Predrill Geomechanical Evaluation Helps in Successful Drilling in High Depletion Wells in a Mature Field, Offshore Sarawak, Malaysia

Abstract

Hydrocarbon-bearing reservoirs in a mature field, offshore Sarawak, Malaysia, contains multiple reservoir cycles interbedded with weak shale and coal formations. Years of production from these reservoirs caused significant pressure depletion, as well as reduced fracture gradient and a narrower drilling mud weight window. An improperly weighted mud may induce wellbore instability in weaker, but normally pressured, formations or mud losses in the heavily depleted reservoirs. Globally, mud losses are considered the most expensive well control operation incidents. Earlier drilling campaigns in ths field encountered numerous wellbore instability incidents; hence, a study was conducted to develop an understanding of the drilling issues and assess the stability of the heavily depleted reservoirs. Collaboration between the drilling and geomechanics teams facilitated better understanding of the stability challenges and helped in mitigating risk related to wellbore instability. A field scale geomechanical model was developed and validated using data from exploration and development wells from different phases of drilling in the study area. The stress path factor (SPF), which determines the reduction in fracture gradient with pore pressure depletion is crucial for defining drilling mud windows, is difficult to constrain in the absence of measured formation fracturing data in virgin and depleted reservoirs. A mud loss event in the depleted zone from a recent drilled well and regional information were used to estimate the range of SPF in the study area. Recorded bottom hole pressures from pressure while drilling (PWD) data suggested that the maximum equivalent circulating density (ECD) recorded was close or within the depleted section. The loss event was associated with reduced fracture gradient due to depletion from its pre-depleted range. This paper describes how geomechanical evaluation with effective well drilling practices and fit for purpose-drilling fluids have helped drilling through depleted reservoirs with ECD management. At the end, it shows a comparison of the predrill wellbore stability mud weight estimates with the actual mud weights used to successfully drill and complete the planned wells.