Bridging the Gap: Highly Depleted Reservoir Entry in a Mature HPHT Field

Abstract

This case study shows how three highly depleted reservoirs with large negative drilling windows, previously considered un-drillable, were safely entered by combining Managed Pressure Drilling (MPD) and drill-in liner technologies with a novel losses response strategy. The challenge was to bridge the gap between the high pore pressure overburden and the low fracture strength reservoir. The reservoir had been depleted by some 12,000psi since production started, creating a 3,000psi negative drilling window. Conventional strategies to prevent losses were deemed unlikely to succeed, and the focus was instead on how best to respond to the near-inevitable onset of total losses. The gap was bridged using a low static mud weight in combination with high applied surface back pressure to give an Equivalent Mud Weight (EMW) suitable for the high-pressure overburden, whilst allowing immediate reduction in bottom hole pressure in the event of total losses on entry into the weakened reservoir. Endurance testing allowed the MPD equipment to be operated outside of its normal pressure envelope. In the reference case, losses would be managed by reducing Surface Back Pressure (SBP) while continuing to drill ahead. In the low-pressure case, Pressurised Mud Cap Drilling (PMCD) would be used to bullhead the well to a lighter mud while drilling ahead, before restoring returns. A contingency plan was in place for managing elevated gas levels in the returns, which was anticipated following a large reduction of bottom hole pressure across the overburden shales. In the extreme case where high gas levels from the shales prevented bringing returns to surface, cementing would also be carried out in Pressurised Mud Cap mode. A drill-in liner was used because many of the scenarios would not permit safe tripping and Wellbore Strengthening material was included in the mud in an attempt to reduce the severity of the losses. Total losses were seen on two of the three wells, and all three wells were successfully completed. This paper will discuss the technology and techniques used along with the planning and procedures required to enable successful well construction in this challenging environment.