Optimization of Underbalanced Drilling Operations to Improve Well Productivity
Authors: Yurkiv F.J. , Churcher P.L. , Edmunds A.C. , Wasslen R.D. , Schmigel K.A. 
Published: March 2010

Abstract
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 bottom-hole 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.

Introduction
During conventional drilling operations the hydrostatic pressure exerted by the column of drilling fluid is, by design, greater than the formation pressure of the zone being penetrated. This overbalanced pressure differential promotes both drilling mud filtrate and particle invasion into the target formation and can reduce well productivity. Fortunately, stimulation techniques can reach beyond damaged zones in most cased wellbore applications to provide access to undamaged reservoir. Horizontal well bores, however, are generally more susceptible to drilling induced damage than vertical wellbores due to their increased exposure time to drilling fluids and increased circulating pressure losses as horizontal length increases(1). Many horizontal well applications also involve underpressured or pressure depleted reservoirs, and are frequently completed open hole. Damage removal in these wells is often costly.
Underbalanced drilling (UBD) has emerged as an effective technique to reduce weIIbore damage, particularly when drilling horizontally into underpressured formations. UBD refers to the drilling process involving downhole circulation systems designed with a hydrostatic pressure lower than the pressure of the reservoir section being penetrated. Properly designed, the process results in a continuous and controlled influx of formation fluids into the wellbore while drilling. Although both the magnitude of wellbore damage and the impact that damage has on well productivity are difficult to quantify in most horizontal wells, laboratory evaluations(2) have measured the effect that overbalanced, balanced, and underbalanced conditions can have on formation damage. These tests have demonstrated that underbalanced pressure conditions can significantly reduce the potential for formation damage both y mud solids blocking pore throats and by filtrate invasion. Field tests(3) have similarly identified benefits of UBD.