Responses of Wayang Windu Geothermal Reservoir To Over 20 Years of Production

Wayang Windu is one of the largest geothermal fields in Indonesia that has been operating for more than 20 years and has evolved since the early development in 1996 to date. The field is transitional between vapor-dominated and liquid-dominated reservoir, where the steam cap is more developed toward the north of the field. Commercial operations commenced in 2000 for Unit-I (110 MW) and in 2009 for Unit-II (117 MW). During the 20 years of operation, the production is mainly located in the northern and central part of the field while the injection takes place in the lower topography toward the south. In the early production, the steam cap and deep brine reservoir were produced proportionally in an area of 10 km2. After 3 years of initial production, however, the poor performance of the brine wells in the northern area has led the field to be developed with 90% of the total production from the northern steam cap. After 20 years of production, a total pressure drop of 18 and 10 bar are observed in the northern steam cap and liquid reservoir, respectively. In the meantime, the liquid pressure is relatively stable in the central, mainly due to limited production. As of today, the northern steam cap, covering an area of 7 km2, is seeing a pressure drop at a rate of around 0.8 bar/year with steam production equivalent to 200+ MW generation. On the other hand, the rate of pressure drop is roughly at 0.5 bar/year in the deep brine reservoir. Up to date, the injection in the southern area has been reliable and providing adequate pressure support with no adverse impact to production wells. Along the 20-year journey, the Wayang Windu Geothermal Field has been facing some challenges like other geothermal fields of similar age around the world. Silica scaling in the wellbore in dry steam wells is one of the key challenges. Regular mechanical cleanouts have been the preferred method to manage the issue; however, a more efficient and cost effective way of preventing the scaling is currently being evaluated, including the use of a downhole scale inhibitor. In addition, having a sustainable production from the deep brine reservoir is another challenge that the Wayang Windu Geothermal Field is facing. Efforts to better characterize the permeability, especially in the deep brine reservoir, is currently underway to enhance the deep production. The increased deep production is to sustain the long-term steam cap performance with the objective of not only prolonging the production plateau at the current generation level, but also providing an expansion opportunity for the field and maximizing its value.