oa Underbalance Drilling in Tight Gas Reservoirs

Deep, tight reservoirs face significant appraisal and development challenges. In particular, it can be<br>difficult proving the presence and mobility of sufficient quantities of gas to make the reservoir<br>economically viable. At the same time, drilling costs are extremely high. In this context, underbalanced<br>drilling (UBD) provides a number of benefits: first, it enables the operator to proof the presence of<br>producible quantities of gas while the well is being drilled. Underbalanced drilling also can minimize<br>formation damage and maximize the rate of penetration. This can result in significant savings of drilling<br>and completion costs relative to conventional drilling. However, not all reservoirs are suitable for UBD<br>as there is much greater risk of mechanical wellbore instabilities relative to wells drilled overbalanced.<br>Hence, geomechanical analyses prior to drilling are of particular importance in order to evaluate the<br>feasibility of UBD operations.<br>In the past, the stability of UBD wells has been analyzed using conventional approaches, simply by<br>extending these to stress states in which immediately after the well is drilled one effective principal<br>stress (the radial stress) is tensile; undrained conditions are assumed to develop instantaneously at<br>the wellbore. This approach leads to very conservative predictions, with the result that many wells that<br>would be candidates for UBD are drilled overbalanced.<br>To apply a less conservative approach, a new analytical model to predict the stability of underbalanced<br>wells has been developed. Based on the recognition that rocks have scale-dependent strengths, the full<br>stress concentration is not developed until some time after the bottom of the well is some distance<br>below the point of interest, and that fluid flow into the advancing wellbore leads to a zone of locally<br>lower pore pressure that extends beneath the drill-bit, it provides rapid assessments of the limit of safe<br>underbalance as a function of drilling rate. The model predicts the regions within which spalling and<br>breakouts will occur. One consequence is that higher permeability leads to the ability to drill both<br>faster and with a larger underbalance. A second is that smaller hole sizes are predicted to be easier to<br>drill underbalanced; in cases where there is a high risk of wellbore collapse of the full-sized well this<br>suggests that drilling an initial pilot well followed by enlargement to full size may mitigate the risk of collapse.