ABSTRACT: Control of Fractured-Reservoir Permeability by Spatial and Temporal Variations in Stress Magnitude
and Orientation
TEUFEL, LAWRENCE W., and JOHN C. LORENZ, Sandia National Laboratories, Albuquerque, NM
It is commonly assumed that the horizontal permeability anisotropy of a naturally fractured reservoir will be elongate along the dominant trend of the subsurface, natural fracture system. Although this can be demonstrated to apply in many simple geologic settings, this predictive concept must commonly be modified for stress changes caused by the post-fracture geologic history of the reservoir, including local variations in stress magnitude
and orientation caused by structures. In cases where local stresses and fractures are superimposed on regional stresses and fractures, the fractures that are parallel to the in situ maximum horizontal stress may provide the dominant control on reservoir permeability. This can occur even if the stress-parallel fractures are significantly fewer in num
er than fractures that trend oblique to the maximum horizontal stress, especially where the stress anisotropy is high. The set of fractures which is open and conductive may change with position around a structure, as a function of the local stress variations. Knowledge of subsurface fracture trends must therefore be used in conjunction with knowledge of the in situ stress orientation and
magnitude
to predict horizontal permeability anisotropy. Fractured reservoir permeability may also change over the life of a reservoir, as perturbations in the in situ stress--caused by changes in pore pressure during drilling and production activities--create changes in the three-dimensional effective stress field, and thus in fracture conductivity. High-angle fractures aligned with the maximum horizont
l stress will have the smallest decline in conductivity. These conclusions are supported by field and core data, well tests, and production histories of several reservoirs.
AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)