Abstract
In order to improve the producing technique of shale oil and gas, it is important to evaluate the in-situ stress states and the extent of hydraulically induced fractures in the rocks. In this study, a hydraulic fracturing test was conducted on Onnagawa formation to measure the in-situ stress state. Onnagawa formation mainly composes siliceous mudstone, and is the regional source rock and oil reservoir. In addition, Onnagawa formation is expected as a potential shale play. The vertical borehole of 78 mm diameter was drilled from the surface to 40 m deep for the test. The hydraulic fracturing test zones were selected based on the core samples and the images taken by a borehole camera. We conducted the tests at more than 10 zones and obtained reliable results at 2 zones. Both zones are carbonate concretion layers which are interbedding the siliceous mudstones. The stress measurement with high accuracy could be conducted by using the high stiffness hydraulic fracturing apparatus. In addition, diameter core deformation analysis (DCDA) was conducted on the core samples to check the principal stress direction. The hydraulic fracturing test results are as follows: the maximum stresses were from 0.99 to 1.13 MPa, and the minimum stress was 0.86 MPa, respectively. The maximum principal stress direction was N70W. In this test, it became clear that it was difficult to make artificial fractures in the siliceous mudstone layers due to water leaking through the pre-existing fractures. However, by using the sleeve fracturing test we conducted in the siliceous mudstone layers, the maximum principal stress direction in N50W was obtained. Moreover, by using DCDA the principal stress directions from N73W to N100W are also obtained in the siliceous mudstone and the carbonate concretion. The principal stress directions indicate similar trend with the directions (NWW-SEE) of the crustal strains in Akita based on the triangulation survey results.
