模拟地层条件下泥岩三轴应力实验及其油气意义

摘要

泥岩的岩石力学性质既关系到了它能否成为良好的盖层，也关系到它能否成为优质的页岩气储层。为了明确地层条件下泥岩岩石力学性质的变化特征及其影响因素，选取四川盆地南部典型的志留系龙马溪组泥岩，进行了20（常温），50，100，130℃四个温度点，每个温度点在0，10，20，40，60 MPa五种围压条件下的20个三轴压缩实验，获得了泥岩岩石力学参数与温度和围压的关系。实验表明：在60 MPa围压范围内，泥岩的弹性模量、泊松比、抗压强度和残余强度随着围压的增加而增加，基本呈线性关系；在130℃温度范围内，泥岩的弹性模量、泊松比、抗压强度的变化与温度变化没有明显的相关性，不同温度下，同一围压条件下，泥岩的岩石力学参数保持相对一致。这说明泥岩的岩石力学性质主要受埋藏深度（围压）的影响，温度变化对其影响不大。从岩石的应力－应变曲线上还可以看出，随着埋深的增加，即使是晚成岩阶段的泥岩也有从脆性向塑性转变的趋势。根据岩石峰值抗压强度和残余抗压强度的数学关系，计算出川南志留系龙马溪组泥岩脆延转变的临界深度约为4200～4400 m。作为盖层时，超过该临界深度，泥岩不容易破裂，具有优越的封闭性；作为页岩气储层，超过该临界深度，泥岩的可压性变差。%Mechanical properties of mudstone largely determines both their quality as cap rocks and the quality as shale gas reservoirs.In this study,the typical mudstone in the Silurian Longmaxi Formation in southern Sichuan Basin was se-lected for triaxial mechanical tests,so as to understand the variations of mechanical properties of the mudstone and their affecting factors.A total of 20 sets of experiments were carried out at confining pressures of 0,10,20,40,and 60 MPa, and at temperatures of 20,50,100,and 130℃,respectively.The relationships between the mechanical properties of mud-stone,temperature,and confining pressure were established.The results showed that there is basically a positive linear re-lationship between the confining pressure and elastic modulus,Poisson’ s ratio,compressive strength,and residual strength of the mudstone within the confining pressure range of less than 60 MPa.There is no clear correlation between tempera-ture and those mechanical properties of mudstone within the temperature range of lower than 130℃.However,those me-chanical parameters of mudstone remained relatively consistent under the same confining pressure and different tempera-tures,indicating that burial depth ( confining pressure) ,instead of temperature,determines the mechanical parameters of mudstone.The stress-strain curves of mudstone also indicated that even the mudstone at the late diagenetic stage tends to transit from brittle to ductile with the increase of buried depth.According to the relationship between the peak and the re-sidual compressive strengths,the critical burial depth for brittle-ductile transition of the Silurian mudstone in Sichuan Ba-sin is determined approximately at 4 200-4 400 m.Beyond this critical depth,the mudstone is less prone to fracture and thus has greater sealing ability as cap rocks,while has poor fracability as shale gas reservoirs.