Evaluating Permeability Enchancement Using Electrical Techniques

John W. Pritchett

关键词: BUILDUP;    DRILLING;    ECONOMICS;    ELECTRIC POWER;    ELECTRICITY;    FLUID FLOW;    FRACTURES;    GEOMETRY;    GEOTHERMAL SYSTEMS;    HOT WATER;    HYDROTHERMAL SYSTEMS;    INJECTION WELLS;    PERMEABILITY;    POWER GENERATION;    SHEAR;    STIMULATION;    SWEEP EFFICIENCY;    TECTONICS;    WATER;    WELLHEADS;   

DOI  :  10.2172/1013695
RP-ID  :  final report
PID  :  OSTI ID: 1013695
Others  :  TRN: US201117%%5

美国|英语

来源: SciTech Connect

【 摘 要 】

Enhanced Geothermal Systems (EGS) development projects involve the artificial stimulation of relatively impermeable high-temperature underground regions (at depths of 2-4 kilometers or more) to create sufficient permeability to permit underground fluid circulation, so that hot water can be withdrawn from production wells and used to generate electric power. Several major research projects of this general type have been undertaken in the past in New Mexico (Fenton Hill), Europe, Japan and Australia. Recent U.S. activities along these lines focus mainly on stimulating peripheral areas of existing operating hydrothermal fields rather than on fresh 'greenfield' sites, but the long-term objective of the Department of Energy's EGS program is the development of large-scale power projects based on EGS technology (MIT, 2006; NREL, 2008). Usually, stimulation is accomplished by injecting water into a well at high pressure, enhancing permeability by the creation and propagation of fractures in the surrounding rock (a process known as 'hydrofracturing'). Beyond just a motivation, low initial system permeability is also an essential prerequisite to hydrofracturing. If the formation permeability is too high, excessive fluid losses will preclude the buildup of sufficient pressure to fracture rock. In practical situations, the actual result of injection is frequently to re-open pre-existing hydrothermally-mineralized fractures, rather than to create completely new fractures by rupturing intact rock. Pre-existing fractures can often be opened using injection pressures in the range 5-20 MPa. Creation of completely new fractures will usually require pressures that are several times higher. It is preferable to undertake development projects of this type in regions where tectonic conditions are conducive to shear failure, so that when pre-existing fractures are pressurized they will fail by shearing laterally. If this happens, the fracture will often stay open afterwards even if injection subsequently ceases. The principal barrier to EGS utilization for electricity generation is project economics. Costs for geothermal electricity obtained from conventional hydrothermal systems are just marginally competitive. Unless and until the costs of routinely and reliably creating and exploiting artificial subterranean fracture networks that can deliver useful quantities of hot fluid to production wells for long periods of time (years) are reduced to levels comparable to those of a conventional geothermal development project, EGS will be of little interest to the electrical power industry. A significant obstacle to progress in projects of this general type is the difficulty of appraising the properties (geometry, fluid transmissivity, etc.) of the fracture(s) created/re-opened by injection. Sustainability of power production is critically dependent upon reservoir thermal sweep efficiency, which depends in turn on the geometry of the fracture network and its interconnections with the various production and injection wells used to circulate fluid underground. If no permeable connections are created between the wells, fluid flow will be too slow for practical utility. If the connections are too good, however (such as a production/injection well pair connected by a single very permeable fracture), production wellhead temperatures will decline rapidly. Unless the permeable fractures created by hydrofracturing can be accurately mapped, the cost of subsequent trial-and-error drilling to try to establish a suitable fluid circulation system is likely to dominate project economics and render EGS impractical.