Estimating hydraulic conductivity using extended leak-off test conducted during drilling large-diameter borehole

Extended leak-off test (XLOT) is one of the in-situ tests, routinely conducted to evaluate integrity of the cased and cemented wellbores during deep borehole drilling, as well as in situ hydraulic properties at a casing shoe depth.

We introduce results of the XLOT conducted in a large diameter borehole, which is drilled for installation of deep borehole based geophysical monitoring system to monitor micro-earthquakes and fault behavior of major linearments in the subsurface. The borehole was planned to secure a final diameter of 200 mm (or more) at a depth of ~1 km deep, with 12" diameter wellbore to intermediate depths, and 7-7/8" (~200 mm) to the bottom hole depths.

We drilled first the 12" diameter borehole to approximately 504 m deep and installed API standard 8-5/8" casing, then cemented the annulus between the casing and bedrock. Then we carried out the XLOT, for several purposes such as confirming casing and cementing integrity, as well as estimating in-situ stress and hydraulic conductivity at the casing shoe depth. To that end, we drilled 4 m length interval to directly inject water and pressurize into the rock mass using the upper API casings. During the XLOT, we recorded flow rates and interval pressures in real time. Based on the logs, we tried to analyze hydraulic conductivity of the test interval, and compare the results with previously reported hydraulic properties measured in other ways.