This study presents the use of a leak-off test (LOT) to estimate mudstone permeability, in which mudstone plays a role as cap-rock formation in a CO2 sequestration test site, offshore southeastern Korea. LOT is a borehole injection test conducted at an open-hole section below casing shoe mainly for assessing borehole/casing integrity and frac-gradient needed for further drilling. During LOT, pressure and volume of injected water are recorded as a function of time that allow us to estimate rock permeability at the open-hole section. In this study, we run LOT at 4 m open-hole section (test interval) below casing shoe (700 m below seafloor) within the cap-rock. Based on recorded pressure and volume of injected water at surface, we assume that the pressure at the open-hole section is equal to the surface pressure plus hydrostatic water column pressure, and that water volume flowing-out through the open-hole section is equal to injected volume minus pressure-induced volume expansion along the casing throughout the depth. Accordingly, we derive a new analytic equation that suits for the LOT configuration based on Darcy's law. The equation relates permeability to the slope of pressure-water volume, which aims at solving problems relevant to transient flow. Based on the derived equation, the mudstone permeability at borehole-scale is estimated to be at an order of ~10–17 m2, which is higher than that at core-scale. We speculate that relatively high permeability might be due to large-scale rock volume tested, which needs further investigation.

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