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COLORADO SCHOOL OF MINES, CO, USA

Multiphysics Simulations Help Track
Underground Fluid Movements

Multiphysics simulations are helping geophysicists at the Colorado School of Mines develop
a way to track underground fluid flows and to better map and understand subsurface
formations and dynamics. One potential application: Tracking the fracking fluids used in
unconventional natural gas and oil extraction.

by GARY DAGASTINE

Geophysicists characterize underground       FIGURE 1. Illustration of the cement block used in hydraulic fracturing experiments. (1)
formations and events by detecting and       Cement block, (2) Gold electrodes, (3) Acoustic sensors, (4) Plastic plate with top array of
analyzing acoustic waves that propagate      16 channels of electrodes, (5) Plastic plate with back array of 16 channels of electrodes,
to the Earth’s surface from seismic          (6-8) High-pressure fluid-injection tubes, (9) Other holes (no injection).
activity. These waves arise from earth
movements caused by natural events           previously-worked oil reservoirs.             the pore water with these excess charges
such as earthquakes, from underground        	 Fracking is a particularly critical and     is responsible for an electrical current
explosions deliberately set to explore       timely issue because of the growing           density on the order of few milliamperes
geological features, and from the            use of the technique, in which fluid          per square meter.
hydraulic fracturing, or fracking, of tight  is injected into the ground at high           	 Because the current varies according
formations with the goal of increasing       pressure to fracture low-permeability         to the kinds of formations the water
permeability.                                shale formations to extract the gas and       passes through, the hope is that useful
	 Although acoustic waves can travel         oil in them. It is vital to know where        inferences can be made based on these
long distances, they have major              fracking fluid is going so that the fracking  electrical signals, which could be detected
limitations in providing details about       process can be optimized, and to avoid        by a network of electrodes placed on the
formation properties. They can’t be used     contamination of shallow aquifers.            ground surface and/or in boreholes.
to directly identify and track the liquids                                                 	 “This method is analogous to
flowing through them, called pore water.     èGEOELECTRIC SIGNALS                          electroencephalography—EEG. In EEG,
Current research at the Colorado School                                                    electrical signals are generated at the
of Mines (CSM), however, suggests that       Subsurface electromagnetic disturbances       synapses between the neurons and
electromagnetic disturbances that can        stem from the net deficiency of electrical    recorded on the scalp. EEG has been a
occur in association with seismic events     charges on the surfaces of minerals. This     key method to understand how the brain
might provide this missing information.      deficiency is compensated for by excess       is working,” said André Revil, a leading
	 Electromagnetic waves don’t propagate      charges in the pore water. The flow of
as far as acoustic waves, but theoretical
models and laboratory experiments
conducted with the aid of numerical
multiphysics simulations show they
can identify and track pore water. The
evolving technique, based on work
pioneered by Russian and Japanese
researchers, opens the door to the
complementary use of acoustic and
electromagnetic analyses to create a more
comprehensive view of the underground
world than we now have.
	 Such a seismoelectric capability would
enable better monitoring of shallow
earthquakes along tectonic faults and
active volcanoes. It also would lead to
better tools for the safe development
of unconventional energy resources
via fracking, and for more effective
secondary/enhanced oil recovery in

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