6:15 PM - 7:30 PM
[SCG59-P06] Measurements of elastic wave velocity of Aji granite on triaxial compression fracture test
Keywords:elastic wave velocity, geothermal fluid reservoir, hydraulic fracturing, dilatancy
We used Aji granite which is formed into cylindrical shape as a specimen and tried three measurement systems. First method is pulse reflection method which place piezoelectric device on the top of specimen. Second method is transmission method (σ1 direction) which place piezoelectric device on the top and bottom of specimen. Third method is transmission method (σ3 direction) which place piezoelectric device on the lateral face of specimen directly. We measured elastic wave velocity under the confining pressure 10 to 200MPa using these methods. In all methods, we could find increase of elastic wave velocity due to compaction through increasing confining pressure. However, in pulse reflection method and transmission method (σ1 direction), we could'nt calculate the velocity under low confining pressure in which porosity is high and in long specimen because of attenuation of pulse. So these two methods are not well-suited on the purpose of measurement in fracturing process which use specimen of 40mm long. On the other hand, in transmission method (σ3 direction), although we cannot reuse piezoelectric device because it is attached directly, it is possible to minimalize the pulse attenuation.
From the results mentioned above, We used the transmission method (σ3 direction) and measured elastic wave velocity of Aji granite in fracture process. This experiment is conducted under confining pressure 20MPa and displacement rate 0.01mm/min using loading system by servo control of inter-vessel deformation fluid-flow apparatus. Specimen is Aji granite which is prepared into a cylindrical shape, which diameter and lengths is 20mm and 40mm, and also which is processed to attach the piezoelectric device.
Increasing of elastic wave velocity was observed until about one-fifth of fracture stress, and then over one-third of it increasing shifted to decreasing. Firstly, increasing of the velocity means closure of micro-cracks in existence. Then gradually, the changes of the velocity start to decrease due to formation of new cracks in the specimen. This effect of formation of crack is more strengthen and the velocity decreases rapidly. Therefore it is possible to explain this decreasing of the velocity by dilatancy effect. Increasing of Vs is affected strongly by closure of cracks extended to σ3 direction because direction of vibration of S-wave is normal to maximum compressional axis in this experiment.