Hydrostatically Underbalanced Managed Pressure Cementing Enables Zonal Isolation in Narrow-Margin HPHT Exploration Well

Abstract

As more and more HPHT & DW wells are drilled to explore or exploit reservoirs with narrow pore pressure (PP) / fracture gradient (FG) windows, the luxury of maintaining significant hydrostatic overbalance during the drilling and cementing operations or of being able to maintain hydrostatic overbalance at all, is being challenged. Managed Pressure Cementing (MPC) is relatively a new cementing technique using Managed Pressure Drilling (MPD) equipment and processes allows the wellbore to be displaced with a hydrostatically underbalanced mud after landing the liner string, then cement with a hydrostatically underbalanced spacer & cement slurry while applying dynamically controlled surface back pressure through MPD set up. MPC was the chosen approach to mitigate the risks when cementing the 9-7/8in liner in a hydrostatically underbalanced condition and applying surface backpressure (SBP) using an automated MPD system to bottom hole pressure between the highest pore pressure and the lowest fracture pressure of the well. To run the 9-7/8in liner, it was determined by simulation that three (3) step mud circulations were required at 1650m, 2280m and 2909m to change the MW from 17.0ppg to 15.2ppg despite the Pore Pressure is 16.68ppg at 2909m. A SBP as high as (850 psi) is applied to maintain the ECD within the operating window for liner circulation with roll over mud and subsequent cementing operation. When cementing the 9-7/8in liner the density of all the fluids were designed at 15.2 ppg to minimize the ECD downhole. Hence, the variation in ECD is solely attributed to the frictional pressures, which inevitably makes the rheology hierarchy play a greater role for an efficient mud removal. Application of comprehensively engineered cementing and MPD techniques resulted in flawless cementation result. No losses or any gain were observed, zero gas migration, and liner top isolation packer was successfully pressure tested and inflow tested. The successful use of MPC in a HPHT exploration well located in offshore Malaysia and drilled by a jack up rig, has delivered significant value to the project and Malaysian cementing experience in general, providing confidence for further applications of this technique and technology. MPC has become the primary technology enabler for efficaciously delivering such challenging well to its planned total depth without compromising the well design and integrity. The progressive method presents to be a safe and technically viable process, enabling the well to be drilled, cased and cemented which would otherwise not be feasible by conventional methods. This has secured the future development prospect of the field by demonstrating the capability to perform drilling to deeper reservoir targets and cementing within a narrow operating windows. The triumph of MPC is dictated by strenuous pre-operation design process, detailed risk assessment with multiparty mitigation plan and communication resulting in an accurate modeling, operational execution thus ultimately, a successful cement barrier. The key aspect is adherence to conventional HPHT cement job design best practices with specific focus on achieving good rheology hierarchy between the fluids pumped downhole to ensure good mud removal hinged around comprehensive and vital computer modeling of ECD envelope with the correct inputs.