Determining the mechanism of post-hydraulic fracturing evolution of fracture-pore space in a heterogeneous reservoir under the influence of the Rehbinder effect

Abstract

This study investigates the post-hydraulic fracturing evolution of the fracture-pore space in a heterogeneous layered reservoir under the influence of the Rehbinder effect. Engineering models of dual porosity, fractal geometry of fracture networks, physicochemical mechanics of the Rehbinder effect, and empirical data on the influence of surface active substances (SASs) on permeability after hydraulic fracturing (HF) have so far developed separately. Therefore, there was a lack of a single model that would connect the geometry of the branched fracture network with the porosity and permeability of the reservoir. In this work, those four directions have been synthesized into one model: the initiation of the primary crack at grain contacts is described by the Griffiths-Irwin criterion, and a secondary fractal ("herringbone") branching model for granular and thin-layered rocks has been built. Through the adsorption opening of the pore space, this geometry is related to the kinetics of subcritical crack growth and surfactant adsorption and, therefore, to the effective porosity φ(t) and anisotropic permeability k∥(t) / k⊥(t). The model was tested on a three-component material “sand-siltstone-shell rock”: by a laboratory triaxial experiment and a stochastic (Monte Carlo, 300 realizations) digital experiment of the same composition with neural network smoothing. As a result, it was found that with increasing surfactant concentration, the critical crack initiation pressure decreases, and the maximum opening and front movement speed increase, reaching saturation when the surfactant concentration approaches 0.30%. At the same time, the permeability increases by 112–224% in 48 h, while the porosity changes by only 0.04–0.20 percentage points. The effective volume function Veff(t) and the criterion for reasonable duration of the technological pause have been proposed – the moment of saturation Veff(t) when its increment dVeff/dt falls below the predefined technological threshold. This makes it possible to reasonably determine the moment of completion of post-hydraulic fracturing space evolution and the start of production without additional costs