Wellbore Stability Problems in Deepwater Gas Wells

摘要

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 style="font-family:Verdana;">in-situ style="font-family:Verdana;"> 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, style="font-family:Verdana;">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.