Drilling around salt structures means coping with a wide range of stress and pore pressure conditions, sometimes over a relatively short vertical distance. Zones with exceptionally low shmin, with open fractures, with overpressured fluids, or with exceptionally high sHMAX can be encountered. It is often possible to make semi-quantitative predictions of stress orientations and magnitudes based on the geological history of the salt structure emplacement, the general tectonic regime, and the displacement history of the sediments around the structures. For example, in the case of a diapiric structure that has pierced through overlying strata, the outward thrust placed on the sediments surrounding the dome shaft imprints the region surrounding the shaft with a highly compressive radial stress and a low tangential stress. These stress regimes not only affect drilling strategies and tactics around salt structures, they also affect completion approaches involving perforation placement, hydraulic fracture design, and horizontal well placement.
1. INTRODUCTION
Salt structures in extensional sedimentary basins are associated with large hydrocarbon deposits. Traps may be found in the anticlinal structures and normal fault blocks above piercement and non-piercing salt domes, in updip traps in upwarped and occasionally overturned strata that terminate against the salt dome flank in the piercement region, in gentle flank anticlines, or under salt tongues (Figs 1 & 2). Accessing these resources presents problems including massive lost circulation, fractured shale sloughing, serious gas cutting of mud, and so on. These problems are related to current stress state, gas migration, and rock properties and fabric alteration arising from large deformations. Issues during salt drilling are discussed elsewhere [1].
Initial drilling of an anticlinal structure above a Gulf of Guinea salt dome in the 1990's resulted in 92 lost drilling days because of an exceptionally low shmin value, a MW window<0.05 density units, and
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Figure 1: Shallow Salt Dome with Trap Locations
massive lost circulation that re-initiated at each attempt to continue advancing. Similar though less severe lost-time cases are reported wherever extensive drilling takes place around salt structures, and the magnitude of the problems are larger the younger and less consolidated the sediments.
Salt dome growth is complex; structures may grow and punch through sediments (piercement), others develop while sediments are emplaced around them, others deform strata above so that thinned anticlines form, and so on. Even in "simple" stress conditions (e.g. pure extensional regimes for millions of years), salt ridges, domes, and isolated salt pods now unconnected to underlying sources of salt can form.
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Figure 2: Drilling Beneath Salt Tongues in Deep Water
In listric fault conditions such as the Gulf of Mexico (GoM), offshore equatorial West Africa and eastern South America, salt tongues linked to deep salt deposits through a "stock" can form (Fig 2), much like laccolith structures observed in igneous rocks. These structures have recently become exploration targets because of better sub-salt seismic imaging.
In thick sedimentary salt beds, mobilization can be observed at all stages of deformation, such as in the Kungurian Salt in the Pre-Caspian Basin.
