TY - JOUR A1 - Mardoukhi, Ahmad A1 - Mardoukhi, Yousof A1 - Hokka, Mikko A1 - Kuokkala, Veli-Tapani T1 - Effects of strain rate and surface cracks on the mechanical behaviour of Balmoral Red granite JF - Philosophical Transactions of the Royal Society of London, Series A : Mathematical, Physical and Engineering Sciences N2 - This work presents a systematic study on the effects of strain rate and surface cracks on the mechanical properties and behaviour of Balmoral Red granite. The tensile behaviour of the rock was studied at low and high strain rates using Brazilian disc samples. Heat shocks were used to produce samples with different amounts of surface cracks. The surface crack patterns were analysed using optical microscopy, and the complexity of the patterns was quantified by calculating the fractal dimensions of the patterns. The strength of the rock clearly drops as a function of increasing fractal dimensions in the studied strain rate range. However, the dynamic strength of the rock drops significantly faster than the quasi-static strength, and, because of this, also the strain rate sensitivity of the rock decreases with increasing fractal dimensions. This can be explained by the fracture behaviour and fragmentation during the dynamic loading, which is more strongly affected by the heat shock than the fragmentation at low strain rates. KW - split Hopkinson pressure bar KW - rock KW - granite KW - dynamic loading KW - fractal dimension KW - surface cracks Y1 - 2017 U6 - https://doi.org/10.1098/rsta.2016.0179 SN - 1364-503X SN - 1471-2962 VL - 375 IS - 2085 PB - Royal Society CY - London ER - TY - JOUR A1 - Hofmann, Hannes A1 - Zimmermann, Günter A1 - Farkas, Márton Pál A1 - Huenges, Ernst A1 - Zang, Arno A1 - Leonhardt, Maria A1 - Kwiatek, Grzegorz A1 - Martinez-Garzon, Patricia A1 - Bohnhoff, Marco A1 - Min, Ki-Bok A1 - Fokker, Peter A1 - Westaway, Rob A1 - Bethmann, Falko A1 - Meier, Peter A1 - Yoon, Kern Shin A1 - Choi, JaiWon A1 - Lee, Tae Jong A1 - Kim, Kwang Yeom T1 - First field application of cyclic soft stimulation at the Pohang Enhanced Geothermal System site in Korea JF - Geophysical journal international N2 - Large-magnitude fluid-injection induced seismic events are a potential risk for geothermal energy developments worldwide. One potential risk mitigation measure is the application of cyclic injection schemes. After validation at small (laboratory) and meso (mine) scale, the concept has now been applied for the first time at field scale at the Pohang Enhanced Geothermal System (EGS) site in Korea. From 7 August until 14 August 2017 a total of 1756 m(3) of surface water was injected into Pohang well PX-1 at flow rates between 1 and 10 l s(-1), with a maximum wellhead pressure (WHP) of 22.8 MPa, according to a site-specific cyclic soft stimulation schedule and traffic light system. A total of 52 induced microearthquakes were detected in real-time during and shortly after the injection, the largest of M-w 1.9. After that event a total of 1771 m(3) of water was produced back from the well over roughly 1 month, during which time no larger-magnitude seismic event was observed. The hydraulic data set exhibits pressure-dependent injectivity increase with fracture opening between 15 and 17 MPa WHP, but no significant permanent transmissivity increase was observed. The maximum magnitude of the induced seismicity during the stimulation period was below the target threshold of M-w 2.0 and additional knowledge about the stimulated reservoir was gained. Additionally, the technical feasibility of cyclic injection at field scale was evaluated. The major factors that limited the maximum earthquake magnitude are believed to be: limiting the injected net fluid volume, flowback after the occurrence of the largest induced seismic event, using a cyclic injection scheme, the application of a traffic light system, and including a priori information from previous investigations and operations in the treatment design. KW - Cyclic soft stimulation (CSS) KW - induced seismicity KW - risk mitigation KW - enhanced geothermal systems (EGS) KW - granite KW - Pohang (Korea) Y1 - 2019 U6 - https://doi.org/10.1093/gji/ggz058 SN - 0956-540X SN - 1365-246X VL - 217 IS - 2 SP - 926 EP - 949 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Mardoukhi, Ahmad A1 - Mardoukhi, Yousof A1 - Hokka, Mikko A1 - Kuokkala, Veli-Tapani T1 - Effects of test temperature and low temperature thermal cycling on the dynamic tensile strength of granitic rocks JF - Rock mechanics and rock engineering N2 - This paper presents an experimental procedure for the characterization of the granitic rocks on a Mars-like environment. To gain a better understanding of the drilling conditions on Mars, the dynamic tensile behavior of the two granitic rocks was studied using the Brazilian disc test and a Split Hopkinson Pressure Bar. The room temperature tests were performed on the specimens, which had gone through thermal cycling between room temperature and - 70 degrees C for 0, 10, 15, and 20 cycles. In addition, the high strain rate Brazilian disc tests were carried out on the samples without the thermal cyclic loading at test temperatures of - 30 degrees C, - 50 degrees C, and - 70 degrees C. Microscopy results show that the rocks with different microstructures respond differently to cyclic thermal loading. However, decreasing the test temperature leads to an increasing in the tensile strength of both studied rocks, and the softening of the rocks is observed for both rocks as the temperature reaches - 70 degrees C. This paper presents a quantitative assessment of the effects of the thermal cyclic loading and temperature on the mechanical behavior of studied rocks in the Mars-like environment. The results of this work will bring new insight into the mechanical response of rock material in extreme environments. KW - granite KW - dynamic loading KW - high strain rate KW - fractal dimension KW - low KW - temperature KW - split Hopkinson pressure bar Y1 - 2020 U6 - https://doi.org/10.1007/s00603-020-02253-6 SN - 0723-2632 SN - 1434-453X VL - 54 IS - 1 SP - 443 EP - 454 PB - Springer CY - Wien ER -