CN102518417A

CN102518417A - Method for determining output volume of hydrofracture of shale gas reservoir

Info

Publication number: CN102518417A
Application number: CN2011104219936A
Authority: CN
Inventors: 赵海峰; 陈勉; 金衍; 张伟
Original assignee: China University of Petroleum Beijing
Current assignee: China University of Petroleum Beijing
Priority date: 2011-12-16
Filing date: 2011-12-16
Publication date: 2012-06-27

Abstract

The invention relates to a method for determining the smallest output volume of staged fracturing modification of a shale gas horizontal well in the field of petroleum engineering, which aims to guarantee that natural cracks are communicated high-efficiently in hydrofracture operation and a highly dense pattern cracking system is formed. Clear fracture liquid is used for brittle shale fracture, the highly dense pattern cracking system is formed after manual fracturing modification, and is essentially different from two symmetric wing cracks formed by fracturing by the aid of traditional high-viscosity fracturing liquid, and an existing fracturing design model based on rock quasi-static mechanics analysis difficultly describes features of shale cracks. A rock fracture dynamics method is used to build a pattern cracking system forming model, the relationship of construction output volume factors, natural factors including crustal stress, natural crack occurrence, reservoir rock mechanics parameters and the like and crack extension is built, and accordingly the method for calculating the smallest critical output volume is realized.

Description

Method for determining shale gas reservoir hydraulic fracturing discharge capacity

Technical Field

The invention relates to a method for determining the minimum displacement of staged fracturing modification of a shale gas horizontal well in the field of petroleum engineering, which is used for ensuring that natural fractures are efficiently communicated in hydraulic fracturing operation to form a highly dense reticular fracture system.

Background

Unconventional oil and gas is an important strategic continuation of fossil energy, with the development of shale gas having attracted considerable attention in the international energy industry in recent years. Shale gas belongs to unconventional natural gas, is natural gas with biological cause and/or thermal cause in an organic-rich shale stratum in an adsorption and/or free state, and has geological characteristics of self-generation and self-storage, adsorption and accumulation, hidden accumulation and the like. Compared with the conventional natural gas, the shale gas development has the advantages of large resource potential, long exploitation life and long production period. Most of gas-producing shale has wide distribution range, large thickness, common gas content and huge shale gas resource amount, so that the shale gas well can produce gas at a stable rate for a long time, the general exploitation life can reach 30-50 years and even longer, and the shale gas well has great commercial value.

The shale gas reservoir conditions belong to fractured hypotonicity/ultra-hypotonicity, and staged clear water fracturing modification (no sand or a small amount of sand) of a horizontal well is a technical means for effectively developing shale gas. At present, 85% of shale gas development wells in the United states are horizontal wells and multi-stage fracturing, and if 5-7 stage fracturing is adopted in part of development wells in Woodford shale by the New field company in the United states, the shale gas yield increasing effect is obvious. However, the geological and mechanical characteristics of the shale gas reservoir are greatly different from those of conventional oil and gas resources, and the fracture-making mechanism and the fracture influence factors of the fracturing are not clearly recognized, so that quantitative scientific basis is lacked for the design and implementation of the fracturing.

The brittle shale fracturing adopts clear water fracturing fluid, a highly dense reticular fracture system is formed after manual fracturing transformation, the system is essentially different from two symmetrical wing-shaped fractures formed by the traditional high-viscosity fracturing fluid, and the shale fracture characteristics are difficult to describe by the existing two-dimensional, three-dimensional simulation and full three-dimensional fracturing design model based on rock quasi-static force analysis. Fracture propagation based on shale fracturing belongs to dynamic fracture of brittle materials, a penman adopts a rock fracture dynamics method to establish a forming model of a reticular fracture system, and a key construction parameter, namely a minimum displacement calculation method, is given.

Disclosure of Invention

The invention aims to provide a method for determining the minimum displacement of the staged fracturing reformation of a shale gas horizontal well. The method adopts a rock fracture dynamics method to establish a forming model of a net-shaped fracture system, and establishes the relationship between the construction displacement factor, the natural factors such as the ground stress, the natural fracture occurrence, the reservoir rock mechanics parameters and the like and the fracture extension, so that the minimum critical displacement calculation method is obtained.

The object of the invention is achieved by considering a certain stage of fracturing of a horizontal well, the plan view is shown in fig. 1, the well axis is along the direction of least ground stress. The hydraulic fracture extends along the ground stress direction near the well bore, a left primary branch and a right primary branch are formed after the hydraulic fracture meets the natural fracture, the primary branch fracture is turned after extending to the end part of the primary branch fracture and continues to extend along the ground stress direction, and the primary branch fracture forms two secondary branch fractures after meeting the natural fracture. By analogy, the hydraulic fractures and the natural fractures are staggered with each other to form a highly dense reticular fracture system.

If the fracturing discharge capacity is small, the pressure of the hydraulic fracture is lower than the pressure required for steering after communicating with the natural fracture, and the steering process can be described by a fracture statics method along with the gradual rise of the injection pressure of the fracturing fluid until steering occurs at the left end or the right end. However, in order to form complex network fractures, it is desirable that the left and right fractures simultaneously satisfy the diversion condition, and the pressure is higher than the pressure required by quasi-static propagation, and fracture propagation must be described by adopting a fracture dynamics method.

Establishing a coordinate system by taking the well axis direction as X, the maximum ground stress direction as Y and the vertical direction as Z, assuming that the inclination angle of the natural fracture is beta and the trend forms an included angle theta with the well axis, taking the nth grade branch fracture as an example, and the unit normal vector of the fracture surface is taken as

n＝[cosβsinθ，cosβcosθ，sinβ] (1)

The positive stress acting on the fracture surface is

In the formula sigma_h、σ_HAnd sigma_vRespectively, horizontal minimum ground stress, horizontal maximum ground stress, and overburden pressure. When the fracturing fluid is filled in the natural fracture, dynamic stress intensity factors of the ends of the left branch fracture and the right branch fracture are respectively calculated by adopting a superposition principle, as shown in figure 2, the length of the natural fracture is recorded as 2a, and the natural fracture is divided into a by the intersection point of the hydraulic fracture and the natural fracture₁、a₂(a₁＜a₂) Two sections.

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As can be seen from the formula (3), the dynamic stress intensity factor of the point A is smaller than that of the point B, and the point B is automatically satisfied only if the point A satisfies the crack turning condition,

K_A＝K_Id (4)

in the formula K_IdIs the rock dynamic fracture toughness constant, but is related to the loading rate. The crack internal pressure p required for crack steering can be determined by the expressions (2) to (4), and the discharge capacity can be determined according to the crack internal pressure condition. N-th stage branch fracture discharge Q_nIn relation to the pump displacement Q

Q_n＝Q/2ⁿ (5)

This assumption is appropriate for elongated shale hydraulic fractures, considering that the fracturing fluid flow is one-dimensional laminar flow along the length of the fracture, ignoring flow in the width, height directions within the fracture and complex flow at the fracture joint points, ends. Based on the plate flow principle, the pressure p (y) and the discharge Q in the fracture of the nth branch can be obtained_nThe general relationship of (a) to (b),

wherein mu is the viscosity coefficient of the fracturing fluid, E is the elastic modulus of the stratum, h is the fracture height (generally equal to the thickness of the reservoir), and rho is the density of the fracturing fluid; strictly speaking P (y) is a function of the distribution along the slit length, F (y), P (y) are approximately linear distribution functions, the gradient of pressure variation is small for the elongated slits, and the mean values F ≈ 2.607 and P ≈ 84.53 are taken here.

The minimum displacement required for simultaneous diversion of the branch fractures can be determined by the equations (2) to (6).

Drawings

The drawings are only for purposes of illustrating and explaining the present invention and are not to be construed as limiting the scope of the present invention. Wherein,

FIG. 1: horizontal well fracture extension plan view

FIG. 2: calculating dynamic stress intensity factor of branch crack end

Detailed Description

In order to more clearly understand the technical features, objects, and effects of the present invention, embodiments of the present invention will now be illustrated.

1. And (3) figuring out the mechanical parameters and the natural fracture occurrence of the shale stratum rock, including the ground stress, the elastic modulus, the dynamic fracture toughness, the natural fracture inclination angle, the included angle between the natural fracture strike and the well axis, the half length of the natural fracture, the fracturing fluid density, the viscosity coefficient, the reservoir thickness and the fracture branch grade number. For example, a shale reservoir depth of 2170 m, geostress σ_h＝34.8MPa、σ_H47.7MPa and sigma_v56.5MPa, stratum elastic modulus E20000 MPa, dynamic fracture toughnessβ＝30θ is 60 °, a is 6m, a in fig. 1₂＝10m，μ＝0.001Pa·S，ρ＝1000kg/m³，h＝5m，n＝3。

2. The positive stress acting on the fracture surface was calculated from the formulas (1) and (2), and the result here was 42.64 MPa.

3. The pressure p in the crack required for crack diversion was calculated from the equations (3) and (4), and here was 44.07 MPa.

4. Calculating the n-th branch fracture discharge Q by the formula (6)_nHere 0.56m³In terms of a/minute.

5. The pump displacement Q, here 4.48m, is calculated from equation (5)³In terms of a/minute.

Claims

1. Fracture dynamics model of shale fracture extension, its characterized in that: the hydraulic fracture extends near a shaft along the ground stress direction, a left first-level branch and a right first-level branch are formed after encountering a natural fracture, the first-level branch fracture turns after extending to the end part of the first-level branch fracture and continues to extend along the ground stress direction, the first-level branch fracture forms two second-level branch fractures after encountering the natural fracture, and the rest is done to form more branch fractures; since the left and right branches extend simultaneously, at pressures higher than those required for quasi-static extension, a fracture dynamics approach must be used for modeling.

2. The method for calculating the pressure in the shale fracture extension fracture is characterized by comprising the following steps: when the fracturing fluid is filled in the natural fracture, the natural fracture is divided into two sections by the intersection point of the hydraulic fracture and the natural fracture, the dynamic stress intensity factors of the ends of the left branch fracture and the right branch fracture are respectively calculated by adopting a superposition principle, and the fracture internal pressure calculation formula is established by taking the rock dynamic fracture toughness as a fracture extension criterion.

3. The method for determining the shale hydraulic fracturing discharge capacity is characterized by comprising the following steps: according to the flowing of the fracturing fluid which is one-dimensional laminar flow along the length direction of the fracture, the flowing in the width direction and the height direction in the fracture and the complex flowing of the fracture connecting point and the end part are neglected, the quadratic function relation between the pressure in the fracture and the discharge capacity of the nth-stage branch fracture can be obtained according to the flat plate flowing principle, and then the minimum discharge capacity required by the hydraulic fracturing operation is determined according to the pressure in the fracture required by the extension of the shale fracture.