LaBrecque, D. , Brigham, R., Denison, J., Murdoch, L., Slack, W., Liu, Q. H., Fang, Y., Dai, J., Hu, Y., Yu, Z., Kleinhammes, A., Doyle, P., Wu, Y., Ahmadian, M.,
The goal of this project is to develop techniques for monitoring hydraulic fractures in reservoirs by injecting electrically conductive, dielectric, or magnetically permeable proppants. The contrasts between the properties of the proppants and the subsurface provided the basis for imaging using geophysical methods. The initial experiments focused on a series of small, shallow fractures; however, the goal of the project is to develop methods applicable to oil-field fractures.
The project began by screening different proppant types using laboratory and numerical analyses that have been ongoing by researchers at the Advanced Energy Consortium (AEC). This work identified Loresco coke breeze and steel shot as materials that could create significant electrical or magnetic contrasts with most geological formations. These proppants were tested by creating hydraulic fractures in a shallow field setting consisting of highly weathered residual saprolite near Clemson University in western South Carolina. Six hydraulic fractures were created in highly monitored cells by injecting the contrasting proppants at a depth of approximately 1.5 m. This created sub-horizontal fractures filled with proppant approximately 10 mm thick and extending 3 to 5 m in maximum dimension.
Each cell had a dense array of electrodes and magnetic sensors on the surface, as well as four shallow vertical electrode arrays that were used to obtain data before and after hydraulic fracturing. Net vertical displacement, cores and trenching were used to characterize the fracture geometries.
Hydraulic fracture geometries were estimated by inverting pre- and post-injection geophysical data using various codes. Data from cores and excavation show that the hydraulic fractures formed a saucer-shape with a preferred propagation in the horizontal direction. The geophysical inversions generated images with remarkably similar form, size, and location to the ground truth from direct observation. Displacement and tilt data appear promising as a constraint on fracture geometry.
LaBrecque, D. J., LaBrecque, D., Casale, D., and Brigham, R.
Multi-Phase Technologies is developing a new tool for detecting voids near boreholes. The new tool uses a combination of magnetometric resistivity (MMR) and single-hole electrical resistivity tomography (ERT) to provide three-dimensional characterization of the volume around a borehole. The goal is to provide an improved method of detecting and delineating tunnels and voids within fractured rock environments. One challenge is to develop a system to provide electrical contact in open, “dry” holes. From a single borehole, ERT alone, provides basic background information around the borehole but cannot provide any directional information. Jointly inverting scalar information from ERT with vector MMR measurements allows the system to uniquely image 3D targets. This talk discusses the prototype design and results of the initial system tests of the new tool. There are a number of design challenges in the development of the tool. One issue is that the system uses total-field fluxgate magnetometers. Measuring the total field allows the system to determine the orientation of the magnetometer within the hole. However, for MMR the system must measure induced fields of the order of a few hundred picoTesla in the presence of the Earth field of a tens of thousands of nanoTesla. Any movement of the sensor during the measurement creates transients on the individual components of the magnetometer that dramatically overwhelm the small induced fields.
LaBrecque, D. J., Flinchum, B., Brigham, R., Pendrigh, N., Sirles, P., and Ivancie, P.
Old mine workings present both physical and environmental hazards. Detecting and locating tunnels and voids can be an important but extremely difficult problem in many remediation projects. The targets are often small (∼2 m in width) and located deep in the surface (hundreds of meters). The subsurface nature of these old mining sites is generally complex and variable, containing faults, fractures, multiple rock types and altered zones. For this application, geophysicists find themselves facing the daunting task of trying to pick targets hundreds of meters deep with meter scale accuracy. Furthermore, it is often important to locate a specific, deep, tunnel; old mine sites are often riddled with tunnels at multiple levels.
In previous work the authors showed the successful use of surface Mise-a-la-Masse (MALM) and cross-hole electrical resistivity tomography (ERT) to successfully locate voids at the site of the former Captain Jack Mine near Ward, Colorado. Recent work at the site has provided additional insight into the use of these technologies and shown the promise of a relatively new method, borehole magnetometric resistivity (BMR) at delineating and locating tunnels. The new technique has the advantage of providing an estimated direction and distance to a tunnel and approximate strike direction of the tunnel all from a single dry or water filled borehole.
LaBrecque, D. J., Casale, D., Flinchum, B. and Brigham, R.
Although the electrical resistivity tomography method has seen relatively wide-spread use in small-scale sites; its application to mid-to large scale sites has been far more limited. This is in part due to limitations in the hardware/methodology commonly used for ERT surveys. Most of these surveys use multi-conductor cables to wire electrodes back to a central receiver/transmitter unit. These cable systems start to become both expensive and unwieldy for electrode separations of more than a few meters. These cable systems are also difficult to apply in the presence of obstacles, building, roads, rivers, etc. Getting sufficient signal-to-noise ratios in a safe and economical fashion becomes increasingly difficult as the survey scale increases.
Our approach is a distributed resistivity system consisting of many transceivers that can communicate with each other through a low-power distributed wireless network. Figure 1 shows an overview of the major components in a wireless unit. Each transceivers has its own microprocessor, multi-channel receiver channels, a global positioning (GPS) module for timing and position information, and a 400 watt transmitter all powered by a high-discharge battery. To compensate for potentially slow communications, the local microprocessor does most of the data processing, and can perform a series of pre-scheduled tasks so that the central controller needs only to download schedules to each unit rather than detailed control information, and uploads processed data rather than raw data. This greatly reduces the need for rapid communications and high data throughput. To improve the signal-to-noise ratio the system allows multiple transceivers to transmit current simultaneously. Although this adds additional complexity to the design of field surveys, it also allows us to tailor the current flow patterns for improved sensitivity to deep structures.
Comparison of Multi-Source data with more traditional ERT surveys at the Astor Pass Geothermal site in Nevada are used to show the capabilities and challenges in applying the Multi-Source approach.
MPT PARTNER PAPERS:
Mohsen Ahmadian1, 2*, Douglas LaBrecque3, Qing Huo Liu4, William Slack5, Russell Brigham3, Yuan
Fang4, Kevin Banks6, Yunyun Hu4, Dezhi Wang4, Runren Zhang4
1. Advanced Energy Consortium
2. Bureau of Economic Geology, The University of Texas at Austin, USA
3. Multi-Phase Technologies, LLC
4. Department of Electrical and Computer Engineering, Duke University
5. FRx Inc.
6. Inversion Technologies, Inc.
In April 2017, the Advanced Energy Consortium (AEC) successfully completed data collection
for a proof-of-concept demonstration of remote mapping of hydraulically fractured networks
using electromagnetic (EM) proppant additives and a variety of EM tools and configurations.
This field-pilot demonstration was conducted at the Devine Test Site, located approximately 50
miles southwest of San Antonio, Texas, and managed by the Bureau of Economic Geology
(Bureau) at The University of Texas at Austin. The objective of the ongoing integrated research
program is to develop a remote EM-imaging technique for hydraulically fractured networks in
order to obtain a higher-resolution image of proppant distribution (lateral/vertical extent and
azimuth), which current technologies, such as microseismic, do not allow. The current study is a
more in-depth follow-up to a series of shallow field tests that the AEC conducted in 2015 near
Clemson University in South Carolina. This paper details the special aspects of the Devine Test
Site that make it a unique asset for benchmarking EM-based hydraulic-fracture mapping tools
Results from the Devine Test Site demonstrate that a measurable and noticeable EM anomaly
was detectable with both time-domain and frequency-domain induced polarization methods. EM-
inversion results were consistent with analysis of surface tiltmeter results but diverged
significantly from passive seismic responses obtained during the hydraulic-fracturing process.
The site will be cored at multiple locations over the next few months, after which accuracy of
models and methods will be validated. Future opportunities for collaboration on this highly
validated benchmarked site are discussed.