Depletion-induced Stress Orientation Changes in Bounded Reservoirs

Day-Lewis, Amy D. F.¹, Mark D. Zoback¹ (1) Stanford University, Stanford, CA

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 Mexico and the North Sea.