HR: 1340h
AN: MR33A-0139    [Abstracts]
TI: Examining the role of Microcracks in Modelling the Permeability Evolution of Crustal Rocks at Elevated Hydrostatic Pressure.
AU: * Benson, P M
EM: phil.benson@utoronto.ca
AF: University College London, Gower Street, London, WC1E 6BT United Kingdom
AU: * Benson, P M
EM: phil.benson@utoronto.ca
AF: University of Toronto, Lassonde Institute, 170 College Street, Toronto, ON M5S 3E3 Canada
AU: Schubnel, A
EM: alexandre.schubnel@utoronto.ca
AF: University of Toronto, Lassonde Institute, 170 College Street, Toronto, ON M5S 3E3 Canada
AU: Meredith, P G
EM: p.meredith@ucl.ac.uk
AF: University College London, Gower Street, London, WC1E 6BT United Kingdom
AU: Young, P
EM: paul.young@utoronto.ca
AF: University of Toronto, Lassonde Institute, 170 College Street, Toronto, ON M5S 3E3 Canada
AB: A key consequence of the presence of void space within rock is its significant influence upon fluid transport properties. This observation is fundamental to our understanding of crustal evolution and energy resource management, for example the efficient recovery of hydrocarbon and water resources, and the safe disposal of hazardous waste. However, the processes responsible for porosity formation are diverse, ranging from depositional processes such as sedimentary sorting and grain alignment, through diagenetic processes such as compaction and cementation, to deformational processes such as microcracking. The porosity that evolves from the superposition of these processes over time may therefore have a complex geometry or fabric. In addition, many of these processes have an inherent directionality which may lead to anisotropy of the void space, and all have been shown to play important roles in influencing the fluid transport properties of rock. The measurement of permeability at elevated pressures and the calculation of permeability from other data (such as elastic wave velocity) remains non-trivial. In particular, in order to test models that predict such relations, ideally both elastic wave velocity and permeability should be measured simultaneously. In this study, we use a novel apparatus in order to measure elastic wave velocities (P and S) contemporaneously with permeability and porosity for three rock types, a high porosity sandstone (Bentheim), a tight sandstone (Crab Orchard), and a microcracked granite (Takidani). This laboratory data is then used with permeability models of Gu‚guen and Dienes and Kozeny-Carman to investigate the role that void space of differing apertures imparts on the measured permeability of different rock types. Using the Kachanov non-interactive effective medium theory, measured elastic wave velocities are inverted using a least square fit, permitting the recovery of crack density evolution with increasing hydrostatic pressure. This crack density data is then used as input to the microcrack-permeability model of Gu‚guen and Dienes. The classic Kozeny-Carman approach of Walsh and Brace is also applied to the measured permeability data via a least square fit, in order to extract tortuosity data. We successfully predict the evolution of permeability with pressure for direct comparison with the laboratory measurements, and report data that compares and contrasts the changes of permeability with hydrostatic pressure in rock where microcracks dominate, as compared to rocks in which equant pores dominate. Additionally, we illustrate how these properties are affected by anisotropy of the rock types via the measured anisotropic fabrics in each rock (either through microcrack preferential alignment or crossbedding). These combined experimental and modelling results illustrate the importance of understanding the details of how rock microstructures change in response to an external stimulus in predicting the simultaneous evolution of rock physical properties.
DE: 5104 Fracture and flow
DE: 5112 Microstructure
DE: 5114 Permeability and porosity
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005