基于数字岩心的页岩储层岩石细观损伤机制研究

Study on meso-damage mechanism of shale reservoir rock based on digital cores

岩石损伤对储层的力学性能及渗流特性等具有显著影响。为研究页岩储层岩石的细观损伤机制,基于CT扫描试验重构三维数字岩心,基于单轴压缩试验及巴西劈裂试验获取岩石力学参数,结合岩石塑性损伤本构模型开展了单轴压缩数值模拟。结果表明:(1)页岩受压时,损伤从孔隙密集区开始发展,后损伤值升高,损伤范围扩展,直到各损伤区贯通,形成剪切斜裂缝。(2)压缩损伤和拉伸损伤存在不同的应力阈值,页岩受压时,压应力先达到压缩损伤的应力阈值,压缩损伤最先产生;拉伸损伤在压缩损伤较大处产生,且表现出滞后性,其损伤体积小于压缩损伤。(3)研究区页岩的孔隙等效直径分布范围为0~200μm,其中,等效直径为10~40μm的孔隙对孔隙度的贡献最大。随着孔隙度的增加,峰后强度的下降速度变缓,岩石延性逐渐增大。(4)孔隙空间分布对页岩的损伤扩展形式及破坏形式存在极大影响,引入分形维数表征孔隙空间分布的复杂程度,发现孔隙的分形维数仅在孔隙度较大时才影响岩石的峰值强度,相同孔隙度下,分形维数越大,峰值强度越低;分形维数对岩石弹性模量的影响极小;孔隙度和分形维数对岩石的损伤体积均存在较大影响,孔隙度和分形维数越大,损伤体积越大。

Rock damage has a significant impact on the mechanical properties and seepage characteristics of reservoirs.In order to study the meso-damage mechanism of reservoir shale,CT scanning test was carried out to reconstruct three-dimensional digital cores.Subsequently,rock mechanical parameters were obtained based on the uniaxial compression test and the Brazilian test,and the uniaxial compression numerical simulation was carried out by using the rock plastic damage constitutive model.The results show that:(1)When the shale is compressed,the damage starts from the pore dense area,then the damage value increases,and finally the damage range expands until each damage area penetrates,forming shear oblique cracks.(2)There are different stress thresholds for the compression damage and the tensile damage.When the shale is compressed,the compressive stress first reaches the stress threshold of the compression damage and the compression damage occurs at once.The tensile damage,showing hysteresis,occurs where the compression damage is larger,and its damage volume is smaller than that of the compression damage.(3)The distribution range of the pore equivalent diameter of shale is 0–200μm,and the pores with an equivalent diameter of 10–40μm contribute the most to porosity.As the porosity increases,the decrease rate of the post-peak strength decreases and the ductility of the rock gradually increases.(4)The spatial distribution of pores has a great influence on the damage propagation form and the failure form of shale.Fractal dimension is introduced to characterize the complexity of the pore structure.The fractal dimension affects the peak strength of rock only when the porosity is large.Under the same porosity,the larger the fractal dimension,the lower the peak strength.The fractal dimension has little influence on the elastic modulus of rock.Both the porosity and the fractal dimension have a great influence on the damage volume of rock.The larger the porosity and the fractal dimension,the larger the damage volume.