Abstract

Although hydraulic fracturing has been used for several decades in the stimulation of hydrocarbon reservoirs, a thorough understanding of the interwoven phenomena is still lacking, especially in the phenomena is still lacking, especially in the case of porous media. This paper describes a two-dimensional numerical approach allowing the study of the fluid/rock skeleton interaction. The elastic stress analysis is coupled with the transient fluid-flow phenomenon. By introducing a failure criterion based on the critical stress intensity factor, quasi-static situations can easily be studied and instability conditions outlined.

This approach was used to interpret fracture initiation, fracture reopening as well as fracture closure after shut-in. The pressure/time curves generated for each of these phenomena clearly depend on the percolation characteristics of the fracturing fluid and the contact time as well as on the intrinsic properties of the rock mass.

Introduction

During hydraulic fracturing of geological formations, fluid losses occur through the matrix and the preexisting cracks, leading to a change in the surrounding preexisting stress field due to this process. The mass balance between the injected fluid volume, the fluid losses in the rock mass and the volume required for fracture growth is not independent. To date, considerable research has been directed toward the understanding of the fracture geometry, and relatively few attempts have been made to realistically study the influence of fluid flow and heat transfer.

The importance of treating the hydraulic fracturing process as a coupled poroelastic phenomenon has been mentioned several times in the literature from different points of view. The interaction between the pore fluid and the elastic rock skeleton definitely affects the stability of the crack growth. Not only can the pore pressure change a catastrophic failure mechanism to a quasi-static propagation phenomenon, but an increase in pore propagation phenomenon, but an increase in pore pressure can also cause unanticipated fracture pressure can also cause unanticipated fracture initiation. In addition, the leakoff or its restraint by pore-pressure buildup will obviously reduce or increase the efficiency of the fracturing operation.

One of the problems in suitable fracture modeling is tracking the topology of the moving boundary. Automatic and movable mesh generation using the Boundary Integral Method is one approach to this problem. Other problems include the intrinsic difficulty in dealing with the poroelastic formulations. poroelastic formulations. This paper outlines a numerical approach in which total coupling of the transient fluid mechanism to the elastic stress distribution, due to a propagating fracture, has been achieved. At present, the model is limited to two-dimensional, pseudostatic situations. The numerical technique pseudostatic situations. The numerical technique is based on the Body Force Method which will now be briefly described.

NUMERICAL APPROACH

The Body Force Method (later referred to as BFM) is a Boundary Integral Equation Method (BIEM). It stems from superpositions of the fundamental solution of a point-load force acting in an infinite elastic medium. In order to satisfy the boundary conditions encountered during the initiation phase of hydraulic fracturing, the BFM was extended by considering the problem of a point load force acting in an infinite plate containing a circular hole. The crack itself is represented by a series of equal and opposite forces acting along the faces of the fracture.

The transient pore pressure distribution is obtained using a Line Source Flow Distribution Method (later referred to an LSFDM) which considers the superpositions of the fundamental solution for flow distribution resulting from a point source in a two-dimensional infinite medium.

p. 129

This content is only available via PDF.
You can access this article if you purchase or spend a download.