A fully coupled hydro-mechanical model for the modeling of coalbed methane recovery

Most coal seams hold important quantities of methane which is recognized as a valuable energy resource. Coal reservoir is considered not conventional because methane is held adsorbed on the coal surface. Coal is naturally fractured, it is a dual-porosity system made of matrix blocks and cleats (i.e fractures). In general, cleats are initially water saturated with the hydrostatic pressure maintaining the gas adsorbed in the coal matrix. Production of coalbed methane (CBM) first requires the mobilization of water in the cleats to reduce the reservoir pressure. Changes of coal properties during methane production are a critical issue in coalbed methane recovery. Indeed, any change of the cleat network will likely translate into modifications of the reservoir permeability. This work consists in the formulation of a consistent hydro-mechanical model for the CBM production modeling. Due to the particular structure of coal, the model is based on a dual-continuum approach to enrich the macroscale with microscale considerations. Shape factors are employed to take into account the geometry of the matrix blocks in the mass exchange between matrix and fractures. The hydro-mechanical model is fully coupled. For example, it captures the sorption-induced volumetric strain or the dependence of permeability on fracture aperture, which evolves with the stress state. The model is implemented in the finite element code Lagamine and is used for the modeling of one production well. A synthetic reservoir and then a real production case are considered. To date, attention has focused on a series of parametric analyses that can highlight the influence of the production scenario or key parameters related to the reservoir

Composition, Microstructures, and Petrophysics of the Mozumi Fault, Japan: In Situ Analyses of Fault Zone Properties and Structure in Sedimentary Rocks from Shallow Crustal Levels

[1] We characterize the chemical, microstructural, and geophysical properties of fault-related rock samples from the 80–100 m wide Mozumi fault zone, north central Honshu, Japan. The fault is exposed in a research tunnel 300–400 m below the ground, and we combine geological data with borehole geophysical logs to determine the elastic and seismic properties of the fault zone. Detailed mapping within the tunnel reveals that the fault zone consists of two zones of breccia to foliated cataclasites 20 and 50 m thick. Two narrow (tens of centimeters wide) principal slip zones on which most of the slip occurred bound the central fault zone. The dominant deformation mechanisms within the fault zone were brittle fracture, brecciation, slip localization, plastic deformation, and vein formation in a sericite-calcite rich matrix. Clay alteration patterns are complex within the fault zone, with clay-rich fault breccia enriched in smectite, illite, and kaolinite relative to the kaolinite and illite dominant in the host rock. Whole rock geochemical analyses show that the fault-related rocks are depleted in Fe, Na, K, Al, Mg, and Si relative to the host rock. The fault zone exhibits depressed electrical resistivity values by 10–100 ohm m relative to the wall rock, values of Vp and Vs values that are ~0.30 and ~0.40 km/s (10–20%) less than protolith values. The spontaneous potential logs indicate that the fault zone has increased freshwater content relative to formation waters. Wellbore-based measurements of Vp and Vs in fault-related rocks to enable us to calculate values of Young\u27s modulus from 16.2 to 44.9 GPa and Poisson\u27s ratio for the fault zone of 0.263 to 0.393. The protolith has Young\u27s modulus of 55.4 GPa and a Poisson\u27s ratio of 0.242. Lowest calculated values of Young\u27s modulus and highest calculated values of Poisson\u27s ratio correspond to fault breccia with increased porosity, high fluid content, and low resistivity values. Taken together, these data show that the shallow portion of the Mozumi fault consists of a complex zone of anastomosing narrow slip zones that bound broad zones of damage. Fluid-rock alteration and deformation created altered fault-related rocks, which have resulted in overall reduced interval velocities of the fault zone. These data indicate that seismic waves traveling along the interface or internally reflected in the fault zone would encounter rocks of differing and reduced elastic properties relative to the host rocks but that in detail, material properties within the fault may vary