Abstract

The paper presents numerical modeling of experimental scale study of hydraulic fracture initiation and propagation for creation of Enhanced Geothermal Systems. The displacement discontinuity method, which is a variant of the boundary element method, is used to model the rock matrix deformation and stress distribution around the fracture surface. Parabolic crack tip elements are used to account for square root variation of the fracture front displacement and stresses. Newtonian fracturing fluid flow is modeled using the standard Galerkin’s Finite Element Method. The fracture initiation and propagation process are addressed following the linear elastic fracture mechanics. The hydraulic fracture simulation process presents a complex numerical problem in which physical processes involved such as rock matrix deformation, fracture fluid flow, and fracture propagation are interdependent. The fracture aperture strongly influences the fluid flow behavior inside the fracture, as the fluid velocity is a function of the fracture aperture, and the fluid pressure influences rock deformation process. Hence, these processes of the fluid flow, the fracture deformation, and the fracture propagation are solved in a coupled manner using sequential iterative approach till the convergence is achieved. First, details of the mathematical model and methodology are presented. The model is then tested against some known analytical and semi-analytical solutions. Finally, the hydraulic fracturing results from a true triaxial EGS experimental cell developed at Colorado School of Mines are used to validate numerical model results.

1. INTRODUCTION

Hydraulic fracturing is considered the primary means of creating functional geothermal reservoirs at sites where the permeability of rock is too limited to allow cost effective heat recovery. The natural and hydraulically created fractures provide conductive paths to the stored hydrocarbons or thermal energy in the reservoir rocks to the wellbore thereby increasing the production rates. Stimulation technology and methodology as used in the oil and gas industry for sedimentary formations are well developed; however, they have not sufficiently been demonstrated for the Enhanced Geothermal Systems (EGS) reservoir creation. Insufficient data and measurements under geothermal conditions make it difficult to directly translate experience from the oil and gas industries to EGS applications. Creation of the EGS reservoirs requires an improved fracturing methodology, rheologically controllable fracturing fluids, and temperature hardened proppants.

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