Effect of modern state of stress on flow-controlling fractures: a misleading paradigm in need of revision

摘要

Comparisons of stress directions and orientations of flow-controlling fractures show that open fractures in the subsurface are not necessarily parallel to maximum compressive stress (S_(Hmax)) and that fractures perpendicular to this direction may be open. Moreover, sealed fractures parallel to S_(Hmax) are numerous. Parallelism of S_(Hmax) and open fractures is not good evidence, by itself, that modern-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.