Evaluation of Open Hole Hydraulic Stimulation in Naturally Fractured Geothermal Reservoir – A Field Case Study

Abstract

Hululais Field, 30 km north to Bengkulu City, is projected to generate 2 x 55 MW. The reinjection strategy in Hululais targets the permeability from fractures within the rock caused by northward and northwest-southeast structures in Hululais. The reinjection well also considers the fact that the fluid flows north-northwest from the upflow zone below the Suban Agung - Bukit Gedang to the outflow zone in Semelako and the ideal distance of 3-4 km from production zone. However, during drilling operation indicates a low permeability reservoir with evidence of partial loss circulation; hence, a stimulation to enhance the permeability is conducted through long open-hole water injection. The interaction between pre-existing natural fractures and hydraulic stimulation may exhibit complex phenomenon where fracture slippage and dilation along fracture networks increase the aperture and hydraulic conductivity. This paper presents the practical approach to design, surveillance, and evaluate the performance of hydraulic stimulation in a naturally fractured geothermal reservoir. It is by integrating the analysis between geophysical, geological, and reservoir engineering knowledge. Lithological and fracture analysis combine with stress analysis are used to identify the zone where the occurrence of fracture slippage is high. During stimulation, surveillance techniques using Hall Plot and Derivative Hall Plot in real-time is proven to deliver quick insight about the permeability enhancement. Also, pre and post hydraulic stimulation evaluation using several methods such as step rate test, pressure, temperature, and spinner analysis along with pressure transient analysis is presented.