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Geothermal Exploration and Drilling

Learn what the National Laboratories are doing to assist the geothermal industry in finding and operating geothermal fields by developing a more thorough understanding of known geothermal resources and new innovative techniques for finding "hidden" geothermal systems.

• Reconnaissance for Hidden Resources
• Basin and Range Exploration
• Innovative Geothermal Exploration Techniques
• 3-D Magnetotelluric Modeling
• Geothermal Resource Exploration and Definition Solicitation
• High-Temperature Instrumentation
• Wellbore Integrity and Lost Circulation
• Hard-Rock Drill Bit Technology
• Cost Database and Simulators
• Diagnostics-while-Drilling (DWD)
• Acid-Resistant Cements
• Cement Structural Response Analysis

Reconnaissance for Hidden Resources

Organization: Lawrence Livermore National Laboratory
Our goal is to evaluate techniques that allow the exploration of large areas, to identify specific locales that might contain new geothermal resources. The twenty-year-old USGS survey of geothermal resources in the US estimated that the undiscovered resource was substantially larger than the known resource. Most of the known-resource areas have since been explored or tested, but very little exploration is targeted towards the undiscovered resource. We will study ways to identify targets for exploration of hidden resources. Two methods have potential for covering large areas and detecting anomalies associated with those systems. The DOE-funded Hyperspectral Imaging Project has demonstrated airborne geobotanical remote sensing at Long Valley. It detected surface effluents, historical signatures, subtle hidden faults and the botanical effects of low level chemical and thermal emissions that, in other locations, might indicate hidden geothermal systems. A combination of satellite-based techniques, including can detect localized strain around hydrothermal systems, and regional strain that may show where faults are favorably oriented to maintain vertical permeability. We will evaluate how effective these methods may be in finding targets for detailed exploration of hidden geothermal systems.

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Basin and Range Exploration

Organization: Idaho National Engineering and Environmental Laboratory
The Basin and Range is thought to comprise the largest geothermal exploration province in the United States, but unanswered questions limit the ability of operators to successfully explore for geothermal resources at low cost and with a high success ratio. A better understanding of the geologic setting and the native state geophysical signature of geothermal systems in the Basin and Range is essential for successful B&R exploration. Completing the analogue study of the Dixie Valley geothermal system will enhance operators' understanding of Basin and Range systems. This task will consist of:

• Correlation of new subsurface data (well logs released by Caithness Operating Company) with surface mapping and regional geophysical data,

• Expansion of the area mapped during 2000 and 2001 to include the northern termination of the Stillwater Fault.

• Assistance with the planning and interpretation for the separately funded, high-resolution aeromagnetic survey to be done by the USGS. This will be an application of a technique proven to be successful in the delineation of the intra-basin faults of the Albuquerque Basin to a practical exploration problem.

We are also planning simulation studies to examine the native state geophysical signature of typical Basin and Range geothermal reservoirs, using models developed previous work. These studies will combine native state reservoir modeling with geophysical modeling in order to assist in B&R exploration via understanding characteristic geophysical signatures. Where appropriate, these numerical studies will be validated through field testing, using geophysical data on existing geothermal fields.

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Innovative Geothermal Exploration Techniques

Organization: Lawrence Berkeley National Laboratory
To assist the geothermal industry in finding and operating geothermal fields we must develop (1) a more thorough understanding of known geothermal resources and (2) new innovative techniques for finding "hidden" geothermal systems. The project will take an integrated approach by calling on geophysical, geochemical, geologic, and remote sensing techniques as potential tools for expanding exploration capabilities for U.S. geothermal industries. Listed below are tasks that will be undertaken in FY2002. Some of these are feasibility studies others represent application of existing technologies.

• Rapid resource evaluation via airborne EM and gravity principal component analysis (Ki-Ha Lee, G. M. Hoversten, LBNL).

• Imaging geothermal reservoir signatures using high resolution satellite observations (D. Vasco, LBNL).

• Simulation of coupled subsurface and subaerial CO2 gas emissions for design of instrumentation and survey strategies for locating hidden geothermal systems (C. Oldenburg, LBNL).

• Isotope geochemistry applied to locating and characterizing "non-conventional" surface manifestations of hidden geothermal systems (B. M. Kennedy, LBNL).

• Field case studies to evaluate earlier exploration efforts and to identify new approaches to assess U.S. geothermal resources (M. Lippmann, LBNL).

• Evaluation of new 3-D magnetotelluric data acquisition systems and imaging algorithms for geothermal resource exploration (G. M. Hoversten, LBNL).

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3-D Magnetotelluric Modeling

Organization: Sandia National Laboratories
Electrical and electromagnetic (EM) methods are currently used in geothermal exploration to detect subsurface resistivity patterns that indicate geothermal resources. Of the EM methods, the magnetotelluric (MT) method is the most effective and commonly used tool used in geothermal exploration and is finding increasing application in development. By mapping the geometry of the 200 to 2000m thick, conductive argillic alteration that normally lies over and adjacent to high temperature geothermal systems, MT is used to target wells and assess reservoir generation capacity. Despite recent significant progress in EM data collection and processing, significant issues regarding MT data interpretation still act as barriers to routine use by the geothermal industry. Development of standard data sets to act as test beds for imaging schemes will provide the needed standards for testing MT interpretation. This type of synthetic data set has proved invaluable for the interpretation of 2D, and now 3D, seismic methods used in the petroleum industry. Sandia will participate in a large-scale, collaborative 3D MT modeling study, similar in concept to the recent synthetic reflection seismic 3D SEG/EAGE Modeling Project (collaborative between DOE and the petroleum industry). Consulting and industry partners are providing their own funding for the project. Sandia has already developed unique 3D EM finite difference modeling and inversion codes that will be applied to the construction of realistic synthetic data sets for three types of geothermal resource targets: volcanic, fault-type, and basin reservoir scenarios. The volcanic model will reflect the resistivity geometry typical of a distributed permeability, 230-3300 C geothermal reservoir beneath an andesitic-silicic volcanic massif, like Medicine Lake, California. The fault model will be representative of single fault zone geothermal reservoirs similar to Dixie Valley, Nevada. The basin model will reflect important features of large sediment-hosted geothermal fields such as the Salton Sea, California. Sandia's role is to compute the synthetic data sets using input from industry. Sandia possesses not only the operating algorithms necessary for this activity, but the high-end computing resources required. Industry partners and academic institutions will use these data sets to validate and test various imaging and interpretation schemes. The end product of this study will be imaged and interpreted synthetic data sets that provide the information necessary for greatly enhanced use of MT in the geothermal industry.

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Geothermal Resource Exploration and Definition Solicitation

Organization: Sandia National Laboratories
This project is in support of the cost-shared (80% DOE, 20% industry) exploration projects that will lead to the definition of new geothermal resources. This project has the primary objective of developing collaborative, interactive (Industry/DOE) efforts to support the exploration leading to the definition of these new geothermal resources. Ultimately, this will lead to an increase of the electrical power from geothermal resources in geographically diverse locations. This project started in FY00 with an initial solicitation that is continuing on in FY02. A new solicitation in FY02 will provide additional resources. The work scope consists of funding and management of seven current cost-shared cooperative agreements with industry participants and the development of a new solicitation for additional projects. DOE/AL awards the cooperative agreements and administers those awards while Sandia ($225K FY02) provides technical oversight, technical support, and assures projects are progressing appropriately.

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High-Temperature Instrumentation

Organization: Sandia National Laboratories
Measurement-while-drilling, well logging, and reservoir characterization require electronic packages that can survive the hot, corrosive environment of geothermal wells. High-temperature electronics will provide optimized drilling controls and improve reservoir definition, thus reducing geothermal drilling costs by reducing the number of wells needed to produce a geothermal reservoir. There is, therefore, significant incentive for improving the temperature rating of instruments for downhole use in geothermal wells. Sandia's program in high-temperature instrumentation focuses on developing sensor systems and controllers using unshielded electronics (not requiring a Dewar) that can be exposed to high temperatures for indefinite periods. The major thrust of this effort is silicon-on-insulator (SOI) technology, which has been demonstrated both in the laboratory and in the field. The Sandia program focuses its development effort to aid small geothermal operators in exploiting this new technology. There are no geothermal operators (operating within the U.S.) that could undertake this development effort without DOE's program assistance. The activities for FY02 are categorized into four separate thrust areas.

• Test newly developed SOI components, qualify them for geothermal use, and report on their status to the geothermal industry. We will hold an open workshop to describe this technology and will invite all US geothermal service companies.

• Provide complete documentation on prototype SOI logging tool designs and assembly. This will include completion of drawings in enough detail that a company could build its own tool, and a manual describing assembly and operation of the tool.

• Provide initial prototypes to industry for evaluation. We have targeted four US geothermal service companies (Welaco, Pruett Industries, Hot Hole Tools and Prime Directional Services) that want to build tools using SOI technology. We will provide each company with a low-temperature (inexpensive) version of the basic tool, which will help them learn to program and make good electronic measurements, and one fully functional set of high-temperature electronics. Each company will then build its first tool using these HT electronics and will evaluate the tools in field use. Each company will report the results of their test to Sandia.

• Continue ongoing consultation with the six companies working on funded SBIR projects involving high-temperature electronics. Those companies and their projects are: E-Spectrum (HT Universal Data Logger); LEL Corp. (fiber optical pressure and temperature tool); PhotoSonic (HT steam quality tool); Linear Measurements (HT crystal clock oscillator and pressure sensor); Silicon Designs (HT inclination sensor); and Sigma Labs (HT large value capacitors needed for all HT geothermal tools).

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Wellbore Integrity and Lost Circulation

Organization: Sandia National Laboratories
Lost circulation (LC) occurs when formation-fluid pressure is less than the fluid column pressure in the wellbore, so that some of the drilling fluid escapes into the formation instead of recirculating back up the well annulus. Lost circulation is often accompanied by further loss of wellbore integrity including sloughing, caving, washing out, or bridging. These phenomena are persistent in geothermal drilling, are very expensive — often accounting for 10-20% of the total cost for drilling a typical geothermal well — and cause many additional drilling problems such as stuck drill pipe, damaged bits, slow drilling rates, and collapsed boreholes.

These problems are being addressed in four general ways:

1. the optimum application of current technology;

2. the development of new tools to quickly detect and characterize lost circulation zones before they cause additional problems;

3. the development and demonstrate of advanced treatment materials; and

4. the search for revolutionary ways to provide wellbore integrity. Given the limitation on funding in FY02, we plan to focus our attention on the latter two areas.

We will work with wellsite service companies, polyurethane-grouting contractors, and polyurethane distributors to develop the infrastructure needed to commercialize polyurethane grouting. Currently we are discussing the commercialization of polyurethane grout with BJ Services, Lang Exploratory Drilling, Baroid, and Green Mountain. A simpler polyurethane grouting system that can be applied deeper and at hotter temperatures than those applied in the successful field test at Rye Patch will be developed. Work is being done to "down select" and optimize to the best currently available polyurethane formulation including evaluating one-part vs. two-part formulations. Each new material will be tested in the lab before being taken to the field. In addition to "down selecting" to the best polyurethane formulation for near term commercialization, an improved simpler devilry system will be developed, e.g., fewer parts, less manpower, faster pumping rates, lower pressures, and, if possible, a squeeze job technique that does not require a packer. After the development and lab testing of the next generation prototype system, we will demonstrate it in a geothermal well of opportunity. Work will be begun both in-house and by contractors, to develop high temperature formulations that can be used to plug loss zones encountered while drilling through depleted reservoirs. This work will start by evaluating formulations already identified as building blocks for high-pressure, high-temperature polyurethane.

A systems study of wellbore integrity will be done. We will assess the state-of-the-art in wellbore lining, particularly with respect to commercial or near-commercial systems that might be employed with minimal modification from their conventional oil and gas applications. These include:

1. casing drilling, in which casing is used as the drill string, thereby emplacing itself as the hole is drilled

   ;

2. expandable tubulars, which provide a method to run a length of casing through an already-cemented casing string and then to expand it to a larger diameter that will provide a partial lining or "patch" to the wellbore; and

3. continuous wellbore lining with some sort of chemical, perhaps polyurethane, that is deposited as the drill bit passes and then hardens into a permanent or temporary lining.

This assessment will comprise detailed investigation of existing technologies and definition of their strengths and weaknesses for geothermal application, as well as extensive data collection to define the most common and severe geothermal well problems that might be mitigated or avoided by a successful system. We will strive to combine lab tests, data collection from the geothermal industry, in-house and contract analysis, and cooperation/cost-share with service companies that have promising existing technology.

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Hard-Rock Drill Bit Technology

Organization: Sandia National Laboratories
Background: Hot, hard, abrasive, and fractured rock formations are routinely encountered when drilling geothermal wells. Consequently, rock penetration rates are generally very low, and bit life is extremely limited. Both of these problems contribute significantly to the cost of geothermal wells. If penetration rates and bit life could both be doubled from their current typical levels, well costs could be reduced by about 15%.

Historically, synthetic-diamond drag bits have been viewed as inadequate for geothermal drilling applications; as a result, roller-cone bits have been used preferentially. However, the inherent cutting efficiency and lack of moving parts of drag bits make them a very promising prospect for future increases in penetration rate and bit life in hard formations. In fact, geothermal drilling tests in the Imperial Valley, California, have already demonstrated a high (55 ft/hr) average rate of penetration (ROP) and extended bit life (about 1000 feet) in a mixed drilling interval that included igneous formations. Furthermore, since drag-bit technology is not as mature as roller-bit technology, there is greater potential for making significant improvements in drag-bit performance.

The hard-rock drill bit technology project is a national-laboratory/industry/university cooperative research and development effort aimed at producing drag cutters and bits capable of more economical drilling in geothermal formations. Our mission is to coordinate this overall effort and to maintain and apply state-of-the art expertise and capabilities for technical consulting, analysis, and laboratory testing. In-house facilities include the Hard-Rock Drilling Facility (HRDF), the Linear Cutter Test Facility (LCTF), and the recently completed Harmonic Excitation Fixture (HEF), which simulates downhole drillstring vibration conditions.

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Cost Database and Simulators

Organization: Sandia National Laboratories
A detailed understanding of drilling costs and their sources is necessary to focus efforts on work that has the highest payoff for the geothermal industry. For this purpose, SNL has developed a spreadsheet-based cost model for estimating geothermal drilling costs and their origins. A method for evaluating the impact of research is also needed. Actual well-cost data are necessary to validate the model and measure the success of the program. The validated model will then become useful for investigating where technology development will lead to cost reductions. Our approach in FY02 is to improve the Sandia cost model by making it more user-friendly and by making the cost coefficients more realistic.

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Diagnostics-while-Drilling (DWD)

Organization: Sandia National Laboratories
Drilling is an essential and expensive element in the exploitation of geothermal energy for power production. Improving the drilling process is critical, but research targets are not obvious because the technology is very mature. Diagnostics-while-Drilling (DWD) addresses drilling improvement both by reducing the cost of conventional drilling processes and by providing a revolutionary new capability. The central concept of DWD is a closed feedback loop, carrying data up and control signals down, between the driller and tools at the bottom of the hole. Upcoming data will give a real-time report on drilling conditions, bit and tool performance, and imminent problems. The driller can use this information to either change surface parameters (weight-on-bit, rotary speed, mudflow rate) with immediate knowledge of their effect, or to send control signals back to active downhole components. DWD will reduce costs, even in the short-term, by improving drilling performance, increasing tool life, and avoiding trouble. Cost analyses indicate that DWD technology would reduce the bus-bar cost of geothermally generated electricity by up to 30%, depending on well depth, well productivity, and the type of geothermal reservoir.

Task 1 - DWD Proof of Concept: Conduct a Proof-of-Concept (POC): Conduct test to demonstrate the value of real-time near-bit data for greatly enhancing drilling rate-of-penetration and overall bit performance in hard formations. This will convince the drilling industry of the value of DWD and stimulate the flow of private resources into the development of an economical high-speed data link for geothermal drilling applications.

Task 2 - Field Ready DWD: Begin development to bring a DWD system to such a point of ruggedness, reliability, and convenience that the geothermal industry will use it in drilling operations.

Task 3 - Acoustic Telemetry Tool: Complete development of the low-temperature version of a prototype acoustic telemetry tool and test it in an actual field situation.

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Acid-Resistant Cements

Organization: Brookhaven National Laboratory
Calcium aluminate phosphate (CaP) cement was developed at BNL in collaboration with Unocal and Halliburton Energy Services as CO2-resistant cement for wells exposed to temperatures up to 280°C and was successfully used to complete geothermal wells in Indonesia. The cement was commercialized under the trade name "ThemaLock Cement" by Halliburton Energy Services in 1998 and was used to complete many geothermal wells in the U.S. and Japan in 1999-2000. Consequently, this technology received a "2000 R&D 100 Award". In 2001, emphasis focused on improving two properties of this CaP cement: its acid resistance in surface groundwater at ~ 90°C; and its toughness-associated mechanical behaviors. The latter property prevents the development of stress cracking during production of superheated geothermal steam and fluid. BNL and Halliburton undertook 6-month acid-exposure tests of newly formulated Al2O3-rich CaP cement systems and showed that weight loss by acid erosion was 20% less than in conventional CaP cement. In a short-term autoclave test at 280°C, carbon- and corundum-based fibers were identified as potential reinforcements for CaP cement. The fracture toughness of composite cements reinforced with these fibers satisfied the criteria of 0.06 MN/m3/2, corresponding to an improvement of ~ 2.7 fold over that of non-reinforced ones. BNL will evaluate the effectiveness of two anti-acid admixtures in conferring greater acid resistance, water-dispersible silicon emulsions and alkaline metal hexafluoro compounds. The physico-chemical factors contributing to lessening the acid erosion of the immersed cement will be investigated. These factors will include phase identification and the development of any microstructure in the cement bodies. The newly formulated cements will be delivered to Halliburton for their independent evaluation before beginning field trials. BNL will investigate the durability of cement composites reinforced with the two high potential fibrous materials, carbon and corundum. Our focus will be on monitoring the changes in toughness-related properties as a function of autoclave exposure time for up to 6 months. Also, we will explore the development of microstructure and chemical alterations at the interfaces between fibers and cement matrix. Some exposed specimens will be sent to Halliburton for post-test analyses.

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Cement Structural Response Analysis

Organization: Brookhaven National Laboratory
FY2001 investigations led to the conclusion that traditional guidelines for well cements are deficient, especially when it comes to the cement mechanical property requirements. Current studies from the oil and gas industry are reaching similar conclusions. This simply demonstrates that selection of cements used for completion of all types of wells must be based on rigorous engineering analysis. Research has clearly shown that tensile strength is also critical for the performance of the well. Two types of failures are of interest, namely, tensile failure for weak far-field stresses and shear failure in the presence of compressive far-field stresses. These two fundamental failure modes will be investigated to see their relevance to cement mechanical properties. Improved material definitions will be obtained through the characterization of conventional, lightweight, and fiber-reinforced cements. Some of the fiber-reinforced cements tested thus far appear very promising for enhancing tensile capacity. There is a need to design and verify optimum formulations for all operating conditions and any transient loadings experienced by the well during its design life. Continuation of the work will tackle these issues, especially material behavior at elevated temperatures associated with geothermal wells. We will test fiber-reinforced and lightweight cements to obtain detailed material properties and behavior, and will perform durability tests on fiber-reinforced cements. We will then model the described conditions by detailed finite element analysis, using the descriptive material models for fiber-reinforced and lightweight cements determined experimentally.

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