超临界CO_(2)对致密碳酸盐岩力学特性影响

Effect of Supercritical CO_(2)on Mechanical Properties of Tight Carbonate Rocks

采用超临界CO_(2)破致密碳酸盐岩具有破裂压力低,易形成复杂缝特点,但其作用的力学机理尚未清晰。选用马家沟组致密白云岩样,采用高温高压超临界CO_(2)饱和流体法,研究了岩样在超临界CO_(2)+地层水的流体中浸泡不同时间后的物性、声波响应、岩石力学特性和破裂形态特征。结果表明,随着浸泡时间增加,化学溶蚀作用引起溶蚀孔径增大,岩样孔隙度、渗透率增加,而声波速度、动态和静态岩石力学参数均下降;当浸泡时间大于1.0 d后,岩样物性参数增大和力学强度降低明显;随着浸泡时间增加,岩样破裂形态由单一低角度剪切缝向高角度剪切缝、共轭缝和剪切、张型复合缝等复杂形态发展;压裂短时间内(小于1.0 d)超临界CO_(2)+地层水形成的弱酸对岩石力学强度劣化程度有限。

The supercritical CO_(2)fracturing has the characteristics of low fracture pressure and easy formation of complex fractures in tight carbonate rock,but the mechanical mechanism of the effect of supercritical CO_(2)on tight carbonate rocks is not clear.The physical properties,acoustic response,rock mechanical properties and fracture morphology of the tight dolomite samples from Majiagou Formation were studied by using the high temperature and high pressure supercritical CO_(2)saturated fluid method,namely the samples were immersed in supercritical CO_(2)+formation water for different time.The results show that with the increase of soaking time,the dissolution pore size and the porosity and permeability of the rock sample increase due to chemical corrosion,and the corresponding acoustic velocity,dynamic and static rock mechanical parameters all decrease.When the soaking time is longer than 1.0 d,the physical property parameters of the rock sample increase and the mechanical strength decreases obviously.With the increase of soaking time,the fracture morphology of rock samples develops from single low angle shear joint to high angle shear joint,conjugate joint and shear and tension composite joint.In short fracturing time(61.0 d),the weak acid formed by supercritical CO_(2)+formation water has limited deterioration degree to rock mechanical strength.