The dependence of permeability on effective stress from flow tests at hot dry rock reservoirs at Rosemanowes (Cornwall) and Fenton Hill (New Mexico)

Two, 3-km-deep boreholes have been drilled into hot (approximately 200/sup 0/C) graphite in northern New Mexico in order to extract geothermal energy from hot dry rock Both boreholes were hydraulically fractured to establish a flow connection Presently this connection has a large flow impedance which may be improved with further stimulation Fracture-to-borehole intersection locations and in situ thermal conductivity were determined from flowing temperature logs In situ measurements of permeability show an extremely strong dependence upon pore pressure--the permeability increased by a factor of 80 as the pressure was increased 83 bars (1200 psi) An estimate of the minimum horizontal earth stress was derived from fracture extension pressures and found to be one-half the overburden stress

Preliminary assessment of a geothermal energy reservoir formed by hydraulic fracturing

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 meters and are separated by approximately 30 meters. 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. Here, an analysis of these experiments is presented in terms of a mathematical model that includes the variable rock permeability and porosity, the connection between 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.

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Interpretation of the pressure and flow data for the two fractures of the Los Alamos hot dry rock (HDR) geothermal system

Pressure-transient testing of a hydraulically fractured geothermal reservoir in low-permeability crystalline basement rock has involved constant rate injection and pressure buildup tests under a wide range of field conditions for a number of fractured regions. Following conventional reservoir analysis methods, data are treated in terms of a transient diffusion equation that relates fluid flow and pressure levels in the main fracture system, associated joints, and the matrix permeability. Pressure-flow data are compared to type curve solutions of the diffusion equation for various flow geometries. The following points are considered in detail: (1) the limits on the fracture geometry, aperture and diffusing areas as determined from the diffusion parameters; (2) the parameters (flow impedance, diffusivity) of the flow-through systems are related to those governing the pressure inflation of the main fractures; (3) the relationship of the rock properties to the reservoir compressibility, effective porosity and permeability are discussed. In particular, laboratory experiments show that the flow properties of all sizes of cracks from large single fractures to the microstructure are pressure dependent if the fluid pressure is near the confining stress; and (4) the competition of flow into the various types of porosity (main fractures, joints, and microstructure) and the effectmore » on the interpretation of type curves are discussed.« less

P. Kruger

Papers

An analysis of the pressure transient testing of a man-made fractured geothermal reservoir *