracture

Further, differences in fracture and matrix backflow capabilities

As a further limitation, the difference in the number of times the fluid reaches the fracture and matrix

Owing to adopting the above-mentioned technical scheme, compared with the prior art, the technical progress achieved in the present invention is:

The calculation method provided by the invention can accurately, comprehensively and real-time acquire various parameters of the artificial fractures of the tight rock core and their variation laws during the experiment. The calculated fracture parameters include: fracture opening, permeability, matrix fracture conductivity ratio, matrix fracture permeability ratio, matrix fracture sweep multiple ratio, and the variation law of fracture properties with pressure gradient is obtained. The calculation method is simple and the result is accurate.

The calculation method of the present invention is suitable for various parameters of fractures with a dip angle (0-15o), and these parameters are useful for indoor research on the laws of material diffusion, mass transfer and seepage in the dense matrix-fracture dual medium system, as well as liquid injection and gas injection to enhance production. The yield process is realistic.

Description of drawings

Fig. 1 is the schematic diagram of the core section of the present invention;

Fig. 2 is the schematic diagram of the core in Example 1 of the present invention;

3 is a schematic diagram of the experimental apparatus used in Example 1 of the present invention;

Fig. 4 is the relation diagram of crack opening and confining pressure in Example 1 of the present invention;

Fig. 5 is the relation diagram of the ratio of fracture and matrix permeability and confining pressure in Example 1 of the present invention;

6 is a graph of the relationship between fracture permeability and confining pressure in Example 1 of the present invention;

Fig. 7 is the relation diagram of the difference between fracture and matrix flow conductivity in Example 1 of the present invention;

Fig. 8. Relationship diagram of the difference in the number of times of fluid swept into fractures and matrix in cores.

Detailed ways

The present invention will be further described in detail below through specific embodiments. It should be understood that the described embodiments are only used to illustrate the present invention, but not to limit the present invention.

In the present invention, the tight cores with fractures are divided into upper layers of fractures, fractures, and lower layers of fractures in order from top to bottom, which are numbered 1 to 3 in sequence, see FIG. 1 .

Embodiment 1-8 Calculation method of artificial fracture parameters of tight core

为15％、裂缝倾角θ＝0°。Take a tight core with artificial fractures, its diameter D is 3.8cm and length is 9.0cm, and it is determined by the liquid measurement method. Referring to Fig. 2, it can be seen that the matrix permeability km of the core is _0.13mD , and the matrix porosity is 0.13mD.

is 15%, and the fracture inclination angle θ=0°.

为7.36mD，通过致密岩心的流体为水，流体粘度μ为1。The tight core with artificial fractures was taken and placed in the experimental device. The schematic diagram of the experimental device is shown in Figure 3. The annular pressure P around the core and the average permeability of the fractured core were measured.

is 7.36mD, the fluid passing through the tight core is water, and the fluid viscosity μ is 1.

In this calculation method, the fluid flows in one direction and passes through the core horizontally, including the following calculation processes in sequence:

其中，h为裂缝开度；First, the average flow rate of fluid in the fracture

Among them, h is the crack opening;

其中，k_f为裂缝渗透率；When using Darcy's formula,

where k _f is the fracture permeability;

Simultaneously formula (1) and formula (2), we can get

其中，V_i为任一层致密岩心的体积，k_i为任一层致密岩心的渗透率；Secondly, the average permeability of tight cores is represented by the method of core volume weighting

Among them, _Vi is the volume of any layer of tight cores, and _ki is the permeability of any layer of tight cores;

Substituting Equation (3) into Equation (4), Equation (5) can be obtained, and the crack opening h is obtained, namely:

where k ₁ = _{k 3} =km , k ₂ =k _f ;

Substitute the measured corresponding parameter values into formula (5), and calculate the crack opening h.

Finally, the obtained fracture opening h is substituted into equation (3) to obtain the fracture permeability k _f .

The ratio R _k of the fracture and matrix permeability of the core is set as

Substitute the matrix permeability km = 0.13mD and the obtained fracture permeability _{k f to} obtain the ratio R _k of the fracture and matrix permeability of the core.

Since the total flow q is the sum of the flow q _i of each layer of tight cores, that is, q=∑q _i ------ Equation (7);

From formula (7), υA=∑υ _i A _i ------ formula (8), where υ is the total flux, υ _i is the flux of any layer of tight cores, and A is the Total cross-sectional area, A _i is the cross-sectional area of any layer of tight cores;

Combining Darcy's formula with formula (8), we can get

Among them, _ki is the permeability of any layer of core;

Since the fractures in the tight core are artificial fractures, the flow rate of the upper layer of the fracture is equal to the flow rate of the lower layer of the fracture,

According to formula (7), formula (9) and Darcy's formula, the matrix flow in the upper layer of the fracture and the matrix flow in the lower layer of the fracture are:

Among them, k ₁ and k ₃ are both equal to the matrix permeability k _m , both of which are 0.13 mD. Substitute the corresponding parameter values from other measurements. According to equations (7), (9) and Darcy's formula, the fracture flow rate is :

Substitute into Equation (10), Equation (11) and the corresponding measured parameter values and crack opening h=13.73 μm to Equation (12),

由于在计算致密岩心的裂缝和基质导流能力的差异R_c过程中上下抵消，因此，无需测定。Pressure gradient along the length of the tight core

No measurement is required because of the up and down cancellation in the calculation of the difference in fracture and matrix conductivity R _c for tight cores.

The difference R _c =55.19 of the fracture and matrix conductivity of the tight core is obtained.

Finally, according to Equation (13), the corresponding parameter values are substituted to obtain the difference N _sweep = 17896 in the number of times the fluid sweeps the fractures and the matrix in the core.

Embodiments 1-8 are respectively the calculation methods of the artificial fracture parameters of tight rock cores, and their steps are the same as the above-mentioned steps, and the difference is only in the difference of the technical parameters, and the details are shown in Table 1:

Table 1 List of various process parameters when the inclination angle θ=0° in the embodiment 1-8

According to Table 1, the relationship between crack opening and ring pressure is drawn. Referring to Figure 4, it can be seen that as the ring pressure increases, the crack opening gradually becomes smaller, and the decreasing range gradually decreases;

According to Table 1, the relationship between the ratio of fracture and matrix permeability and the ring pressure is drawn. See Figure 5. It can be seen that with the increase of ring pressure, the ratio of fracture and matrix permeability gradually decreases, and the decreasing range gradually decreases;

According to Table 1, the relationship between fracture permeability and ring pressure is drawn. Referring to Figure 6, it can be seen that with the increase of ring pressure, the fracture permeability gradually decreases, and the decreasing range gradually decreases;

According to Table 1, the relationship between the fracture and the matrix conductivity difference is plotted, see Figure 7, it can be seen that with the increase of the ring pressure, the difference between the fracture and the matrix conductivity gradually becomes smaller, and the decreasing range gradually decreases;

According to Table 1, the relationship between the number of fluid swept fractures and the matrix in the core is drawn. Referring to Fig. 8, it can be seen that as the annular pressure increases, the difference in the number of fluid swept fractures and the matrix in the core gradually becomes smaller, and the decreasing range gradually decreases. .

The calculation method in this embodiment is suitable for measuring various parameters of cores without angular fractures. These parameters are useful for indoor research on the law of material diffusion, mass transfer and seepage in the dual medium system of tight matrix-fracture, as well as the enhancement of oil recovery by liquid injection and gas injection. The process is realistic.

Example 9-11 Calculation method of artificial fracture parameters of tight core

Embodiments 9-11 are respectively the calculation methods of artificial fracture parameters of tight rock cores, and their steps are basically the same as those of embodiment 1, and the difference is only in the difference of technical parameters. For details, refer to Table 2 for details:

List of various process parameters in Table 2 Examples 9-11

The contents of other parts of Examples 9-11 are the same as those of Example 1.

The calculation methods of Examples 9-11 are suitable for measuring various parameters of fractures at corresponding angles (0-15o). These parameters are useful for indoor studies on the laws of material diffusion, mass transfer and seepage in the dense matrix-fracture dual-medium system, as well as fluid injection and injection. The process of gas enhanced oil recovery has practical significance.

Embodiments 1-11 are only preferred embodiments of the present invention, not other forms of limitation made by the present invention, and any person skilled in the art cannot use the above-mentioned technical content as inspiration to change or remodel to equivalent changes. Equivalent embodiments, as long as they do not depart from the technical essence of the claims of the present invention, simple modifications, equivalent changes and modifications made to the above embodiments still belong to the scope of protection of the claims of the present invention.

Claims (3)

1. A method for calculating artificial fracture parameters of a compact rock core is characterized in that the compact rock core with artificial fractures to be calculated is divided into three layers, namely an upper layer of the fractures, a lower layer of the fractures and an upper layer of the fractures from top to bottom, the three layers are marked as i being 1-3, on the premise that fluid flows in a single direction and horizontally passes through the compact rock core, the fracture angle theta and the matrix porosity of the compact rock core are calculated

Diameter D, matrix Permeability k_mAverage permeability of

A pressure gradient in the length direction of the compact core

And a fluid viscosity μ of a fluid passing through the dense core; the method for calculating the artificial fracture parameters of the compact rock core comprises the following steps:

average flow velocity of fluid in the fracture

Wherein h is the crack opening; and when the expression is expressed by the darcy formula,

wherein k is_fIs the crack permeability;

combining vertical type (1) with formula (2) to obtain

The average permeability of the compact core is expressed by a core volume weighting method

Wherein, V_iVolume, k, of any layer of the densified core_iThe permeability of any layer of the compact rock core is obtained;

substituting formula (3) for formula (4) yields:

substituting the crack opening h obtained according to the formula (5) into the formula (3) to obtain the crack permeability k_f；

The total flow rate q is the flow rate q of each layer of the compact rock core_iSum, i.e. q ═ Σ q_iFormula (7);

from the formula (7), υ A ═ Σ υ_iA_i-formula (8) wherein upsilon is the total flux, upsilon_iFlux of any layer of the compact core, A is the total cross-sectional area of the compact core, A is_iThe cross section area of any layer of the compact rock core;

the simultaneous darcy formula and the formula (8) to obtain

Wherein k is_iThe permeability of any layer of core;

the flow rate of the upper layer of the fracture is equal to the flow rate of the lower layer of the fracture,

and then according to the formula (7), the formula (9) and the Darcy formula, the matrix flow of the upper layer of the crack and the matrix flow of the lower layer of the crack are as follows:

according to the formula (7), the formula (9) and the Darcy formula, the flow rate of the crack

Further, differences in fracture and matrix flowback capabilities

Difference in the number of times the fluid swept the fracture and the matrix

2. The method for calculating the parameters of the artificial tight core fracture as claimed in claim 1, wherein the fracture angle θ is 0-15 °.

3. The method for calculating the artificial fracture parameters of the dense core according to claim 1 or 2, wherein the ratio of the fracture permeability to the matrix permeability is calculated as follows:

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