Laboratory characterisation of shale properties

Abstract

Shale gas has become a significant resource play in the USA over the past few years and companies are now evaluating the shale gas potential of many sedimentary basins, including several onshore basins within Australia. The renewed focus on rock sequences that have hitherto largely been ignored has necessitated the development of workflows and methods for characterising shales. Along with the deployment of new methods comes the need for interpretation frameworks in order to understand properties such as rock source quality, mechanical properties and production performance from a diverse range of measurements. Laboratory characterisation of rock properties is an important part of any resource evaluation and for shale gas, specific properties of importance include silt content, organic matter abundance and type, static and dynamic mechanical properties (brittleness), micro/macro-fabrics, porosity, permeability, petrophysical properties and anisotropy. Here we introduce a workflow for systematic shale characterisation in the laboratory with a number of examples to illustrate and discuss the application to reservoir evaluation in shale gas plays. A suite of shales from a number of sedimentary basins around the world was collected and characterised with a full suite of non-destructive petrophysical methods before destructive geomechanical testing was performed. For each sample, a representative portion was analysed for quantitative mineralogy using XRD and XRF, and clay chemical reactivity via cation exchange capacity (CEC) and grain size by centrifugation. For many samples, surface area and Mercury Injection Capillary Pressure (MICP) for porosity and pore throat distribution were also performed and used to predict permeability from models available in the literature. Several imaging techniques including Scanning Electron Microscopy (SEM) and X-ray Computed (micro-)Tomography (X-ray CT) at low and high resolution were performed. Shale strength has previously been shown to be related to CEC, which is inversely proportional to silt content. Anisotropy of shale properties is both intrinsic and stress-induced. Dielectric properties are related to water content at high frequency and dispersion in the dielectric constant is directly related to CEC of clays in particular and hence rock strength. Stress-induced anisotropy of elastic properties was found to be dependent on the orientation of microfabrics with respect to the maximum principal stress direction. Low and high field nuclear magnetic resonance can be used to distinguish clay-bound and free water as well as adsorption of organic components and to screen for wettability. High and low field NMR techniques are combined to show that illitic shales tend to be strongly water wet while the presence of kaolinitic clays imparts a tendency for shales to become oil wet with likely consequences for oil/gas recovery strategy, production flow efficiency and drilling design.