Are the Shallow Oil Reservoirs of South-Central Kentucky Undergoing Natural, Microseismic Deformation?
RUTLEDGE, JAMES, Nambe Geophysical, Inc., contractor at Los Alamos National Laboratory, Los Alamos, NM
In a series of downhole, seismic-monitoring tests conducted from 1993-1995
in Clinton County, Kentucky, it was shown that microearthquakes are temporally
and spatially associated with oil production from low-porosity, fractured,
carbonate rocks at <600 m depth. Gross changes in production rate correlate
with microearthquake event rate changes. Hypocenters and first- motion
data have revealed previously undetected, low-angle thrust faults above
and below the currently-drained depth intervals. Production history, well
logs and drill tests indicate the seismically-active faults or fractures
are previously-drained intervals that have subsequently recovered to hydrostatic
pressure via brine invasion. Computed storage volumes and correlations
of production intervals with the seismically-active faults indicate the
oil reservoir in the study area is primarily a set of compartmentalized,
low-angle thrust faults.
State of stress determined from composite focal mechanisms indicates
a near-surface thrust regime. The seismic behavior is consistent with poroelastic
models that predict slight increases in compressive stress above and below
currently-drained volumes. Estimated extraction-induced stress changes
outside currently-drained volumes are very small (<0.008 MPa). The pressure
cycling and partial replacement of oil with denser brine along the seismically-active
faults preceding current, adjacent production may have driven the faults
closer to failure (shear slip). The small magnitudes of production-induced
stress changes implies the productive fractures are critically stressed
for shear failure within the pre-existing, background state of stress and,
further, that similar, potentially-productive fractures may be naturally
active at lower rates with no production activity.
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