The development of horizontal drilling and hydraulic fracturing has enabled the extraction of hydrocarbons from fine-grained sedimentary rocks. This type of unconventional resource has gained much attention for most oil companies all over the world. Understanding petrophysics and the gas-water-rock interactions is essential in guaranteeing the effective recovery of hydrocarbons from shale and other tight reservoirs. But shales and tight rocks are commonly highly heterogeneous with behaviors tiered in multiple scales [
Fluid transport in such complex media cannot be easily determined with conventional techniques, which are successfully applied for reservoir rocks. At micro-/nanoscales, the interplay between fluids and the pore wall is no longer negligible. The mechanisms of diffusion, slip flow, and sorption all significantly affect the transport of geofluids [
As contemporary petroleum exploration activities have focused on unconventional resources, the traditional experimental methods and numerical simulation tools are sometimes no longer effective. We confront numerous challenges to accurately describe the petrophysics of fine-grained sedimentary rocks in unconventional tight and shale reservoirs.
R. Beloborodov et al. experimentally characterize the dielectric properties of fluid-saturated artificial shales. Because of the high dielectric contrast between water and hydrocarbons, the producible layers of reservoir rocks and surrounding media can be effectively distinguished. In this paper, the authors investigate the frequency-dependent dielectric properties of artificial shale rocks prepared from silt-clay mixtures via mechanical compaction.
T. Han et al. present a theoretical model for the anisotropic dielectric properties of artificial shales. Their model is based on the theoretical assumption of differential effective medium models for any number of mineral grain components aligned in any directions and is shown to be independent of the mixing order. By incorporating a measured orientation distribution function of the clay particles and by inverting the dielectric properties of the artificial sample composed of clay and brine, their model is capable of modeling the frequency-dependent anisotropic dielectric properties of artificial shales.
Gas shales and tight reservoirs exhibit extremely low permeability. This low permeability makes it difficult to apply traditional transport modeling approaches, such as Darcy’s law, to shales [
S. Huang et al. proposed a comprehensive apparent permeability model to consider the multiple transport mechanisms in shale gas reservoirs. The specific mechanisms include viscous flow, slip flow, Knudsen diffusion, and surface diffusion. In their model, the pore diameter and mean free path of gas molecules are corrected by considering the adsorption layer and dense gas effect.
Y. Zeng et al. develop a modified apparent permeability model to describe gas flow in shale gas reservoirs. The apparent model integrates viscous flow, Knudsen diffusion, and gas desorption. They additionally consider a macroseepage model of multistage fractured horizontal wells accommodating multiple gas flow mechanisms to predict the dynamic pressure and production performance.
Hydrocarbon extraction from tight reservoirs is feasible using multiple-fractured horizontal wells. The technology to create multiple-fractured horizontal wells creates a complex stimulated reservoir volume (SRV) with induced fractures proximal to the hydraulic fractures. J. Wang et al. focus on the geometric properties of this stimulated reservoir volume in tight reservoirs and develop a new semianalytical model to analyze the well bottom pressure response. The calculations are helpful to understand the dynamic characteristics of multiple-fractured horizontal wells and the performance of the stimulated reservoir volume.
Hydraulic fracturing is one of the key methods for the effective development of unconventional reservoirs. In the process of hydraulic fracturing, a significant volume of fracturing fluid is injected into the reservoir. However, much of this fracturing fluid is retained in the formation after flow-back [
We appreciate the significant contributions of the various authors to this special issue and their perseverance during the publication process. We also thank the many anonymous reviewers who helped evaluate and contribute to these papers.