From plugging lost-circulation cross-flow zones to wellbore integrity
Transactions - Geothermal Resources Council
Fixing each lost-circulation zone as it is encountered before drilling ahead has been standard practice because of the technologies historically available to drillers. In recent years, however, there have been significant developments in wellbore integrity technology including a) reactive and shear-setting plugs for lost-circulation / cross-flow control, b) the use of dual-tube reverse-circulation rigs to drill severe lost-circulation zones, and c) alternative emplacement techniques for primary cementing (reverse circulation and "tremmie" pipe). These and other new techniques are allowing lost circulation mitigation strategies to change dramatically. Instead of fixing each lost-circulation zone as it is encountered, drillers can consider the bigger question, "How do we get the next casing cemented in with minimal lost time and additional cost?" The Wellbore Integrity Program at Sandia National Laboratories began with the development of polyurethane grouting as an advanced lost-circulation / cross-flow plug mitigative measure. While a technology specifically focused on plugging lost-circulation zones may be the only sure way of mitigating the effects of severe cross flows and to minimize overall drilling costs, there is a need to take a broad system perspective that considers how lost circulation impacts well design, drilling ahead, casing, primary cementing, etc. The ultimate goal is not controlling lost-circulation, but rather maintaining wellbore integrity. In this paper, we describe the evolution of the Sandia National Laboratories program from developing polyurethane grouting to alleviate lost-circulation zones to ensuring wellbore integrity. We examine several possible new technologies, compare their distinct advantages to current methods, identify the factors inhibiting their use, and investigate ways in which they can be integrated into future drilling. The successful polyurethane grouting at the Rye Patch geothermal field in northern Nevada is interpreted as a guide for how to plug cross flows. Other potential approaches that have been utilized to successfully plug cross flows are discussed, and a roadmap to a comprehensive Wellbore Integrity Program is proposed.