Hydraulic Stimulation Techniques Applied to Injection Wells at the Salak Geothermal Field, Indonesia

Abstract

Abstract The Salak field is the largest developed geothermal resource in Indonesia. Steam production levels have been maintained at or above the rated power plant capacity for 15 years through periodic infill drilling and injection realignment. The reservoir management strategy for Salak involves injecting separated brine into deep wells on the margins of the reservoir where permeability is known to be low. A technique that has been used to enhance permeability in new injection wells combines hydraulic and thermal fracturing. This involves complex rock/fluid interactions and heat transfer relationships that must be considered in order to design appropriate injection rates, pumping times, and injection temperatures. This paper describes the techniques used to characterize candidate wells and interpret surveillance data collected during long-term cold water injection experiments. Analyzing continuous measurements of well injectivity along with pressure transient data taken periodically shows the evolution of enhanced permeability during stimulation. A high degree of success has been achieved using these techniques at Salak, with significant improvements in well injection capacity. These results provide additional alternatives for managing injection to increase the ultimate heat recovery from the reservoir. Introduction The Salak geothermal field is located within a protected forest about 60 km south of Jakarta. Geothermal operations are conducted pursuant to contracts with Pertamina (the Indonesian National Oil Company) and PLN (the Indonesian National Electrical Utility Company). The first exploration and appraisal wells were drilled at Salak from 1982 through 1986 and the first 110 MW (2 x 55 MW) power plants started commercial operation in March 1994. Between 1995 and 1997, four additional power plants (4 x 55 MW) were built at Salak. PT. Indonesia Power, a subsidiary of PLN, operates Unit 1–3 while Chevron Geothermal Salak, Ltd. operates Units 4–6. In 2002 the output of Units 4–6 was increased to 65.6 MW each bringing the total generation capacity of Salak to 377 MW and making it one of the six largest geothermal fields in the world. Formulating effective injection strategies has proven to be the most challenging aspect of field management. The initial development plan required injecting separate brine which is significantly below original reservoir temperature in fairly close proximity to production wells. A surveillance program including tracer tests, chemical monitoring, microseismic monitoring and pressure-temperature surveys of individual wells has been carried out since production commenced in order to monitor the impact of injection. As the breakthrough of chemical and thermal fronts was observed, injection was gradually moved to the field margins. This has resulted in expansion of the area available for drilling infill production wells while several former injection wells have been converted to production after allowing sufficient time for thermal recovery. Robust data gathering and monitoring programs have played an important role in realigning injection to optimize heat recovery from the reservoir (Acuña, et al., 2008).