Strategic Reservoir Characterisation Manoeuvre in a Clay Sensitive, Pressure-Depleted and Geomechanically Complex Gas Reservoir in Offshore Peninsular Malaysia

Abstract

Acquiring subsurface data in a highly unstable and geomechanically complex formation pose unusual challenges to adequately characterize the reservoir from production and completion perspective. The traditional way of acquiring data through LWD or wireline logging might be operationally constrained by severe hole break-out, mud losses and high differential sticking due to pressure depletion across the reservoirs. This condition warrants a specific and critical approach to optimally design the data acquisition plan to minimize subsurface uncertainties to meet the gas production target from the field. This paper will highlight customized techniques to successfully acquire good quality of subsurface logging and pressure data, technical and operational considerations involved and some of the best practice applied to operate within the threshold limit amidst the time-dependent hole instability culprit. Detailed technical plan, rigorous pre-job simulation and sensitivity analysis set the high tone to acquire critical subsurface log and pressure data prior to completion. Customizing LWD drilling dynamic parameters such as mud circulation rate, real time data transfer rate, pressure drawdown and build-up time, controlling the logging speed in ROP and optimizing the mud design are among the approaches taken to obtain fit-for-purpose data in a very narrow operating limit amidst the borehole instability issues. Acquiring real time image from LWD density tool enables fracture and break-out evaluation validating the existing geomechanical model in this field and enhance the understanding of causes for hole instability issues leading to real time well trajectory optimization and updating. Theoretically, open hole log data can be also complemented by cased hole pulsed neutron and production logging acquisition to formulate reservoir properties, gas water contact and estimated shut-in reservoir pressure through prolific log processing methodology and interpretation procedures. Post data acquisition activities along the problematic reservoir interval and incorporating all the interpretation outcomes, the reservoir connectivity and pressure communication are confirmed throughout the field area. Even though the main target reservoirs in the newly drilled well could not be completed and produced due to operational issues, the case presented in this paper will provide valuable lessons learned in designing and operating a well in the highly complex geomechanical area and strategizing data acquisition program to minimize the subsurface uncertainties. Best practices in customizing LWD open hole and wireline cased hole data acquisition to fully characterize the formation including downhole CO2 gas presence set the pioneering guideline in this region and become the critical benchmark for future well operation. In conclusion, securing adequate and reliable subsurface data in a highly unstable and geomechanically complex formation will justify for a strategic approach and customized plan to obtain much needed insights for sustainable gas production and delivery from the field. The first integrated and comprehensive approach combining both open hole and cased hole logging capabilities in Peninsular Malaysia region are presented in this paper setting a commendable runway for fit-for-purpose operation and optimization.