Feed Zones Characterization Based on the Intensity of Faults and Fractures to Reduce the Uncertainty of the Future Field Development in Ulubelu Geothermal Field, Lampung, Indonesia

Abstract

Located at the southern tip of Sumatra, geological structures in Ulubelu Geothermal Field strongly depicts the consequence of dextral movement of the Great Sumatran Fault along with the volcanic activity related structures. It produces northwest – southeast (NW-SE) alignments that can be clearly observed on the morphology. Although the NW-SE structures is obvious on the surface, the influence of these structures such as faults and fractures to the permeability in Ulubelu Geothermal Field is still uncertain. The study is conducted by integrating borehole image logging data with Production Logging Tool (PLT) from eight wells to assess the impact of these structures to the permeability of each feed zones. The study reveals permeability of the feed zones can be divided further into four ranks based on the intensity of the faults and fractures. The predominantly northwest – southeast (NW-SE) faults and conductive fractures intersected by the wells are the main factor to the fluid flow contribution from the feed zones. It is evidenced that fault zone gives the highest flow contribution. When the fault is absent the permeability deteriorates and leaving the intensity of conductive fractures contributes to some degree, mainly contributes between a half and a third compared to the fault contribution. By plotting the feed zones elevation with its rank, the feed zones are located differently on the western and the eastern part of the field due to fault control. The identification of these feed zones and its controlling factor will greatly reduce the uncertainty for future field development and plan the effective make-up wells.