Application of Controlled-Influx Mud Cap Drilling Techniques in Exploiting the Marine Pre-Salt Carbonates in Santos Basin: Case Study

Abstract

The pre-salt Coquina Carbonate beds in the Santos Basin are the prime focus of major oil producers in the region. In addition to the thick, unstable salt layers posing borehole stability and casing issues, a primary challenge in the appraisal of the faulted Coquina reserves is the handling of immense downhole losses in the weaker and occasionally fractured carbonate layers just below the salt strata. Although the bottomhole pressure limitations posed by conventional drilling have been eliminated by Managed Pressure Drilling attempts, the transition has also brought along a new set of technical challenges such as: Instability in wells while tripping and high fluid losses while restoring controlFailed attempts to regain control in loss zones by pumping lost circulation material/cement into fracturesDifficulty in zonal isolation, even with Managed Pressure CementingPore plugging and reservoir damage due to placement of cement as lost circulation materialWellbore depletion and differential sticking tendency during shut-in timeLimitations on the use of Positive Displacement Motors due to additional pressure fluctuations The paper reviews the above challenges as planning and technological gaps suppressing the production prospects and investigates the application of Pressurized Mud Cap Drilling and Constant Bottomhole Pressure techniques with controlled Gas-Influx as an effective approach in mitigating the constraints by aligning the bottomhole pressure and the pore pressure. A case study for immense downhole losses in the reservoir interval has been presented. The results of feasibility analysis on reported case histories depict that circulating out the influxes while drilling with improved Surface Back Pressure systems minimizes losses, improves completion quality and reduces shut-in gaps.