Geothermal Well Stimulation Experiments and Evaluation

Abstract

ABSTRACT Republic Geothermal, Inc., and its subcontractors have planned and executed six experimental fracture stimulation treatments under the Department of Energy-funded Geothermal Reservoir Well Stimulation Program (GRWSP). The program, begun in February 1979, is ultimately to include eight full-scale field hydraulic and chemical stimulation experiments in geothermal wells. This paper describes the six treatments completed to date. Two stimulation experiments were performed at Raft River, Idaho, in late 1979. This is a naturally fractured, hard rock reservoir with a relatively low geothermal resource temperature (290°F). A planar hydraulic fracture job was performed in Well RRGP-5 and a dendritic, or reverse flow, technique was utilized in Well RRGP-4. In mid-1980, two stimulation experiments were performed at East Mesa, California. The stimulation of Well 58-30 provided the first geothermal well fracturing experience in a moderate temperature (350°F) reservoir with matrix-type rock properties. The two treatments consisted of a hydraulic fracture of a deep, low-permeability zone and a dendritic fracture treatment of a shallow, high-permeability mud/cement-damaged zone in the same well. In early 1981, an acid etching stimulation treatment was performed in the Ottoboni State 22 well located in The Geysers geothermal area of California. The treatment involved the injection of 476 bbl of 10% HF-5% HCT acid behind a 476 bbl slug of high viscosity crosslinked gel fluid. This technique was intended to take advantage of the fluid mobility differences to etch discrete flow channels, or fingers, in the fracture faces. A 7,600 bbl hydraulic fracture treatment was also performed in early 1981 in the Baca 23 well of the Redondo Creek area of New Mexico. The stimulation interval was in the upper part of the Bandelier Tuff, a 450°F interval in which the well had not encountered productive natural fractures. This treatment utilized a large cooling water prepad and temperature resistant proppants, i.e., sintered bauxite and resin-coated sand. 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, fracture mapping with geophones, and numerical models. This combination of evaluation techniques yielded an interpretation of fracture geometry and productivity enhancement. A discussion of the pre-stimulation and post-stimulation data and their evaluation are provided for each experiment in this report. Five of the six stimulation experiments were at least technically successful in stimulating the wells. The two fracture treatments in East Mesa 58-30 more than doubled the producing rate of the previously marginal producer. The two fracture treatments in Raft River and the one in Baca were all successful in obtaining significant production from previously unproductive intervals. However, these three treatments failed to establish commercial production due to deficiencies in either fluid temperature or flow rate or both. The acid etching treatment in the well at The Geysers did not have any material effect on producing rate. Evaluations of the field experiments to date have suggested improvements in treatment design and treatment interval selection which offer substantial encouragement for future stimulation work.