Publications

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During the initial phase of this SubTER project, we conducted a series of high resolution seis- mic imaging campaigns designed to characterize induced fractures. Fractures were emplaced using a novel explosive source, designed at Sandia National Laboratories, that limits damage to the borehole. This work provided evidence that fracture locations could be imaged at inch scales using high-frequency seismic tomography but left many fracture properties (i.e. per- meability) unresolved. We present here the results of the second phase of the project, where we developed and demonstrated emerging seismic and electrical geophysical imaging tech- nologies that characterize 1) the 3D extent and distribution of fractures stimulated from the explosive source, 2) 3D fluid transport within the stimulated fracture network through use of a contrasting tracer, and 3) fracture attributes through advanced data analysis. Focus was placed upon advancing these technologies toward near real-time acquisition and processing in order to help provide the feedback mechanism necessary to understand and control frac- ture stimulation and fluid flow. Results from this study include a comprehensive set of 4D crosshole seismic and electrical data that take advantage of change detection methodologies allowing for perturbations associated with the fracture emplacement and particulate tracer to be isolated. During the testing the team also demonstrated near real-time 4D electri- cal resistivity tomography imaging and 4D seismic tomography using the CASSM approach with a temporal resolution approaching 1 minute. All of the data collected were used to develop methods of estimating fracture attributes from seismic data, develop methods of as- similating disparate and transient data sets to improve fracture network imaging resolution, and advance capabilities for near real-time inversion of cross-hole tomographic data. These results are illustrated here. Advancements in these areas are relevant to all situations where fracture emplacement is used for reservoir stimulation (e.g. Enhanced Geothermal Systems (EGS) and tight shale gases).

A 20 foot long, 10 inch inner diameter combustion tube facility has been developed at Purdue’s Zucrow Laboratories for performing fundamental investigations on flame acceleration and deflagration-to-detonation transition (DDT). The facility can be used to test gaseous, liquid, or solid fuels in a variety of tube configurations. The fuel/oxidizer mixture is ignited at the closed end of the tube, and the flame propagation velocity and local pressure are measured using ion probes and pressure transducers located along the length of the tube. Experiments have been conducted for a range of gaseous and liquid fuels in the unobstructed tube with an open end, and with a single orifice plate installed mid-way down the tube length. The relative performance in terms of maximum flame velocity and acceleration and the potential for DDT are compared for the different fuels. Preliminary scaling analysis is performed to explore trends in the data and to investigate the relative influence of important physical parameters on the flame acceleration.

Nitrous oxide and ethylene appear extremely promising as a bipropellant mixture for rocket propulsion systems. Earlier work with this bi-propellant mixture suggested that at high initial pressures, a deflagration will undergo extremely rapid acceleration and achieve transition to detonation in very short distances. To study the flame acceleration and DDT behavior as a function of initial pressure, tests were carried out in a closed, large L/d combustion tube and detonation pressures and velocities were measured. These mixtures were ignited using a high voltage spark ignition system and the results were compared to those which used a nichrome wire igniter. In addition, similar tests with ethylene-oxygen were performed in the same combustion tube for comparison. The detonation run-up distances from the nitrous oxide and oxygen tests and the dependence of run-up distance on initial pressure were determined and compared with those values from the nichrome wire igniter tests.

In the past several years nitrous oxide has been more widely considered as a “safe”, clean oxidizer for rocket propulsion systems, that was useable as both a monopropellant or as a bipropellant. Therefore, the present work investigated the use of nitrous oxide and ethylene as a bi-propellant mixture at elevated pressures. Earlier work with this bipropellant mixture suggested that a steady detonation can be established in a combustion tube of larger L/d ratio and the associated flame acceleration prior to DDT could be studied more accurately. In order to use this bi-propellant mixture in a pulsed detonation engine quick flame acceleration is essential and this leads to a transition to detonation in a short duration of time. To study the flame acceleration and DDT behavior, tests were carried out in a combustion tube with L/d = 68 and detonation pressures were recorded using high pressure (100,000 psia) transducers. The detonation velocities were determined based on the time instances of measured pressure peaks and the distances between the transducers. Additionally, the pre compression observed in the combustible mixture before transitioning to a detonation was also studied and described in this paper. Finally, the run-up distances from these tests were determined and compared with values for different fuel-oxidizer mixtures from literature.

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During the initial phase of this Department of Energy (DOE) Geothermal Technologies Office (GTO) SubTER project, we conducted a series of high-energy stimulations in shallow wells, the effects of which were evaluated with high resolution seismic imaging campaigns designed to characterize induced fractures. The high-energy stimulations use a novel explosive source that limits damage to the borehole, which was paramount for change detection seismic imaging and re-fracturing experiments. This work provided evidence that the high-energy stimulations were generating self-propping fractures and that these fracture locations could be imaged at inch scales using high-frequency seismic tomography. While the seismic testing certainly provided valuable feedback on fracture generation for the suite of explosives, it left many fracture properties (i.e. permeability) unresolved. We present here the methodology for the second phase of the project, where we are developing and demonstrating emerging seismic and electrical geophysical imaging technologies that have been designed to characterize 1) the 3D extent and distribution of fractures stimulated from the explosive source, 2) 3D fluid transport within the stimulated fracture network through use of a contrasting tracer, and 3) fracture attributes through advanced data analysis. Focus is being placed upon advancing these technologies toward near real-time acquisition and processing in order to help provide the feedback mechanism necessary to understand and control fracture stimulation and fluid flow.

During the initial phase of this SubTER project, we conducted a series of high resolution seismic imaging campaigns designed to characterize induced fractures. Fractures were emplaced using a novel explosive source that limits damage to the borehole. This work provided evidence that fracture locations could be imaged at inch scales using high-frequency seismic tomography but left many fracture properties (i.e. permeability) unresolved. We present here the methodology for the second phase of the project, where we will develop and demonstrate emerging seismic and electrical geophysical imaging technologies that characterize 1) the 3D extent and distribution of fractures stimulated from the explosive source, 2) 3D fluid transport within the stimulated fracture network through use of a contrasting tracer, and 3) fracture attributes through advanced data analysis. Focus will be placed upon advancing these technologies toward near real-time acquisition and processing in order to help provide the feedback mechanism necessary to understand and control fracture stimulation and fluid flow.

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