The Strategy of Reinjection Drilling at Lumut Balai Geothermal Field, Sumatera Selatan, Indonesia

Abstract

Lumut Balai geothermal field is in a volcanic setting, precisely located in the Lumut Tua caldera which has a diameter of 9 km with elevations ranging from 913 - 1800 masl. Production wells with the lowest elevation are at 994 masl elevation. By considering the distance of the reinjection zone with the closest production zone as far as 3-4 kilometers, then the surface elevation difference between the production and reinjection well, the permeability target comes from primary and secondary permeability, so the strategy is to drill reinjection well in the north of the production zone. Eight reinjection wells were drilled to meet the need for injection capacity of 4000 tons/hour from the planned generation of 2 x 55 MWe in Lumut Balai. The requirement of 4000 tons/hour can be achieved from 8 (eight) reinjection wells in 4 (four) reinjection clusters. Of the eight reinjection wells, four of them produced injection capacities more than 1000 tons/hour. The large injection capacity is obtained from drilling in reinjection wells that are drilled inside the Lumut Tua caldera. The wells in the caldera, the loss zone is found at elevations 0 - (-200) masl, whereas for wells outside the caldera, it is very difficult to obtain permeability. The permeability found in drilling is estimated from the primary permeability obtained from the layering of metasediment rocks. So that, there is no large permeability that has connection with subsurface. Very much different from the conditions of subsurface permeability within the caldera, where the structure on the surface correlates with the subsurface structure and is also highly developed. With the result that, the drilling wells in the caldera are better than outside.