A review of transport mechanisms and models for unconventional tight shale gas reservoir systems

Abstract

Abstract Gas transport in unconventional reservoirs is highly complex by differing greatly from those in conventional reservoirs with low porosity and permeability. Shale incorporates both organic and inorganic matter which by their nature are discrete. A striking peculiarity of the shale matrix is the pore structures occurring mainly in sizes ranging from a few to hundreds of nanometers. Certain interactions taking place between fluid-pore wall influence drastic changes in the physical properties of fluid, and thereby incentivize different mass transfer mechanisms coupled simultaneously as continuum flow, slip flow, transition flow of bulk gas flow, and absorb gas surface diffusion. Historically, flow behavior in porous media can be denoted by an apparent permeability parameter as the coupling result of gas transport in the organic and inorganic pores. Three main approaches of estimating apparent permeability are applied in the literature, namely experiment, numerical, and analytical methods; every method has its merits and demerits. Among them, analytical methods have been developed in cognizance of critical underlying factors of nanoporous media, and were found to be relatively easy to use, precise, and sufficiently instantaneous compared to other available methods of approximating gas flow behavior. The present work aims to systematically review those analytical models with a special focus on fundamental flow mechanisms in tight shale reservoirs. Different model classifications and dependence parameters are discussed in detail. The influence of pore cross-section shapes on fluid flow capacity in the shale matrix was elaborated as well. On account of the existing studies, models differ greatly in the way they capture the actual physics of tight gas reservoir systems. The review therefore concluded that fluid transport models should comprehensively incorporate multifaceted effects of a reservoir to be considered apt for accurate predictions of the recoverable gas resources in shales.