Drilling through salt sections requires that the particular properties of salt, its creep behaviour and high solubility, be recognized and incorporated in the drilling plan. Salt is a viscous material and creeps under differential stress; the creep rate is a strong function of both temperature and stress difference (actually underbalance between the mud pressure and the vertical stress). A simple model approach to account for these effects in a reasonably quantitative manner is described.
Problems encountered in drilling through salt include hole closure leading to stuck tools, differential dissolution of beds of carnallite, bischofite and other halides, encountering stiff and non-viscous stringers in salt strata, and exiting salt into non-salt rocks, always a challenging phase of the drilling. Strategies for successful salt drilling involve recognizing salt closure behavior, stresses, and adjusting drilling fluid density and temperature to minimize problems. Casing design issues in salt are also discussed.
1. INTRODUCTION
Large oil and gas reservoirs are associated with salt structures. Domal structures in the Gulf of Mexico (GoM - Jurassic salt emplaced during the Tertiary), Williston basin (Mid US Continent Devonian age) the North Sea (Zechstein age salt emplaced in the Cretaceous), Iran (Zagros salt plugs, which in some areas outcrop), Brazilian and West African offshore basins, and other areas, provide targets for exploratory oil and gas drilling. Sub-salt resources are found in the GoM salt tongue regions, in large areas in Kasakhstan (Kashagan and Tengiz), and in other areas. These may involve drilling through as much as 1500-2500 m of salt to depths of 5-9 km.
]Drilling through salt is rapid if there are few nonsalt beds. Typical ROP of 15 to 40 m/hr means that a 1000 m section can usually be drilled in two or three days with a PDC bit. ROP is important because speed minimizes hole closure from creep. Salt is essentially impermeable, so the effect of drilling fluid density (MW) on ROP is small. MW management can be used to control closure rate while sustaining reasonable penetration rates. However, high MW carries risks of lost circulation in non-salt zones, and this risk must be properly managed through knowledge of stresses. Salt does not present as serious drilling problems as fractured shale, but there are challenges such as washouts, rapid borehole closure, mud weight control issues, and casing placement decisions. Subsalt overpressure or pressure reversion may exist, and extensive rubble or sheared zones are common underneath salt tongues or adjacent to diapirs. It may be difficult to decide where salt ends and nonsalt sediments start: salt-infilled rubble zones and salt with 30-40% non-salt shale and sand inclusions can exist within salt beds, or at the boundaries of salt structures. However, most drilling problems within salt are managed relatively easily by considering salt properties during planning and drilling. Issues arising in drilling around salt structures are discussed elsewhere [1].
Salt is found as salt tectonics structures (domes, ridges, salt tongues, pillows...) as undeformed bedded sedimentary salt, and as mixed domains, as in the GoM, PreCaspian Basin, South Atlantic margin basins (Brazil, Angola...), Canadian Scotian Shelf and the Central Graben area and more southern parts of the North Sea.
