Depletion-induced
Stress Orientation Changes in Bounded Reservoirs
Day-Lewis, Amy D. F.1, Mark D.
Zoback1 (1)
It is well known that uniform depletion
in a laterally-extensive, homogeneous reservoir will induce uniform changes in
the magnitudes of the horizontal principal stresses without any change in the
stress directions. Using a simple, analytical model we show how the presence of
an impermeable fault (or other impermeable boundary) in the reservoir can cause
the horizontal principal stresses to reorient. Depletion on one side of the
boundary will induce a uniaxial, normal traction on the boundary. Superposition
of this traction onto the background stress state results in new horizontal
principal stress orientations. The amount of apparent rotation depends on 1)
the pore pressure change relative to the original horizontal differential
stress, 2) the orientation of the fault relative to the original maximum
horizontal principal stress direction, and 3) the poroelastic properties of the
reservoir. If the pore pressure drop is similar to, or greater than, the
initial horizontal differential stress, and the bounding fault is at a high
angle to the original direction of maximum horizontal stress, then the amount
of horizontal principal stress reorientation can be quite large, in excess of
45 degrees. One geologic environment in which such conditions are likely to be
encountered is overpressured, normal faulting regions where a small horizontal
differential stress is expected and large amounts of depletion could occur. We
successfully use the model to explain observed stress directions in depleted
fields in
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California