Fracture patterns in graywacke outcrops at The Geysers geothermal field

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The Geysers geothermal field covers an area of more than 35,000 acres and represents one of the most significant steam fields in the world. The heterogeneous nature of the reservoir, its fracture network and non-sedimentary rock distinguish it from ordinary sandstone reservoirs in terms of reservoir definition and evaluation (Stockton et al. 1984). Analysis of cuttings, record of steam entries, temperature and pressure surveys and spinner logs have contributed to an understanding of the subsurface geology and rock characteristics of the Geysers. Few conventional electrical log data are available for the main body of the reservoir. It is generally believed ... continued below

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Sammis, Charles G.; An, Lin Ji & Ershaghi, I. January 1, 1991.

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The Geysers geothermal field covers an area of more than 35,000 acres and represents one of the most significant steam fields in the world. The heterogeneous nature of the reservoir, its fracture network and non-sedimentary rock distinguish it from ordinary sandstone reservoirs in terms of reservoir definition and evaluation (Stockton et al. 1984). Analysis of cuttings, record of steam entries, temperature and pressure surveys and spinner logs have contributed to an understanding of the subsurface geology and rock characteristics of the Geysers. Few conventional electrical log data are available for the main body of the reservoir. It is generally believed that while the fractures are the main conducts for fluid transport through the reservoirs, tight rocks between the major fractures contain the bulk of the fluid reserves. No independent measurement of liquid and vapor saturation can be made from the existing downhole tools. Pressure depletion in The Geysers geothermal field has become a major concern to the operators and utility companies in recent years. Plans for further development activities and future field management are contingent upon accurate computer modeling and definition of the field. The primary issues in reliable characterization of The Geysers field are the role of the rock matrix in holding liquid reserves and providing pressure support, the nature of fracture network, extent of liquid saturation in the reservoirs and injection pattern strategies to maximize heat recovery. Current modeling of The Geysers field is done through the use of general purpose geothermal reservoir simulators. Approaches employed include treating the reservoir as a single porosity equivalent or a dual porosity system. These simulators include formulation to represent transport of heat, steam and water. Heterogeneities are represented by spatial variations in formation or fracture permeability-thickness product, porosity or fluid saturations. Conceptual models based on dual porosity representations have been shown to duplicate the history. Prediction of future performance is, however, not reliable because of uncertainties in assumptions of the initial state of the reservoir, Specifically, several different initial state conditions have led to a fairly good match of the historical data. Selection of the exact initial conditions is a major dilemma. In dual porosity models, the complex nature of fracture network is formulated by a systematic, well-organized set of orthogonal fractures. Also, the exact nature of matrix-fracture interaction, and the role of adsorption and capillarity in pressure support are not well-defined.

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  • Proceedings, sixteenth workshop on geothermal reservoir engineering, Stanford University, Stanford, CA, January 23-25, 1991

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  • Report No.: SGP-TR-134-15
  • Grant Number: None
  • Office of Scientific & Technical Information Report Number: 887490
  • Archival Resource Key: ark:/67531/metadc876080

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  • January 1, 1991

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  • Sept. 21, 2016, 2:29 a.m.

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  • Dec. 5, 2016, 4:30 p.m.

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Sammis, Charles G.; An, Lin Ji & Ershaghi, I. Fracture patterns in graywacke outcrops at The Geysers geothermal field, article, January 1, 1991; United States. (digital.library.unt.edu/ark:/67531/metadc876080/: accessed June 24, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.