Theory of thermal recovery from a spherically stimulated hot dry rock reservoir

A conceptual model is presented to describe thermal recovery from a semi-infinite hot dry rock (HDR) geothermal reservoir containing an equidimensional permeable zone. Transient behavior may be represented uniquely by five dimensionless parameters. Steady production temperatures are always highest for a host medium bounded by a proximal constant temperature surface and lowest for an insulated boundary. Boundary effects are insignificant for reservoir burial depths up to an order of magnitude greater than the reservoir radius. A threshold behavior in time is evident for very large reservoir throughput. This boundary behavior describes, in dimensionless time, the maximum rate at which thermal depletion may occur. This state is evident for large dimensionless throughput magnitudes corresponding directly with high circulation rates within the reservoir. Predictions compare favorably with results from a 300-day circulation test at the Fenton Hill Geothermal Energy Site, New Mexico. -from Author