Abstract
We used a ring-shear apparatus to examine the perpendicular permeability of sands with different mineral compositions to evaluate fault behavior around gas hydrate reservoirs. The effect of effective normal stress on the permeability of two sand types was investigated under constant effective normal stresses of 0.5–8.0 MPa. Although Toyoura sand and silica sand No. 7 mainly comprise quartz, silica sand No. 7 contains small amounts of feldspar. For Toyoura sand, the permeability after ring-shearing dramatically decreased with increasing effective normal stress up to 3.0 MPa, then gradually decreased for stresses over 3.0 MPa, whereas the permeability after ring-shearing of silica sand No. 7 rapidly decreased with increasing effective normal stress up to 2.0 MPa. Although the relationships between the permeability after ring-shearing and effective normal stress for both sands could be expressed by exponential equations up to 3.0 MPa, a more gradual change in slope was shown for Toyoura sand. The permeabilities of both sands were almost equal for effective normal stresses over 3.0 MPa. The mineralogical properties of the small amount of feldspar in the sample indicate that both mineralogy and original grain size distribution affect the fault permeability and shear zone formation.
Highlights
Gas hydrates in sediments are expected to be developed as a next-generation energy resource, which would affect the future development of agriculture, construction, industry, and human life.In the exploitation of methane hydrates, the production methods of methane gas from methane hydrate layers by depressurisation, thermal stimulation, and inhibiter injection have been proposed [1,2,3,4].For all methods, the gas and water permeability of methane hydrate-bearing sediments are important factors in estimating the efficiency of methane gas production
Several studies used a ring-shear apparatus with large-displacement shearing to measure permeability evolution [11,12,13,14,15,16], because a ring-shear device can shear a specimen with a large displacement that is similar to natural faults with the same direction of movement for the mobile half of the specimen relative to the stationary half [17,18,19,20,21,22,23,24]
Where q is the flow rate of the pore fluid, μ is the viscosity of the pore fluid, L is the length of the water flow (m), DP is the difference in pressure after initial compaction (Pa), and A is the cross-sectional area of the specimen (m2)
Summary
Gas hydrates in sediments are expected to be developed as a next-generation energy resource, which would affect the future development of agriculture, construction, industry, and human life.In the exploitation of methane hydrates, the production methods of methane gas from methane hydrate layers by depressurisation, thermal stimulation, and inhibiter injection have been proposed [1,2,3,4].For all methods, the gas and water permeability of methane hydrate-bearing sediments are important factors in estimating the efficiency of methane gas production. Previous studies found that permeability evolution due to particle crushing is related to porosity reduction [11,12,13,14,15,16]. These studies have not completely clarified the relationship between permeability and effective normal stress after large-displacement ring-shearing under overburden pressure, which is less than approximately 10 MPa, in methane hydrate reservoir zones. It is not obvious to research the influence of similar grain size and the sand mineralogy on fault permeability under the normal stress conditions of methane hydrate reservoirs
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