The stability of submarine gas hydrates is largely dictated by pressure and temperature, gas composition, and pore water salinity. However, the physical properties and surface chemistry of deep marine sediments may also affect the thermodynamic state, growth kinetics, spatial distributions, and growth forms of clathrates. Our conceptual model presumes that gas hydrate behaves in a way analogous to ice in a freezing soil. Hydrate growth is inhibited within fine‐grained sediments by a combination of reduced pore water activity in the vicinity of hydrophilic mineral surfaces, and the excess internal energy of small crystals confined in pores. The excess energy can be thought of as a “capillary pressure” in the hydrate crystal, related to the pore size distribution and the state of stress in the sediment framework. The base of gas hydrate stability in a sequence of fine sediments is predicted by our model to occur at a lower temperature (nearer to the seabed) than would be calculated from bulk thermodynamic equilibrium. Capillary effects or a build up of salt in the system can expand the phase boundary between hydrate and free gas into a divariant field extending over a finite depth range dictated by total methane content and pore‐size distribution. Hysteresis between the temperatures of crystallization and dissociation of the clathrate is also predicted. Growth forms commonly observed in hydrate samples recovered from marine sediments (nodules, and lenses in muds; cements in sands) can largely be explained by capillary effects, but kinetics of nucleation and growth are also important. The formation of concentrated gas hydrates in a partially closed system with respect to material transport, or where gas can flush through the system, may lead to water depletion in the host sediment. This “freeze‐drying” may be detectable through physical changes to the sediment (low water content and overconsolidation) and/or chemical anomalies in the pore waters and metastable presence of free gas within the normal zone of hydrate stability.

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

Despite its widespread use in petrophysics, tortuosity remains a poorly understood concept. Tortuosity can have various meanings when used by physicists, engineers or geologists to describe different transport processes taking place in a porous material. Values for geometrical, electrical, diffusional and hydraulic tortuosity are in general different from one another. Electrical tortuosity is defined in terms of conductivity whereas hydraulic tortuosity is usually defined geometrically, and diffusional tortuosity is typically computed from temporal changes in concentration. A better approach may be to define tortuosity in terms of the underlying flux of material or electrical current with respect to the forces which drive this flow. Unsteady transport processes, including diffusion, can be described only by a population of tortuosities corresponding to the different flow paths taken by particles traversing the medium. In measurements of steady flow (e.g., those normally used to obtain resistivity or permeability), information about particle travel times is lost, and so the multiple values of tortuosity are homogenised. It can be shown that the maximum amount of information about pore structure is embedded in transport processes that combine advective and diffusive elements. Most existing formulations of tortuosity are model-dependent, and cannot be correlated with independently measurable pore-structure properties. Nevertheless, tortuosity underpins the rigorous relationships between transport processes in rocks, and ties them with the underlying geometry and topology of their pore spaces. Tortuosity can be redefined in terms of the energetic efficiency of a flow process. The efficiency is related to the rate of entropy dissipation (or isothermally, energy dissipation) with respect to a simple, non-tortuous model medium using the postulates of non-equilibrium thermodynamics. Through Onsager’s reciprocity relation for coupled flows it is possible to inter-relate efficiency for pairs of transport processes, and so go some way towards unifying tortuosity measures. In this way we can approach the goal of predicting the value of one transport parameter from measurements of another.

Visual observation of gas hydrates at the microscopic scale in synthetic porous media provides unequivocal visual evidence that clathrates can form in systems without the presence of a free-gas phase. Hydrates were formed from a soluble liquid hydrate former (tetrahydrofuran, C₄H₈O), from free gas (CH₄), and from dissolved gas (CO₂). Clathrates were found to form within the center of pore spaces, rather than on grain surfaces. Cementation of grains only occurred in regions of a small grain size, or where a large proportion of pore space was filled with hydrate. However, even at high clathrate saturations, a thin film of free water persisted on grain surfaces. The results have important implications for the potential cementing effect of hydrates on sediments, and thus for sediment permeability, slope stability, and seismic interpretation of hydrate-bearing sediments.