真三轴条件下砾岩力学特性试验

Experiment on mechanical properties of conglomerate rocks under true triaxial loading

塔里木盆地库车山前构造带油气资源丰富,开发潜力巨大,地质条件复杂且发育有巨厚砾石层,砾石体积分数高、强度大、分布范围广,导致钻遇岩层非均质性强、地应力高,极大地提高了钻井难度,降低了破岩效率,严重制约了开发进程。为研究不同载荷状态下砾岩的强度及破坏特征,采用砾岩露头开展真三轴试验,并对试样进行CT扫描。结果表明:三向载荷作用下砾岩的破坏过程及其强度特征受水平主应力影响显著,主要表现为脆性破坏,强度随水平主应力呈线性增长,而真三轴弹性模量受水平主应力影响较小,且其平均值小于单轴弹性模量;受砾岩非均质性和组分强度差异影响,基质的变形导致砾石颗粒承受力矩并形成剪切破坏和裂纹绕砾发展,最终导致砾岩的宏观破坏。

Kuqa region in Tarim Basin China has a great potential for oil and development.However,it has multiple thick layers of conglomerate rocks with high strength and hardness,posing a great challenge to well drilling.In this study,in order to analyze the strength and failure mechanisms of the conglomerate rocks under different loading states,a true triaxial experiment was carried out using outcrop rock samples of the conglomeratein the region.A CT technique was used to scan the rock specimens in the triaxial experiments to reveal the failure process of the rock.The experimental results show that the failure process and strength characteristics of conglomerate rock under true triaxial loadings are significantly affected by horizontal principal stress with the rock being subjected mainly to brittle failure.The strength of the rock increases linearly with horizontal principal stress,while its elastic modulus,lower than the uniaxial elastic modulus,is less affected by the horizontal principal stress.Because of the heterogeneity and various conglomerate components(i.e.gravels) with different strength,the deformation of the matrix rock can lead to different loadings of forces on the gravel particles,which can induce shear failure and cracks around the gravels,eventually leading to the macro failure of the conglomerate rock.