Reservoir Engineering of Shallow Fault-Charged Hydrothermal Systems

Abstract

Many of the low-to-moderate temperature (< 15OOC) hydrothermal resources being developed in the United States occur in near-surface aquifers. These shallow thermal anomalies, typical of the Basin and Range and Cascades are attributed to hydrothermal circulation. The aquifers are often associated with faults, fractures, and highly complex geological settings; they are often very limited in size and display anomalous temperature reversals with depth. Because of the shallow depth and often very warm temperatures of these resources, they are attractive for development of direct-use hydrothermal energy projects. However, development of the resources is hindered by their complexity, the often limited manifestation of the resource, and lack of established reservoir engineering and assessment methodology. In this paper a conceptual model of these systems is postulated, a computational model is developed, and reservoir engineering methods (including reservoir longevity, pressure transient analysis, and well siting) are reevaluated to include the reservoir dynamics necessary to explain such systems. Finally, the techniques are applied to the Sumnville, California hydrothermal anomally.