Theoretical and Experimental Study of Water Loss in Shale Matrix : A Nonequilibrium Thermodynamics-based Two-Phase Flow, Damage Chemo-poroelastic Investigation

Abstract

Hydraulic fracturing (HF) is the most common technology in the development of shale reservoirs. It consumes large amounts of chemically treated water of which 60- 95% is often not recovered in flowback operations. This massive water loss can lead to significant environmental concerns and cause many economic and technical issues for operators. Despite past research, a comprehensive theoretical model coupling micro-scale mechanisms involved in the problem of water loss in shale is still lacking. In this dissertation, a constitutive theory is developed using non-equilibrium thermodynamics and continuum mechanics which couples fluid flow with the mechanics of chemically active materials in the shale matrix. First, a micro-scale experimental investigation of damage behaviour in shale is performed under the coupled effect of stresses and fluid interactions. The observations are used to develop a novel two-phase flow, damage chemo-poroelastic constitutive model. Further experimental investigations are conducted to assess the extent to which these mechanisms contribute to the water loss phenomenon in the shale matrix and provide inputs to the model. To accomplish this objective, several techniques such as spontaneous imbibition, contact angle measurement, X-ray micro-computed tomography (micro-CT), neutron-CT, and pore pressure transmission tests are utilised. Next, the COMSOL Multiphysics platform is used to numerically solve the coupled set of partial differential equations using the finite element method. Numerical results provide novel insights into the poromechanical behaviour of water and gas saturated shales. They indicate that, where chemical swelling stresses are negligible, shale’s poroelastic and chemo-poroelastic responses are similar unless the micro-structural deterioration of the matrix with time is considered. This implies that, even when the swelling stress is low, the chemical effects – acting to lower the rock strength – induce significant changes in the stress distribution in the matrix and modify the fluid flow paths. The micro-structural deterioration occurs only in the water-saturated region increasing its storage capacity thus significantly contributing to the water loss. The main contributions of this research are the theoretical development and modelling of two-phase flow, damage chemo-poroelastic behaviour of shale as well as the novel experimental investigation of involved physical and chemical processes. The research findings provide valuable insights into the chemo-poromechanics coupling in shale matrix and its important role in causing water loss.