Changing the Safe Drilling Window With Invert Emulsion Drilling Fluids

Abstract

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 132207, ’Changing the Safe Drilling Window With Invert Emulsion Drilling Fluids: Advanced Wellbore Stability Modeling Using Experimental Results,’ by Terry Hemphill, SPE, Halliburton; William Duran, SPE, Saudi Aramco; and Younane Abousleiman, SPE, Vinh Nguyen, SPE, Minh Tran, SPE, and Son Hoang, U. of Oklahoma, originally prepared for the 2010 CPS/SPE International Oil and Gas Conference and Exhibition in China, Beijing, 8-10 June. The paper has not been peer reviewed. 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 often is the case for wells having high deviation angles, and is of particular concern in extended-reach-drilling (ERD) wells. With increasing stepouts, the drilling equivalent circulating density (ECD) continues to climb with increasing measured depth (MD) while the true-vertical depth (TVD) remains fairly constant. Introduction The drilling of challenging wells often is 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 commonly is 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 often is not equal to the density measured under ambient conditions, especially when drilling with invert-emulsion drilling fluids (IEFs). In the modeling of stresses around an intact wellbore, the safe drilling window becomes more narrow with increasing hole angle. Specifically, a deviated wellbore profile requires increased EMW to prevent collapse and reduced EMW to initiate fractures. Hence, in ERD wells, the safe drilling window can become very narrow at the higher angles of deviation. In the drilling of ERD wells having long departures, the ECD will continue to increase with increasing MD, as the frictional pressure produced while circulating increases with hole length while the TVD does not increase by much, if at all. The effect of ECD in these kinds of wells serves to increase the operating EMW to the point where the upper bound of the safe drilling window can be violated and initiation of wellbore fracturing is predicted. Consequently, much effort has been focused in recent years to widen the safe drilling window safely without sacrificing wellbore stability.