Wellbore Strengthening – Where Field Application Meets Theory

Abstract

Abstract In recent years there has been a growing awareness of the importance of Wellbore Strengthening Material (WSM) for sealing drilling-induced and natural fractures both in permeable and impermeable zones. This paper details the application of a software tool designed specifically for determining the optimum blend of WSM to drill the formations of the challenging Cashiriari field in Peru. Several theories have been proposed to explain techniques which are in vogue for mechanically strengthening wellbores so as to control fracture propagation: Hoop Stress Enhancement ("Stress Cage") theory involves increasing the near-wellbore stress, which is usually carried out by opening fractures to tighten the wellbore and sealing the fractures immediately in that open state; "Fracture Closure Stress" theory utilizes high-fluid-loss materials to plug and tighten existing fractures; and "Fracture Propagation Resistance" theory involves isolation of the fracture tip. The design and successful utilization of a blend of WSM based on the opening and sealing of fractures suggests that these three theories are, in many respects, complementary. Any theory explaining a particular problem is ultimately judged by its practical application. Cashiriari field, known for expected mud losses to both natural and drilling-induced fractures in primarily permeable formations, is where this wellbore strengthening application took place. The optimum blend of particulates, designed using wellbore-strengthening (WBS) software, was applied consistently on the wells drilled by the operator in the Cashiriari field in the 2009-2010 campaign. Fracture resistance was increased up to 0.8 lb/gal above the minimum horizontal stress for drilling and up to 1.68 lb/gal for running casing and cementing operations. Specialized logs showing the presence of WSM inside fractures were an important tool for optimization of the WSM blend. WSM design software takes in account a multitude of drilling parameters and rock properties to predict the fracture width and estimate the required size distribution and concentration of the WSM particulates. Consequently, the WBS solution reduced lost circulation as a primary contributor of costly and time-consuming lost circulation incidents in the Cashiriari field. Further, it is anticipated that as the WBS approach is applied more widely, the incidence of lost circulation events will be greatly reduced.