CN106226217A

CN106226217A - A kind of descend the method for fracture opening, device and application thereof definitely

Info

Publication number: CN106226217A
Application number: CN201610540163.8A
Authority: CN
Inventors: 曾联波
Original assignee: China University of Petroleum Beijing
Current assignee: China University of Petroleum Beijing
Priority date: 2016-07-11
Filing date: 2016-07-11
Publication date: 2016-12-14
Anticipated expiration: 2036-07-11
Also published as: CN106226217B

Abstract

本发明提供了一种确定地下裂缝张开度的方法，具体为：确定裂缝检测区域，测量深度为x的地下裂缝所处区域的静岩围压，按照公式e_i＝a^f(x)+b计算地下裂缝张开度e_i；其中，f(x)代表裂缝深度为x时所处区域的静岩围压；a、b均为数学拟合而成的拟合系数。本发明提供的技术方案，为确定地层围压条件下裂缝张开度、孔隙度和渗透率提供了可行途径，能够定量地恢复天然裂缝张开度、天然裂缝孔隙度和天然裂缝渗透率在不同地层围压条件下的真实分布情况，为客观合理地评价天然裂缝在致密低渗透储层中的储集、渗流作用和贡献大小提供了有效手段。The invention provides a method for determining the opening degree of underground cracks, specifically: determining the crack detection area, measuring the static rock confining pressure of the area where the underground cracks with a depth of x are located, and calculating according to the formula e _i = a ^f(x) + b The underground fracture opening e _i ; where, f(x) represents the static rock confining pressure in the area where the fracture depth is x; a and b are fitting coefficients obtained by mathematical fitting. The technical scheme provided by the present invention provides a feasible way for determining fracture opening, porosity and permeability under formation confining pressure conditions, and can quantitatively restore natural fracture opening, natural fracture porosity and natural fracture permeability under different formation confining pressure conditions It provides an effective means for objectively and reasonably evaluating the accumulation, seepage and contribution of natural fractures in tight low-permeability reservoirs.

Description

A method, device and application for determining the opening degree of underground fractures

技术领域technical field

本发明涉及石油勘探领域，具体涉及一种确定地下裂缝张开度的方法、装置及其应用。The invention relates to the field of petroleum exploration, in particular to a method, device and application for determining the opening degree of underground fractures.

背景技术Background technique

天然裂缝地下开度、孔隙度和渗透率的定量评价是反映天然裂缝所起作用及其贡献大小的3个核心参数，也是裂缝油气藏开发所需的关键参数。岩心资料是表征天然裂缝定量参数的最可靠、最直接和最常用的途径。但岩心取到地表以后由于地层围压的释放，存在一个岩心膨胀的过程，会导致地表岩心上天然裂缝的张开度远远大于其在地下的真实张开度，因而通过地表岩心描述得到的裂缝孔隙度和渗透率会远远大于地下的真实值，这样势必会造成天然裂缝作用的错误评价，得到的天然裂缝参数不能客观地反映它们在地层围压条件下的真实情况。Quantitative evaluation of underground opening, porosity and permeability of natural fractures are the three core parameters that reflect the role and contribution of natural fractures, and are also key parameters required for the development of fractured reservoirs. Core data is the most reliable, direct and common way to characterize the quantitative parameters of natural fractures. However, after the core is taken to the surface, due to the release of the formation confining pressure, there is a process of core expansion, which will cause the opening of the natural fractures on the surface core to be much larger than the real opening of the underground. Therefore, the fracture porosity and The permeability will be far greater than the real value of the underground, which will inevitably lead to a wrong evaluation of the natural fractures, and the obtained natural fracture parameters cannot objectively reflect their real conditions under the confining pressure of the formation.

因此，在进行天然裂缝参数的定量表征时，必须要恢复至地层围压条件下的裂缝真实参数，主要是裂缝的张开度、裂缝的孔隙度和裂缝的渗透率3个参数，这样才可以客观地定量评价天然裂缝对致密低渗透油气储层的作用和贡献大小，正确地指导裂缝性致密低渗透油气藏的勘探和合理开发。Therefore, when performing quantitative characterization of natural fracture parameters, it is necessary to restore the real parameters of the fracture under the confining pressure of the formation, mainly the three parameters of fracture opening, fracture porosity, and fracture permeability, so that it can be objectively Quantitatively evaluate the role and contribution of natural fractures to tight and low-permeability oil and gas reservoirs, and correctly guide the exploration and rational development of fractured tight and low-permeability oil and gas reservoirs.

发明内容Contents of the invention

本发明的目的是提供一种确定地下裂缝张开度的方法，并应用该方法确定裂缝在地下的张孔隙度和渗透率，以指导实际生产应用。The purpose of the present invention is to provide a method for determining the opening degree of underground fractures, and use the method to determine the open porosity and permeability of fractures in the ground, so as to guide actual production and application.

在裂缝的几个重要参数中，由于岩心上天然裂缝的产状和规模在地表和地下几乎没有发生变化，但天然裂缝的张开度由于钻井岩心取得地表以后围压释放发生的膨胀发生了很多的变化。因此，本发明首先提供了一种确定地下裂缝张开度的方法，所述包括以下步骤：确定裂缝检测区域，测量深度为x的地下裂缝所处区域的静岩围压，按照公式I计算地下裂缝张开度e_i：Among the several important parameters of fractures, the occurrence and scale of natural fractures on the core hardly change between the surface and underground, but the opening degree of natural fractures changes a lot due to the expansion of the confining pressure released after the drilling core is obtained from the surface . Therefore, the present invention at first provides a kind of method for determining the opening degree of underground cracks, which includes the following steps: determining the crack detection area, measuring the static rock confining pressure in the area where the underground cracks where the depth is x, and calculating the opening degree of underground cracks according to formula 1 e _i :

e_i＝a^f(x)+b I；e _i = a ^f(x) + b I;

所述公式I中，f(x)代表裂缝深度为x时所处区域的静岩围压；a、b均为拟合系数。用上述公式，就能够得到不同地层围压条件下的裂缝地下张开度。In the formula I, f(x) represents the static rock confining pressure of the region where the fracture depth is x; a and b are fitting coefficients. Using the above formula, the underground opening of fractures under different formation confining pressure conditions can be obtained.

所述公式I中，f(x)代表某一深度的静岩围压，与裂缝的埋藏渗透、裂缝产状、裂缝中的流体压力、地应力方向与大小等因素有关。具体而言，所述f(x)的计算公式为：In the formula I, f(x) represents the static rock confining pressure at a certain depth, which is related to factors such as the burial and penetration of fractures, the occurrence of fractures, the fluid pressure in fractures, the direction and magnitude of in-situ stress. Specifically, the formula for calculating f(x) is:

$f f ((x x)) = = \frac{μ μ}{11 - - μ μ} H h f f (({σ σ}_{v v})) sin sin θ θ + + {Hρ Hρ}_{s the s} cos cos θ θ + + H h f f (({σ σ}_{H h})) sin sin θ θ cos cos α α + + H h f f (({σ σ}_{h h})) sin sin θ θ cos cos α α - - {γHρ γHρ}_{00}$

其中，μ为泊松比；H为埋藏深度；f(σH)、f(σv)、f(σh)分别水平最大主应力、垂向应力和水平最小主应力随深度变化的梯度；ρ₀为流体容重；θ、α分别为裂缝倾角以及裂缝走向与水平最大主应力方向的夹角；γ为孔隙流体压力系数。以上各参数均可采用本领域常规方法测量、确定。Among them, μ is Poisson’s ratio; H is the burial depth; f(σH), f(σv), f(σh) are the gradients of horizontal maximum principal stress, vertical stress and horizontal minimum principal stress with depth; _ρ0 is Fluid bulk density; θ and α are the fracture dip angle and the angle between the fracture strike and the horizontal maximum principal stress direction; γ is the pore fluid pressure coefficient. Each of the above parameters can be measured and determined by conventional methods in the art.

所述公式I中，a、b为拟合系数，可根据地区裂缝特点依经验人为确定或采用数学拟合的方法获得。In the formula I, a and b are fitting coefficients, which can be manually determined according to the characteristics of regional fractures or obtained by mathematical fitting methods.

本发明同时提供了一种确定地下裂缝张开度的装置，所述装置包括加载系统(1)、压力控制系统(2)和测量处理系统(3)；The present invention also provides a device for determining the opening degree of an underground fracture, the device comprising a loading system (1), a pressure control system (2) and a measurement processing system (3);

所述加载系统(1)的中心为样品放置单元；所述样品放置单元的外周为围压加载单元；The center of the loading system (1) is a sample placement unit; the periphery of the sample placement unit is a confining pressure loading unit;

所述压力控制系统(2)包括地层围压控制单元以及轴向负荷控制单元；The pressure control system (2) includes a formation confining pressure control unit and an axial load control unit;

所述测量处理系统(3)用于参数的测量、计量和实验数据的后期处理。The measurement processing system (3) is used for parameter measurement, metering and post-processing of experimental data.

其中，加载系统(1)包括岩心样品的放置系统和样品周围的围压加载系统两部分，能够保证不同围压条件下岩心裂缝开度的动态变化；压力控制系统(2)包括样品受到的地层围压控制以及轴向负荷的控制两部分，能够保证不同地层围压和地应力的加载；测量处理系统(3)包括岩样中流体变化、变形变化等系列参数的精确计量和实验数据的后期处理系统，得到在不同地层围压条件下的裂缝开度变化规律。利用图1的装置进行的物理模拟，可以得到在不同静岩围压条件下岩心上天然裂缝张开度的变化规律。Among them, the loading system (1) includes two parts: the placement system of the core sample and the confining pressure loading system around the sample, which can ensure the dynamic change of the crack opening of the core under different confining pressure conditions; The two parts of confining pressure control and axial load control can ensure the loading of different formation confining pressure and ground stress; the measurement and processing system (3) includes the accurate measurement of a series of parameters such as fluid changes and deformation changes in rock samples and the post-processing of experimental data. The processing system is used to obtain the variation law of fracture opening under different formation confining pressure conditions. Using the physical simulation performed by the device in Fig. 1, the variation law of the opening degree of natural fractures on the core under different static rock confining pressure conditions can be obtained.

本发明同时保护所述方法或装置在确定地下裂缝孔隙度中的应用，具体包括以下步骤：The present invention simultaneously protects the application of the method or device in determining the porosity of underground fractures, specifically comprising the following steps:

(1)测量地下裂缝的张开度e_i以及地表裂缝的张开度e_0i，按照公式II计算地下裂缝张开度与地表裂缝张开度的比例系数A_i；(1) Measure the opening degree e _i of underground cracks and the opening degree e _0i of surface cracks, and calculate the proportional coefficient A _i of the opening degree of underground cracks and the opening degree of surface cracks according to formula II;

${A A}_{i i} = = \frac{{e e}_{i i}}{{e e}_{00 i i}} - - - - - - I I I I;;$

(2)测量地表各条裂缝的长度L_i及其裂缝所占的总面积A_s，按照公式III计算地表岩心裂缝的孔隙度 (2) Measure the length L _i of each fracture on the surface and the total area A _s occupied by the fractures, and calculate the porosity of the surface core fractures according to formula III

${φ φ}_{f f} = = ((\frac{11}{{A A}_{s the s}})) {Σ Σ}_{i i = = 11}^{n no} {L L}_{i i} \cdot \cdot {e e}_{00 i i} - - - - - - I I I I I I;;$

所述公式III中，n代表样本中裂缝的总条数；In said formula III, n represents the total number of cracks in the sample;

(3)将所述A_i和代入公式IV中，计算得到地下裂缝的孔隙度φ'_f；(3) combine the A _i and Substitute into Formula IV to calculate the porosity φ' _f of the underground fracture;

φ'_f＝A_i×φ_f IV。φ' _f =A _i ×φ _f IV.

本发明还保护所述方法或装置在确定地下裂缝渗透率中的应用，包括以下步骤：The present invention also protects the application of the method or device in determining the permeability of underground fractures, including the following steps:

${A A}_{i i} = = \frac{{e e}_{i i}}{{e e}_{00 i i}} - - - - - - I I I I;;$

(2)测量地表裂缝的长度L_i及其所占面积A_s，按照公式V计算地表岩心裂缝的渗透率K_ff；(2) Measure the length _{Li and the occupied area A s} _of the surface fracture, and calculate the permeability K _ff of the surface core fracture according to formula V;

${K K}_{f f} = = C C \cdot \cdot \frac{11}{{A A}_{s the s}} \cdot \cdot {Σ Σ}_{i i = = 11}^{n no} {e e}_{00 i i}^{33} \cdot \cdot {L L}_{i i} - - - - - - V V;;$

所述公式III中，n代表样本中裂缝的总条数；C代表拟合系数；In the formula III, n represents the total number of cracks in the sample; C represents the fitting coefficient;

(3)将所述A_i和K_f代入公式VI中，计算得到地下裂缝的渗透率K'_f；(3) Substituting said A _i and K _f into formula VI, calculates the permeability K' _f of the underground fracture;

${K K}_{f f}^{' '} = = {A A}_{i i}^{33} \times \times {K K}_{f f} - - - - - - V V I I . .$

本发明提供的方法及装置，为确定地层围压条件下裂缝张开度、孔隙度和渗透率提供了可行途径，能够定量地恢复天然裂缝张开度、天然裂缝孔隙度和天然裂缝渗透率在不同地层围压条件下的真实分布情况，为客观合理地评价天然裂缝在致密低渗透储层中的储集、渗流作用和贡献大小提供了有效手段。The method and device provided by the present invention provide a feasible way for determining fracture opening, porosity and permeability under formation confining pressure conditions, and can quantitatively restore natural fracture opening, natural fracture porosity and natural fracture permeability under different formation confining pressures. The real distribution under these conditions provides an effective means for objectively and reasonably evaluating the accumulation, seepage and contribution of natural fractures in tight low-permeability reservoirs.

附图说明Description of drawings

图1为实施例1所述装置的示意图；Fig. 1 is the schematic diagram of the device described in embodiment 1;

图2是实施例2所得不同静岩围压下的裂缝开度分布关系图；Fig. 2 is the crack opening degree distribution diagram under different static rock confining pressures obtained in embodiment 2;

图3是实施例2所得地表岩心裂缝张开度的分布频率图；Fig. 3 is the distribution frequency figure of embodiment 2 gained surface core fracture openness;

图4是实施例2所得岩心裂缝张开度恢复至地层围压条件下的分布频率图；Fig. 4 is the distribution frequency diagram under the condition of formation confining pressure that the crack opening degree of the rock core obtained in Example 2 is restored;

图5是实施例2所得地表岩心裂缝孔隙度的分布频率图；Fig. 5 is the distribution frequency figure of surface core fracture porosity obtained in embodiment 2;

图6是实施例2所得岩心裂缝孔隙度恢复至地层围压条件下的分布频率图；Fig. 6 is the distribution frequency diagram under the condition that the core fracture porosity obtained in Example 2 is restored to the formation confining pressure;

图7是实施例2所得地表岩心裂缝渗透率分布频率图；Fig. 7 is the surface core fracture permeability distribution frequency figure obtained in embodiment 2;

图8是实施例2所得岩心裂缝渗透率恢复至地层围压条件下的分布频率图。Fig. 8 is a distribution frequency diagram of the core fracture permeability obtained in Example 2 under the condition of recovering the formation confining pressure.

具体实施方式detailed description

以下实施例用于说明本发明，但不用来限制本发明的范围。The following examples are used to illustrate the present invention, but are not intended to limit the scope of the present invention.

实施例1Example 1

本实施例提供了一种确定地下裂缝张开度的装置，如图1所示，所述装置包括加载系统(1)、压力控制系统(2)和测量处理系统(3)；This embodiment provides a device for determining the opening degree of underground cracks, as shown in Figure 1, the device includes a loading system (1), a pressure control system (2) and a measurement processing system (3);

实施例2Example 2

本实施例利用实施例1提供的装置得到的不同静岩围压下的裂缝开度分布关系图(图2)，反映了随着地层围压(深度)增加，裂缝的张开度呈负指数函数递减的变化规律。This embodiment uses the device provided in Example 1 to obtain the fracture opening distribution diagram (Fig. 2) under different static rock confining pressures, reflecting that as the formation confining pressure (depth) increases, the fracture opening decreases with a negative exponential function change rule.

具体为：Specifically:

确定裂缝检测区域，测量深度为x的地下裂缝所处区域的静岩围压，按照公式I计算地下裂缝张开度e_i：Determine the fracture detection area, measure the static rock confining pressure in the area where the underground fracture is located at a depth of x, and calculate the opening degree e _i of the underground fracture according to formula I:

e_i＝a^f(x)+b I；e _i = a ^f(x) + b I;

所述公式I中，f(x)代表裂缝深度为x时所处区域的静岩围压；所述f(x)的计算公式为：In the formula 1, f (x) represents the static rock confining pressure of the region when the fracture depth is x; the calculation formula of the f (x) is:

$f f ((x x)) = = \frac{μ μ}{11 - - μ μ} H h f f (({σ σ}_{v v})) s the s i i n no θ θ + + {Hρ Hρ}_{s the s} c c o o s the s θ θ + + H h f f (({σ σ}_{H h})) s the s i i n no θ θ c c o o s the s α α + + H h f f (({σ σ}_{h h})) s the s i i n no θ θ c c o o s the s α α - - {γHρ γHρ}_{00}$

其中，μ为泊松比；H为埋藏深度；f(σH)、f(σv)、f(σh)分别水平最大主应力、垂向应力和水平最小主应力随深度变化的梯度；ρ₀为流体容重；θ、α分别为裂缝倾角以及裂缝走向与水平最大主应力方向的夹角；γ为孔隙流体压力系数；以上各参数均采用本领域常规方法测量确定；Among them, μ is Poisson’s ratio; H is the burial depth; f(σH), f(σv), f(σh) are the gradients of horizontal maximum principal stress, vertical stress and horizontal minimum principal stress with depth; _ρ0 is Fluid bulk density; θ and α are the fracture dip angle and the angle between the fracture strike and the horizontal maximum principal stress direction; γ is the pore fluid pressure coefficient; the above parameters are measured and determined by conventional methods in the field;

所述a、b采用数学拟合的方法获得；The a and b are obtained by mathematical fitting;

经常规方法测量，裂缝在地表的张开度主要为150-400微米，平均为307.0微米；该地区地表裂缝张开度的分布频率如图3所示；Measured by conventional methods, the opening degree of cracks on the surface is mainly 150-400 microns, with an average of 307.0 microns; the distribution frequency of surface crack openings in this area is shown in Figure 3;

采用本实施例提供的方法计算，该地区地下裂缝的张开度主要为120-320微米，平均为241.2微米，与地表裂缝张开度相比，地下裂缝的张开度平均下降21.5％；该地区地层围压条件下的地下裂缝的张开度分布频率如图4所示；Calculated by the method provided in this embodiment, the opening degree of underground fractures in this area is mainly 120-320 microns, with an average of 241.2 microns. Compared with the opening degree of surface cracks, the opening degree of underground fractures decreases by 21.5% on average; under the formation confining pressure condition in this area The distribution frequency of the opening degree of underground fractures is shown in Fig. 4;

将所述地下裂缝的张开度e_i以及地表裂缝的张开度e_0i代入公式II，计算地下裂缝张开度与地表裂缝张开度的比例系数A_i；Substituting the opening degree e _i of the underground cracks and the opening degree e _oi of the surface cracks into formula II to calculate the proportional coefficient A _i of the opening degree of the underground cracks and the opening degree of the surface cracks;

${A A}_{i i} = = \frac{{e e}_{i i}}{{e e}_{00 i i}} - - - - - - I I I I;;$

测量地表各条裂缝的长度L_i及其裂缝所占的总面积A_s，按照公式III计算地表岩心裂缝的孔隙度 Measure the length L _i of each fracture on the surface and the total area A _s occupied by the fractures, and calculate the porosity of the surface core fractures according to formula III

将所述A_i和代入公式IV中，计算得到地下裂缝的孔隙度φ'_f；The A _i and Substitute into Formula IV to calculate the porosity φ' _f of the underground fracture;

φ'_f＝A_i×φ_f IV；φ' _f = A _i × φ _f IV;

经常规方法测量，该地区地表裂缝孔隙度主要为0.6％-1.4％，平均为1.02％；该地区地表裂缝孔隙度的分布频率如图5所示；Measured by conventional methods, the surface fracture porosity in this area is mainly 0.6%-1.4%, with an average of 1.02%. The distribution frequency of surface fracture porosity in this area is shown in Figure 5;

采用本实施例提供的方法计算，该地区地下裂缝的孔隙度主要为0.4％-1.2％，平均为0.81％，与地表裂缝孔隙度相比，地下裂缝孔隙度平均下降20.6％；该地区地下裂缝孔隙度在地层围压条件下的分布频率如图6所示；Calculated by the method provided in this example, the porosity of underground fractures in this area is mainly 0.4%-1.2%, with an average of 0.81%. Compared with the porosity of surface fractures, the porosity of underground fractures decreases by an average of 20.6%. The distribution frequency of porosity under formation confining pressure is shown in Fig. 6;

按照常规方法测量地表裂缝的长度L_i及其所占面积A_s，按照公式V计算地表岩心裂缝的渗透率K_f；Measure the length L _i and the occupied area A _s of surface fractures according to conventional methods, and calculate the permeability K _f of surface core fractures according to formula V;

所述公式V中，n代表样本中裂缝的总条数；C代表拟合系数；In the formula V, n represents the total number of cracks in the sample; C represents the fitting coefficient;

将所述A_i和K_f代入公式VI中，计算得到地下裂缝的渗透率K'_f；Substituting said A _i and K _f into formula VI, calculates the permeability K' _f of underground fractures;

${K K}_{f f}^{' '} = = {A A}_{i i}^{33} \times \times {K K}_{f f} - - - - - - V V I I;;$

经常规方法测量，该地区地表裂缝的渗透率主要为40-100豪达西，平均为87.07豪达西；该地区地表裂缝渗透率分布频率如图7所示；Measured by conventional methods, the permeability of surface fractures in this area is mainly 40-100 Hao Darcy, with an average of 87.07 Hao Darcy; the distribution frequency of surface fracture permeability in this area is shown in Figure 7;

采用本实施例提供的方法计算，该地区地下裂缝的渗透率主要为10-25豪达西，平均为19.4豪达西，与地表裂缝渗透率相比，地下裂缝的渗透率平均下降77.8％；该地区地下裂缝渗透率在地层围压条件下的分布频率如图8所示。Calculated by the method provided in this embodiment, the permeability of underground fractures in this area is mainly 10-25 Hao Darcy, with an average of 19.4 Hao Darcy. Compared with the permeability of surface fractures, the permeability of underground fractures decreases by 77.8% on average; The distribution frequency of underground fracture permeability in this area under formation confining pressure is shown in Fig. 8.

上述结果反映裂缝在地下的张开度、孔隙度和渗透率都比地表下降许多，只有恢复到地下的裂缝参数，才能够客观地反映和评价裂缝对致密低渗透储层的作用及贡献大小。The above results show that the opening degree, porosity and permeability of fractures in the underground are much lower than those on the surface. Only the fracture parameters restored to the underground can objectively reflect and evaluate the role and contribution of fractures to tight low-permeability reservoirs.

本发明可以通过成像测井测料、密闭取心资料来检测参数的准确性。所得地下裂缝的张开度、裂缝的孔隙度和裂缝的渗透率，可以客观地定量评价天然裂缝对致密低渗透油气储层的作用和贡献大小，正确地指导裂缝性致密低渗透油气藏的勘探和合理开发。The present invention can detect the accuracy of parameters through imaging well logging data and sealed coring data. The obtained underground fracture opening degree, fracture porosity and fracture permeability can objectively and quantitatively evaluate the role and contribution of natural fractures to tight and low-permeability oil and gas reservoirs, and correctly guide the exploration and rationalization of fractured tight and low-permeability oil and gas reservoirs. develop.

虽然，上文中已经用一般性说明、具体实施方式及试验，对本发明作了详尽的描述，但在本发明基础上，可以对之作一些修改或改进，这对本领域技术人员而言是显而易见的。因此，在不偏离本发明精神的基础上所做的这些修改或改进，均属于本发明要求保护的范围。Although, the present invention has been described in detail with general description, specific implementation and test above, but on the basis of the present invention, some modifications or improvements can be made to it, which will be obvious to those skilled in the art . Therefore, the modifications or improvements made on the basis of not departing from the spirit of the present invention all belong to the protection scope of the present invention.

Claims

1. A method for determining the degree of opening of underground cracks, characterized in that, comprises the following steps: determining the crack detection area, measuring depth is the static rock confining pressure in the area where the underground cracks of x are located, and calculating the degree of opening of underground cracks e _i according to formula 1:

e _i = a ^f(x) + b I;

In the formula I, f(x) represents the static rock confining pressure of the region where the fracture depth is x; a and b are fitting coefficients.

2. method according to claim 1, is characterized in that, the computing formula of described f (x) is:

f f ((x x)) = = \frac{μ μ}{11 - - μ μ} H h f f (({σ σ}_{v v})) s the s i i n no θ θ + + {Hρ Hρ}_{s the s} c c o o s the s θ θ + + H h f f (({σ σ}_{H h})) s the s i i n no θ θ c c o o s the s α α + + H h f f (({σ σ}_{h h})) s the s i i n no θ θ c c o o s the s α α - - {γHρ γHρ}_{00}

Among them, μ is Poisson’s ratio; H is the burial depth; f(σH), f(σv), f(σh) are the horizontal maximum principal stress, vertical stress and horizontal minimum principal stress gradient respectively; ρ0 is the fluid bulk density; _θ , α are the fracture dip angle and the angle between the fracture strike and the horizontal maximum principal stress direction; γ is the pore fluid pressure coefficient.

3. The method according to claim 1 or 2, characterized in that the coefficients a and b are manually determined according to the characteristics of regional fractures or obtained by mathematical fitting.

4. A device for determining the opening degree of underground cracks, characterized in that the device comprises a loading system (1), a pressure control system (2) and a measurement processing system (3);

The center of the loading system (1) is a sample placement unit; the periphery of the sample placement unit is a confining pressure loading unit;

The pressure control system (2) includes a formation confining pressure control unit and an axial load control unit;

The measurement processing system (3) is used for parameter measurement, metering and post-processing of experimental data.

5. The method according to any one of claims 1 to 3 or the application of the device according to claim 4 in determining the porosity of underground fractures, is characterized in that it comprises the following steps:

(1) measure the opening degree e _i of underground cracks, and measure the opening degree e _0i of surface cracks, and calculate the ratio coefficient A _i of the opening degree of underground cracks and the opening degree of surface cracks according to formula II;

{A A}_{i i} = = \frac{{e e}_{i i}}{{e e}_{00 i i}} - - - - - - I I I I;;

(2) Measure the length L _i of each fracture on the surface and the total area A _s occupied by the fractures, and calculate the porosity of the surface core fractures according to formula III

{φ φ}_{f f} = = ((\frac{11}{{A A}_{s the s}})) {Σ Σ}_{i i = = 11}^{n no} {L L}_{i i} \cdot \cdot {e e}_{00 i i} - - - - - - I I I I I I;;

In said formula III, n represents the total number of cracks in the sample;

(3) combine the A _i and Substitute into Formula IV to calculate the porosity φ' _f of the underground fracture;

φ' _f =A _i ×φ _f IV.

6. The application of the method according to any one of claims 1 to 3 or the device according to claim 4 in determining the permeability of underground fractures, characterized in that it comprises the following steps:

{A A}_{i i} = = \frac{{e e}_{i i}}{{e e}_{00 i i}} - - - - - - I I I I;;

(2) Measure the length _{Li and the occupied area A s} _of the surface fracture, and calculate the permeability K _f of the surface core fracture according to formula V;

{K K}_{f f} = = C C \cdot &Center Dot; \frac{11}{{A A}_{s the s}} \cdot &Center Dot; {Σ Σ}_{i i = = 11}^{n no} {e e}_{00 i i}^{33} \cdot &Center Dot; {L L}_{i i} - - - - - - V V;;

In the formula III, n represents the total number of cracks in the sample; C represents the fitting coefficient;

(3) Substituting said A _i and K _f into formula VI, calculates the permeability K' _f of the underground fracture;

{K K}_{f f}^{' '} = = {A A}_{i i}^{33} \times \times {K K}_{f f} - - - - - - V V I I . .