库车坳陷泥岩盖层脆韧性破裂特征数值模拟

Numerical simulation of the brittle-ductile fracture characteristics of the mudstone caprock in Kuche Depression

随着埋藏深度增加,岩石经历脆性、脆-韧性、韧性3个变形阶段。以库车坳陷泥岩盖层为例,通过三轴力学试验,在围压为5、10、20、40 MPa下得到峰值强度、残余强度、弹性模量、泊松比、黏聚力、摩擦系数等岩石力学参数,应用离散元数值模拟方法进行了相同围压的模拟,模拟结果与三轴压缩试验吻合度高。利用该组参数进行较高围压(72、90、120、140 MPa)条件下的模拟,模拟结果拟合出Mohr-Coulomb破裂包络线、残余强度包络线,结合Byerlee的摩擦滑动定律,预测出盖层脆韧性转换的临界值:72 MPa是脆性到脆-韧性转换的临界点;140 MPa是脆-韧性到韧性转换的临界点。根据地应力梯度25 kPa/m,推测吉迪克组泥岩盖层在深度达到5600 m时,理论上不容易被错断,封闭效果较好。

With the rise of the buried depth,the rocks undergo three deformed stages:brittleness,brittleness-toughness and toughness.Taking the mudstone caprock in Kuqa Depression as an example,with the help of the triaxial mechanics test,under the conditions of 5,10,20,40 MPa,the rock mechanics parameters such as the peak intensity,residual intensity,elastic modulus,poison's ratio,cohesive force,friction coefficient and so on are obtained,and moreover by means of discrete element numerical simulating method,the simulation of the same confining pressure was applied,the alignment between the simulated results and the triaxial compression test was pretty high.The parameters in this group were made full use to simulate for rather high confining pressures:72,90,120 and 140 MPa,the broken envelope and residual strength envelopes of Mohr-Coulomb were fitted through the simulated results.Combined with Byerlee's friction and sliding laws,the critical value of the brittle-ductile transition of the caprock was predicted:72 MPa was the critical transformed point from the brittleness to brittleness-toughness;and moreover 140 MPa is the critical transformed point from the brittleness-toughness to toughness.According to the geostress gradient of 25 kPa/m,it was speculated that when the mudstone caprock depth of Jidike Formation reaches 5600 m,it is theoretically not easy to be wrongly broken and has much better sealing property to the oil and gas.