煤岩脆性特征压痕法评价及储层改造意义

Brittleness evaluation of coal rock based on indentation method and the significance of coal reservoir stimulation

利用压痕法(indentation method)对山西阳泉矿区新景煤矿3号煤和内蒙古二道岭矿区深部2-1煤厘米级尺寸试样进行静态多级加载试验研究,获取两区煤岩的维氏硬度与断裂韧度参数值,其中阳泉矿区新景煤矿3号煤维氏硬度均值为305.60 N/mm2,二道岭矿区2-1煤维氏硬度均值为418.10 N/mm2;断裂韧度方面,阳泉矿区新景煤矿3号煤均值为0.18 MPa·m1/2,二道岭矿区深部2-1煤均值为0.30 MPa·m1/2,二道岭矿区深部煤岩表现出高硬度、断裂韧性的属性,同时利用2者比——煤岩脆性指数表征煤岩脆性性状:阳泉矿区新景煤矿3号煤脆性指数平均为1.34μm-1/2,二道岭矿区深部2-1煤平均为1.55μm-1/2,整体上二道岭矿区2-1煤脆性更强。压痕试验现象表明阳泉矿区新景煤矿3号煤压后裂缝为两翼对称主干裂缝,压头撤下卸载后裂缝残留宽度仍较大;而二道岭矿区深部2-1煤压后裂缝为弥散式多裂缝形态,卸载后裂缝残留宽度较小,但压头附近煤岩破碎效率较高。结合压痕法试验结果与能量耗散原理,认为脆性较弱的阳泉矿区新景煤矿3号煤随着压头载荷增大,压后原始裂缝与残留裂缝比降低,材料回弹效应较弱。同时随着压头载荷的增加煤岩弹性形变所需的能量降低而塑性形变能量逐渐升高,说明高载荷施加条件下阳泉矿区新景煤矿3号煤破坏以塑性形变为主;而对于脆性较强的二道岭矿区深部2-1煤而言,随着压头载荷增大,压后原始裂缝与残留裂缝比例升高材料回弹效应增强。随着压头载荷的增加煤岩塑性形变能降低,而弹性形变耗能逐渐升高,表明高载荷施加下二道岭矿区深部2-1煤破坏以弹性形变形式为主。结合上述理论认识,认为在脆性较弱的构造软煤压裂改造中应以高裂缝净压力进行加载,提升残留裂缝空间减小弹性形变能耗比例;而在脆性较强的原生结构硬煤压裂中以较低的裂缝净压力加载,以维持塑性形变耗能的比例,规避卸载后压裂裂缝的回弹恢复,保障裂缝的导流能力。

Multistage static loading experimental study of No.3 coal of Yangquan mining area and No.2-1 coal Erdaoling deep mining area were conducted by indentation method(Im)to obtain the Vickers hardness and fracture toughness of coal rock and parameter values,The mean value of Vickers hardness of No.3 coal in Xinjing coal mine of Yangquan mining area is 305.60 N/mm2,and that of No.2-1 coal in Erdaoling deep mining area is 418.1081 N/mm2.In terms of fracture toughness,the mean value of No.3 coal in Xinjing coal mine in Yangquan mining area is 0.18 MPa·m1/2,and the mean value of No.2-1 coal in Erdaoling deep mining area is 0.30 MPa·m1/2.The coal rock in Erdaoling deep mining area shows the properties of high hardness and fracture toughness.The brittleness of coal rock was characterized by the brittleness index:the brittleness index of No.3 coal in Xinjing coal mine of Yangquan mining area is 1.34μm-1/2 on average,and brittleness index of No.2-1 coal in Erdaoling deep mining area is 1.55μm-1/2 on average.The phenomenon of indentation test shows that the post-compression crack of No.3 coal in Xinjing coal mine of Yangquan mining area is a symmetrical trunk crack with two wings,and the residual width of the crack is still large after the indenter is removed and unloaded.However,the fractures of No.2-1 coal in Erdaoling deep mining area are in the form of diffuse multi-fracture.After unloading,the residual width of fractures is small,but the crushing efficiency of coal rock near the indenter is high.Based on the results of indentation test and the principle of energy dissipation,it is concluded that No.3 coal in Xinjing coal mine of Yangquan mining area is weak brittle,the ratio of cracks to residual cracks decreases and the material rebound effect weakens with the increase of indenter load.With the increase of indenter load,the energy required for the elastic deformation of coal rock decreases slightly,while the energy required for plastic deformation increases gradually.However,with the increase of indenter load,the ratio of cracks to residual cracks increases,and the material rebound effect increases,the plastic deformation energy of coal rock decreases,while the energy dissipation of elastic deformation increases gradually.It is considered that in the fracturing of soft coal with weak brittleness,it should be loaded with high net fracture pressure to improve the residual fracture space and reduce the proportion of elastic rebound energy dissipation.In the fracturing of hard coal with high brittleness,the fracture net pressure is applied at a lower rate to maintain the energy dissipation ratio of plastic deformation,avoid the rebound recovery of fracture after unloading,and ensure the conductivity of fracture.