岩芯成饼单元安全度三维数值试验及地应力反馈分析

THREE-DIMENSIONAL NUMERICAL TEST OF ELEMENT SAFETY DEGREE OF ROCK CORE DISCING AND FEED-BACK ANALYSIS OF GEOSTRESSES

岩芯饼化是深部及高应力地质环境特有的现象,通过岩芯饼化来推测地应力有重要的理论和工程意义。首先模拟不同应力对岩芯饼化厚度的影响,并综合三维弹性数值模拟和带拉伸的莫尔-库仑强度准则,提出岩芯饼化"潜在破坏面"单元安全度的计算方法,研究不同地应力对岩芯潜在破坏面的影响。模拟结果表明,在水平应力一定的条件下,随着竖向地应力的变化,竖直岩芯潜在破坏面单元安全度、剪切破坏和拉伸破坏模式都发生变化。单元安全度方法能够直观显示岩芯潜在破坏面安全度的分布,并量化地反映地应力与岩芯破坏模式之间的对应关系。采用单元安全度方法,对美国Williston盆地Bakken页岩储层的前期勘测地应力和观测岩芯分析,取得相一致的计算结果,说明利用该方法分析与识别地应力是可行的。

Rock core discing is a kind of unique phenomena that occurs in environment with high geostresses.Both practical observation and theoretical analysis state that there is an inner relation between geostresses and the degree of rock core discing.Because it is difficult to measure geostresses in above environment,it has remarkable meanings in estimating geostresses by rock core discing.There is a complex mechanical process in rock discing,whose calculation formula and theory are very immature.The paper simulates how different geostresses affect the degree of rock core discing,stating that different geostresses induce different failure patterns of rock core,and the discing can be induced by the action of either shearing or tension.Then the paper proposes a calculation method of safety degree of elements located in potential failure surface of rock core combining elastic finite element method and Mohr-Coloumb criterion with tension limitation.The element safety degree distribution of potential surface can be displayed by element safety degree method,and can reflect the relation between geostresses and discing thickness.Based on the relation,the observation of rock core can be used to feed-back analyze original geostresses.By simulation based on element safety method,corresponding to different vertical stresses while the horizontal stresses being fixed,the safety of element degree distribution and the failure pattern of rock core are gotten.Using the method to analyze the geostress and rock core of Bakken formation in Williston Basin in USA,the results of calculation and observation are identical,which shows the method is feasible.