Fracture aperture: A review on fundamental concepts, estimation methods, applications, and research gaps

Among all rock-fracture parameters, fracture aperture plays an especially crucial role in a range of engineering applications, such as controlling reservoir behavior during production and storage, drilling management, and groundwater recharge and exploitation. However, major challenges persist in the accurate measurement, inference and modelling of fracture aperture. In this work, we offer an overview on the definition, research history, and applied significance of fracture aperture, and we review previous studies on fracture aperture presenting a data compilation illustrating how aperture varies against other fracture parameters, based on analyses of 48 datasets. The analysis of the integrated dataset yields correlations between fracture aperture and other fracture and geomechanical parameters (e.g., Young's modulus, Poisson ratio, spacing, length, height). In the current body of literature, data exist that were mainly collected through experimental, field and modelling studies and analyzed in different ways to derive insights in the association between aperture and other selected parameters. There exist various methods to measure rock-fracture aperture, whose determination is important particularly for the collection of data required to build discrete fracture network models, for the purpose of investigating fluid flow and geomechanical behavior in underground and surface conditions. Examples of these measurement methods include: the study of outcrops and cores, analysis of conventional and advanced well-log data to understand fracture networks in near-bore region; development of artificial intelligence algorithms to constrain or predict fracture aperture based on other known surface or subsurface data. In general, fracture aperture demonstrates significant correlations with a number of other fracture parameters, such as fracture length, fracture density and tectonic stresses, and with related geological variables. Some of these attributes can be readily constrained by means of cost-effective approaches and surface or subsurface studies including outcrop investigations, seismic data, well-log and core analyses. Finally, based on the current state of knowledge, avenues for future work are outlined.