ABSTRACT
Since the initial hydraulic fractures were established at the Fenton Hill site of the Los Alamos HDR project, several pressurization and flow experiments have been performed. The fractures are in granite at a depth of approximately 2900 metres, and are separated by approximately 30metres. The flow experiments were planned to establish the water losses to the surrounding rock, determine pressure and stress dependent rock properties, and to characterize the fracture system in terms of extent, volume and the interconnection' between the fractures. The present paper presents an analysis of these experiments in terms of a mathematical model that includes the variable rock permeability and porosity as given by simple models, and the effects of the earth stresses. These features are incorporated into the diffusion equation for the pore pressure. The experimental relationship of pressure and flow in the two fractures is examined.
INTRODUCTION
The initial hydraulic fractures were established at the Fenton Hill site of the Los Alamos Hot Dry Rock project in October of 1975 (Blair, et al., 1976). The fractures are in granite at a depth of approximately 2900 metres. A fracture system was established at each of two boreholes, referred to as EE-1 and GT-2. Microseismic ranging and other experiments indicate that, at a depth of 2900 metres, the boreholes are separated by as little as 10 metres but the fractures may be separated by as much as 30 or 50metres with more than one fracture associated with EE-1. As is the case in other formations, the fractures are expected to be nearly planar and perpendicular to the direction of the least principle earth stress (S3). Previous analyses of the system and the present one suggest that the total system consists of two planar, parallel fractures. However, the possibility that one fracture system (EE-1) consists of two distinct fractures has not been confirmed by the present analysis. Many pressurization and flow experiments have been performed on each fracture system and on the combined system. These have provided considerable data on the interdependence of pressure, total flow, flow between the fractures, and flow into the surrounding formation. These data are complicated by changes occurring in the fracture geometry under pressurization. The main effects being the extension of the areas of the fractures. The data indicates that, once a fracture is established, it remains open with a constant area and a width sufficient to allow flow into the fracture with low impedance. The response of the flow to pressure then is governed by the permeation through the porous rock surrounding the fractures. The present analysis is based on the one and two-dimensional time dependent diffusion equations governing the flow of water and hence the transient pore pressure (P) in the granite. At early times in the pressurization of either fracture the flow is one-dimensional; at a time determined by the rock properties and the dimensions of the system, the extent of the pore pressure field becomes comparable to the dimensions of the fractures and the two dimensional equation would be necessary. In this case the fractures are assumed to have cylindrical symmetry about an axis parallel to S3 and to have radii determined by equating the areas to that of circles.
