Publications

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Chemical tracers are commonly used to characterize fracture networks and to determine the connectivity between the injection and production wells. Currently, most tracer experiments involve injecting the tracer at the injection well, manually collecting liquid samples at the wellhead of the production well, and sending the samples off for laboratory analysis. While this method provides accurate tracer concentration data, it does not provide information regarding the location of the fractures conducting the tracer between wellbores. The goal of this project is to develop chemical sensors and design a prototype tool to help understand the fracture properties of a geothermal reservoir by monitoring tracer concentrations along the depth of the well. The sensors will be able to detect certain species of the ionic tracers (mainly iodide) and pH in-situ during the tracer experiment. The proposed high-temperature (HT) tool will house the chemical sensors as well as a standard logging sensor package of pressure, temperature, and flow sensors in order to provide additional information on the state of the geothermal reservoir. The sensors and the tool will be able to survive extended deployments at temperatures up to 225 °C and high pressures to provide real-time temporal and spatial feedback of tracer concentration. Data collected from this tool will allow for the real-time identification of the fractures conducting chemical tracers between wellbores along with the pH of the reservoir fluid at various depths.

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New generations of high-temperature (HT) sensors and electronics are enabling increased measurement speed and accuracy allowing collection of more accurate and relevant data by downhole tools. Unfortunately, this increased capability is often not realized due to the bottleneck in the uplink data transmission rates due to poor signal characteristics of HT wireline. The objective of this project is to enable the high transmission rate of raw data from downhole tools such as acoustic logging tools and seismic measurement devices to minimize the need for downhole signal processing. To achieve this objective, Sandia has undertaken the effort to develop an asymmetric high-temperature (HT), highspeed data link system for downhole tools capable of operating at temperatures of 210°C while taking advantage of existing wireline transmission channels. Current data rates over HT single-conductor wireline are limited to approximately 200 kbps. The goal system will be capable of transmitting data from the tool to the surface (uplink) at rates of > 1Mbps over 5,000 feet of single-conductor wireline as well as automatically adapt the data rate to the longer wirelines by adapting modern telecommunications techniques to operate on high temperature electronics. The data rate from the surface to the tool (downlink) will be significantly smaller but sufficient for command and control functions. While 5,000 feet of cable is the benchmark for this effort, improvements apply to all lengths of cable.

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Muons are subatomic particles that can penetrate the earth’s crust several kilometers and may be useful for subsurface characterization. The absorption rate of muons depends on the density of the materials through which they pass. Muons are more sensitive to density variation than other phenomena, including gravity, making them beneficial for subsurface investigation. Measurements of muon flux rate at differing directions provide density variations of the materials between the muon source (cosmic rays and neutrino interactions) and the detector, much like a CAT scan. Currently, muon tomography can resolve features to the sub-meter scale.

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Understanding the connectivity of fracture networks in a reservoir and obtaining an accurate chemical characterization of the geothermal fluid are vital for the successful operation of a geothermal power plant. Tracer experiments can be used to elucidate fracture connectivity and in most cases are conducted by injecting the tracer at the injection well, manually collecting liquid samples at the wellhead of the production well, and sending the samples off for laboratory analysis. This method does not identify which specific fractures are the ones producing the tracer; it is only a depth-averaged value over the entire wellbore. Sandia is developing a high-temperature wireline tool capable of measuring ionic tracer concentrations and pH downhole using electrochemical sensors. The goal of this effort is to collect real-time pH and ionic tracer concentration data at temperatures up to 225 °C and pressures up to 3000 psi.

Environments relevant to geothermal energy exploration frequently exceed the temperatures and pressures commonly experienced by downhole tools in the oil and gas industry. As such, pushing the boundaries with geothermal tool development can often necessitate exceeding manufacturer specifications for temperature and pressure of individual circuit components. High-temperature circuit designers often must dedicate considerable time and resources to determine if a component exists that they may be able to knead performance out of to meet their requirements. In light of this difficulty, Sandia National Laboratories has initiated a program funded by the Geothermal Technologies Office at the US Department of Energy to compile and make available an empirically determined, practical dataset of select high-temperature component performances beyond specification. Detailed here are the efforts surrounding geothermal temperature characterization of commercially available HT-Flash memory modules made by Texas Instruments (SM28VLT32-HT) and preliminary results of 3 commercial solid tantalum capacitors. Flash evaluation boards were modified for high temperature application and read, write and erase functionality were tracked as well as prolonged data retention at various temperatures well beyond datasheet specifications. It was observed that each flash function has a different maximum operation temperature above specification. As temperature increases, erase, write, and then read functions successively fail. Within duration and temperature limits, functionality of each operation returns after cooling back below its threshold value. Importantly for logging tools, after cooling the flash modules in this study still retain all memory previously written. Flash lifetime at temperature was examined at several temperatures by 1000hr duration tests in the oven with new writes and periodic full memory reads throughout the test. To test the capacitors, capacitance and equivalent series resistance were tracked over a 1000hr test at 260°C. Results of MatLab fault analyses are described for each aspect of this study to facilitate out-of-spec high temperature tool design.

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High-temperature geothermal exploration requires a wide array of tools and sensors to instrument drilling and monitor downhole conditions. There is a steep decline in component availability as the operating temperature increases, limiting tool availability and capability for both drilling and monitoring. Several applications exist where a small motor can provide a significant benefit to the overall operation. Applications such as clamping systems for seismic monitoring, televiewers, valve actuators, and directional drilling systems would be able to utilize a robust motor controller capable of operating in these harsh environments. The development of a high-temperature motor controller capable of operation at 225°C significantly increases the operating envelope for next generation high temperature tools and provides a useful component for designers to integrate into future downhole systems. High-temperature motor control has not been an area of development until recently as motors capable of operating in extreme temperature regimes are becoming commercially available. Currently the most common method of deploying a motor controller is to use a Dewared, or heat shielded tool with low-temperature electronics to control the motor. This approach limits the amount of time that controller tool can stay in the high-temperature environments and does not allow for long-term deployments. A Dewared approach is suitable for logging tools which spend limited time in the well however, a longer-term deployment like a seismic tool [Henfling 2010], which may be deployed for weeks or even months at a time, is not possible. Utilizing high-temperature electronics and a high-temperature motor that does not need to be shielded provides a reliable and robust method for long-term deployments and long-life operations.

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