SOME INFLUENCES OF STRATIGRAPHY AND STRUCTURE ON RESERVOIR STRESS ORIENTATION

The azimuth of maximum horizontal stress in a reservoir can vary signi ficantly with depth and with position on a subsurface structure. We pr esent and discuss evidence from field data for such variation and demo nstrate both analytically and with finite-element modeling how such ch anges might take place. Under boundary conditions of uniform far-field displacement, changes in stratigraphic layering can reorient the prin cipal stress direction if the formation is intrinsically anisotropic. If the formation stiffness is lower perpendicular to bedding than para llel to bedding (as is often the case in layered geologic media), an i ncrease in dip will reduce the component of compressive stress in the dip azimuth direction. Folds can reorient principal stresses because f lexural strain varies with depth and position. Compressive stress perp endicular to a fold axis increases with depth at the crest of an antic line and decreases with depth at the limb. When the regional stress an isotropy is weak, this change in stress magnitude can reorient the loc al principal stress directions. Numerical simulations of such effects gave results consistent with changes in stress orientation at the Cymr ic and Lost Hills oil fields in California as observed via shear-wave polarization analyses and tiltmeter surveys of hydraulic fracturing. K nowledge of such variation of stress direction with depth and structur al position is critical for drilling, completions, hydraulic fracture, and well pattern designs.