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FRACKING MONITORING

casing. The cube was instrumented with
32 nonpolarizing silver/silver chloride
electrodes for voltage measurement, 16
each on the top and one side of the block,
plus six acoustic emission sensors mounted
on three sides (see Figure 2).

èEXPERIMENTS VALIDATED
THE SIMULATIONS

During the tests, electrical signals were
detected that corresponded to fluid leaks
along the well seal (see Figure 3), as were
bursts of acoustic emissions and fluid
pressure changes.
	 “We used a two-step process to invert
the electrical data to pinpoint the
position of these leaks,” Revil said. “First,
we applied a deterministic least square
algorithm to retrieve the source current
density at a given point in the block at
a given time. Then, we used a genetic
algorithm, or probability sampling, to
refine the position of the source current
density.
	 “The results of the inversion were in
excellent agreement with the location
of the wells in question, and also with
the acoustic emissions in the vicinity
of the wells,” he said. “This showed
us definitively that passively recorded
electric signals can be used to monitor
fluid flow along wells during leakages.
It also suggests they might be able to
monitor fluid flow in numerous other
applications that involve hydromechanical
disturbances.”

èTHE NEXT STEP: FIELD                          FIGURE 3. Forward-modeled voltage distribution of a dipole for one event (A and B),
TRIALS                                         Spatial location of the dipole within the concrete block (C and D), Close-up of the dipole
                                               location (E and F), showing an off-vertical orientation of the dipole moment.
The next step is to develop field trials to
investigate this technique further, from a                                     “That level of data
few meters to several kilometers in scale.
If all goes well, long-sought solutions to                                               fusion has never been
important problems may be at hand, such                                                  done before. It is a
as development of aquifer monitoring                                                     tremendously exciting
and safety systems; better ways to                                                       time to be working in the
assess the integrity of old, plugged, and                                                area of geophysics.”
abandoned wells; and perhaps even the
ability to characterize fractured rock                                                         –ANDRÉ REVIL, COLORADO SCHOOL
systems via the movement of the fluids
within them. The ultimate goal, Revil                                                                                    OF MINES
said, is to combine the electric data with
pressure and acoustic data for a fully
integrated analytical capability.
	 “That level of data fusion has never
been done before,” Revil concluded. “It
is a tremendously exciting time to be
working in the area of geophysics.” v

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