Geothermal Reservoir Pressure Requirements for Production

Abstract

Rogers is an engineering company, and as such our interest in reservoir engineering is concerned with the production aspect. Reservoir testing and production management techniques practiced today are largely rule-of thumb. We are hoping to learn from you such parameters as proper well spacing and wellbore sizing for a given reservoir which will provide optimum production and life of the reservoir. We cannot contribute much in this area other than perhaps to ask the questions which may stimulate research and to relate some of our observations as to what we find is required to satisfy our needs. For example, in making reservoir modeling studies we feel it is very important that the chemistry of the system be considered, particularly where it is indicated that production will be affected by flashing in the reservoir.

There have been instances where well production has been limited by reservoir permeability rather than by wellbore diameter. When such wells are produced to their maximum capacity, the pressure drawdown in the reservoir exceeds the gas evolution pressure, and Flashing occurs in the reservoir. If the reservoirs are in shale or limestone (carbonate) formations, calcite precipitations can occur at the point of flashing. Well flow may cease entirely due to loss in permeability. This siituation has occurred in some of the wells drilled in Casa Diablo. Flashing in the formation could have been prevented by operating the wells at a higher back pressure, but this would have reduced the well flow to uneconomic production rates.