In Situ Permeability and Scale Dependence of an Active Accretionary Prism Determined From Cross‐Borehole Experiments

Abstract

Permeability controls fluid flow in the Earth's crust and affects a wide range of processes including advective transport and pore pressure generation. However, in situ measurements of permeability are few, especially in active tectonic settings or at scales relevant to regional flow. We analyze formation fluid pressure records from oceanic boreholes in the Nankai accretionary prism offshore southwest Japan, focusing on unexpected responses to drilling operations conducted at boreholes ~100 m to the northeast. We develop a 2‐D numerical model of transient fluid flow and conduct a parametric grid search to define hydraulic diffusivity. A value of 0.19–0.46 m2/s (corresponding to a permeability of 9.8 × 10−13 to 2.4 × 10−12 m2) yields the best fit to observed pressure responses. Together with laboratory measurements on core samples and drillstrem tests reported in previous studies, our analysis indicates a strong scale dependence of permeability, likely reflecting the presence of permeable faults and fractures.

In situ hydromechanical responses during well drilling recorded by distributed fiber-optic strain sensing

“…DSS could be also deployed in studying natural processes involving hydromechanical responses, such as at seismogenic structures (e.g., faults) related to earthquake occurrences (Kinoshita and Saffer, 2018) (Guglielmi et al, 2020).…”

In situ hydromechanical responses during well drilling recorded by fiber-optic distributed strain sensing

“…However, if the pulse is shortened, the light pulse energy and thus the signal intensity of the backscatters are lowered, and the measurement accuracy becomes low at positions distant from the input end. For overcoming the limitations of conventional COTDRs, in the new tunable wavelength coherent optical time-domain reflectometer (TW-COTDR) method, the tunable wavelength distributed feedback laser and chirp signals by frequency sweeping and modulation methods are used to shorten laser light pulses while simultaneously ensuring sufficient pulse intensity (Kishida et al, 2014;Koyamada et al, 2009). To enhance the intensity of chirped signals and suppress the range side lobe, Gaussian amplitude modulation is performed.…”

“…The value was obtained from a prior calibration measurement, which was conducted using the tensile tester with a displacement gauge. We used an optical interrogator NBX-SR7000 (Neubrex Co., Ltd., Japan) with TW-COTDR function in this study (Kishida et al, 2014). The instruments can provide high measurement accuracy (0.5 µε) and spatial resolution (5 cm), allowing for the monitoring of very small strains over long distances (∼ 25 km) in a distributed manner.…”

Multidisciplinary database of permeability of fault zones and surrounding protolith rocks at world-wide sites

“…The scale of permeability tests is important to differentiate, because the rock matrix permeability can be 2 to 4 orders of magnitude smaller than the bulk permeability of the same rock that includes the macroscopic fracture networks in low-porosity brittle rocks 86,87 . In high-porosity weak rocks the fractures add some permeability component 88 , but the fluid-induced transient fracture dilation effects are also important. The relative effect of macroscopic fractures become more important in rocks of lower porosity and larger density 89 .…”