A Review of the Geothermal Reservoir Well Stimulation Program

Republic Geothermal, Inc., and its subcontractors have planned and executed four experimental fracture stimulation treatments under the Department of Energy-funded Geothermal Reservoir Well Stimulation Program (GRWSP). The 2-year program, begun in February 1979, is Ultimately to include six full-scale field hydraulic and chemical stimulation experiments in geothermal wells. This paper describes the overall program and the four treatments completed to date. The GRWSP is organized into two phases. Phase I consists of literature and theoretical studies, laboratory investigations, and numerical work. The main purpose of this work is to establish the technological bases for geothermal well stimulation design. Phase I1 will include the planning, execution, and evaluation of six well stimulation treatments which utilize the technology developed in Phase I. Two stimulation experiments were performed at the Raft River, Idaho, known geothermal resource area (KGRA) in late 1979. This is a naturally fractured, hard rock reservoir with a relatively low geothermal resource temperature 149 C {+-} (300 F{+-}). A conventional planar hydraulic fracture job was performed in Well RRGP-5 and a ''Kiel'' dendritic, or reverse flow, technique was utilized in Well RRGP-4. In mid-1980, two stimulation experiments were performed at the East Mesa, California, KGRA. The stimulation of Well 58-30 provided the first geothermal well fracturing experience in a moderate temperature, 177 C {+-} (350 F{+-}), reservoir with matrix-type rock properties. The two treatments consisted of a conventional hydraulic fracture of a deep, low-permeability zone and a mini-frac ''Kiel'' treatment of a shallow, high-permeability zone in the same well. The stimulation experiment results to date were evaluated using short-term production tests, conventional pressure transient analysis, interference pressure data, chemical and radioactive tracers, borehole acoustic televiewer surveys and numerical models. This combination of evaluation techniques yielded an interpretation of fracture geometry and productivity enhancement. However, the evaluation of artificially induced fractures in naturally fractured formations was found to lead to possibly non-unique solutions. In all the field experiments, artificial fractures were created and well productivity was increased. A discussion of the prestimulation and poststimulation data and their evaluation are provided for each experiment in this report