Abstract
The analysis of wellbore stability in deepwater gas wells is vital for effective drilling operations, especially in deepwater remote areas and for modern drilling technologies. Wellbore stability problems usually occur when drilling through hydrocarbon formations such as shale, unconsolidated sandstone, fractured carbonate formations and HPHT formations with narrow safety mud window. These problems can significantly affect drilling time, costs and the whole drilling operations. In deepwater gas wells, there is also the possible of gas hydrate problems because of the low temperature and high pressure conditions of the environment as well as the coexistence of gas and water inside the wellbore. These hydrates can block the mud line, surface choke line and even the BOP stack if no hydrate preventive measures are considered. In addition, the dissociation of these hydrates in the wellbore may gasify the drilling fluid and reduce drilling mud density, hydrostatic pressure, change mud rheology and cause wellbore instabilities. Traditional wellbore stability analysis considered the formation to be isotropic and assumed that the rock mechanical properties are independent of in-situ stress direction. This assumption is invalid for formations with layers or natural fractures because the presence of these geological features will influence rock anisotropic properties, wellbore stress concentration and failure behavior. This is a complicated phenomenon because the stress distribution around a wellbore is affected by factors such as rock properties, far-field principal stresses, wellbore trajectory, formation pore pressure, reservoir and drilling fluids properties and time. This research work reviews the major causes of wellbore stability problems in deepwater gas wells and outlines different preventive measures for effective drilling operation, because real-time monitoring of drilling process can provide necessary information for solving any wellbore stability problems in a short time.
Highlights
Deepwater exploration is a difficult and highly risky operation due to problems such as wellbore instability that occur while drilling
Most formations especially shale formations drilled with oil-based drilling fluids perform better than those drilled with water-based fluids but some researchers have reported that oil-based drilling fluids are not the best solution to the problems that occur while drilling shale gas wells and different alternatives have been proposed
Wellbore instability problems in deepwater exploration and production are expected when drilling through shale, unconsolidated sandstone and fractured carbonate formations as well as HPHT formations with narrow drilling safety margin
Summary
Deepwater exploration is a difficult and highly risky operation due to problems such as wellbore instability that occur while drilling. Willson et al, [10] wrote an SPE article on Wellbore-Stability Challenges in the Deepwater Gulf of Mexico: Case History Examples from the Pompano Field They discussed the problems of drilling depleted sands, wellbore instability at high angles, and the hazards associated with drilling subsalt extended-reach wells. They outlined the effect of shale bedding plane failure on wellbore stability and the possible angle of attack for stabled rilling operations in weak bedding planes They developed a robust tool to account for the conditions needed to identify and drill through weak bedding planes, the rock compressive strength for weak bedding planes, minimum mud pressure to prevent shear failure and wellbore slip as well as suitable attack angle. It can lead to stuck pipe and possible loss of well
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