Abstract: Geomechanics Approach to Management of Naturally Fractured Reservoirs

Lawrence W. Teufel, John C. Lorenz

Fractures are present in almost all hydrocarbon reservoirs, but it is only when fractures form an interconnected network that their affect on fluid flow becomes important. Fractures not only enhance the overall permeability of many reservoirs, they also create significant permeability anisotropy. Knowledge of the orientation and magnitude of horizontal permeability anisotropy has major economic importance in developing and managing a reservoir. Geologists commonly assume that the horizontal permeability anisotropy of a naturally fractured reservoir will be elongate along the dominant trend of subsurface natural fracture systems. Although this can be demonstrated to apply in many simple geologic settings, this predictive concept must commonly be modified for stress changes caused by th post-fracture geologic history of the reservoir, including local variations in stress magnitude and orientation caused by structures. In cases where the 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 number than fractures that trend oblique to the maximum horizontal stress, especially where the stress anisotropy is high. The set of fractures that is open and conductive may change with position around a structure as a function of local stress variations. Knowledge of subsurface fracture trends must be used in conjunction with knowledge of the i -situ stress orientations and magnitudes to predict horizontal permeability anisotropy. Fracture reservoir permeability may also change over the life of a reservoir because perturbations in stress state, caused by drilling, production, and waterflood activities, create changes in the three-dimensional effective stress field, and thus in fracture conductivity. High-angle fractures aligned with the local maximum horizontal stress will have the smallest decline in conductivity as the reservoir is produced. These conclusions are supported by core analyses, in-situ stress measurements, well tests, and production histories of several reservoirs, including naturally fractured reservoirs in North Sea chalk, carbonates in Texas and Alaska, and sandstones in Colorado and Wyoming.

AAPG Search and Discovery Article #90985©1994-1995 AAPG Distinguished Lecture