Defining Acceptable Loss Rates in MPD Operations with Statically Underbalanced Fluids: Operational Experience and Detailed Methodology

Abstract Fluid lost circulation events are a challenging aspect during drilling operations. These events occur due to preexisting natural fractures and/or induced fractures, permeable formations, faults, depleted zones, excessive annular pressure, and elevated Equivalent Circulating Density (ECD). Undesired outcomes such as influx, hole instability, or stuck pipe can be triggered by mud losses. Particularly of interest, when utilizing Managed Pressure Drilling (MPD) with hydrostatically underbalanced fluid, or unknown pore pressure, it becomes critical to measure the magnitude of losses without altering the surface back pressure and flow rates. In these cases, mud losses may lead to a reduction in annulus friction pressure and subsequently decrease ECD, potentially masking pressure imbalances in the wellbore. Thus, while using MPD, when losses are observed, maintaining stable back pressure enhances the chances of achieving balance. Evaluating allowable bottom hole pressure reduction versus the predicted overbalance is essential, as a drop in annulus friction could cause the resultant ECD to be underbalanced. Misinterpreting this situation might lead to catastrophic outcomes, such as crossflow. Effective monitoring, timely assessment of loss rates, and careful consideration of drilling parameters are crucial in mitigating risks associated with mud losses during MPD operations, ensuring the safety and efficiency of the drilling process. This paper provides an overview of the risks associated with losses on MPD with hydrostatically underbalanced fluid and presents a method for determining during planning phase an acceptable loss rates involving evaluating factors like annulus flow rate, mud properties, surface back pressure, and interval length. The proposed method involves assessing the worst-case scenario by considering planned ECD and maximum predicted pore pressure. Hydraulic simulations are presented to aid in quantifying the drop in annulus flow due to losses, helping prevent underbalanced conditions. Additionally, considerations for assessing loss rates and operational procedural considerations are also introduced.