|Evaluation of Deep Subsurface Resistivity Imaging for Hydrofracture Monitoring
||Last Reviewed 6/14/2016
The goal of this project is to quantify how well an in-situ measurement of bulk electrical resistivity using the new method of Depth to Surface Electromagnetic (DSEM) imaging can be related to the changes in rock properties and fluid propagation that occur as a result of hydraulic fracturing.
GroundMetrics, Inc. (GMI), San Diego, CA, 92123
Global Microseismic Services, Inc. (GMS)
Berkeley Geophysics Associates, Ltd.
Encana Oil & Gas, USA
Approximately 45 percent of the world’s recoverable natural gas reserves are classified as unconventional. Worldwide, the share of unconventional gas production is projected to increase from the current 14 percent to 32 percent. Increasing production from new, tight shale resources is projected to result in the U.S. overtaking Saudi Arabia as the world’s largest producer of liquid fuels (oil, natural gas, and biofuels) as early as 20131.
Hydraulic fracturing (fracking) has enabled commercial production from unconventional formations. However, fracking is more expensive than the conventional methods used to produce gas and oil, and fracked wells exhibit a much faster decline in production than conventional wells. Furthermore, there are environmental concerns with the amount of water required, pollution of groundwater reservoirs, triggering of earthquakes, and release of methane into the atmosphere. A key concern of the general public is hydrofracturing out of the formation and into the groundwater table.
Unconventional wells exhibit highly variable production in a given area, and often the majority of gas or oil produced comes from only a few of the fracturing stages, resulting in more extensive fracturing operations than are really needed and excess proppant being pumped into the formation. These inefficiencies indicate that the eventual destination of the injected fluids used in reservoir stimulation is poorly understood.
1BP Energy Outlook 2030 http://www.bp.com/genericarticle.do?categoryId=2012968&contentId=7083149, accessed 1/27/2013.
Seismic methods are used to locate hypocenters and, via the tomographic fracture image method, produce images of entire fracture networks. However, the underlying data represent the fracture of the host rock. In contrast, if successful, the proposed DSEM method will image the presence of hydrofracturing fluid in the new pore spaces and quantify the resulting increase in porosity. The following project impacts and benefits are anticipated:
- Reduced cost and use of fracture fluid by reducing the number of fracture stages.
- Improved recovery and reduced environmental impact via improved mapping of fracture propagation.
- Reduced cost from replacing high cost aspects of a microseismic survey with electromagnetic elements. Extension of microseismic methods to formations where they currently are problematic and provide inadequate information.
- Developing and demonstrating ways to monitor hydrofrac height growth.
Accomplishments (most recent listed first)
- GMI has begun iterations of both types of inversion methods to test effectiveness.
- GMI has identified forward modeling and approximate inversion as the most appropriate way to further refine the inversion.
- GMI has begun the complicated data inversion procedure and produced a preliminary image.
- GMI has completed the data processing in the frequency domain for the six sensors closest to the region fractured with brine, and for the three sensors furthest away. The standard deviation of the data over the 48 hour duration of the test was less than the target of 10 pV/Am.
- GMI has completed the data processing in the time domain for all 76 sensor channels of the sensor array for 1.5 hours of data acquired before the target stages were fractured and for 1.5 hours after fracturing all target stages. The standard deviation of the difference in the late time data for post- and pre-frac datasets was on the order of 100 pV/m.
- GMI has successfully conducted the first borehole-based electromagnetic survey during a commercial hydrofracturing procedure.
- Initial processing has shown noise levels of some sensors in the field to be equal to their lab performance, an exceptional result for field operations.
- GMI has successfully arranged access to a commercial fracking procedure and has performed a reconnaissance survey showing that the noise levels during fracking would not be too high to conduct our measurements.
- GMI has implemented a computer program to calculate the surface electric field change caused by a fracture disturbing the subsurface distribution of electric current produced by a steel borehole casing in an anisotropic layered earth. The code is finite element, 3D, and calculates the full EM solution (electric and magnetic field components). Discussions with industry and academic experts indicate this is the first code capable of incorporating the small scale of a borehole casing into a reservoir scale model.
- Initial verification of the new 3D EM code was completed by comparing it to published solutions for a split borehole casing communication system, and to a prior 2D DC code that modeled a casing. Good agreement was seen for both comparisons.
- GMI has applied the new 3D EM code to calculate the signature of a hydrofracture produced by current flow along a casing comprising a horizontal section connected to a vertical section and located in a layered earth, thereby meeting the first program milestone.
- Desired improvements to GMI data recorders have been made, including the addition of very high precision internal voltage reference for in-field calibration, and a simplification of all operating software to a single code module. Most of the improved receiver and data acquisition hardware has been received from the manufacturers and has passed required in-house acceptance tests.
- Operational improvements to receiver, data acquisition, and ancillary survey hardware were successfully field tested at a site in the California desert.
- An approach has been defined for the DSEM signal modeling.
- Fracture images have been provided by GMS.
- A list of requirements has been drawn up for central monitoring of the performance of all data recording units.
Current Status (June 2016)
The is currently working to refine the inversion to 3D/produce preliminary images. These remaining project activities are ongoing as GMI is close to completing the remaining project work.
Project Start: October 1, 2013
Project End: April 30, 2016
DOE Contribution: $2,472,641
Performer Contribution: $941,191
NETL – William Fincham (firstname.lastname@example.org or 304-285-4268)
GroundMetrics – Dr. Michael Wilt (email@example.com or 858-381-4147)
Quarterly Research Progress Report [PDF-1.65MB] January - March, 2014
Quarterly Research Progress Report [PDF-794KB] October - December, 2013