Geomechanical Optimisation with Managed Pressure Drilling for Successfully Penetrating Challengeable Depleted Reservoirs in Offshore Sarawak, Malaysia

摘要

Drilling new development or infill wells in a mature shallow water field, offshore Sarawak, Malaysia, is challenging. Reservoir depletion reduces the fracture gradient to a level such that drilling and cementing operations risk hydraulically fracturing the wellbore. To compound matters, intra-reservoir claystone sections are weak and require higher mud weights to maintain wellbore stability. As a result, the operational mud-weight window becomes very narrow, and drilling operations will be challenging using conventional drilling technologies. Without prior quantification of the operational mud window there is an increased risk of wellbore instability, lost circulation, and even loss of the hole. A geomechanical study was undertaken for the field to understand the evolution of the fracture gradient due to depletion of the multiple sandstone reservoirs, the drilling risks associated with the large amounts of depletion, and to quantify operational mud-weight windows. The study used drilling and wireline log data from offset wells to derive the in situ stresses, formation pressures, and rock strength properties. The results show that a tight drilling mud window exists through many of the depleted reservoirs, but these narrow mud-weight windows could be widened by casing off interim reservoirs that have undergone high depletion. The results were integrated into the final drilling plan. As a consequence, the managed pressure drilling was optimised to cope with narrow mud-weight windows, improving the capability of drilling the new well to total depth. This paper describes the practical geomechanical workflow that assists in understanding the drilling challenges in the depleted shallow water field and how the results should be adopted in well planning and drilling operations. The integrated geomechanical study and managed pressure drilling practice provide a feasible approach to mitigate drilling risks and minimise drilling costs. This approach can play a significant role in the well planning and drilling management of challenging depleted reservoirs.