测井多参数两向量法识别页岩气地质“甜点”

摘要

为了给页岩气水平井地质导向钻井目标层的优选提供地质依据,以四川盆地威远地区下寒武统筇竹寺组页岩地层为研究对象,提出了测井多参数两向量法识别页岩气地质“甜点”的方法.该方法首先利用测井曲线计算页岩有机碳含量、脆性指数和含气饱和度;然后优选敏感反映好、中、差页岩气层的测井地质参数,结合试井资料构建页岩气层样本集;再由好、差两类页岩气层样本计算第一、第二判别向量C、K及其对应的综合指标R1、R2,建立好、中、差页岩气层判别图版和分类特征投影函数F;进而根据判别结果优选出页岩气地质“甜点”作为地质导向钻井的目标层,并进行了现场检验.结果表明:①据该区筇竹寺组页岩地层测井地质参数拟合/改进的有机碳含量、脆性指数、含气饱和度的公式可用于相关参数的计算;②依据页岩气层判别图版与特征函数F确定页岩气地质“甜点”层段判别标准所得到的结果直观可靠:③现场实践中,采用该方法实时准确划分了页岩地层的目标层和导向标志层,准确率由采用传统的多元线性逐步判别法的72.6％提高到了现在的87.4％.结论认为,该方法应用于研究区页岩气地质“甜点”的判别,所得结果合理可靠、准确度高.%For providing a geological basis for the target layer optimization of geosteering drilling of horizontal shale gas wells,with the shale formation of Qiongzhusi Fm,Lower Cambrian in the Weiyuan area,Sichuan Basin,as the research subject,it was proposed to identify geological "sweet spots" of shale gas by a two-vector method based on logging multi-parameter.In this method,the total organic carbon (TOC),brittleness index (BI) and gas saturation (Sg) of shale are firstly calculated using the logging curves.Secondly,logging and geological parameters that can sensitively reflect good,medium and poor shale gas layers are preferentially selected,and then combined with well test data,the sample set of shale gas layers is constituted.Thirdly,the first and second discriminant vectors (C,K) and their corresponding comprehensive indexes (R1,R2) are calculated based on the samples of good and poor shale gas layers,and then the discrimination chart of good,medium and poor shale gas layers is plotted and the projective function of classification characteristic (F) is established.And finally,based on the discrimination results,the geological "sweet spots" of shale gas is selected as the target layer of geosteering drilling,and verified on site.It is demonstrated that the TOC,BI and Sg formulas fitted/improved on the basis of the logging and geological data of Qiongzhusi Fm shale in this area can be used in the calculation of related parameters.Besides,the discrimination criterion of geological "sweet spots" of shale gas according to the shale gas layer discrimination chart and the characteristic function (F) can provide intuitive and reliable discrimination results.Finally,when this method is applied to the LWD field test,target layers and steering markers are classified accurately in real time,and the precision ratio rises to 87.4％ from 72.6％,the precision ratio of the traditional multi-element linear stepwise discrimination method.To sum up,this method is applicable to the identification of geological "sweet spots" of shale gas and the results are reasonable,reliable and accurate.