Casing Program and Well Trajectory Design for a Geothermal Well Based on Stratigraphic Analysis of the Malili Sheet: A Conceptual Study in the Parara Geothermal Prospect, South Sulawesi, Indonesia

Abstract

Recent geopolitical instability in the Middle East, accompanied by significant increases in global oil and gas prices, has further exposed the vulnerability of fossil fuel-dependent energy systems. Concurrently, global commitments toward decarbonization and sustainable development have accelerated the transition toward renewable energy sources. Among the available alternatives, geothermal energy offers a reliable and low-carbon solution, especially in Indonesia, where substantial geothermal potential remains underdeveloped despite favorable geological conditions. Nevertheless, the advancement of geothermal energy development requires technically reliable exploration and drilling approaches to address subsurface complexity and reduce risks. In this context, this study presents a conceptual design of casing, drilling mud system, and well trajectory for a frontier geothermal exploration well in the Parara Geothermal Prospect, Indonesia. The design was developed based on regional stratigraphic interpretation derived from the Malili Geological Sheet, surface manifestation data, and regional structural information. The prospective target zone is conceptually estimated to occur at depths of approximately 1000-1500 m, primarily within the ultramafic units of the Wasuponda Complex, which may function as secondary geothermal reservoirs via tectonic fracturing and hydrothermal alteration. Formation temperature was estimated using a regional geothermal gradient of 40-45 °C/km, while initial formation pressure was approximated using a hydrostatic pressure approach. The Lahendong geothermal field was employed as a conceptual analogue to infer the subsurface geological framework (e.g., fracture-controlled permeability zones), while the LOKA-1 and TONDON-1 wells were used to infer regional drilling operational risks. A four-stage casing design, interval-based drilling mud program, and deviated well trajectory were proposed to improve the probability of intersecting permeable fracture zones. 3D visualization was utilized to illustrate the integrated relationship between well trajectory, casing placement, and interpreted stratigraphy. The results provide an initial technical framework for geothermal exploration drilling evaluation in the Parara prospect. Although the proposed parameters require further validation, the results are expected to reduce exploration uncertainty and facilitate future geothermal development in the Parara prospect.