Analysis of Growth and Interaction of Multiple Hydraulic Fractures

Abstract

Abstract A general-purpose numerical formulation is presented for the incremental quasistatic stable presented for the incremental quasistatic stable growth and interaction of fluid driven fractures in reservoir structures. Multiple fractures emanating from multiple wellbores are allowed to evolve on arbitrarily curved trajectories, dictated by non-uniform stress fields arising from interaction of fractures with one another and with material/stress variations. The model simulates fracture growth by simultaneously solving for stresses, displacements and pressure distributions associated with the fracturing configuration at each timestep. The elastostatic computation employs a surface integral method, while the coupled fluid flow and crack opening modelling involves a weak starting assumption of (instantaneous) self-similarity in the opening displacement profile. A search scheme is used to determine the directions and amounts of growth at the end of each timestep. Several sample fracture growth patterns are presented to illustrate the generality and applicability of the simulator, which is verified by comparison with many of our laboratory, experiments. The overall scheme provides an efficient, direct and convenient tool for design of multiple fracturing treatments, whenever these begin to be used for creation of suitable underground fracture networks (e.g., for enhanced petroleum recovery or solution mining). Introduction There is no existing numerical simulator in the resource extraction industry which can predict the curvilinear growth of a fracture system during a (hydraulic) fracturing treatment. In fact, it is not even widely recognised that the stress fields of separate fractures propagating from adjacent boreholes can be used to influence each other and dictate a final linkage trajectory which would otherwise be impossible (e.g., due to the dominance of tectonic stress polarisation). This study presents the first analysis to describe both the mechanical interaction of two or more fractures growing in a reservoir, and even the first simulation to trace the reorientation associated with initiation at an angle to the eventual preferred direction, viz. that dictated by tectonic stress orientation/variation, stratification, inclusions, pore-pressure distribution, etc. (see Fig. 1). Although apparently not seriously considered to date by the industry, this curving evolution of (multiple) fractures has dominant influence: for instance, in near-wellbore growth (including competing wings and the potential for shear-induced propping); in efforts to achieve horizontal fractures where vertical is preferred; in the design of fracture intersection schemes to kill blow-outs; and in the vital question of creating linked fracture networks (e.g., to form more stable and efficient plane sweeps rather than line drives from, perhaps five-spot, arrays of injection and production wells in enhanced recovery operations). production wells in enhanced recovery operations). A comprehensive computer program has been developed, for which the formulations and numerical implementation are described below. This model, which we call MULTIFRAC, not only provides a capability to solve the vast array of associated elasticity problems (most previously inaccessible, but including a variety of established solutions for verification), but it also allows these fracture geometries to evolve in typical reservoir structures, driven by fluid flow from any number of wellbores. Sample configurations have been tested in the laboratory, and have verified the realism/accuracy of the MULTIFRAC program.