Comparisons of stress directions and orientations of flow-controlling fractureshow that open fractures in the subsurface are not necessarily parallel to maximum compressire stress (SHm?) and that fractures perpendicular to this direction may be open. Moreover, sealed fractures parallel to Snma? are numerous. Parallelism of SHm? and open fractures is not good evidence, by itself, that modem-day stress controls the orientation of open fractures. A determining factor for fluid flow is the degree of mineral cement within fractures, which is a function of fracture size and the rock's diagenetic history. In most subsurface opening-mode fracture systems, fractures are partly filled with cement deposited at the time of fracturing. This cement forms strong mineral bridges that prop the fracture open. The remaining part of the fracture may be open or filled with cements precipitated after fractures ceased opening. For the many reservoirs in which opening-mode fractures are the key flow pathways, cement patterns rather than stress data may provide the insight needed to determine which fractures are open to fluid flow.
A widely held belief in the petroleum industry is that modem state of stress determines which natural fractures are important for fluid flow at the reservoir scale. Fractures that strike parallel to present-day maximum horizontal compression (SHm?) and perpendicular to the horizontal direction of current minimum compression (Shm?n) are inferred to have a greater likelihood of being open (e.g. Queen & Rizer 1990, Parks & Gale 1999). A recent survey of operators and industry structural geologists overwhelmingly rated present-day SHm? as the factor dictating strike of open fractures (AAPG Reservoir Deformation Group, 1999 pers. comm.). In seismic shear-wave analysis a basic assumption is that open fractures are preferentially oriented by the current stress field acting on the rock mass (Crampin 1987). In production analysis engineers frequently assume that fractures are the most compliant part of the rock mass and are susceptible to closure with increasing effective normal stress (Warpinski et al. 1991).
A wide spectrum of rock-mechanics observations and model studies demonstrate compliant fractured rock masses and individual joints (nonmineralized fractures) (Barton et al. 1985). An opening-mode fracture is expected to close when pore pressure falls below the minimum stress, unless there is some mismatch or propping of the fracture. However, these contemporary views derive in part from influential studies that show that critically stressed faults in crystalline rock are the locus of fluid flow (Barton et al. 1995). The contrary view (e.g. Dyke 1995) that natural-fracture aperture and permeability are not highly sensitive to changes in effective normal stress has largely been neglected. Considering the importance of natural fracture systems in this era of deep directional drilling and challenging fracture targeting, we examine the validity of the contemporary stress/open fracture paradigm and its implications for exploration risk and reservoir management.