AN ANALYSIS OF THE PRESSURE TRANSIENT TESTING OF A MAN-MADE FRACTURED GEOTHERMAL RESERVOIR *

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