Abstract
A two-dimensional model of a hydraulic fracture propagating in a weakly consolidated, highly permeable reservoir rock during a waterflooding operation is described in this paper. The model recognizes the essential differences that exist between this class of fractures and conventional hydraulic fracturing treatments of oil and gas wells, namely: (i) the large-scale perturbations of pore pressure and the associated poroelastic effects caused by extended injection time; (ii) the extremely small volume of fluid stored in the fracture compared with the injected volume; and (iii) the leakage of water from both the borehole and the propagating fracture. The model consists of a set of equations encompassing linear elastic fracture mechanics, porous media flow and lubrication theory. Three asymptotic solutions applicable at different time regimes are found theoretically, and numerical results are obtained from the discretized governing equations. The solution reveals that the injection pressure does not evolve monotonically, as it increases with time in the early time radial-flow regime but decreases in the late time fracture-flow regime. Thus, the peak injection pressure does not correspond to a breakdown of the formation, as usually assumed, but rather to a transition between two regimes of porous media flow. However, this problem exhibits an extreme sensitivity of the time scales on a dimensionless injection rate $\mathcal {I}$ . If $\mathcal {I} \lessapprox 1$ , the time to reach the peak pressure could become so large that it cannot be observed in field operations, i.e. the fracture remains hydraulically invisible. Finally, it is found that poroelasticity significantly affects the response of the system, by increasing the injection pressure and delaying the time at which the peak pressure takes place.
Highlights
Waterflooding is a well-developed petroleum engineering technique used to increase oil recovery from hydrocarbon-bearing rocks
The theoretical model described in this paper suggests instead that the large peak pressure is linked to a transition of the flow pattern in the porous medium, caused by the moving boundary represented by the propagating crack
This paper has described a 2-D model of a hydraulic fracture propagating in a poroelastic medium
Summary
Waterflooding is a well-developed petroleum engineering technique used to increase oil recovery from hydrocarbon-bearing rocks (de Swaan 1978; Weijermars, van Harmelen & Zuo 2016). The efficiency of water injection treatments to stimulate production is predicated in part on the initiation and propagation of hydraulic fractures at producer wells to ensure a more efficient sweep of the reservoir (van den Hoek & Mclennan 2000; Sharma et al 2000; Noirot et al 2003). This fracture allows the injected fluid to leak through the crack surfaces, which eventually leads to the development of a linear flow pattern around the borehole-fracture system. These observations are counter-intuitive since the tensile strength should be so small in weak rocks that the breakdown pressure estimated according to the Haimson & Fairhurst (1967) criterion could in principle be approximated by the pressure required to reach an effective tensile hoop stress at the borehole wall
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