The Bowland Shale is a Carboniferous formation of Asbian to Yeadonian age located in Northern England. It is estimated to have a shale gas section with more than 5,000 ft thickness holding over 1300 TCF of total original gas in place. Drilling in the Bowland Basin started in August 2010 and by the end of 2011, three vertical wells (PH-1, GH-1 and BS-1) were drilled to a depth of 8,860 to 10,500 ft with objective of logging and coring the potential shale gas formations including Upper Bowland, Lower Bowland, Hodder Mudstone and Sabden Shale. All the drilled wells encountered several borehole stability problems, such as tight-hole, pack-off, overpull and excessive cutting, causing significant non-productive time (NPT) during drilling. Specifically, in GH-1 and BS-1, side-tracking was required to reach the target depth which imposed significant cost to the project. Careful investigation of the recorded drilling problems showed that majority of them were associated with formation collapse due to insufficient drilling fluid pressure. Fluid losses also occurred in some of the formations due to either too high of downhole pressure or presence of critically stressed natural fractures. These incidents implied that the applied casing design and mud weight program were not appropriate for the current-day state of stress and rock properties.
A comprehensive experimental and analytical geomechanical study was carried out to develop a reliable borehole stability model that can firstly explain the observed drilling incidents and secondly provide guidance for design and drilling of the planned wells. The plan was to drill a S-shape appraisal well (PNR-1) in the Preston New Road exploration site to log and core the Bowland Shale sequence and select the optimum landing depths for subsequent horizontal sections (PNR-1z and PNR2) to be completed for multi-stage hydraulic fracturing. The study recognized intrinsic shale anisotropy as a primary causative factor for borehole stability issues and formation collapses in the offset wells. Other important factors were identified to be the abnormal pore pressure regime and the presence of a tectonic strike-slip stress regime with large horizontal stress anisotropy. The anisotropy of the Bowland Shale was characterized in both laboratory and field scales, and anisotropic wellbore stability models were developed for offset and planned wells. As a result of this study, the PNR-1, PNR-1z and PNR2 wells were successfully drilled and completed with no notable borehole stability problems despite the presence of narrow operating mud weight window in several stratigraphic intervals. This paper presents a summary of the conducted borehole stability analysis aiming at risk-free and successful drilling of horizontal wells in the Preston New Road exploration site with emphasis on the effect of shale anisotropy.