Mass transport within the fracture-matrix systems of unconventional shale reservoirs: application to primary production and EOR in Eagle Ford

Abstract

The average primary oil recovery factor of Eagle Ford shale is around six percent; thus, a considerable amount of oil will be left behind after primary production. A major technique to enhance oil production in Eagle Ford could be gas injection because waterflooding does not seem plausible. This thesis evaluates the potential of gas injection enhanced oil recovery (EOR) using dual-porosity compositional modeling. The modeling effort focuses on three areas: transport mechanism at the matrix fracture interface, rock deformation effect on the phase behavior in the matrix pores, and multi-phase rate transient analysis (RTA). The complex nature of the fluid system and its transport in unconventional shale reservoirs requires robust computation codes that clearly reflect the physics of mass transport and thermodynamic phase behavior calculations. In this thesis, I have addressed this issue and have developed a new implicit method which relies on partial molar volume to decouple the transport equations into a pressure-composition solution, followed by a straight forward flash calculation that provides unambiguous crossing of phase boundaries, quantification of phase saturations, and phase compositions. Specifically, this formulation maintains a strong connection to the underlying physics. The pressure-composition code is an implicit numerical solution technique and is formulated for the dual-porosity reservoirs and for application in stimulated shale reservoirs. The model also includes diffusion mass transport for studying diffusion of components across the fracture-matrix interface in conjunction of with wet-gas EOR in shale reservoirs. The conclusion is that diffusion mass transport across fracture-matrix interface is a major gas-EOR mechanism even when advective flow via Darcy velocity is inactive (i.e., zero pressure gradient) or is at a very low level (i.e., very low permeability and low-pressure gradient). The new model includes a geomechanical component to study the effect of the pore pressure change and associated rock deformation on shale reservoir performance, and a pore-confinement, thermodynamic component to account for the shift in the phase envelop in nano-scale shale reservoir pores. Here, the model results indicate that fracture pore deformation during production provides additional driving force for hydrocarbon recovery. Finally, the new model was used to evaluate the rate transient analysis (RTA) in shale reservoirs where is reservoir production characteristics is highly composition-dependent. The results indicate that RTA is also applicable in stimulated shale reservoirs (i.e., dual-porosity) and RTA results provides an accurate value for the effective stimulated reservoir rock.