Selecting Wells for Underbalanced Coiled Tubing Drilling in Deep and Tight Reservoirs

Abstract Drilling conventional horizontal wells in deep, tight reservoirs using conventional methods poses several challenges. Recently, underbalanced coiled tubing drilling (UBCTD) has emerged as a promising alternative, particularly for smaller borehole diameters. UBCTD offers notable advantages, such as enhanced rates of penetration, reduced formation damage, and the potential to achieve natural production without stimulation. However, the practical criteria for determining the suitability of UBCTD for specific wells remain underexplored. To identify suitable wells for UBCTD, we have developed comprehensive workflow to provide foresight and guidance. The workflow focuses on determining whether a well should be drilled using UBCTD before the commencement of drilling operations. This workflow consists of several critical steps: (1) diagenetic rock typing; (2) natural fracture prediction; (3) 3D geomechanics modeling; (4) extracting rock properties and in-situ stresses along the planned well trajectory from the 3D geomechanics model; (5) breakdown pressure envelope calculation. If the calculated breakdown pressure is lower than the wellhead can safely provide and the rock quality is deemed inadequate for natural production, conventional drilling methods will be selected, followed by hydraulic fracturing treatments after drilling. Conversely, if the breakdown pressure is relatively high and diagenetic rock typing indicates favorable conditions for natural production, UBCTD may be recommended for drilling. The developed workflow and criteria have been applied retrospectively to analyze conventional wells that experienced challenges during hydraulic fracturing, as well as wells drilled using UBCTD. Field case studies will be presented to validate the effectiveness of the workflow and criteria.