Publications

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Construction technology and durability of geothermal wells have significantly improved in the last 30 years resulting in wells that both cost less and have longer mean times to failure. In developed geothermal areas, well design and drilling practice are mature, but drilling problems typically still account for an additional 30% to the cost of a well. However, new drilling practices and technologies are constantly being introduced, such as dual-tube reverse-circulation (DTRC) drilling. Some of these have the potential to reduce geothermal drilling costs even further. Other areas where drilling technology is evolving include lost-circulation control, drill rigs, and cementing.

A series of field tests sponsored by Sandia National Laboratories has simultaneously demonstrated the hard-rock drilling performance of different industry-supplied drag bits as well as Sandia's new Diagnostics-While-Drilling (DWD) system, which features a novel downhole tool that monitors dynamic conditions in close proximity to the bit. Drilling with both conventional and advanced ("best effort") drag bits was conducted at the GTI Catoosa Test Facility (near Tulsa, OK) in a well-characterized lithologic column that features an extended hard-rock interval of Mississippi limestone above a layer of highly abrasive Misener sandstone and an underlying section of hard Arbuckle dolomite. Output from the DWD system was closely observed during drilling and was used to make real-time decisions for adjusting the drilling parameters. This paper summarizes penetration rate and damage results for the various drag bits, shows representative DWD display data, and illustrates the application of these data for optimizing drilling performance and avoiding trouble.

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We have concluded a laboratory study to evaluate the survival potential of polymeric materials used for lost circulation plugs in geothermal wells. We learned early in the study that these materials were susceptible to hydrolysis. Through a systematic program in which many potential chemical combinations were evaluated, polymers were developed which tolerated hydrolysis for eight weeks at 500 F. The polymers also met material, handling, cost, and emplacement criteria. This screening process identified the most promising materials. A benefit of this work is that the components of the polymers developed can be mixed at the surface and pumped downhole through a single hose. Further strength testing is required to determine precisely the maximum temperature at which extrusion through fractures or voids causes failure of the lost circulation plug.

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Sandia National Laboratories and Security DBS have collaboratively examined the hard-rock drilling performance of a conventional drag-bit design that was run in conjunction with field tests of Sandia's prototype Diagnostics-While- Drilling (DWD) system for acquiring real-time downhole and surface data. This effort constituted the first two phases of work under the terms of a multi-partner Cooperative Research and Development Agreement (CRADA) that has been established between Sandia and four bit manufacturers for the purpose of developing and demonstrating "best effort" drag bits that are capable of drilling difficult formations such as those commonly found at geothermal energy production sites. For both CRADA phases completed to date, the test bit (Security DBS, Model PD 5) drilled in the same well-characterized hard lithologic interval at the GTI Catoosa Test Facility near Tulsa, OK. In each case, extensive time-resolved downhole and surface data were acquired with the DWD system. During Phase 1, an experienced driller controlled the drilling parameters only on the basis of standard rig instrumentation readings. For Phase 2, one or more drilling engineers continuously observed the streaming DWD displays and actively guided the drilling process. Significantly different results were achieved in Phases 1 and 2 for penetration rate and bit life, which are reported along with bit damage assessments and representative data from the unique downhole measurement sub that monitored conditions at the bit. This information has supported the development of designs and DWD-based drilling strategies for the "best effort" bits being tested during CRADA Phase 3.

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.

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This report describes development of a system that provides high-speed, real-time downhole data while drilling. Background of the project, its benefits, major technical challenges, test planning, and test results are covered by relatively brief descriptions in the body of the report, with some topics presented in more detail in the attached appendices.

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This proposal provides the rationale for an advanced system called Diagnostics-while-drilling (DWD) and describes its benefits, preliminary configuration, and essential characteristics. The central concept is a closed data circuit in which downhole sensors collect information and send it to the surface via a high-speed data link, where it is combined with surface measurements and processed through drilling advisory software. The driller then uses this information to adjust the drilling process, sending control signals back downhole with real-time knowledge of their effects on performance. The report presents background of related previous work, and defines a Program Plan for US Department of Energy (DOE), university, and industry cooperation.