We monitored AE events accompanied with a failure of Berea sandstone specimen induced by pore pressure increase in a tri-axial test. After the test, several located AE events were compared with X-ray CT images obtained on the fractured specimen. The comparison elucidated that most of the sources are closed to or exactly on the locations of macroscopic fracture planes. From the stress and strain data, we also found that the failure can be explained and predicted by Mohr-Coulomb's criterion and the effective stress theory.
AE (Acoustic Emission) monitoring has been recently often applied to detect behavior of petroleum reservoirs. For example, at oil fields in Ekofisk and Valhall in North Sea and in Clinton County, Kentucky in U.S., AE events have been monitored with operation of EOR (enhanced oil recovery) . EOR is the technology to recover oil and gas from depleted reservoirs by injecting fluid such as water, natural gas, and carbon dioxide. Since AE events are most likely caused with increase of pore pressure by the injected fluid, the AE monitoring helps to understand how the injected fluid penetrates and permeates. In this research, we monitored AE events accompanied with a failure of Berea sandstone specimen induced by pore pressure increase in a tri-axial experiment. Moreover, we compared location of AE sources to fracture planes on X-ray CT (computed tomography) images obtained after the experiment.
In this experiment, we used a cylindrical specimen of Berea sandstone, measuring 38mm in diameter and 76mm in height, as shown in Figure 1. P-wave velocities of the specimen measured along X-, Y-, and Z-directions were 2.46, 2.48, 2.46 km/s, respectively. This indicates that the specimen is almost isotropic. Since a permeability test conducted on the specimen indicated a large permeability, 92.1md, the specimen was expected to be completely saturated in a few seconds after applying pore pressure in the tri-axial test. To monitor AE events, twelve cylindrical piezoelectric elements, 5 mm in diameter and 6.7 mm long, having resonance frequency of 300 kHz, were placed on the surface of the specimen. After above preparations, the specimen covered by silicon rubber with pedestals was set in a pressure cell, and tri-axial test was conducted. After the tri-axial test, X-ray CT images were obtained on the fractured specimen with a medical CT scanner (TOSHIBA Aquilion16). Through using the CT scanner, we can observe cracks inside of the specimen with leaving silicon rubber covering the specimen, only with removing piezoelectric elements and strain gauges that interrupt X-ray due to their high density.
Fig.2 shows change of axial pressure (σ1), confining pressure (σ3), pore pressure (P), and strains (ε1, ε3 and εvol) along the elapsed time in this experiment. The procedure of this experiment is shown as follows.
At the first step from the beginning to the point (1) in Fig.2, the specimen was set in a pressure cell, and then the axial pressure was applied slightly.