Optimization of water injection into vapor-dominated geothermal reservoirs

Water injection into a vapor-dominated geothermal reservoir is an effective method of sustaining steam production from the field. Injection puts additional water to the reservoir and raises the prevailing reservoir pressure. This process improves the field's productivity. However, the increased pressure also increases the water retention capacity of the reservoir rocks through the effects of adsorption and capillary condensation. Due to the significant costs associated with water injection programs, optimizing injection not only involves maximizing the energy yield from the resource but also the present worth of the project. Two crucial parameters that need to be established are: 1) how much to inject; and, 2) when to inject it. This study investigated the optimal design of these parameters. It was found that comparable energy yield can be attained for injection programs that are initiated at various stages of the field's development. Higher injection rates are desirable when the injection program starts later in the productive life of the field. Considering the economics of the project, it is best to implement the injection program during the later stages of the field's development. This way, a greater fraction of the injectate can become available for production and at the same time optimize the present worth of the project.