A poromechanical model of hydraulic fracturing volumetric opening

Abstract

In the present study, the hydraulic fracturing volumetric opening is referred to the volumetric opening in per unit bulk volume of rocks, in fact, the increment of porosity due to hydraulic fracturing, including the pore volumetric opening and fracture volumetric opening. The former is an idealized representation of elastic volumetric opening of widely distributed microcracks, fissures, and pores due to porous flow, and the latter is the volumetric opening of newly generated trunk fracture due to breakdown of rock skeleton. The poromechanical model is a set of mechanical formulas to model the evolutions of these volumetric openings in a hydraulic fracturing process, so that to give a proper evaluation of hydraulic fracturing openings in RVE scale. Firstly, a three-staged hydraulic fracturing process is proposed, based on which, staged expressions of volumetric openings are established using the theory of poroelasticity and the damage model of cohesive breakdown. Then, the precise evolutions of these volumetric openings are obtained by incorporating hydraulic fracturing propagation regimes into those expressions aforementioned. Therefore, the poromechanical model proposed in the present study is bridging the gaps between the two classic theories, the poroelasticity and hydraulic fracture in a RVE scale. As examples, four limiting cases of hydraulic fracturing are chosen to show how to use this model to calculate the hydraulic volumetric openings. These limiting cases are corresponding to the four limiting propagation regimes of hydraulic fracturing, the storage-viscosity regime, leak-off-viscosity regime, storage-toughness regime, and leak-off-toughness regime.