ABSTRACT:
The oil and gas industry is exploring reservoirs under increasingly difficult geological conditions. Accessing these reservoirs requires drilling through unconsolidated formations, faulted rocks, rubble zones and salt structures. The formations not only have highly overpressured pore pressure and abnormal insitu stresses, but also have very low fracture gradients. With extremely narrow drilling mud window, drilling engineers experience a high frequency of wellbore stability problems during well construction, causing substantial non-productive time in drilling operations. To keep wellbore from shear failure and tensile failure in this narrow mud weight window, wellbore strengthening is crucially important. This paper presents a new method to strengthen wellbore by reducing both rock shear and tensile failures. This novel method is based on the premise that building an impermeable boundary condition at wellbore wall can avoid drilling mud invasion into pore spaces of the formation. This reduces pore pressure increase near the wellbore, and also reduces effective tangential stress and increases effective radial stress. Poroelastic solution with permeable and impermeable boundary conditions is developed to analyze pore pressure and drilling fluid interaction and wellbore stresses changes induced by drilling. Numerical study of this poroelastic solution shows that for the same mud weight the wellbore is stable for the impermeable mud condition; however, it is unstable for the permeable case.
1 INTRODUCTION
A considerable number of oil and gas reservoirs in the world are located in unconsolidated sands or in the naturally fractured formations. These complicated formations usually have not only highly overpressured pore pressure and abnormal in-situ stresses, but also very low fracture gradients. Consequently, this causes an extremely narrow safe drilling mud window (Figure 1), and drilling engineers experience a high frequency of wellbore stability problems during well construction, inducing substantial non-productive time in drilling operations.
