Characteristics of acoustic emission during deformation and failure of typical reservoir rocks under triaxial compression: An example of Sinian dolomite and shale in the Sichuan Basin
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摘要: 随着二氧化碳地质封存、深部地热开采、地下储气库建设、页岩气开发、二次驱油/驱气等工业应用的快速发展,与地下流体注入有关的诱发地震活动呈现一定的增加趋势.利用声发射实验观测油气田典型岩石在三轴压缩条件下变形破坏过程与声发射活动特征,对研究注水诱发地震过程有着重要意义.本文利用四川盆地现场采集的震旦系白云岩及页岩,采用实验室声发射技术观测研究岩石三轴压缩变形破坏过程中地震波速度等物性参数及声发射事件时空分布特征.实验结果表明:震旦系白云岩及页岩在变形破坏过程中均有一定的声发射活动.根据声发射定位结果,声发射主要集中在破坏前后的较短时间内,页岩的层理面为结构弱面,控制最终破坏面的形态及声发射特征.根据应力-应变结果,白云岩在压缩的后期阶段有一定的扩容现象,但页岩在整个压缩阶段均没有明显扩容现象.研究结果表明四川盆地较古老的白云岩及页岩具有脆性破坏特征,地下流体注入容易诱发微震活动,形成裂缝,有利于页岩气压裂开采.微震活动有利于监测裂缝的发生发展,但同时在页岩气开发及二氧化碳地质封存时应采取相应的预防控制措施进行安全合理的储盖层管理,避免灾害性诱发地震的发生.Abstract: With rapid development of carbon dioxide geological storage, exploitation of deep geothermal energy and shale gas, construction of underground oil/gas reservoirs, and enhanced oil/gas recovery, the underground-fluid-injection (UFI) induced seismicity is widely observed. Such seismicity depends on the injection pressure, lithology of reservoir and trap, tectonic stress, and development of cracks. Research of the role of these factors on different scales is crucial to understand of the occurrence of UFI induced earthquakes. Investigation of mechanical properties of reservoir rocks under hydraulic conditions and triaxial compression from laboratory experiments can improve our understanding of the mechanism of UFI induced seismicity. The dolomite and shale are typical sedimentary rocks in the Sichuan Basin, China, which is known as a representative area with UFI induced seismicity in China. In order to make clear the geomechanical conditions of UFI induced seismicity, this paper presents some results of a triaxial compression laboratory study of the process of deformation and failure and the characteristics of acoustic emission (AE) in Sinian dolomite and shale collected from the Sichuan Basin.The experiments were performed in a high pressure vessel of a loading system. First, the confining pressure was increased slowly to 10 MPa, which is equal to the stress conditions existing at depth ~1 km in the subsurface. Second, when the confining pressure was kept constant, the axial stress was increased at a constant rate of 2 MPa/min until the specimen fractured. The AE events were monitored by a high-speed multi-channel waveform recording system. In addition, the ultrasonic transmission measurement was periodically applied to monitor the P velocity and amplitude change induced by the confining pressure and axial stress during the process of loading. Finally, a 3D X-ray CT scanning was carried out for the broken specimen to reconstruct the fault geometry and the heterogeneous structure. The combination of strain-stress data and associated AE is used to discuss the mechanical performance of the specimens.The experimental results can be summarized as follows: (1) The P velocities of both dolomite and shale increase in the initial loading stage. When the axial loading exceeds 30 MPa, the P velocities of two samples keep constant. It indicates that there is little micro-crack in both two samples. (2) Both dolomite and shale demonstrate brittle behavior. (3) The strength of dolomite is 324 MPa. The sample is compressed in the axial direction and dilated in the circumferential direction under the initial axial loading. However, when the axial stress reaches 280 MPa, the volumetric strain is dominated by the circumferential strain, contributing to the increase of the sample volume, which is known as rock dilatancy. (4) During the experiment of dolomite, there are two significant stress drops. The main fracture forms as soon as the second stress drop occurs. According to the location of AE hypocenters, there are two fracture planes which are located in the lower part of the sample. In addition, there are a lot of foreshocks prior to the first stress drop. (5) The strength of shale is 197 MPa. During the entire loading stage, the axial strain plays a dominant role in the volumetric strain, leading to the decrease of the sample volume. (6) During the experiment of shale, there are several stress drops. The foreshocks of shale are relative lower than those of dolomite. The location of AE hypocenters is along the foliation, indicating the dominate role of foliation on the mechanical performance of shale. (7) A great amount of AE events happen during the dynamic fracturing and aftershocks take place in both rock samples. (8) The foliation orientation with respect to the direction of the principal stress is 15°, which is considered to be the most favorable angle. Such oriented foliation plays a key role in determining the peak strength, the precursory behavior preceding the final fracture, the spatiotemporal distribution of AE events and the geometry of the faulting plane. (9) The comparison between the AE distribution and X-ray CT scan images shows an excellent agreement between the location of AE hypocenters and the position of the macroscopic shear band and the final narrow fault.This paper describes the rock fracture tests of dolomite and shale from the Sichuan Basin, China under a triaxial compression. The experimental results indicate that the brittle failure behavior of both dolomite and shale is the key factor for understanding the extremely high level of UFI induced seismicity and hydraulic fracturing for shale gas exploitation. The activities of seismicity themselves have a positive effect on monitoring of the initiation and growth of cracks. In addition, the formation of the fractures can enhance the oil/gas recovery and improve the sequestration potential of carbon dioxide. At the same time, an advanced injection management plan is very required for the aforementioned geoengineering to avoid damaging events.
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Key words:
- Triaxial compression /
- Dolomite /
- Shale /
- Acoustic emission /
- Fluid injection /
- Induced seismicity
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