Vertical or Horizontal? - Hydraulic Fracture Geometry as a Make or Break to a Tight Gas Field in the Western Sichuan Basin

Development of unconventional tight gas fields require effective and optimal placement of hydraulic fractures that enhance insitu permeability and improved accessibility to reservoirs. Optimally, hydraulic fractures are expected to have sufficient vertical height growth to connect stacked gas-bearing and horizontally more permeable reservoir packages while being contained to stay away from water-rich intervals. In this paper, a case study is presented on the complex hydraulic fracture geometry in a highly over-pressured and tectonically stressed tight gas field in the Western Sichuan Basin. Difficulties in formation breakdown and proppant placement, pump truck failure, continuous high treating pressures, screen-out, and casing shear are encountered during the hydraulic stimulation of the Xujiahe members of the upper Triassic Xujiahe formation. These difficulties are preceded by larger-than-overburden breakdown pressures and ISIPs during diagnostic pre-fracture injection tests, which imply that hydraulic fracture may have initiated and propagated horizontally. Instead of concluding with a purely horizontal fracture geometry, evidence supports that the hydraulic fractures, as delineated by different fracture monitoring techniques, may form a complex fracture network characteristic of T- and I-shape geometry. The main causes for such complex fracture geometry and limited fracture height are investigated and studied. A fit-for-purpose zone selection and perforation strategy is implemented which avoids high water saturation, less fraccable and highly laminated intervals. The findings and experience underline the importance of integrating geology, geomechanics, and fracture diagnostics to understand hydraulic stimulation effectiveness and to improve operational performance in the unconventional tight gas field development.