Evaluation of reservoir characteristics using well data of sibayak geothermal field, North Sumatra, Indonesia

The Sibayak geothermal field is located in northern part of Sumatra Island, Indonesia. Exploration surveys were conducted during 1989-1997 when ten exploration wells were drilled. Repeated temperatures logging in the wells are analyzed for initial formation temperature using a Horner plot. Then estimated highest formation temperatures are plotted on the activity index curve1) for quantitative evaluation of the thermal potential of wells. Initial reservoir pressures are also determined from a pivot point of pressure profiles. The results show the maximum formation temperature in the Sibayak reservoir reaches up to 310°C. A few wells located in the central part in Sibayak are evaluated to be of Type A of activity index (AI) suggesting high thermal potential. On the other hand, wells at southern part belong to Types C and D of AI. These low grades of thermal potential may be caused by low temperature water inflow along the caldera rim.