非对称载荷下各向异性砂岩半圆弯曲断裂特性的离散元研究

Granular discrete element research on semi-circular bending fracture characteristics of anisotropic sandstone under asymmetric loading

储集层资源往往赋存于各向异性的节理岩体中,其断裂模式较为复杂,仅根据以往的工程经验难以进行高效开采。为了更好地认识各向异性储层的断裂特征,并实现对裂缝扩展形态控制和预测,从而更为科学地对储层进行采集,利用离散元方法,开展非对称载荷下各向异性砂岩半圆弯曲断裂特性的研究,通过调整底部支座的不对称度,可以实现岩体从纯Ⅰ型到纯Ⅱ型断裂的控制。主要结论如下:(1)半圆弯曲数值试样的峰值荷载表现出明显的各向异性,随节理角度增长,主要呈现出"W"和"增大–平稳–增大"的分布形式。随非对称系数增加,主要呈现出"急降–平稳"的趋势。(2)对于破坏模式,试样破坏模式随切缝倾角和节理分布方式不同,主要表现为偏移式、顺层式、切层–顺层式和复合型切层式4种破坏形式。随着非对称系数增加,试样内应力场发生改变,试样破裂路径逐渐沿支座移动方向发生偏离。(3)获取基于J积分计算求解得出的无量纲几何因子,并基于此设计了纯Ⅱ型、纯Ⅰ型和复合型的加载方案。由几何因子求解所得试样断裂韧度随节理变化呈现出明显的各向异性,而随非对称系数增加,主要呈现出"急降–平稳"的趋势。(4)受节理的影响,Ⅰ型断裂起裂角分布在0°~36.9°范围,Ⅱ型断裂起裂角分布在47.68°~76.12°范围,并非与理论值完全相同。

Reservoir resources often occur in anisotropic jointed rock mass and fracture mode of anisotropic reservoirs is relatively complex,which make it difficult to carry out efficient exploitation only based on engineering experiences. To better understand the fracture characteristics of anisotropic reservoirs,to realize the control and prediction of fracture expansion morphology,and hence,to collect reservoirs more scientifically,this paper uses the discrete element method(DEM) to study fracture characteristics of the anisotropic sandstone by semi-circular bending test under asymmetric loading. By adjusting the asymmetric degree of the bottom bearing, the control of rock mass fracture from pure Ⅰ mode to pure Ⅱ mode can be achieved. The main results are as follows:(1) The peak load of the semi-circular bending samples shows obvious anisotropy,showing distribution forms of "W" and "increasing-stable-increasing" with increasing the joint angle,and a trend of "precipitous drop-stable" with increasing the asymmetric coefficient.(2) SCB samples with different notch angles and joint distributions show different failure modes mainly including offset type,bedding type,cut-bedding type and composite bedding type. With increasing the asymmetric coefficient,the stress field in the samples changes,making fracture path of the samples deviate along the movement direction of the bearing.(3) The fracture toughness of the samples obtained by solving the geometric factor,which is obtained based on the calculation of J-integral and by which 3 kinds of asymmetric loading ways of pure Ⅱ mode,pure Ⅰ mode and compound mode are designed,shows obvious anisotropy with the change of the joint angle and presents a trend of "precipitous drop and stable" with increasing the asymmetric coefficient.(4) Impacted by the joints,the initial crack angles of pure Ⅰ mode and pure Ⅱ mode are respectively distributed in the range of 0°–36.9°and 47.68°–76.12°,which are not exactly the same as the theoretical values.