一种基于应力-应变特征的岩石脆性指数评价改进方法

An improved method for evaluating rock brittleness index based on stress-strain curve characteristics

脆性指数作为岩石力学评价的重要指标之一,对准确评价岩石脆性特征具有重要意义。选取四川盆地新场气田须家河组五段的 21 块岩石样品,对其开展室内三轴岩石力学实验,分别模拟了样品在围压 0 MPa,12 MPa,22 MPa,32 MPa 下的三轴饱水力学和声学测试,基于岩石应力-应变曲线分析不同岩石脆性特征参数的差异,分别计算基于杨氏模量的脆性因子(BYm)、基于泊松比的脆性因子(Bμ)、基于可恢复应变与总应变的脆性因子(BRε)、基于屈服强度与抗压强度的脆性因子(BRσ),并对比、分析了现有的基于应力-应变曲线的脆性评价方法,进而综合室内岩石力学测试结果建立了脆性指数 Bn 与BYm,Bμ,BRε,BRσ的函数关系式,通过多元回归方法建立新的脆性指数模型,对岩石样品的脆性进行评价。结果表明:传统的脆性指数评价方法在区分岩石样品脆性时效果不佳;改进后的模型能够更好地区分研究区高脆性泥岩、低脆性泥岩和砂岩。该研究结果对于准确评价岩石脆性特征具有指导意义。

As one of the important indexes for rock mechanics evaluation,the evaluation of brittleness index is of great significance to rock characteristics. 21 rock samples of the fifth member of Xujiahe Formation of Xinchang Gas Field in Sichuan Basin were selected to carry out laboratory triaxial rock mechanics experiments. Triaxial satura- tion hydraulics and acoustic tests of samples under confining pressure of 0 MPa,12 MPa,22 MPa and 32 MPa were simulated respectively. Based on the stress-strain curve characteristics,the differences of different brittleness characteristic parameters were analyzed. The brittleness factors such as BYm,Bμ,BRε and BRσ respectively based on Young's modulus,Poisson's ratio,recoverable strain and total strain,and yield strength and compressive strength,were calculated. The existing brittleness evaluation methods based on stress-strain curves were compared and analyzed,and the functional relationships of brittleness index Bn with BYm,Bμ,BRε and BRσ were established according to the laboratory rock mechanics test results. A new brittleness index model was established by multiple regression method to evaluate the brittleness of rock samples. The results show that the conventional brittleness index evaluation method is not effective in distinguishing brittleness of rock samples,and the improved model can better distinguish high brittle mudstone,low brittle mudstone and sandstone in the study area. The study results have guiding significance for accurately evaluating the brittleness characteristics of rocks.