不同沉积年代泥岩卸荷力学特性试验研究

Experimental study of unloading mechanical behaviour of mudstone in different geological era

鄂西渝东地区蕴藏着丰富的油气资源,纵向主要分布有侏罗系和志留系两套泥岩盖层,区域经历的抬升剥蚀过程会导致盖层破裂,影响油气保存。选取沉积于侏罗系沙溪庙组、桐竹园组以及志留系龙马溪组的3个典型区块泥岩,开展10~50MPa 5种围压下的三轴卸荷力学试验,探索卸荷对泥岩力学特性影响。研究表明:随卸荷围压增大,3类泥岩的卸荷效应均增强,侏罗系泥岩体现在卸荷变形增大,志留系泥岩主要体现在沿卸荷方向的侧向膨胀;对比常规三轴压缩试验中岩石破裂时的应力状态,卸荷得到的泥岩强度均上升,内摩擦角更大,泥岩侧压破裂系数呈负指数分布,随围压增大而减小,并且沉积时间短浅的泥岩在抬升剥蚀过程中更容易产生破坏;岩石破裂形态由岩石组构与应力环境共同决定,沙溪庙组泥岩的裂纹形态为单一剪切断面破坏,并且断口平缓,而桐竹园组泥岩与龙马溪组泥岩则随围压升高,由单一剪切断面向共轭剪切破坏过渡,其中桐竹园组泥岩断口平滑,龙马溪组泥岩断口则呈现锯齿状。

Western Hubei-Eastern Chongqing region is rich in oil and gas resources, and the cap rocks are Jurassic mudstone and Silurian mudstone from top to bottom. However, this region has been suffered long term uplift and erosion, which may result in cap rock generating fractures. To investigate mechanical properties of mudstones during geological evolution, three typical tectonic mudstones deposited in the Jurassic Shaximiao formation, Tongzhuyuan formation and Silurian Longmaixi group, are selected to conduct triaxial unloading tests under five different confining pressures from 10 to 50 MPa. The results show that the unloading effect of these three mudstones is enhanced with the increase of unloading confining pressure, and the Jurassic mudstone increases in unloading deformation, the Silurian mudstone mainly exhibits the lateral expansion along the unloading direction. In comparison with the stress state of rock failure in the normal triaxial compression test, under unloading condition, the unloading strength of the mudstone and the internal frictional angle are increased. The mudstone lateral fracture coefficient is negative exponential distribution and decreases with the increase of the confining pressure. The mudstone with short sedimentary history is more prone to damage during the uplift and denudation process. The rock fracture pattern is governed by the rock structure and the stress environment. The mudstone of Shaximiao formation presents a single shear section failure, and the fracture is gentle, while the fractures of the mudstones of Tongzhuyuan formation and Longmaxi formation transit from a single shear cross section to the conjugate shear with increasing the confining pressure. The fracture of Tongzhuyuan formation mudstone is smooth, but the fracture of Longmaxi formation mudstone is jagged.