Wellbore Stability Challenges in the Deep Water, Gulf of Mexico: Case History Examples from the Pompano Field

Abstract

Abstract Several wellbore stability challenges are faced when drilling in deep water. Overburden sediments are typically weak and overpressured; pore-frac windows are therefore narrow; salt bodies may have to be penetrated; rubble zones may exist adjacent to the salt bodies; reservoir formations may be depleted, with consequent risks of lost circulation and differential sticking. Extended reach wells, required to access satellite reserves, require close monitoring of ECDs when drilling. The pore-frac drilling window may be further complicated by changes in water depth existing over the length of the well path being drilled. All these challenges have arisen in the various wells drilled at BP's Pompano field in the deepwater Gulf of Mexico. This paper describes the wellbore stability challenges faced. Citing well case history examples, the paper describes the experiences gained in tackling these challenges through the pre-well analyses and post-well observations. The paper concludes by providing guidelines and recommendations for data collection, pre-planning activities, drilling practices and real-time ECD / wellbore stability management that should be implemented to eliminate non-productive time when drilling in these challenging environments. Introduction Drilling costs in deepwater fields can be high, even for development wells. Infill drilling and exploration from existing facilities to access satellite reserves pose particular challenges. Well costs must be kept as low as practical to realize the full net present value of these typically smaller hydrocarbon accumulations. For these reasons, technical investment in the pre-planning and execution phases of development well drilling is essential to maximize and extend field production and profitability. This paper describes issues of relevance to wellbore stability, as they pertain the BP's Pompano Field in the deepwater Gulf of Mexico (GoM). Wellbore Instability Challenges Faced in Deepwater Pore pressure and fracture gradient prediction Historically, the challenge of principal concern when drilling deepwater wells, particularly the early exploration wells, has been the prediction of overpressures and fracture gradients. The narrow pore pressure and fracture gradient (PPFG) windows have necessitated multiple casing strings to reach the target reservoir formations. Errors in predicting PPFG windows have in the past resulted in the failure of wells. This has been of particular importance in sub-salt wells where the poor seismic quality may make it impossible to accurately predict pore pressure from seismic velocities. Here recourse may be made to basin modeling to extrapolate pore pressures into sub-salt regions after achieving suitable calibration at extra-salt locations. Much has been written on the prediction of pore pressure and fracture gradient prior to drilling e.g., 1, and this general topic will not be considered in any great detail in this paper. However, one challenge faced in the design of extended reach wells in deep water is the case where a varying water depth exists along the well profile. This can commonly occur in the GoM, for example, when drilling in the vicinity of the Sigsbee Escarpment, where changes in water depth of over 2000 feet can occur over lateral distances of about 2 miles. The deepwater GoM fields, Mad Dog and Atlantis, both underlie the Sigsbee Escarpment. Though the principal production facilities will be located on top of the escarpment in shallower water (ca. 4420 feet water depth), some extended reach wells will be drilled into areas of the field laying in deeper water (up to 6500 feet water depth)2. One-dimensional predictions of pore pressure and fracture gradient are evidently inappropriate in the design of such wells. To accurately predict pore pressures and fracture gradients, the varying water depth has to be taken into consideration. The lower fracture gradient (relative to that for a shallower water vertical well profile) existing over the long tangent section of extended reach wells drilled from the escarpment to access deeper water reserves is an important aspect in the well design. In this case, an extra casing string is usually required in the tangent section.