An important dataset to emerge from the Wenchuan earthquake Fault Scientific Drilling project is direct measurement of the permeability evolution of a fault zone. In order to provide context for this new observation, we examined the evolution of tidal responses in the nearfield region (within~1.5 fault lengths) at the time of the mainshock. Previous work has shown that seismic waves can increase permeability in the farfield, but their effects in the nearfield are more difficult to discern. Close to an earthquake, hydrogeological responses are generally a combination of static and dynamic stresses. In this work, we examine the well water level data in the region of the large M w 7.9 Wenchuan earthquake and use the phase shift of tidal responses as a proxy for the permeability variations over time. We then compare the results with the coseismic water level pattern in order to separate out the dynamic and static effects. The coseismic water level pattern for observed steps coincident with the Wenchuan mainshock mainly tracks the expected static stress field. However, most of the wells that have resolvable tidal responses show permeability enhancement after this large earthquake regardless of whether the coseismic response for the well water level is increasing or decreasing, indicating permeability enhancement is a distinct process from static poroelastic strain.
Fines migration has
posed a great challenge to gas and water production
in CBM reservoirs, resulting not only in dramatic permeability reduction
but also in excessive wear on equipment. The objective of this study
was to investigate critical flow conditions for massive fines detachment
in the dewatering phase, for the purpose of yielding an improved understanding
of fines detachment mechanisms and their effective control in the
field. First, fines migration experiments under saturated conditions,
including effluent concentration and permeability measurements, were
conducted at elevated pressure gradients on fractured coal samples
with various apertures. Experimental results indicate the existence
of a critical pressure gradient (CPG) for massive fines detachment.
Second, a mathematical model was developed to describe single particle
detachment in the fracture, accounting for the coupling effects of
hydrodynamic and extended-DLVO forces. Effects of fines size and fracture
aperture on fines detachment were analyzed, and CPGs were determined
from the proposed model. Modeling results revealed that the pressure
gradient required for fines detachment first decreased with increasing
fines size, reached a minimum value, and then increased; these minimum
values are defined as CPGs, which exhibit a strong negative correlation
with fracture aperture. CPGs obtained from modeling were slightly
smaller than those determined from experiments, due to the assumptions
of homogeneous surfaces and spherical particles in the model. Finally,
the implications of this research on field-scale fines control in
coal were thoroughly discussed.
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