Modeling and Optimization of Mud Cap Drilling for Undrillable Wells Due to Loss of Circulation

Mud Cap Drilling (MCD) is a technique from Managed Pressure Drilling (MPD), which is used when all conventional attempts fail to solve the loss of circulation. MCD places a very high drilling fluid density, called Mud Cap (MC), in the annulus above the lost zone to prevent the mud from returning to the surface. A sacrificial fluid instead of drilling mud is used to clean the hole and transport the cuttings into the loss formation. This study presents a detailed step-by-step model and design for a well in the Middle East. New equations were developed to calculate the MC height, MC density, and the position of the interface between the MC and the sacrificial fluid. The present model also depicts pump rate optimization and hole cleaning to reach an efficient drilling program. The case study presented within this paper encountered a complete lost zone and was considered as an un-drillable well. The pore and fracture pressures were calculated using drilling data from the offset well. A sensitivity analysis was performed to obtain optimal MC height and density. A complete optimal MCD design and optimal MCD operation for this well is conducted in detail. The results of this study have proved that the proposed model can be utilized as a tool to design and optimize an MCD program.