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.

The adsorption of hydrocarbon (pure CH4 and C2H6) on illitic clay was investigated at temperatures of 333, 363, and 393 K (60, 90, and 120 °C) over a range of pressures up to 30 MPa using grand canonical Monte Carlo (GCMC) simulations. We first discussed the comparability of molecular simulation results with experimental measurements. Our results indicate that molecular simulation results of the excess adsorption are comparable with the experimental measurements if they are both expressed per unit surface area available for adsorption instead of per unit mass. The gas density profiles indicate that the adsorption of CH4 and C2H6 is mainly affected by the clay surface layers. In micropores smaller than 2 nm, the overlapping of the interaction of the simulated pore walls with the gas results in enhanced density peaks. For pore sizes of 2 nm or larger, the overlapping effect is significantly reduced, and the height of the gas density peak close to the surfaces is no longer affected by pore sizes. The maximum excess adsorption of illite for C2H6 is almost twice that for CH4 due to the stronger interaction between illite and C2H6 than between illite and CH4, but the saturation capacity (maximum loading) is the same for both. Our findings may provide some insights into gas adsorption behavior in illite-bearing shales and give some guidance for improving experimental prediction.

No abstract

No abstract

Consolidation of clays tends to result in changes in particle orientation and pore size distribution as well as progressive reduction of porosity and permeability with increasing effective stress. Clay particles are expected to rotate normal to an axial load, thus decreasing flow path tortuosity parallel to the particle alignment direction and increasing tortuosity normal to the particle alignment. This results in the development of anisotropic permeability, such that the horizontal permeability of a consolidated sediment is greater than the vertical permeability at any given porosity. Within any uniform layer, levels of permeability anisotropy are modest. Typically, permeability anisotropy produced by consolidation of natural clays is in the range 1.1–3 and does not reach the high levels predicted by simple models of clay particle reorientation. The discrepancy arises from particle clustering and irregularities in particle packing. Although somewhat higher levels of anisotropy may exist as a consequence of lamination within individual beds, values > 10 that are known to exist on the formation scale are produced by strong contrasts between the permeabilities of interlayered beds. As argillaceous sediments have permeability ranges of many orders of magnitude, apparently subtle lithological layering in a shale unit may lead to a highly anisotropic flow behaviour.