Whilst the step-out lengths of proposed ERD wells are becoming more and more challenging, wellbore stability assurance technologies - both in the pre-planning and execution phases - are developing at an equal pace. In this paper we describe several new developments in theoretical understanding and predictive capability of rock failure surrounding wells drilled at high-angle to bedding that are required to solve the problems encountered in these challenging environments. Rig-site processes for the integration of this new understanding with real-time diagnostic measurement and monitoring provide the means to deliver borehole stability assurance for ERD wells drilled in the most challenging environments.
Introduction
It has been 10 years since the temporary suspension of the extended reach drilling (ERD) program in the Niakuk field, North Slope, Alaska, due to the severe wellbore instability problems in the 8.5-in sections of successive ERD wells. The peer review1 and studies2,3 that were commissioned to investigate these problems highlighted the importance of integrating, in a holistic way, results of wellbore stability prediction, drilling fluid optimization, hydraulics and cuttings transport, operational practices and PWD tool utilization.
Since the Wytch Farm, UK and Niakuk, Alaska, ERD well drilling campaigns in the mid-'90s, there has been a steady progression in the vertical depth and horizontal departure length of ERD wells drilled world wide (Figure 1)4. Wells with horizontal departures in excess of 40,000 feet (12 km) at vertical depths of less than 10,000 feet (3 km) are now being actively considered as a viable way of accessing satellite reserves from existing facilities or, in the case of environmentally-sensitive arctic environments, to develop offshore fields from onshore locations.
A review of the recent SPE conference literature reveals that the challenges of ERD well feasibility planning and execution identified at Niakuk persist to the present day. Notable case history summaries have been presented by ExxonMobil for their Sakhalin-1 development in the Russian Far East. Here the offshore Chayvo field reservoirs are being accessed from an onshore location using ERD wells with reaches of 9 to 11 km5,6. In the Norwegian part of the North Sea, ExxonMobil again are using ERD drilling technology to access multiple independent reservoirs from their Ringhorn development, requiring well departures of up to 8 km7. Elsewhere in the Norwegian sector, Statoil have successfully drilled ERD wells with up to 7593 m (24911 ft) departure from their Visund platform; a record from a floating installation8,9. The reader is particularly directed to these papers, plus their associated references and bibliography, for a recent compilation and discussion of drilling engineering aspects of ERD well construction. In the rest of this paper, the authors will focus on the wellbore stability aspects of ERD wells. Particularly, new understanding and predictive capability for assessing instability in wells drilled at high angles to bedding are presented. Real-time drilling monitoring and operational practices are discussed, as are approaches that can be applied to minimize the risk of incurring wellbore instability problems in extended reach and high-angle wells.
Wellbore instability in ERD wells - what's different about it? One can legitimately question whether wellbore instability in ERD wells differs significantly from instability occurring in near-vertical wells and in high-angle wells of lesser departure. It is the authors' opinion that there are differences in assessing and addressing wellbore instability in ERD wells. The additional considerations are more subtle in relation to conventional high-angle wells, but extra assurance steps are considered necessary. The list below summarizes particular issues that should be addressed when planning ERD wells.
