低阻页岩气层含气饱和度计算新方法

四川盆地涪陵地区焦石坝页岩气田的产层为上奥陶统五峰组—下志留统龙马溪组,其底部一般发育厚度20 m左右、有机质含量和含气丰度均高的优质页岩气层,但是该层段却不具有最高的电阻率数值,较低的电阻率往往会导致利用电法测井信息的Archie等公式计算获得的含气饱和度远远低于实验分析值。为提高页岩气层关键参数的计算精度,开展了寻求利用非电法测井信息计算含气饱和度的方法研究。在深入分析该区页岩气层低阻成因的基础上,剖析了传统电法测井求取含气饱和度的局限性,阐述了利用中子—密度孔隙度重叠计算含气饱和度的可行性。根据页岩气层中子、密度等孔隙度测井响应特征,采用岩心实验刻度,利用密度测井值、密度孔隙度、密度与中子孔隙度的差值,以及通过优化最佳叠合系数的双孔隙度重叠等多种方法建立了页岩气含气饱和度计算模型。该新方法有效避开了电法测井中的低阻因素的影响,从根本上解决了优质页岩气层段含气饱和度计算数值偏低的问题,在实际应用中见到了良好的效果,为该区页岩气储量的准确计算提供了技术支撑。

A new method for calculating gas saturation of low-resistivity shale gas reservoirs

The Jiaoshiba shale gas field is located in the Fuling area of the Sichuan Basin, with the Upper Ordovician Wufeng Fm–Lower Silurian Longmaxi Fm as the pay zone. At the bottom of the pay zone, a high-quality shale gas layer about 20 m thick is generally developed with high organic contents and gas abundance, but its resistivity is relatively low. Accordingly, the gas saturation calculated by formulas (e.g. Archie) using electric logging data is often much lower than the experiment-derived value. In this paper, a new method was presented for calculating gas saturation more accurately based on non-electric logging data. Firstly, the causes for the low resistivity of shale gas reservoirs in this area were analyzed. Then, the limitation of traditional methods for calculating gas saturation based on electric logging data was diagnosed, and the feasibility of the neutron–density porosity overlay method was illustrated. According to the response characteristics of neutron, density and other porosity logging in shale gas reservoirs, a model for calculating gas saturation of shale gas was established by core experimental calibration based on the density logging value, the density porosity and the difference between density porosity and neutron porosity, by means of multiple methods (e.g. the dual-porosity overlay method by optimizing the best overlay coefficient). This new method avoids the effect of low resistivity, and thus can provide normal calculated gas saturation of high-quality shale gas reservoirs. It works well in practical application. This new method provides a technical support for the calculation of shale gas reserves in this area.