A Method To Evaluate Lost-Circulation Materials—Investigation of Effective Wellbore-Strengthening Applications

Abstract

Summary With increased drilling activity in deeper, harsher, and more-complex environments, controlling nonproductive time during drilling is critical to lowering the cost of well construction. Lost circulation (LC), one of the main contributors to nonproductive time, is a widely discussed topic drawing industry attention, as is evident from recent publications and forums/workshops on the topic. The literature on LC reveals several means of mitigation, ranging from the simple use of lost-circulation materials (LCMs) to the use of gunk/chemical sealants and methods for altering near-wellbore stresses (stress caging or wellbore strengthening). There is a subtle difference between LC and wellbore strengthening, and any research should help facilitate a better understanding of the two. Presented previously (Kumar et al. 2010; Savari et al. 2012) were novel insights into LCMs for wellbore-strengthening applications. Continuing the research effort, this paper discusses another approach for designing LCMs to withstand higher wellbore pressures within a fracture. A property of plug-breaking pressure (PBP) was determined for different LCM combinations by use of a permeability-plugging apparatus (PPA) with tapered slots. LCM combinations with differing mechanical properties behaved differently compared with PBP data. Particles, such as ground marble, alone were not able to plug the tapered slot, and hence the PBP was zero. By use of the previous work described by Savari et al. (2012), the addition of resilient graphitic carbon (RCG) helped with plugging the tapered slot, with a PBP of approximately 900 psi. Adding fibers to the LCM combination was also evaluated. Plugging did not improve when fibers were added only to one other type of particulate LCM. But, the combination of ground marble, RGC, and specific fibers was found to provide the most-efficient plugging, with the highest PBP of approximately 2,100 psi. This work confirms that all LCMs are not equal and that the LCM type plays a role in terms of both plugging and “toughness” to better withstand displacement pressures. It also validates that, most of the time, combinations of LCMs work more effectively compared with the use of only one type. The novel PBP data on different LCM combinations can provide a qualitative comparison of their ability to withstand displacement and/or failure pressures when designing LCM combinations in wellbore-strengthening applications.