Changing the Safe Drilling Window with Invert Emulsion Drilling Fluids: Advanced Wellbore Stability Modeling Using Experimental Results

Abstract

In today's increasingly complex well designs, often the range in drilling fluid densities required to prevent hole collapse without fracturing the wellbore, i.e. the safe drilling window, is narrow. This is often the case for wells having high angles of deviation, and is of particular concern in extended-reach drilling (ERD) wells. With increasing step-outs, the drilling equivalent circulating density (ECD) continues to climb with increasing measured depth (MD) while the true vertical depth (TVD) remains fairly constant. The net result can be that as horizontal departure lengths continue to increase, the drilling ECD violates the safe drilling window.In a recent study presented to the industry, the effects of changes in the water phase salinity (WPS) of invert emulsion drilling fluids (IEF) were investigated as a function of rock shear failure for two very different shales: a deepwater West Africa shale and a more-competent Oklahoma shale. After a 3-hr exposure time, the changes in rock strength with different WPS levels were measured directly and results were qualitatively consistent for the two shales.In this paper, the effect of changes in the WPS of IEF on the safe drilling window is demonstrated in an ERD drilling scenario. The modeled failure envelope is first examined using conventional elastic modeling. Poroelastic modeling is then used to show the effect of changes in the safe drilling window resulting from changes in pore pressure. As a further step, changes in the safe drilling window are next shown using porochemoelastic wellbore stability modeling, where the effects of invert emulsion drilling fluid chemistry coupled with changes in pore pressure are introduced. The results show that the safe drilling window can be changed with invert emulsion chemistry, and that there is much to be learned using more advanced poroelastic and porochemoelastic modeling techniques. IntroductionThe drilling of challenging wells is often characterized by a narrow safe drilling window, with the fracture initiation pressure as the upper bound and the wellbore hole collapse pressure as the lower bound. For those cases where the collapse pressures are below the pore pressure, the pore pressure then serves as the safe drilling window lower bound. The safe drilling window is commonly gauged in terms of equivalent mud weight (EMW) and represents the acceptable range of density for maintaining wellbore stability. When the wellbore is circulated, for example during drilling operations, the EMW is equivalent in value to the ECD. When the wellbore is static, the EMW is equivalent in value to the equivalent static density (ESD). Depending on the downhole pressure and temperature conditions, the ESD is often not equal to the density measured under ambient conditions, especially when drilling with IEFs. The general equation for calculation of ECD incorporating circulating pressure drop (ΔP) is given in Equation 1:

Modelling of Time-Dependent Wellbore Collapse in Hard Brittle Shale Formation under Underbalanced Drilling Condition

“…where κ ′ is the hardening parameter, which is influenced by the hydration effect. The Drucker-Pracker yield criterion has been widely used in the problem of wellbore stability in shale formations to describe the elastoplastic behavior of shale [24][25][26][27][28]. In this study, the elastoplastic calculation is performed using the Drucker-Prager criterion.…”