Recent reservoir pressure and steam flow rate declines at The Geysers geothermal field in California have attracted interest in studies of increased cold water injection into this system. in this paper, numerical studies of such injection into a fractured vapor-dominated reservoir are conducted using a two-dimensional radial, double-porosity model. The results obtained indicate that cold water injection into superheated (low-pressure) zones will greatly enhance the productivities of steam wells. Injection into two-phase zones with significant liquid reserves in the matrix blocks does not appear to aid in Steam recovery until most of the original liquid reserves are depleted. Sensitivity studies are conducted over the range of fracture and matrix permeabilities applicable to the Geysers. The sensitivity of the grid size is also conducted, and shows very large grid effects. A fine vertical space discretization near the bottom of the reservoir is necessary to accurately predict the boiling of the injected water. predict the boiling of the injected water
The Geysers geothermal field in northern California has been under exploitation since the early 1960's. The development of the field for electrical power generation progressed at an average rate of 67 MW/yr during the 1970's and accelerated to 150 MW/yr during most of the 1980's, reaching the current generating capacity of approximately 2,100 MW. This rapid development has caused several problems including excessive reservoir pressure and resulting production decline, and high concentrations of noncondensible gases and chlorides in the Steam produced in certain parts of the reservoir. The pressure and well productivity decline is due to excessive pressure and well productivity decline is due to excessive steam withdrawal from the reservoir and can only be halted by reducing the net mass extraction from the system. Although the pressure has declined from 35 bars to below 14 bars since exploitation started, the temperature is still near the initial value of 240 degrees C; the reservoir longevity is limited by the original mass in-place rather than heat.
Under this circumstance, the problem of maintaining production and maximizing energy recovery from the resource production and maximizing energy recovery from the resource becomes critical. Various remedies, including reduction of turbine inlet pressure, steam conservation in operations, and water injection, have been considered or adopted to reduce the rate of decline of steam production and extend the life of the field. The results of analysis, along with field practice, show that reduction of turbine inlet pressure and steam conservation in operations are effective means to increase energy recovery and power production. load-following has also been successfully used in some parts of the Geysers. Experience with injection of cold water into vapor-dominated geothermal reservoirs such as Larderello, Italy and The Geysers indicates that an increase in production rates may result from the boiling of injected water in strongly depleted areas. However, in some cases the enthalpy of steam produced from wells near an injection well declines as a result of premature breakthrough of the injected water.
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