不同类型储层岩石三轴压缩变形破裂与声发射特征研究

INVESTIGATION OF TRIAXIAL COMPRESSION FAILURE AND ACOUSTIC EMISSION CHARACTERISTICS OF DIFFERENT RESERVOIR ROCKS

随着页岩气开采、废水回注和CO_(2)地质封存工程活动的进行,储层应力环境改变诱发地震的问题得到广泛关注,研究储层岩石变形破裂和声发射规律对于理解诱发地震活动具有重要意义。本文选取了页岩、致密砂岩和白云岩开展三轴压缩声发射试验,获取了空间裂缝形态,揭示了不同类型储层岩石的变形破裂过程和声发射特征。结果表明:(1)页岩的扩容应力与峰值应力比最高,其次为白云岩和致密砂岩,表明页岩主要发生脆性破坏。(2)岩石结构对裂缝扩展和强度具有明显影响,页岩层理发育,抗压强度和裂缝形态各向异性显著。当层理角度为0°时,试样发生剪切-拉张复合型破坏。当层理角度为30°和60°时,试样主要发生剪切破坏。当层理角度增大至90°时,试样主要发生拉张破坏。致密砂岩破裂形成剪切主裂缝,白云岩形成两条剪切主裂缝和微裂缝。(3)不同类型储层岩石破裂过程声发射特征差异显著。页岩在扩容应力点附近有少量声发射活动,达到峰值应力时,声发射活动迅速增强。致密砂岩仅在破裂瞬间有少量声发射活动。相比之下,白云岩在裂缝非稳定扩展阶段和峰后阶段声发射活动显著。因此,在工程实践中需要根据微震监测调整施工措施,避免页岩作为储盖层发生脆性破坏和白云岩储层改造诱发地震。

With the development of engineering activities such as exploitation of shale gas,waste water injection and carbon dioxide geological sequestration,the issue of induced seismicity due to the change of stress state of reservoir rocks has aroused wide attention,the investigation of failure process and acoustic emission characteristics is of great significance for the understanding of induced seismicity.A series of triaxial compression acoustic emission(AE)tests were conducted on shale,tight sandstone and dolomite,the spatial fracture geometry were obtained,and the failure process of different reservoir rocks were characterized by using acoustic emission monitoring method.The results suggested that:(1)the ratio of dilation stress to peak stress is highest for shale,followed by dolomite and tight sandstone,indicating that the breakdown of shale is dominated by brittle failure.(2)The rock fabric significantly affects fracture propagation and strength,shale typically has developed laminar structure,the compressive strength and failure types present strong anisotropy.Shear-tensile compound failure occurs and massive shear microfractures tend to form when bedding plane inclination is 0°.For shale samples with bedding plane inclinations of 30°and 60°,shear failure tends to occur during compression process.When bedding plane inclination increases to 90°,tensile fractures are likely to create.Tight sandstone sample produces a relative simple shear fracture,and dolomite sample develops two main shear fractures and multiple microfractures.(3)The acoustic emission characteristics of different reservoir rocks vary significantly during the failure process.The acoustic emission activities of shale samples begin to increase near the dilatation stress point,and the number and energy of acoustic emission events increase rapidly when the peak stress is reached.The tight sandstone sample only has a small amount of acoustic emission activity at the moment of breakdown.By comparison,the acoustic emission activities of dolomite samples are obvious in unstable fracture growth stage and post-peak stage,the amplitude,energy and number of events are relative large.Therefore,construction measures should be adjusted according to the microseismic monitoring results in practical engineering to avoid the brittle failure of shale when served as the cap rock and alleviate the induced earthquakes in the stimulation of dolomite formation.