Nanopores to megafractures: Current challenges and methods for shale gas reservoir and hydraulic fracture characterization

The past two decades have seen a tremendous focus by energy companies on the development of shale gas resources in North America, resulting in an over-supply of natural gas to the North American market in recent years. This shale gas “revolution” was made possible primarily through the application of drilling and completion technologies, particularly horizontal wells completed in multiple hydraulic fracturing stages (multi-fractured horizontal wells, or MFHWs). While these technologies have proven successful in commercializing the resource, imperfect understanding of basic shale gas reservoir properties, and methods used to characterize them, has perhaps led to inefficiencies in shale gas resource development and recovery that can be improved over time with further research. The purpose of the current work is to 1) provide an overview of shale gas storage and transport mechanisms 2) summarize the challenges associated with evaluating key reservoir and hydraulic fracture properties and 3) discuss recent advances by the authors in the area of shale gas reservoir and hydraulic fracture characterization. For the latter topic, advances in multi-scale characterization techniques, from reservoir sample evaluation to production data analysis will be addressed, however an emphasis will be placed specifically on methods to evaluate reservoir and rock properties along the length of the horizontal well to enable selection of hydraulic fracture stage placement and improved well forecasting. Rock cuttings retrieved during drilling are typically the only reservoir samples obtained from horizontal wells, and therefore methods for quantitative assessment of pore structure, gas content, gas-in-place, permeability, fluid-rock interaction, and rock mechanical property assessment will be discussed. In particular, the following recent innovations by the authors are highlighted: 1) use of the Simplified Local Density (SLD) model to account for fluid property alteration from pore confinement, and to predict high-pressure gas adsorption from low-pressure adsorption data collected for small amounts of cuttings samples 2) extraction of permeability/diffusivity from low-pressure adsorption rate data, also collected for small amounts of cuttings samples 3) use of a variable pressure, environmental SEM to assess fluid distribution and micro-wettability to support pore-scale modeling studies 4) estimation of unpropped hydraulic fracture permeability through generation of fractures in core plugs under stress 5) use of microhardness tests to evaluate fine scale changes in “mechanical stratigraphy” and 6) use of sonic core-holders to provide measurements of dynamic rock mechanical properties for shale samples subject to in-situ stress. Modification of diagnostic fracture injection tests (DFITs), a common well-testing technique performed on shales to derive reservoir property and stress information but performed usually only at the toe of horizontal wells, to enable tests to be performed at multiple points along a horizontal well, will be proposed. Finally, advances in production analysis methods to account for effects such as pore confinement, relative permeability, and stress-dependent permeability will be reviewed, as will techniques for extracting hydraulic fracture properties through analysis of flowback data. It is hoped that this summary will provide geoscientists and engineers with a comprehensive overview of shale gas reservoir and hydraulic fracture evaluation challenges and potential solutions, with a view to enabling more efficient shale gas extraction.