Recovery Optimization Strategy for Thin Oil Column Reservoir with Large Gas Cap: Case Study of Gunung Kembang Field

Abstract

Effective exploitation of the thin oil rim in the Gunung Kembang field is particularly challenging because of the huge size of the overlaying gas cap, and the thickness of the oil rim varying between 25 to 40 feet gross pay interval, not to mention the sizeable water aquifer underlying the reservoir. Horizontal wells were implemented since 1992 to enhance oil recovery by reducing gas and water coning, nevertheless, oil recovery still remain around 3%. As the pressure is depleting, horizontal wells revealed to pose higher risk than before, just like in the second stage of horizontal drilling in 2004, where water breakthrough and total loss occurred. The plan to add more horizontal wells to add recovery was coincided with POD commitment to deliver gas by gas cap blow down. This paper presents a team effort to formulate optimization strategy for maximizing oil recovery and revenue while delivering gas as committed in POD. Integrated reservoir characterization that comprises of carbonate depositional study, and reservoir simulation was conducted to best manage not only the drilling location but also number of wells and production strategy for next horizontal wells. Carbonate depositional study using existing logs and core data revealed the best oil potential zone in Baturaja formation in terms of porosity and permeability. Meanwhile, reservoir simulation grid was aligned with the zonation from carbonate deposition model, after that the performance of existing 10 wells was history-matched to develop an improved strategy. Sensitivity analysis conducted demonstrated that locating horizontal oil wells in the upper oil rim near the gas oil contact proved to be the best strategy for depletion of the oil rim. Among five scenarios developed, one final scenario was selected to accommodate both oil recovery optimization and gas cap blow down. This scenario includes eight additional horizontal oil wells while utilizing their produced gas to accommodate gas commitment, without drilling another gas well. Oil recovery is expected to rise to about 8% while gas is being delivered according to POD. Introduction Gunung Kembang Field is located in the South Sumatera Extension area of Medco E&P Indonesia working areas. This field was discovered in 1987 with GK-1 drilling and was put on production in June 1988. This carbonate reservoir has a thin oil column of around 40 ft sandwiched between thick gas cap of about 117 ft and water aquifer with the drive mechanism is mainly gas cap drive with weak water drive. Currently, cumulative production of this field has reached 3.8 MMSTB of oil and 50 BCF of gas with 10 wells, in which 6 of them are horizontal well. Because of apparent thin oil column, vertical well performance were upset by quick gas coning and water coning, thus encouraging the need for horizontal wells. Started in 1991, three horizontal wells were drilled; they are GK-7, GK-9 and GK-10. Excluding the production of GK-10 well which entered poor quality rock in the horizontal section, the cumulative production from other 2 horizontal wells until the end of 2004 is 2.5 MMSTB-indicating the success of horizontal well performance. Further simulation study was conducted by third party to maximize oil recovery in Gunung Kembang field by drilling 5 horizontal wells. Based on those simulation study, another horizontal drilling campaign was executed in 2004 by side-tracking 3 existing vertical wells, GK-1, GK-3, GK-6 to be GK-1 Horizontal Well (HW), GK-3 HW and GK-6 HW. Unfortunately, only GK-1 horizontal is considered successful, meanwhile the other two having problem in early water breakthrough, and total loss. Thus the oil recovery is still remains low at about 3% from total Original Oil in Place.