Proposed Extended-Reach Drilling Project

ABSTRACT This paper presents engineering and research results for a North Sea well designed to be deviated six miles. Every phase of drilling, evaluating, and completing high angle wells was studied utilizing in-house expertise, published literature, and service, company capabilities. This paper summarizes studies in borehole stability, cuttings transport, data acquisition, dril1string design, rig requirements, and well planning. INTRODUCTION Extended-reach drilling1 (ERD) refers to drilling directional wells beyond routine capabilities. When the term was first coined in 19802, the implication was that highly deviated wells must utilize special tools such as MWDs for logging and surveying, oil-based muds for lubricity and hole stability, and top-drives for rotating drillstrings during trips. These "special tools" are now commonly used, and extended-reach wells to three miles displacement are being drilled. Extended-reach wells at displacements beyond three miles provide benefits for developing offshore fields. Four times more reservoir area could be reached by doubling the displacement to six miles. High displacement platform wells may be preferred to subsea completions for those fields in which wells require remedial work and artificial lift. Technical and operational challenges are expected when reaching these greater distances. To tackle the challenges offered by greater displacement drilling, a team of Conoco engineers and research scientists completed a preliminary design for a record extended-reach well. The goal was to reach six miles lateral displacement by 12,000 feet TVD in a central North Sea location. Team members were encouraged to "leave no stone unturned" in their studies. Their knowledge of the assigned subject was to be supplemented by a review of the literature and through meetings with the other team members and vendor representatives. As a result of the mix of research scientists, analysis-oriented engineers, field-oriented engineers, and drilling management, each study project benefitted from a broad range of perspectives. BOREHOLE STABILITY Mechanical borehole stability is a major factor determining ERD limits. In addition to high angles, extended-reach wells have open hole exposed for longer-than-normal periods of time. An inadequate mud system or mud weight profile can result in borehole collapse, shale swelling or sloughing problems which would cause stuck pipe, enlarged wellbores, and other critical conditions. Most borehole stability problems can be avoided if sufficient mud weight is supplied to support the borehole wall. Wilson and Willis4 have credited Mobil's success in high angle drilling in the Statfjord Field to their ability to establish a mud weight profile based on borehole stability theory. According to Fuh5, the roIe of rock mechanics in high angle holes includes analysis of rock stresses around the borehole arid prediction of rock failure under tension and compression. Compressive (shear) failures of the rock surrounding the borehole are the major cause of many borehole problems, experienced as tight spots or stuck pipe during high angle drilling.