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.

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.

The first use of wired pipe in BP was in early pre-commercial field trials in seven Oklahoma wells in 2004to 2005(Reeves et al, 2006. IntelliServ commercialized its networked drillstring or wired pipe with full connectivity to 3 rd party downhole tools in early 2007.BP field trialed the commercial version of wired pipe with a comprehensive suite of BHI's Loging While Drilling (LWD) tools, Measurment while drilling (MWD) and AutoTrak rotary steerable on two Wamsutter (Wyoming) wells in 2007. These wells achieved a number of "industry firsts" including the first use of IntelliServ along string pressure measurements and the first use of real-time high resolution wellbore imaging while drilling.Since then, BP has deployed wired pipe commercially on more than 14 wells in 4 additional locations (Trinidad, North Sea, Colombia, and Deep Water Gulf of Mexico) representing a good cross section of drilling conditions and challenges (including wellbore stability, hole cleaning, BHA vibrations, formation pressure measurements in depleted zones, complex geology and challenging directional requirements). This paper is a summary of the above experiences. It includes a discussion of the challenges and solutions in area such as hardware modifications (e.g. wiring top drives, reamers, jars) deployment, logistics, reliability, pipe handling and other operational modifications, surface connectivity and dataflow to shore. The bulk of this paper discusses wired pipe enabled applications while drilling. Introduction:BP has been involved with IntelliServ since its early days (Jellison et al, 2003). Early field trials of wired pipe were conducted on BP wells in Oklahoma (Reeves et al, 2006). The objective of these trials was to test the mechanical and network reliability of the system. Attempts to use the real-time data were limited. In 2007, two wells in Wamsutter (Wyoming) were used for the "Wired Pipe enabled applications" field trial. The focus of these trials was to explore what the types of real-time applications that could be enabled by the additional data available while running wired pipe, or in others words "what can we do with wired pipe that we can't do with mud pulse". Although the Wamsutter wells themselves were not thought to be sufficiently challenging or high cost to warrant the use of wired pipe on a commercial basis, they did offer the opportunity to test a number of applications which were thought to be of significant use in other more challenging and higher cost locations (mostly offshore).

This paper presents a case study of borehole instability from 4 wellbores on the Gulf of Mexico (GOM) shelf, offshore Louisiana. Logging while drilling (LWD) borehole images are combined with observations of cavings and modeling of borehole shear failure in order to diagnose the mechanisms of instability and thus select the appropriate remedial action. It is observed that instability due to shear failure of intact rock (borehole breakout) can be suppressed by increasing mud weight. However, where pre-existing planes of weakness such as bedding planes and fractures dominate the mechanism of instability, mud weight increases do not necessarily lead to a more stable hole and can in fact further destabilize the wellbore. Introduction Despite considerable effort from the drilling, subsurface and geomechanics communities, many oil wells continue to suffer from wellbore instability problems during drilling. Although instability is quite common, in the majority of cases a considerable amount of uncertainty exists around exactly where, when and why the instability occurred. Unfortunately, it is almost axiomatic that logs will not be run in an unstable wellbore. Direct measurements of the borehole shape and condition which can be obtained from caliper and image logs are therefore rarely acquired in the wellbores where (from a geomechanics point of view) they would be most valuable. Modeling and cavings analysis alone, can leave considerable uncertainty as to the location and to some extent the mechanism of failure. An exception to the axiom can be where LWD image data is acquired. It is still unlikely that LWD imaging tools would be run in a well where significant instability was expected. However, LWD is often acquired in wells that turn out to be less stable than anticipated. In these cases a rare glimpse of the unstable wellbore wall in the early stages of collapse may be captured. This is very useful information, which would normally remain the secret of the well. Mechanisms of wellbore instability Mechanism of mechanical wellbore instability can be grouped in two main classes.Instability due to failure of intact rock (rock which is unbroken and isotropic in strength)Instability due to failure of rock containing pre-existence planes of weakness (bedding planes, fractures, cleavage). Rock containing pre-existing weaknesses such as bedding or cleavage may be intact in the sense that it is unbroken. However, for the sake of this discussion intact is defined as above. The majority of quantitative wellbore stability studies since the 1979 paper by Bradley1 have modeled the wellbore wall as intact rock subject to the stresses imposted from the far-field and the wellbore fluid. This type of failure gives rise to symmetrical breakouts in the wellbore walls. Breakouts can be stabilized by increasing the mud weight, or may stabilize after reaching a certain size under favorable combinations of stress and strength. Breakouts are quite often observed in image and multi-arm caliper log data and are clearly a common cause of wellbore instability. Other mechanisms of instability where pre-existing weaknesses are present do not necessarily stabilize with time or with increased mud weight. Instability due to such mechanisms is therefore rarely calipered or imaged, making the exist location and mechanism of instability uncertain. Consideration of wellbore instability due to pre-existing weaknesses in oil wells is for the most part relatively recent2,3,4,5,6. Evidence of these mechanisms came from observations such as correlations of trouble time with wellbore trajectory and the existence of pre-existing fracture planes, bedding planes and cleavage in cavings. Types of wellbore instability associated with pre-existing weaknesses can be grouped into two classes.

Summary This paper presents a case study of borehole instability from four wellbores on the Gulf of Mexico (GOM) shelf, offshore Louisiana. Logging-while-drilling (LWD) borehole images are combined with observations of cavings and modeling of borehole shear failure to diagnose the mechanisms of instability and, thus, select the appropriate remedial action. It is observed that instability caused by shear failure of intact rock (borehole breakout) can be suppressed by increasing the MW. However, where pre-existing planes of weakness (such as bedding planes and fractures) dominate the mechanism of instability, mud-weight increases do not necessarily lead to a more stable hole and can, in fact, further destabilize the wellbore. Introduction Despite considerable effort from the drilling, subsurface, and geomechanics communities, many oil wells continue to suffer from wellbore-instability problems during drilling. Although instability is quite common, in the majority of cases a considerable amount of uncertainty exists concerning exactly where, when, and why the instability occurred. Unfortunately, it is almost axiomatic that logs will not be run in an unstable wellbore. Direct measurements of the borehole shape and condition that can be obtained from caliper and image logs are, therefore, rarely acquired in the wellbores from which (from a geomechanics point of view) they would be most valuable. Modeling and cavings analysis alone can leave considerable uncertainty regarding the location and, to some extent, the mechanism of failure. An exception to the axiom can be one in which LWD-image data are acquired. It is still unlikely that LWD-imaging tools would be run in a well in which significant instability was expected. However, LWD is often acquired in wells that turn out to be less stable than anticipated. In these cases, a rare glimpse of the unstable wellbore wall in the early stages of collapse may be captured. This is very useful information that would normally remain the secret of the well. In this paper, a case study from the GOM shelf that includes wellbore instability, LWD imaging, cavings observations, and rock-failure modeling is presented. The authors of this paper were present to provide technical support to the drilling activities of this well soon after initial signs of instability were observed. We were fortunate enough to be able to acquire all the data presented below in a timely manner, such that analysis and recommended remedial action could impact drilling operations. A detailed description and analysis of the data form the bulk of this paper; however, we first discuss mechanisms of wellbore instability within the context of pertinent literature on this subject. Mechanisms of Wellbore Instability We propose that mechanisms of mechanical wellbore instability can be grouped in two main classes:Instability caused by failure of intact rock (i.e., rock that is unbroken and isotropic in strength).Instability because of the failure of rock containing pre-existing planes of weakness (e.g., bedding planes, fractures, and/or cleavage). Rock containing pre-existing weaknesses such as bedding planes or cleavage may be intact in the sense that it is unbroken. For the sake of this discussion, however, intact is defined as above. The majority of quantitative wellbore-stability studies since the 1979 paper by Bradley1 have modeled the wellbore wall as intact rock subject to the stresses imposed from the far field and the wellbore fluid. This type of failure gives rise to symmetrical break- outs in the wellbore walls. Breakouts can be stabilized by increasing the MW, or they may stabilize after reaching a certain size under favorable combinations of stress and strength. Breakouts are quite often observed in image and multiarm-caliper log data and are clearly a common cause of wellbore instability. Other mechanisms of instability in which pre-existing weaknesses are present do not necessarily stabilize with time or with increased MW. Instability because of such mechanisms is, therefore, rarely calipered or imaged, making the exact location and mechanism of instability uncertain. Consideration of wellbore instability caused by pre-existing weaknesses in oil wells is, for the most part, relatively recent:2–6 evidence of these mechanisms came from observations, such as correlations of trouble time with wellbore trajectory, and the existence of pre-existing fracture planes, bedding planes, and/or cleavage in cavings. A particularly insightful documentation of both field and laboratory evidence of this mode of failure from fissile shales in the North Sea is presented by Okland and Cook.4 From the data presented in the literature, as well as in this paper and in the author's experience, it seems that types of wellbore instability associated with pre-existing weaknesses can be grouped into two classes:Failure because of the existence of "impermeable" pre-existing weaknesses.Failure because of the existence of "preferentially permeable" planes of pre-existing weaknesses. In the case in which the pre-existing weaknesses are not preferentially permeable, an increase in the MW tends to further support the wellbore wall. An example of this type might be where a single set of bedding planes intersected. Where the mud and filtrate preferentially enter pre-existing planes of weakness, increasing the MW does not add support to the wellbore wall and may increase instability. Networks of pre-existing weakness (i.e., where two sets of weakness, such as bedding planes and fractures, intersect) are probably more likely to be permeable than a single plane of weakness (e.g., just bedding planes). In an extreme case, the body of rock may actually be composed of many discrete rock fragments with no cohesion between them, rather like a pile of rubble or the material seen in brittle/semibrittle fault zones. This type of rock mass could be referred to as a rubble zone - the rock having been effectively turned to rubble. These pre-existing weaknesses could be a combination of fractures, cracks, bedding planes, and cleavage planes. Naturally, fissile rock - such as thinly bedded shale - is likely to be particularly susceptible to becoming rubble where it is affected by faulting.