Zonal Hydraulic Stimulation in the Salak Geothermal Field

Abstract

While hydraulic stimulation with zonal isolation is performed regularly in oil and gas reservoirs, it remains a rare practice in geothermal reservoir management. One of the main reasons for this is a fear of potentially isolating sporadic feeding zones from natural fractures in the reservoir due to casing/cement. In this paper, we review a technical study conducted to test the viability of such zonal stimulation and the actual procedure successfully undertaken in the Salak reservoir during 2012-2013. In 2005, the Salak reservoir management team initiated the Salak Optimization Project to manage the water injection in the Salak geothermal reservoir. One of the strategic decisions was to move brine injection west of the Salak reservoir away from the production area. Consequently, three injectors were drilled west of the main Salak reservoir. All of the injectors unfortunately had uneconomically low injectivity and it was decided to hydraulically stimulate by injecting cold condensate water at high pressure for an extended period without any zonal isolation (bullhead stimulation). These stimulation episodes resulted in mixed outcomes. For the 2012-2013 drilling campaign, the use of zonal Isolation hydraulic stimulation was proposed. Initially, a conceptual geomechanical reservoir model was constructed to study benefits of zonal isolation. The results suggested that the zonal isolation stimulation may improve well injectivity and outperform bullhead stimulation. Subsequently, a well with a combined pre-perforated and blank 10-3/4” and 8-5/8” liner was designed. The blank part was then cemented ~300ft high to create zonal isolation. Furthermore, main injection casing 13-3/8” was successfully installed in one string, which eliminated tie-back (common in geothermal wells), improving its long term reliability. This combination of long string injection casing and zonal isolated liner would then allow a full-opening packer to perform injection at specific zone. Pre-stimulation, the wells’ injection capacity was around 150 kph at 500 psi WHP. After the stimulation, the well injectivity was tested to be 600 kph at 500 psi WHP. This huge improvement has helped to eliminate the necessity of drilling another injection well.