Bullheading optimization study of the PMCD technique

Drilling highly fractured formations, typically fractured carbonates, often experiences kick problems and severe losses of circulation, resulting in a high nonproductive time (NPT). Pressurized mud cap drilling (PMCD) is the most appropriate methodology for such scenarios, being performed by injecting a sacrificial fluid into the annulus, without fluid return to the surface. When a gas kick is detected, the bullheading operation is executed in order to force gas and cuttings back into the formation, also reducing annulus pressure and sealing fractures. However, for a successful operation, the flow rate must be properly selected in order to push gas back into the formation, without causing well damage. The present work aims to solve a nonlinear constrained optimization problem using SQP (Sequential Quadratic Programming) during bullheading operation, ensuring the successful use of the PMCD technique. Simulation studies were carried out and validation tests were performed by employing an experimental facility, which depicts the main characteristics of the oil well drilling process. As a result, the methodology developed is able to provide proper bullheading flow in order to assure safe operation and effective minimization of annulus gas content for carbonate reservoirs, the majority of world oil reserves, characterized as a drilling challenge due to its complexity and heterogeneity nature and where implementing PMCD is the only safe mode of operation.