Laboratory Investigation on Brittle-Ductile Transition and Fracture Evolution in Onagawa Shale by Ultrasonic Wave Velocity and X-ray CT

概要

The fossil fuel dependency of Japan to suffice its energy requirement was about 87.4% in 2017, and Japan is now attempting to reduce it to about 56% by the year of 2030. Still, most of it is about to be imported, and under such circumstances, Onagawa tight oil and gas formation in Akita basin of northeast Japan could be considered as a potential source for economical oil and gas extraction. According to the report “Assessment of Undiscovered Oil and Gas Resources in the Akita Basin Province, Japan, 2018” published by United States Geological Survey there are about 100 million barrels of oil, and another undiscovered 111 million barrels of oil and nearly 2.4 billion cubic meters of natural gas as to be explored. However, these formations are can only be accessed by unconventional methods, that is by horizontal drilling and hydraulic fracturing.

　The most important factor in hydraulic fracturing is the brittleness of the rock, which is estimated by various formulas unique to each formation (Zhang et al., 2016; Mews et al., 2019). However, tight oil and gas formations are not always found in brittle environments. With increasing depth, and burial conditions the shale formations show ductile character (Yuan et al., 2017). Moreover, shales and mud rocks with high clay contents creep as ductile rocks (Kohli and Zoback, 2013; Sone and Zoback, 2013, 2014). Because, the Onagawa formation can be found at depths of about 3~4 km, and it is in a reverse fault regime (Okamura et al., 1995), the approximate overburden stress of about 100 MPa is the least principle stress. Therefore, these formations could easily run in to ductility principally due to their depth. On top of this depth effect, because of the various clay contents, time dependent creep deformation (viscoelastic creep) also lead these formations to isotropic stress conditions, hence to ductile behavior, either inhibiting the fracture propagation or resulting unexpected fracture formations. Apart from shale been found as these tight oil and gas formations, they are also found as caprock formations to reservoir rocks. Hence, it is necessary to investigate the fracture evolution in shale to understand the reservoir volume enhancement and cap rock integrity. As a result, this thesis is compiled of two main sections focusing the estimation of fracture evolution under increasing effective pressure and under increasing differential stress.