Slow slip source characterized by lithological and geometric heterogeneity

Slow slip events (SSEs) accommodate a significant proportion of tectonic plate motion at subduction zones, yet little is known about the faults that actually host them. The shallow depth (<2 km) of well-documented SSEs at the Hikurangi subduction zone offshore New Zealand offers a unique opportunity to link geophysical imaging of the subduction zone with direct access to incoming material that represents the megathrust fault rocks hosting slow slip. Two recent International Ocean Discovery Program Expeditions sampled this incoming material before it is entrained immediately down-dip along the shallow plate interface. Drilling results, tied to regional seismic reflection images, reveal heterogeneous lithologies with highly variable physical properties entering the SSE source region. These observations suggest that SSEs and associated slow earthquake phenomena are promoted by lithological, mechanical, and frictional heterogeneity within the fault zone, enhanced by geometric complexity associated with subduction of rough crust

Methane and Carbon Dioxide Adsorption on Illite

The adsorption of CH₄ and CO₂ onto illitic clay was investigated at the temperatures 298, 313, 328, 358, and 423 K (25, 40, 55, 85, and 150 °C) over a range of pressures up to 50 MPa using grand canonical Monte Carlo (GCMC) simulations. Our simulation results showed spontaneous and exothermic adsorption behavior of illite for CH₄ and CO₂ with enthalpy changes of −3.50 kJ/mol and −25.09 kJ/mol, respectively. Our results indicated that the interlayer counter cations (K⁺) play an important role in CO₂ adsorption. Methane adsorption is mainly affected by the clay surface layers rather than the interlayer counter cations. The density and volume of CH₄ and CO₂ in their adsorbed phase at saturation were extrapolated from the linear portion of the excess adsorption isotherm. The resulting values were compared with available experimental data, and possible factors causing inconsistency were described. We discussed some issues associated with the Langmuir fit to experimental excess adsorption data in the case of low pressures. Our findings may provide some insights into gas adsorption behavior in illite-bearing shales

Characterization of Darai Limestone Composition and Porosity Using Data-Constrained Modeling and Comparison with Xenon K-Edge Subtraction Imaging

Data-constrained modeling is a method that enables three-dimensional distribution of mineral phases and porosity in a sample to be modeled based on micro-computed tomography scans acquired at different X-ray energies. Here we describe an alternative method for measuring porosity, synchrotron K-edge subtraction using xenon gas as a contrast agent. Results from both methods applied to the same Darai limestone sample are compared. Reasonable agreement between the two methods and with other porosity measurements is obtained. The possibility of a combination of data-constrained modeling and K-edge subtraction methods for more accurate sample characterization is discusse

Wave attenuation in partially saturated porous rocks: New observations and interpretations across the scales

Seismic waves propagating in porous rocks saturated with two immiscible fluids can be strongly attenuated. Predicting saturation effects on seismic responses requires a sound understanding of attenuation and velocity dependencies on the fluid distribution. Decoding these effects involves interpreting laboratory experiments, analyzing well-log data, and performing numerical simulations. Despite striking differences among scales, flow regimes, and frequency bands, some aspects of wave attenuation can be explained with a single mechanism — wave-induced pressure diffusion. Different facets of wave-induced pressure diffusion can be revealed across scales

Characterization of Darai Limestone Composition and Porosity Using Data-Constrained Modeling and Comparison with Xenon K-Edge Subtraction Imaging

Data-constrained modeling is a method that enables three-dimensional distribution of mineral phases and porosity in a sample to be modeled based on micro-computed tomography scans acquired at different X-ray energies. Here we describe an alternative method for measuring porosity, synchrotron K-edge subtraction using xenon gas as a contrast agent. Results from both methods applied to the same Darai limestone sample are compared. Reasonable agreement between the two methods and with other porosity measurements is obtained. The possibility of a combination of data-constrained modeling and K-edge subtraction methods for more accurate sample characterization is discussed

Molecular dynamics study of CO2 sorption and transport properties in coal

Crown Copyright © 2016 Published by Elsevier Ltd. All rights reserved. An understanding of gas transport in nano-scale porous media is crucial for many industrial applications, for example, processes associated with CO2 injection, storage and enhanced coalbed methane (ECBM) production. In this study, we carried out combined molecular dynamics (MD) and Grand Canonical Monte Carlo (GCMC) simulations on the transport properties (i.e. self- and transport diffusivities and permeability) of CO2, in a realistic intermediate rank bituminous coal (flexible coal model) at a temperature of 328 K (55 °C) and a range of pressures up to 25 MPa. Self-diffusivity and sorption isotherms of CO2 are obtained directly from the MD and GCMC simulations. The Maxwell-Stefan diffusion model was then applied to correlate the self- and transport diffusivities. The permeability was computed through an integration of the transport diffusivity over the sorption concentration obtained from the simulations. The results show that CO2 self-diffusivity decreases with increasing reservoir gas pressure up to 8 MPa, then increases with pressure due to the interaction between coal and CO2. The transport diffusivity increases with the reservoir gas pressure as a result of an enhanced thermodynamic factor. The simulation results reveal a negative correlation between the sorption-induced coal swelling and CO2 self-diffusivity due to the interaction between CO2 and coal. Rigorous modeling of gas recovery and production thus requires consideration of specific interaction of the gas and coal matrix. Permeability of CO2 exponentially increases with the decreasing reservoir gas pressure, which is comparable with published field data

Slow slip source characterized by lithological and geometric heterogeneity

Slow slip events (SSEs) accommodate a significant proportion of tectonic plate motion at subduction zones, yet little is known about the faults that actually host them. The shallow depth (<2 km) of well-documented SSEs at the Hikurangi subduction zone offshore New Zealand offers a unique opportunity to link geophysical imaging of the subduction zone with direct access to incoming material that represents the megathrust fault rocks hosting slow slip. Two recent International Ocean Discovery Program Expeditions sampled this incoming material before it is entrained immediately down-dip along the shallow plate interface. Drilling results, tied to regional seismic reflection images, reveal heterogeneous lithologies with highly variable physical properties entering the SSE source region. These observations suggest that SSEs and associated slow earthquake phenomena are promoted by lithological, mechanical, and frictional heterogeneity within the fault zone, enhanced by geometric complexity associated with subduction of rough crust