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
【Abstract】 With rapid development of carbon dioxide geological storage,exploitation of deep geothermal energy and shale gas,construction of underground oil/gas reservoirs,and enhancedoil/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 ~1km 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 3DX-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 reaches280 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 bandand 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.
【Key words】 Triaxial compression； Dolomite； Shale； Acoustic emission； Fluid injection； Induced seismicity；
- 【文献出处】 地球物理学报 ,Chinese Journal of Geophysics , 编辑部邮箱 ,2015年03期