涪陵页岩气田焦石坝区块页岩脆性指数地震定量预测

A quantitative seismic prediction technique for the brittleness index of shale in the Jiaoshiba Block, Fuling shale gas field in the Sichuan Basin

为了确定四川盆地涪陵页岩气田焦石坝地区页岩储层相对高脆性层段及压裂有利区,研发了适应于复杂构造应力环境的多参量页岩脆性指数的地震定量预测技术。首先通过对构造、岩石矿物组分、岩心裂缝发育程度、测井、地震等资料的综合分析,明确了构造挤压应力对岩石脆性特征的影响作用;然后在Rickman公式的基础上,引入反映横向剪切应力—应变作用的剪切模量,构建了基于杨氏模量、泊松比、剪切模量×密度的新脆性指数预测模型;最后结合基于矿物组分的弹性岩矿脆性指数法可精确计算全井段脆性指数的优势,形成了适应复杂构造应力环境的多参量页岩脆性指数定量预测技术。应用效果表明:(1)利用该技术预测的结果与未参与建模探井的脆性指数计算结果吻合较好,相对误差在4%以内,证明该技术是可靠的;(2)焦石坝地区上奥陶统五峰组—下志留统龙马溪组优质页岩的脆性指数具有自上而下逐渐增高的趋势且横向上分布稳定,其中五峰组—龙马溪组一段一亚段的脆性指数为最高,是水平井穿行与压裂的最有利层段。

In this paper, a quantitative seismic prediction technique of multi-parameter shale brittleness index suitable for the environment with complex structural stress was developed in order to confirm highly brittle intervals and favorable fracturing zones in the shale reservoirs in the Jiaoshiba Block, Fuling shale gas field, Sichuan Basin. Firstly, the effect of structural compression stress on the brittleness characteristics of rocks were figured out by analyzing structures, mineral composition, development degree of fractures in cores, well logging and seismic data comprehensively. Secondly, according to the Rickman formula, a new brittleness index prediction model based on Young＇s modulus, Poisson＇s ratio and shear modulus × density was established by introducing the shear modulus which reflects lateral shear stress-strain. Finally, the quantitative prediction technique of multi-parameter shale brittleness index suitable for the envi- ronment with complex structural stress was developed by virtue of the superiority of an elastic rock brittleness index method based on mineral composition to accurately calculate the brittleness index of a full hole. Field application shows that this technique is reliable, since its prediction results coincide with the calculated brittleness index of exploratory wells which are not used for modeling, with a rel- ative error margin below 4%; and that the brittleness index of good shale of Upper Ordovician Wufeng Fro-Lower Silurian Longmaxi Fm in the Jiaoshiba Block increases from the top to the bottom and is stably distributed laterally. Particularly, the first sub-member of the first member of Longmaxi-Wufeng Fms is the highest in brittleness index, so it is the most favorable interval for penetrating and fracturing of horizontal wells.