缺失声波条件下的页岩储层地应力测井解释方法

Calculation of the anisotropic in-situ stress for shale gas reservoirs with the absence of sonic log data

针对页岩储层各向异性和现场双极测井数据有限的实际情况,提出了基于加权平均思想,通过中子、密度、伽马等测井资料回归声波数据,并利用横观各向同性模型来提高页岩储层地应力解释精度的方法.利用该方法以美国某页岩区块为例进行了现场地应力计算应用,并结合QEMSCAN技术对地层各向异性程度进行了对比分析.结果表明,该地区各向异性参数Eh/Ev值在1.4～1.9,水平杨氏模量的大小比垂直弹性模量高,且垂直弹性模量更符合垂向的岩性解释结果,垂向泊松比则比水平泊松比要略大,受各向异性影响较小.应用横观各向同性模型地应力值较常规模型偏大,且Blanton法比Higgins法要高,两者同小型压裂数据吻合度较高,也更能反映岩性差异引起的构造应力改变和带来的地应力差异,证明了在各向异性页岩储层中更好的适用性,同时也侧面反映了通过综合测井数据和临井声波资料在回归声波结果上也具有较好的解释精度.

Shale anisotropic characteristics and sparse sonic logging data both challenge the in-situ stress calculation of shale formations.From the composite logging information,including neutron,density,porosity logs,etc.,the inverse of sonic logging data for the wells for lack of logging data will be achievable if sonic logging data from adjacent wells exist.Furthermore,two transverse isotropic stress models are presented to enhance accuracy.In a case of a Shale Gas Block in the USA,the above method was applied in in-situ stress calculation and the QEMSCAN technology was used to analyze the influence of rocks on geomechanical anisotropy.The result shows that the anisotropic parameter Eh/Ev was in the range of 1.4-1.9,the vertical Young＇s modulus was smaller than horizontal Young＇s modulus and accorded with the interpretation result.Furthermore,the vertical Poisson＇s ratio was slightly higher than the horizontal Poisson＇s ratio and was less influenced by anisotropy than the latter.In addition,the stress values obtained by use of the transverse isotropic stress model are bigger than those obtained by the conventional stress model and the stress calculated by use of the Higgins model is smaller than those by the Blanton model.The stress values of the transverse isotropic models well matched the results of in-situ mini frac tests,and can more clearly reflect the variation of tectonic stress and stress differences caused by a variation of lithological properties.In addition,it also reflects the reliability and utility of the sonic logging inverse method.