Underbalanced Drilling of a Horizontal Well in Depleted Reservoir: A Wellbore-Stability Perspective

Summary The need to reduce formation damage and to avoid differential sticking and lost circulation in depleted reservoirs favors the use of underbalanced-drilling (UBD) technology. In highly depleted reservoirs, the pore pressure can be very low, necessitating the use of extremely low-density fluid to achieve an equivalent circulating density (ECD) below the pore pressure. In such situations, the stress redistribution around the wellbore has to be supported mainly by the rock matrix, and limited support that is provided by the mud pressure. Therefore, UBD can dramatically increase the risk of wellbore instability. From literature review, it was found that the techniques and methodologies to carry out wellbore-stability analysis on UBD of horizontal wells are not well documented. This paper presents a wellbore-stability study that was conducted to evaluate the feasibility of using UBD technology to drill a horizontal well in a highly depleted reservoir in Libya. The study started with geomechanical laboratory tests and mechanical Earth Model (MEM) construction to evaluate the in-situ stresses, pore pressure, and mechanical properties of the formations likely to be encountered during the planned UBD campaign. Wellbore-stability analysis was subsequently conducted for the planned horizontal well to be drilled underbalanced using a new practical approach. The analysis revealed a high probability of extensive and severe breakout within the weak zones if penetrated underbalanced and the potential for massive wellbore collapse. Under the guidance of this study, the well plan and drilling designs were amended to minimize drilling risk.