Controls on the intrinsic flow properties of mudrock fractures: A review of their importance in subsurface storage

Abstract

Effective storage and containment of injected fluids, over a range of spatial and temporal scales, is reliant upon the sealing capacity of the lithologies overlying geological stores. Low-permeability mudrocks are considered effective candidates to restrict the migration of injected fluids from the host formation, owing to their low matrix permeabilities (< 10−19 m2). Fluid-conductive fault and fracture systems can threaten seal integrity by creating high permeability pathways (> 10−19 m2), potentially compromising subsurface storage operations. To safeguard and expedite the initialisation of storage projects on an impactful scale, rigorous comprehension of the intrinsic flow properties of fractures in mudrocks is key. The distribution of fractures within fracture networks, and the degree to which these configurations promote interconnectivity, is a primary factor influencing fluid transport. At the individual fracture scale, a fractures ability to transmit fluid is a function of the aperture distribution, which is itself governed by a series of hierarchical controls operating across various scales. Accurate understanding, characterisation and quantification of the physical transport mechanisms and fluid flow dynamics prevalent in rock fractures is frustrated by the existence of heterogeneous aperture distribution, caused by fracture surface roughness. Further hindrances to understanding the fundamental transport properties of fractures stem from our limited knowledge of the breadth and complexity of hydromechanical responses that emerge from the coupling of pore pressure, effective stress and multiphase flow. In this review paper, we have collated and analysed the large body of experimental and theoretical literature pertaining to single- and two-phase fluid transport, and the geomechanical properties of single fractures and fracture networks in relation to fluid conductivity. We focus upon naturally occurring fractures in mudrocks and the current understanding of the physical and transport properties which impact the risks to secure containment in geological reservoirs.