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

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.

Geomechanical modeling of stresses adjacent to salt bodies: Part 1—Uncoupled models

“…One reason is that stresses and fluid pressures adjacent to salt may be highly perturbed relative to their regional values (e.g., Bradley, 1978;Whitson and McFadyen, 2001;Rohleder et al, 2003;Dusseault et al, 2004). Predicting the magnitude of these anomalies is also challenging (e.g., Rohleder et al, 2003;Willson et al, 2003;Willson and Fredrich, 2005).…”

Geomechanics Considerations for Through- and Near-Salt Well Design

“…From a drilling and geomechanics perspective, therefore, salt exits into homogeneous formations should pose little difficulty, except perhaps in the narrow gouge zone potentially present in the case of rapidly down-built sediments adjacent to the diapir. The salt exit drilling experiences from the Pompano field support this classification (see Willson et al 17 for further details). The risk of high-angle well exits resulting in a "parallel to bedding" mode of borehole failure would, therefore, appear low in these instances.…”

A model for extended-reach limit analysis in offshore horizontal drilling based on formation fracture pressure

“…Compared with the terrestrial environment, the deposition rate in the ocean formation is faster, and the abnormal pressure of the formation is generally developed (Willson et al, 2003); therefore, the overburden pressure is usually lower due to the long water column. In addition, offshore drilling tend to have much lower fracture pressure (Rocha et al, 2003a(Rocha et al, , 2003b, leading to a narrower MWW (Zhiming et al, 2006), which is not friction pressure loss gradients, MPa/m ρ b bulk density, g/cm 3 ρ f the equivalent density of the formation fracture pressure, g/cm 3 ρ m mud density, g/cm 3 ρ mix mixture density, g/cm 3 ρ w seawater density, g/cm 3 ρ g grain density, g/cm 3 ρ p pore pressure equivalent density, g/cm 3 ρ fl fluid density, g/cm 3 ϕ porosity, dimensionless σ H maximum horizontal principal stress, MPa σ h minimum horizontal principal stress, MPa σ v the overburden pressure, MPa ω 1 , ω 2 tectonic stress coefficient, dimensionless conducive to simplify the casing program, even leading to the failure of reaching the target stratum .…”