基于VTI介质弹性参数的页岩脆性预测方法及其应用

Shale brittleness prediction based on elastic parameters of VTI media

大量的科学研究表明,页岩的脆性对油气储层的各向异性、储集能力、渗透性和可改造性等具有重要的影响,因此岩石的脆性预测成为了评价页岩气藏的重要内容。实验测试分析也已经证实,页岩的脆性与岩石中的矿物含量和孔隙结构等影响力学性质的因素密切相关。Rickman等(2008)提出了基于弹性参数的脆性系数计算方法,融合了力学参数杨氏模量和泊松比。然而,目前计算杨氏模量和泊松比的方法均以各向同性介质为前提,受此局限,脆性系数计算公式必然不能精确地描述复杂介质中的脆性特征。为此,引入Thomsen(2013)提出的VTI介质弹性参数计算方法,推导出隐含了各向异性参数ε,γ和δ的脆性系数计算公式,从理论上直接把页岩的脆性和各向异性密切地联系了起来,较好地诠释了现实中脆性页岩存在各向异性的岩石物理特性。将该方法应用于四川盆地西部新场-孝泉地区须家河组五段页岩气的开发实践中,结果表明脆性系数高值异常区储层脆性较强,能有利于在优质储层中识别出"脆性甜点"储层。预测结果与现仅有的两口页岩气专层井吻合良好。

Numerous studies show that shale brittleness has great influences on the anisotropy, pooling capacity, permea- bility and fracability of oil and gas reservoirs, thus making the prediction of shale brittleness one of the hot spots in shale gas reservoirs evaluation. Lab analyses also confirmed a close relationship between shale brittleness and several factors like the mineral content and the pore structure that affect mechanical properties of shale rocks. Rickman et al （2008） proposed a method for brittleness coefficient calculation based on elastic parameters. The method combined Young＇ s modulus and the Poisson ratio- two mechanical parameters that can only be calculated with an assumption that the reser- voirs in concern are isotropous. Therefore, the method can not accurately describe the brittleness of complex media. To deal with the problem, we introduced the elastic parameter calculation method of VTI media proposed by Thomsen （2013）, and developed a brittleness calculation formula comprised of anisotropic parameters ε,γ and δ. Theoretically, the formulat links shale brittleness to anisotropy and offered a better way for anisotropic characterization of shale rocks. Appli- cation of the method to the study of the 5th member of the Xujiahe Formation in Xinchang-Xiaoquan gas field of Sichuan Basin indicated that shale reservoirs in areas with abnormally high brittleness coefficients have strong brittleness, thus it can be used to better identify ＂brittle sweet spots＂ within ＂geologic sweet spots＂. The brittleness prediction results coin- cide well with the drilling results of the only two shale gas wells.