Hululais Field Review from Geoscientific Exploration to Exploitation Stage

Geothermal studies in Hululais carried out by Pertamina was started in 1993 with an exploration survey. The detailed geological mapping as the earliest geothermal resource assessment of Hululais was done internally by Pertamina. The geochemical study was done together with the geological study. The geophysical study was carried out using three methods. The gravity survey was run in 1993 for 320 points along 12 trace lines, while the magnetotelluric survey was conducted in 2006 and 2012 for 64 locations with 1-2 km spacing. The microearthquake survey was done in 2012 for 115 days monitoring time by six sensors in 10 km spacing and in 2019. The conceptual model as a product of the integrated study of three geoscientific surveys shows that the upflow zone is interpreted in Suban Agung - Suban Gregok in the southern area. Fumarole, mud pool and steam-heated hot spring are found in this zone. The outflow zone is interpreted towards the north in Semelako area supported by the chloride hot spring. The appearance of these manifestations is possibly controlled by fault structure in Musi Segment zone of Sumatra Fault Zone (SFZ). The heat source comes from granodiorite intrusion below Bukit Beriti Besar - Bukit Gedang Hululais. The cap rock has 500 meters thick in the upflow zone and increases to 800 meters thick towards the outflow zone in the northern part of the study area. The upflow zone was proved by two exploration wells that drilled until 3000 mMD. The first well in Suban Agung has a high temperature reach 300°C with neutral reservoir fluids and good fault structure permeability. The second exploration well that drilled towards outflow zone shows a temperature of 205°C with neutral reservoir fluids but with low permeability. In the exploitation stage, 21 development wells have been drilled for making the update conceptual model. Among them, ten wells are production wells and the remains are reinjection ones. The permeability in the reinjection wells is not so good compared to the production wells. It might be due to secondary mineral filling in the fault structure permeability zone, so this situation causes a problem in injection water to the reservoir. Investigation of the subsurface permeability distribution in the Hululais reservoir is challenging, so further research should be done to answer this challenge.