阅读说明：本技术 一种利用地震资料计算地应力差异系数的方法 (Method for calculating ground stress difference coefficient by using seismic data ) 是由 齐晴 孙振涛 田建华 陈勇 杨勤林 朱博华 胡玮 史飞洲 裴思嘉 于 2018-08-20 设计创作，主要内容包括：本发明提出了一种利用地震资料计算地应力差异系数的方法,其中,所述方法包括以下步骤：S1：基于地震资料建立地层的应力场；S2：拟合地层面的趋势函数,计算所述地层面上点的曲率变形分量,基于所述曲率变形分量,得出地层面的应力张量；S3：基于所述应力张量得出最大水平主应力和最小水平主应力；S4：基于所述最大水平主应力和所述最小水平主应力,得出地应力差异系数。本发明的方法能够利用三维地震资料计算地应力差异系数；同时可以利用大范围的地应力差异系数计算结果划分开发单元,指导水平井部署,可靠性较高。(The invention provides a method for calculating a ground stress difference coefficient by using seismic data, wherein the method comprises the following steps: s1: establishing a stress field of a stratum based on seismic data; s2: fitting a trend function of the ground surface, calculating a curvature deformation component of a point on the ground surface, and obtaining a stress tensor of the ground surface based on the curvature deformation component; s3: deriving a maximum horizontal principal stress and a minimum horizontal principal stress based on the stress tensor; s4: and deriving a ground stress difference coefficient based on the maximum level principal stress and the minimum level principal stress. The method can utilize three-dimensional seismic data to calculate the ground stress difference coefficient; meanwhile, development units can be divided by utilizing the calculation result of the ground stress difference coefficient in a large range, horizontal well deployment is guided, and reliability is high.)

1. A method for calculating a geostress coefficient of variation using seismic data, the method comprising the steps of:

s1: establishing a stress field of a stratum based on seismic data;

s2: fitting a trend function of the ground surface, calculating a curvature deformation component of a point on the ground surface, and obtaining a stress tensor of the ground surface based on the curvature deformation component;

s3: deriving a maximum horizontal principal stress and a minimum horizontal principal stress based on the stress tensor;

s4: and deriving a ground stress difference coefficient based on the maximum level principal stress and the minimum level principal stress.

2. The method of claim 1,

in step S1, a stress field of the formation is established based on the sheet theory by using the formation information, the velocity information, and the density information of the formation, in combination with the deformation geometry equation and the stress-strain relationship.

3. The method of claim 2,

in step S2, a least squares fit is used to the trend function of the formation.

4. The method of claim 3,

in step S3, a maximum horizontal principal stress and a minimum horizontal principal stress are derived from the stress tensor based on the stress moire circle theory.

5. The method of claim 3,

the trend function is set to:

w(x,y)＝a₀+a₁x+a₂y+a₃x²+a₄xy+a₅y²。

6. the method of claim 5,

the curvature deformation component is calculated according to the following formula,

wherein, in the above formula,

w is displacement components on three coordinate axes respectively;

a₃、a₄and a₅Are all coefficients in a trend function;

κ_xand kappa_yRepresenting the curvature deformation components in the x and y directions, respectively;

κ_xyrepresents the xy planeAn inner curvature deformation component.

7. The method of claim 6,

the stress tensor of the formation plane is found according to the following formula,

wherein, in the above formula,

σ_xand σ_yRepresenting positive stress in the x and y directions, respectively;

ν is the poisson ratio, E is the elastic modulus, and t is the formation thickness.

8. The method of claim 7,

the maximum horizontal principal stress and the minimum horizontal principal stress are found according to the following equations,

wherein, in the above formula,

σ_xand σ_yDenotes the positive stress in x and y directions, respectively, tau_xyRepresents the shear stress in the xy plane;

σ_maxrepresents the maximum horizontal principal stress sum σ_minRepresenting the minimum level principal stress.

9. The method of claim 8,

the ground stress difference coefficient K is obtained according to the following formula,

Technical Field

The invention relates to the field of seismic exploration and development, in particular to a method for calculating a ground stress difference coefficient by using seismic data.

Background

With the development of the petroleum industry, unconventional oil and gas resources gradually become the main body of production increase of oil and gas fields in various countries. The shale gas reservoir has wide distribution in the global scope, compared with the conventional oil gas reservoir, the shale gas reservoir has the characteristics of self-generation and self-storage and low porosity and low permeability, and the key of large-scale exploitation of the low porosity and low permeability shale gas reservoir lies in the application of a horizontal well fracturing technology. The shale gas horizontal well is developed, firstly, a target layer is perforated, then fracturing and crack forming are carried out, a propping agent in fracturing fluid plays a supporting role on the pores of a shale matrix, the porosity and permeability of the matrix are improved, and whether the shale reservoir can be fractured into net-shaped cracks is an important factor for increasing the yield of the horizontal well.

CN201710406786 discloses a method for calculating the geostress difference coefficient of a shale gas reservoir, which is characterized in that the geostress difference coefficient △ Ki of the shale reservoir is calculated through parameters such as stratum pore fluid pressure gradient FPG of the shale reservoir, rock density DEN of an overlying stratum of the shale reservoir and the like, the compressibility of the shale reservoir is evaluated according to △ Ki, and the evaluation method describing the compressibility of the shale reservoir is high in coincidence rate is verified through the staged fracturing effect of the shale reservoir at the horizontal section of a horizontal well.

Disclosure of Invention

In order to overcome the defects of the prior art, the invention provides a method for calculating a ground stress difference coefficient by using seismic data. According to the method, the structure information, the speed information and the density information of the stratum are utilized based on seismic data, the stress field of the stratum is established, the stress strain tensor and the curvature tensor of the stratum surface (namely the ground surface) are obtained through calculation, the principal curvature, the principal strain and the principal stress of the stratum surface are solved, the ground stress difference coefficient is finally calculated, the fracturing effect of the shale gas horizontal well is improved, the innovation is achieved, the foundation is laid for the development of the shale gas horizontal well, and the international lead is achieved.

In order to achieve the above object, the present invention provides a method for calculating a ground stress difference coefficient by using seismic data, wherein the method comprises the following steps:

s1: establishing a stress field of a stratum based on seismic data;

s2: fitting a trend function of the ground surface, calculating a curvature deformation component of a point on the ground surface, and obtaining a stress tensor of the ground surface based on the curvature deformation component;

s3: deriving a maximum horizontal principal stress and a minimum horizontal principal stress based on the stress tensor;

s4: and obtaining the ground stress difference coefficient number based on the maximum horizontal principal stress and the minimum horizontal principal stress.

The method as recited above, wherein,

in step S1, a stress field of the formation is established based on the sheet theory by using the formation information, the velocity information, and the density information of the formation, in combination with the deformation geometry equation and the stress-strain relationship.

The method as recited above, wherein,

in step S2, a least squares fit is used to the trend function of the formation.

The method as recited above, wherein,

in step S3, a maximum horizontal principal stress and a minimum horizontal principal stress are derived from the stress tensor based on the stress moire circle theory.

The method as recited above, wherein,

the trend function is set to:

w(x,y)＝a₀+a₁x+a₂y+a₃x²+a₄xy+a₅y²。

the method as recited above, wherein,

the curvature deformation component is calculated according to the following formula,

wherein, in the above formula,

w is displacement components on three coordinate axes respectively;

a₃、a₄and a₅Are all coefficients in a trend function;

κ_xand kappa_yRepresenting the curvature deformation components in the x and y directions, respectively; degree (C)

κ_xyRepresenting curvature deformation components in the xy plane

The method as recited above, wherein,

the stress tensor of the formation plane is found according to the following formula,

wherein, in the above formula,

σ_xand σ_yRepresenting positive stress in the x and y directions, respectively;

ν is the poisson ratio, E is the elastic modulus, and t is the formation thickness.

The method as recited above, wherein,

the maximum horizontal principal stress and the minimum horizontal principal stress are found according to the following equations,

wherein, in the above formula,

σ_xand σ_yDenotes the positive stress in x and y directions, respectively, tau_xyRepresents the shear stress in the xy plane;

σ_maxrepresents the maximum horizontal principal stress sum σ_minRepresenting the minimum level principal stress.

The method as recited above, wherein,

the ground stress difference coefficient K is obtained according to the following formula,

the method comprises the steps of calculating a ground stress difference coefficient by utilizing seismic data, integrating formation information, speed and density information of a stratum, establishing a stress field of the stratum, calculating a stress strain tensor and a curvature tensor of a stratum surface, solving a principal curvature, a principal strain and a principal stress of the stratum surface, and finally calculating the ground stress difference coefficient.

Compared with the prior art, the method of the invention has the following advantages:

(1) the geostress difference coefficient can be calculated by utilizing three-dimensional seismic data;

(2) the development units can be divided by utilizing the calculation results of the ground stress difference coefficient in a large range, the deployment of the horizontal well is guided, and the reliability is high.

Drawings

The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. In addition, the shapes, the proportional sizes, and the like of the respective members in the drawings are merely schematic for facilitating the understanding of the present invention, and do not specifically limit the shapes, the proportional sizes, and the like of the respective members of the present invention. Those skilled in the art, having the benefit of the teachings of this invention, may choose from the various possible shapes and proportional sizes to implement the invention as a matter of case.

FIG. 1 is a flow chart of a method of calculating a geostress coefficient of variation using seismic data in accordance with the present invention;

FIG. 2 is a flow chart of an embodiment of the present invention; and

FIG. 3 shows the prediction result of the difference coefficient of the ground stress of the Longmaxi group in the work area X.

Detailed Description

The details of the present invention can be more clearly understood in conjunction with the accompanying drawings and the description of the embodiments of the present invention. However, the specific embodiments of the present invention described herein are for the purpose of illustration only and are not to be construed as limiting the invention in any way. Any possible variations of the invention, which may be considered to be within the scope of the invention, will occur to those skilled in the art upon studying the disclosure and the accompanying drawings, and the invention will be further described below.

The ground stress generally refers to the internal force existing in the formation rock, the influence degree of the ground stress on the fracture morphology is mainly reflected on the magnitude of the horizontal principal stress difference, and the ratio between the difference between the maximum horizontal principal stress and the minimum horizontal principal stress is defined as a ground stress difference coefficient. When the local stress difference coefficient is smaller, the artificial cracks extend along the direction of the natural cracks, and the original natural cracks are communicated to form network cracks; along with the increase of the ground stress difference coefficient, the ground stress control action is gradually enhanced, the crack gradually expands along the direction vertical to the minimum horizontal main stress, and the crack form is relatively single.

The low gas layer pressure is one of the main characteristics of shale gas reservoir development, shale gas wells usually cannot perform self-blowout, horizontal well development is generally adopted, fracturing reformation of the horizontal well is performed, and artificial cracks are formed in the gas layer under the action of water power so as to improve the flowing capacity of fluid in the gas layer.

The ground stress difference coefficient is defined as the ratio between the difference between the maximum and minimum level principal stresses and the minimum level principal stress. The initiation and propagation of artificial fractures around the wellbore is affected by the remote stress field. When the local stress difference coefficient is smaller, the artificial cracks extend along the direction of the natural cracks, communicate the original natural cracks and form network cracks. When the local stress difference coefficient is larger, the natural cracks expand, the hydraulic cracks directly penetrate through the natural cracks at the intersection points, and continue to expand along the original direction of the maximum horizontal main stress to form two main cracks. Research shows that when the local stress difference coefficient is less than 0.1, reticular cracks are easy to generate. Therefore, the horizontal well should be designed by comprehensively considering the above conditions so as to obtain good fracturing effect and productivity.

Referring to fig. 1, the method for calculating a geostress difference coefficient using seismic data according to the present invention is characterized by comprising the steps of: s1: establishing a stress field of a stratum based on seismic data; s2: fitting a trend function of the ground surface, calculating a curvature deformation component of a point on the ground surface, and obtaining a stress tensor of the ground surface based on the curvature deformation component; s3: deriving a maximum horizontal principal stress and a minimum horizontal principal stress based on the stress tensor; s4: and deriving a ground stress difference coefficient based on the maximum level principal stress and the minimum level principal stress.

In one embodiment, the geostress analysis requires a relatively ideal model: it is assumed that the formation deformation of the formation is entirely stress induced and here the earth medium is isotropic, uniformly continuous and fully elastic. In general, the width and length of the formation undergoing bending deformation are much greater than its thickness, so that the stress state near the structural surface can be simulated using the theory of sheet bending.

And establishing a stress field of the stratum by utilizing the structural information of the stratum, the speed and the density information and combining a deformation geometric equation and a stress-strain relation based on a sheet theory. And fitting a trend function of the stratum surface by using a least square method, solving curvature components of each point, and calculating to obtain a stress strain tensor of the stratum surface (namely the stratum surface). The magnitude of the principal stress can be obtained according to the stress Mohr circle theory. And calculating the ground stress difference coefficient according to the maximum level principal stress and the minimum level principal stress.

In the rectangular coordinate system, the deformation geometric equation is as follows:

wherein, in the above formula,

ε_x、ε_yand ε_zRepresenting positive strain in the x, y and z directions, respectively

γ_xy、γ_yzAnd gamma_xzRepresenting the shear strain in the xy, yz and xz planes, respectively.

u, v and w are displacement components on three coordinate axes respectively

According to the theory of thin plates:

and has the following components:

defining the curvature deformation component as:

thus, the strain component can be written as:

ε_x＝zκ_x,ε_y＝zκ_y,γ_xy＝2zκ_xy(5)

in general, the stress versus strain relationship:

wherein, in the above formula (6),

v is Poisson's ratio and E is elastic modulus

σ_x、σ_yAnd σ_zRespectively representing positive stress in x, y and z directions; .

τ_xy、τ_xzAnd τ_yzRespectively representing the shear stress on the xy, yz and xz planes

From equation (6), the inverse relationship of stress versus strain can be derived as:

wherein the content of the first and second substances,

based on the theory of lamellas, due to σ _z0, so there is:

the relative bulk strain θ is:

or written as:

the stress is expressed as strain, and formula (9) is taken into formula (8) relative to σ_x、σ_yAnd solving to obtain:

finally, the following can be obtained:

thus, there are:

and the thickness of the stratum is t-2 z, wherein when t is the double-journey, z is the depth of the stratum. The stress component on the formation surface is obtained in the formula (13):

according to the stress Mohr circle theory, the main stress can be obtained:

therefore, the strain and stress can be further calculated by only obtaining the curvature of each point. The curvature is usually calculated by fitting a least squares method to the trend function of the formation surface, and then calculating the curvature component of the point thereon.

Setting the function of the undetermined coefficient of the trend surface as:

w(x,y)＝a₀+a₁x+a₂y+a₃x²+a₄xy+a₅y²(16)

fitting the n scatter points into one trend surface,

represents a trend value, Q is the trend value

As an original value w_iTo minimize Q, i.e.:

by solving the system of equations, the trend surface function of the formation can be obtained.

The calculated formula for the curvature of the trend surface is:

solving the equation set can obtain the coefficient a of the fitted surface of the stratum trend surface₃，a₄，a₅The curve at this point can be obtained from the formula (18)And (4) rate.

Corresponding stress parameters can be respectively calculated according to the formula (13), the formula (14) and the formula (18), so that the maximum horizontal principal stress and the minimum horizontal principal stress are obtained.

And calculating the difference coefficient K of the ground stress according to the maximum horizontal principal stress and the minimum horizontal principal stress:

σ in formula (19)_maxAnd σ_minRespectively the maximum and minimum horizontal principal stresses.