Optimization of underbalanced drilling operations to improve well productivity

In excess of 100 horizontal wells have been drilled underbalanced in the Weyburn Unit in S.E. Saskatchewan. initial underbalanced drilling (UBD) operations intermittently placed horizontal wellbores in an overbalanced condition due to large variations in bottomhole pressure while drilling. Pressure fluctuations acting at the sandface were found to have resulted in reduced productivity in many of these wells by forcing drilled fines into productive pore throats. In an effort to reduce formation damage, UBD procedures were modified to better maintain an underbalanced condition and decrease the magnitude of bottomhole pressure surges occurring during drilling operations. Initial steps taken to optimize Weyburn UBD operations included the use of electromagnetic survey tools, the acquisition of bottomhole pressure data and adding on-site UBD technical support. This led to the implementation of basic operational practices such as minimizing survey and connection times, displacement of the drill string past the first float to nitrogen prior to making connections, shutting in the annulus during connections, and adjustment of circulation parameters to account for formation fluid influx penetration rates, and wellbore length. Wells drilled using the adjusted UBD practices resulted in approximately 40% more production, on average, than those wells drilled using the initial UBD practices.