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Geoscience Conference & Exhibition
Innovative Geoscience Solutions – Meeting Hydrocarbon Demand in Changing Times
March 7-10, 2010 – Manama, Bahrain
Underbalance Drilling in Tight Gas Reservoirs
(1) GeoMechanics International, Dubai, United Arab Emirates.
(2) GeoMechanics International, Houston, TX.
Deep, tight reservoirs face significant appraisal and development challenges. In particular, it can be difficult proving the presence and mobility of sufficient quantities of gas to make the reservoir economically viable. At the same time, drilling costs are extremely high. In this context, underbalanced drilling (UBD) provides a number of benefits: first, it enables the operator to proof the presence of producible quantities of gas while the well is being drilled. Underbalanced drilling also can minimize formation damage and maximize the rate of penetration. This can result in significant savings of drilling and completion costs relative to conventional drilling. However, not all reservoirs are suitable for UBD as there is much greater risk of mechanical wellbore instabilities relative to wells drilled overbalanced. Hence, geomechanical analyses prior to drilling are of particular importance in order to evaluate the feasibility of UBD operations.
In the past, the stability of UBD wells has been analyzed using conventional approaches, simply by extending these to stress states in which immediately after the well is drilled one effective principal stress (the radial stress) is tensile; undrained conditions are assumed to develop instantaneously at the wellbore. This approach leads to very conservative predictions, with the result that many wells that would be candidates for UBD are drilled overbalanced.
To apply a less conservative approach, a new analytical model to predict the stability of underbalanced wells has been developed. Based on the recognition that rocks have scale-dependent strengths, the full stress concentration is not developed until some time after the bottom of the well is some distance below the point of interest, and that fluid flow into the advancing wellbore leads to a zone of locally lower pore pressure that extends beneath the drill-bit, it provides rapid assessments of the limit of safe underbalance as a function of drilling rate. The model predicts the regions within which spalling and breakouts will occur. One consequence is that higher permeability leads to the ability to drill both faster and with a larger underbalance. A second is that smaller hole sizes are predicted to be easier to drill underbalanced; in cases where there is a high risk of wellbore collapse of the full-sized well this suggests that drilling an initial pilot well followed by enlargement to full size may mitigate the risk of collapse.