岩石破坏后力学特性及其对天然气开采的影响

Mechanical property of rock post-failure and its influence on natural gas exploitation

地层在失去应力平衡后,岩石将发生破坏,而岩石的岩性差异导致破坏后的力学特性也相差较大。岩石在峰值强度后将产生走向各异的裂缝;若岩石在破坏后产生大量岩粉,将堵塞地层中的喉道和裂缝,导致储层的渗流能力降低。选用四川盆地4个区块的4类岩性的多个岩样进行三轴压缩实验,根据岩样峰值强度后的破坏模式定义了3种类型:Ⅰ型岩样破坏后将产生拉伸裂缝,并且应力突降;Ⅱ型岩样破坏后将产生剪切裂缝,应力缓慢降低;Ⅲ型岩样破坏后将产生拉伸与剪切裂缝,应力往复变化。Ⅰ型与Ⅱ型岩样在破坏后形成的裂缝较为单一,且裂缝面光滑;Ⅲ型岩样在破坏后形成走向各异的裂缝,且产生大量的岩粉,影响天然气开采。针对Ⅲ型岩样破坏后产生大量的岩粉,以煤岩为例,提出采用超前预防处理以及在排采期进行治理的措施。

Rock breaking will occur when the formation loses stress balance. Lithologic difference of rock will generate comparatively large difference of mechanical property. Different strike of cracks will produce after peak strength of rock; if rock breaking produces powdery material, the pore and fracture will be blocked and the seepage capacity of reservoir will decrease. The author chose multiple samples of four types with different lithology from four strata in Sichuan Basin to have tri-axial mechanics test,three types of failure modes after its peak strength are defined： type Ⅰ sample will generate tensile fracture and stress drops suddenly; type Ⅱ sample will generate tensile and shear fracture, and stress changes alternately; type Ⅲ sample will generate shear fracture and stress decreases slowly. The cracks of type Ⅰ and type Ⅱ are homogeneous and the fracture face is smooth; type Ⅲsample will generate massive cracks and powdery, which will affect the gas exploitation. For this powdery material, taking the coal as an example, advanced preventive processing and drainage-mining governance are proposed at this stage.