For unconventional resources exploration and development, hydraulic fracture pattern, geometry and associated dimensions are critical elements in determining if the uneconomically low-permeability reservoirs can be effectively and efficiently stimulated. In principle, hydraulic fracture growth pattern is dominated by the state of stress in the subsurface, and commonly and optimally, hydraulic fractures are expected to grow vertically from deep wells and to have sufficient height growth to connect stacked hydrocarbon-bearing and horizontally more permeable reservoir packages while being contained to stay away from water-rich intervals. While the tectonic setting and in-situ stress conditions for vertical fracture development have been well studied, those for horizontal or complex fractures remain unclear or not well reported. In this paper, a case study is presented on the complex hydraulic fracture in a highly over-pressured and tectonically stressed tight gas field in the Sichuan Basin of southwest China.
During deep well hydraulic stimulation of the primary reservoirs of the Upper Triassic Xujiahe formation in the central western Sichuan Basin, difficulties are encountered in formation breakdown, injectivity establishment, proppant placement, pump equipment shut down, continuous high treating pressures, casing shear, and screen-out. These difficulties are preceded by larger-than-overburden breakdown pressures and ISIPs (Instantaneous Shut-In Pressure) during diagnostic pre-fracture injection tests, which imply that hydraulic fracture may have initiated and propagated horizontally. Different fracture monitoring techniques, however, have indicated that instead of getting purely horizontal fracture geometry, hydraulic fractures may have formed a complex network characteristic of T-shape or I-shape geometry. The main causes for such complex fracture geometry and limited fracture height are investigated and studied.
To quantitatively evaluate the fracturing performance and strategy, two classes of numerical hydraulic fracture models are undertaken. In addition to the LEFM (Linear Elastic Fracture Mechanics) based hydraulic fracture simulation, a fully-coupled finite-element based model is developed that takes into account a full suite of rock mechanical properties of sandstone and shale sequences and geological features that may limit or facilitate the fracture growth. The modeling results indicate that in addition to the typical stress control on the growth of fracture height, contrast in mechanical property and the presence of bed-parallel geological features may act as fracture baffles or barriers that prematurely arrest the vertical growth and facilitate development of horizontal fractures.