11:30 AM - 11:45 AM
[MGI31-04] Reanalysis of in-situ crustal stress measurements in the Izu-Ohshima volcanic observation well
Keywords:in-situ crustal stress, hydraulic fracturing method, borehole, Isu-Ohshima, drilling mud induced vertical fracture, borehole breakout
Starting from 1996 to the end of 1999, a 1 km deep observation well was drilled inside the caldera of Izu-Oshima Volcano (1 km west from the summit of Mt. Mihara, approximately 180 m inside from the western edge of the caldera). Objectives of the drilling included structural exploration of the volcano, thermal interactions around the magma conduit, and the structure and formation history of the caldera and volcano. Inside the borehole, various observations, stress measurements, core collection, and physical loggings were conducted. Such data were reported on the subject of geological structure, stress distribution, physical properties of the volcano, etc. (Watanabe , Volcanological Society 1998, B21; Nakata et al., Volcanological Society 1998, B22; Watanabe, Joint Meeting 1999, Av-023; Nakata et al., Volcanological Society 1999, P44; Omura et al., Volcanological Society 2002, P20; Omura, Volcanological Society 2023, B3-09).
In this presentation, we focused on in-situ crustal stress measurement using the hydraulic fracturing method carried out inside the borehole. This is because recently conventional procedure of hydraulic fracturing method has been improved, and the Geotechnical Society of Japan's standard "Method for initial stress measurement by hydraulic fracturing technique (JGS 3761-2017)" has been established. This presentation is to apply the new standard method to the hydraulic fracturing data at that time. In-situ crustal stress is an important physical parameter that is thought to affect eruption styles such as volcanic fissure eruption. Understanding the crustal stress state accurately is important for volcanic eruption forecast and volcanic disaster prevention.
Hydraulic fracturing was carried out at five locations in a depth interval from 700 to 1000 m, where a homogeneous geological structure was recognized to be underwater sediment containing submarine fan-like turbidites, based on cuttings observations and geophysical loggings. Valid hydraulic fracturing data were obtained at four of these locations. The new standard revises the physical interpretation of crack reopening pressure in hydraulic fracturing. Under the conventional method, once a newly formed crack due to hydraulic fracturing was closed, the crack surfaces were considered to be in close contact. But, under the new standard, the closed crack surfaces are not completely adhered and there are minute irregularities (small fissures), and it is assumed that water has penetrated and pore water pressure is applied on the crack surfaces, just as before the crack was initially formed (Ito and Hayashi, 1994). Application of the reported hydraulic fracturing data at that time has, according to the new standard, showed contradictory results that the maximum horizontal compressive principal stress is smaller than the minimum horizontal compressive principal stress. It is highly likely that there was any incomplete interpretation of the hydraulic fracturing data, resulting in insufficient readings of crack reopening pressure and crack closing pressure. Therefore, we went back to the drawings of the hydraulic fracturing data, converted them into digital images, and obtained the time-pressure time series digital data values. Then, we attempted to read the crack reopening pressure and crack closing pressure again according to the new standards. The results will be presented on site.
Regarding the principal stress direction, in the borehole televiewer images, vertical cracks that occurred in the borehole wall during drilling (fracture cracks generated by mud water pressure as a result of the same principle as hydraulic fracturing, indicating the maximum horizontal compressive principal stress direction) and the borehole breakouts (groove-like depressions caused by stress concentration and compressive failure on the borehole wall, indicating the minimum horizontal compressive principal stress direction) are clearly recognized in several locations in the borehole. Those images indicate the horizontal maximum compressive principal stress direction is northwest-southeast, and it is confirmed that the stress direction is almost the same as the surrounding average tectonic stress field.
I would like to express my gratitude here to Prof. Hidefumi Watanabe (then, Earthquake Research Institute, University of Tokyo), Prof. Takao Ohminato (Earthquake Research Institute, University of Tokyo), and Dr. Yuichi Morita (National Research Institute for Earth Science and Disaster Resilience) for providing data related to hydraulic fracturing at the Izu-Ohshima volcanic observation well.
In this presentation, we focused on in-situ crustal stress measurement using the hydraulic fracturing method carried out inside the borehole. This is because recently conventional procedure of hydraulic fracturing method has been improved, and the Geotechnical Society of Japan's standard "Method for initial stress measurement by hydraulic fracturing technique (JGS 3761-2017)" has been established. This presentation is to apply the new standard method to the hydraulic fracturing data at that time. In-situ crustal stress is an important physical parameter that is thought to affect eruption styles such as volcanic fissure eruption. Understanding the crustal stress state accurately is important for volcanic eruption forecast and volcanic disaster prevention.
Hydraulic fracturing was carried out at five locations in a depth interval from 700 to 1000 m, where a homogeneous geological structure was recognized to be underwater sediment containing submarine fan-like turbidites, based on cuttings observations and geophysical loggings. Valid hydraulic fracturing data were obtained at four of these locations. The new standard revises the physical interpretation of crack reopening pressure in hydraulic fracturing. Under the conventional method, once a newly formed crack due to hydraulic fracturing was closed, the crack surfaces were considered to be in close contact. But, under the new standard, the closed crack surfaces are not completely adhered and there are minute irregularities (small fissures), and it is assumed that water has penetrated and pore water pressure is applied on the crack surfaces, just as before the crack was initially formed (Ito and Hayashi, 1994). Application of the reported hydraulic fracturing data at that time has, according to the new standard, showed contradictory results that the maximum horizontal compressive principal stress is smaller than the minimum horizontal compressive principal stress. It is highly likely that there was any incomplete interpretation of the hydraulic fracturing data, resulting in insufficient readings of crack reopening pressure and crack closing pressure. Therefore, we went back to the drawings of the hydraulic fracturing data, converted them into digital images, and obtained the time-pressure time series digital data values. Then, we attempted to read the crack reopening pressure and crack closing pressure again according to the new standards. The results will be presented on site.
Regarding the principal stress direction, in the borehole televiewer images, vertical cracks that occurred in the borehole wall during drilling (fracture cracks generated by mud water pressure as a result of the same principle as hydraulic fracturing, indicating the maximum horizontal compressive principal stress direction) and the borehole breakouts (groove-like depressions caused by stress concentration and compressive failure on the borehole wall, indicating the minimum horizontal compressive principal stress direction) are clearly recognized in several locations in the borehole. Those images indicate the horizontal maximum compressive principal stress direction is northwest-southeast, and it is confirmed that the stress direction is almost the same as the surrounding average tectonic stress field.
I would like to express my gratitude here to Prof. Hidefumi Watanabe (then, Earthquake Research Institute, University of Tokyo), Prof. Takao Ohminato (Earthquake Research Institute, University of Tokyo), and Dr. Yuichi Morita (National Research Institute for Earth Science and Disaster Resilience) for providing data related to hydraulic fracturing at the Izu-Ohshima volcanic observation well.