Abstract
SUMMARY We present a new type of transducer capable of measuring local pore fluid pressure in jacketed rock samples under elevated confining pressure conditions. The transducers are passive (strain-gauge based), of small size (7 mm in diameter at the contact with the rock and around 10 mm in length), and have minimal dead volume (a few mm3). The transducers measure the differential pressure between the confining fluid and the internal pore pressure. The design is easily adaptable to tune the sensitivity and working pressure range up to several hundred megapascals. An array of four such transducers was tested during hydrostatic pressurization cycles on Darley Dale sandstone and Westerly granite. The prototypes show very good linearity up to 80 MPa with maximum deviations of the order of 0.25 MPa, regardless of the combination of pore and confining pressure. Multiple internal pore pressure measurements allow us to quantify the local decrease in permeability associated with faulting in Darley Dale sandstone, and also prove useful in tracking the development of pore pressure fronts during transient flow in low permeability Westerly granite.
Highlights
Pore fluid pressure is a key physical variable exerting first order controls on properties such as strength, transport properties, and seismic wave speeds in rocks
Since the early 1960s, rock deformation experiments have commonly been conducted with concomitant measurements or control of pore pressure, typically using external servo-controlled pumps connected to the rock samples’ pore space by some length of high pressure tubing
When a constant fluid pressure is imposed at the ends of a specimen, the pore pressure remains homogeneous throughout the specimen provided that the pore pressure diffusion rate in the sample is much larger than the rate of pore pressure change due to pore space dilation/compaction
Summary
Pore fluid pressure is a key physical variable exerting first order controls on properties such as strength, transport properties, and seismic wave speeds in rocks. When a constant fluid pressure is imposed at the ends of a specimen, the pore pressure remains homogeneous throughout the specimen provided that the pore pressure diffusion rate in the sample is much larger than the rate of pore pressure change due to pore space dilation/compaction By contrast, when pore space deformation is too rapid compared to fluid flow rate, pore pressure is expected to vary within the rock, and the externally imposed pressure is no longer representative of the internal state of the sample. Such ‘partially drained’ conditions are typically avoided in laboratory experiments since the pore pressure is unknown and nonuniform within the specimen. Recent laboratory experiments using specific sample arrangements have shown that local pressure measurements provide key information on the dynamics of failure and fault slip (Brantut 2020; Proctor et al 2020) in partially drained conditions
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