Description: The study of geothermal reservoir behavior is presently in a state of change brought about by the discovery that reservoir heterogeneity--fractures in particular--is responsible for large scale effects during production. On the other hand, some parts of a reservoir, or some portions of its behavior. may be unaffected by fractures and behave, instead, as if the reservoir were a homogeneous porous medium. Drilling has for many years been guided by geologists prospecting for fractures (which have been recognized as the source of production), but until recently reservoir engineers have not studied the behavior of fractured systems under production. In the last three years research efforts, funded by the Department of Energy and others, have made significant progress in the study of fractures. The investigations into simulation of fracture flow, tracer analysis of fractured systems, and well test analysis of double porosity reservoirs are all advancing. However, presently we are at something of a conceptual impasse in defining a reservoir as fractured or porous. It seems likely that future directions will not continue to attempt to distinguish two separate reservoir types, but will focus instead on defining behavior types. That is, certain aspects of reservoir behavior may be considered to be generally of the porous medium type (for example, field wide decline), while others may be more frequently fracture type (for example, breakthrough of reinjected water). In short, our overall view of geothermal reservoir definition is becoming a little more complex, thereby better accommodating the complexities of the reservoirs themselves. Recent research results already enable us to understand some previously contradictory results, and recognition of the difficulties is encouraging for future progress in the correct direction.
Date: December 1, 1983
Creator: Horne, Roland N.
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