不同围压下致密砂岩破裂过程声发射特征研究

摘要

Revealing the fracture mechanism of tight sandstone has important guiding significance for the fracturing design of tight oil and gas reservoir and the reconstruction of fracture network.This paper carries out the conventional triaxial compression tests of tight sandstone at different confining pressures.The rock samples are obtained from the Chang-6 reservoir of Yanchang Formation of the Ordos Basin.The influence of confining pressure on the mechanical properties of rock is analyzed.The fracture process of sample is studied by acoustic emission localization technique.The characteristics of acoustic emission location events and the space-time evolution of acoustic emission signals under different confining pressures are analyzed.In addition,CT scan is performed on the sample after rupture.The internal fracture characteristics of samples are observed based on CT slice images.The conclusions include the following.(1 )The space-time evolution of acoustic emission is mainly expressed in the compaction stage under different confining pressures.With the increase of confining pressure,main acoustic emission events are postponed.(2)The effect of confining pressure on accumulative count is mainly manifested in the compaction stage.The accumulative count shows a similar trend on the other stage.Under different confining pressures,the moment when accumulative count increases rapidly can be as a sign of rock fracture.(3)With the increase of confining pressure,the fractured phenomena tends to simple.The complex phenomena dominated by tensile fracture gradually change into a single shear fracture.(4)CT scanning section directly reflects the fracture configuration of the sample.The whole fracture configuration of the sample is consistent with the acoustic emission location.Based on the acoustic emission location,the crack distribution and the interaction process of the crack are further described.The fracture process of sample is studied with acoustic emission technique and CT scanning technique,which is significant for investigating rock mass failure and instability mechanism.%揭示致密砂岩的破裂机制对致密油气储层压裂设计和压裂缝网改造具有重要的指导意义.本文采用鄂尔多斯盆地延长组长6储层致密砂岩试样,开展了不同围压下的常规三轴压缩试验,分析了围压对岩石力学性质的影响.采用声发射定位技术研究了试样破裂过程,分析了不同围压下声发射定位事件的信号特征及其时空演化序列.此外,对破裂后试样进行了CT扫描,基于CT切片图像观测了试样内部破裂特征.得到以下几点认识:(1)不同围压下,声发射时空演化差异主要表现在压密阶段.随着围压的增加,声发射事件主要发生时段后移.(2)围压对声发射特征参数累计振铃计数的影响主要表现在压密阶段,其他阶段累计振铃计数呈相似变化趋势.不同围压下均可将累计振铃计数快速增加的瞬间作为岩石即将破裂的标志.(3)随着围压的增加,岩石破裂形态趋于简单化,由拉张破裂为主的复杂形态逐渐转变为单一的剪切破裂形态.(4)CT扫描切片直观地反映了试样的破裂形态,与声发射定位所得试样整体破裂形态相吻合,并且在声发射定位的基础上进一步刻画了裂纹分布情况与各裂纹相互作用过程.采用声发射定位技术与CT扫描双重方法,研究试样破裂过程,对于深入研究岩石破裂机制具有一定的意义.