Fracture Effectiveness and its Influence on Gas Productivity in Ultra-deep Tight Sandstone Reservoirs of the Keshen Gas Field, Tarim Basin

Fracture effectiveness plays a key role in gas productivity of ultra-deep tight sandstone reservoirs, Kuqa depression, Tarim Basin. Based on cores, thin sections, well logging, well testing and production data, the study evaluated fracture effectiveness and illustrated its impacts on gas productivity. High-angle and vertical shear fractures are the most important types. Distribution of effective fractures shows great heterogeneous. Fracture effectiveness is influenced by tectonism, diagenesis and in-situ stress. Earlier fractures or fractures in close to gypsum rock are easier to be filled. Completely filled fractures can be reopened under late tectonism. Dissolution improves local fracture effectiveness. Minerals spanning fracture surfaces protect fracture effectiveness from late compression. Fractures filled with calcite can be activated by acidification. Effective fractures parallel to maximum horizontal principal compressive stress direction show larger aperture. Overpressure can decrease the effective normal stress to maintain fracture effectiveness. With exploitation, decline in pore pressure reduces fracture effectiveness. Linear density, aperture, and strike of effective fractures influence gas productivity. Effective fractures greatly enhance matrix permeability. Therefore, more abundant and larger aperture fractures are always corresponded to higher productivity. However, effective fractures also facilitate late water invasion, especially, both mutually parallel. Intense water invasion leads to rapidly declines in productivity.