低阻气层测井识别和评价新方法——以鄂尔多斯盆地东胜气田为例

随着鄂尔多斯盆地东胜气田天然气开发步入中后期，先后出现了一些低电阻率气层（以下简称低阻气层）井。这些低阻气层与水层的电性差异很小，由于缺乏足够的分析化验数据，并且对其成因机理的认识尚不清楚，因而给气层识别带来了困难、对储层产能评价和预测造成了不利的影响。为了给东胜气田后期天然气高效开发提供可靠的技术支撑，在总结该区低阻气层测井响应特征的基础上，从分析成因机理入手，开展了低阻气层常规测井识别方法研究，提出了测井曲线重叠法和四孔隙度差值/比值法等新方法。研究结果表明：①低阻气层成因除了受到泥质分布形式、黏土矿物阳离子附加导电性以及钻井液侵入因素的影响之外，主要还有下述两种影响类型；②一种是由于高孔隙度、高渗透率及高可动水饱和度引起的低阻气层；③另一种则是由于复杂孔隙结构引起的高束缚水（毛细管水）饱和度，束缚水中主要是毛细管水，毛细管水又形成了较好的导电网络，造成储层呈现低电阻率特征。结论认为，采用所提出的常规测井方法来识别低阻气层，解释符合率可以达到83%，识别效果较好。

A new method for logging identification and evaluation of low-resistivity gas layers: A case study of the Dongsheng Gasfield in the Ordos Basin

As the natural gas development in the Dongsheng Gasfield of the Ordos Basin steps into middle and late stages, some wells have successively drilled into low-resistivity gas layers, the electrical property of which is rarely different from that of water layers. There is not sufficient analysis and experiment data on low-resistivity gas layers and their genetic mechanisms have not been understood clearly, which brings about difficulties to their identification and results in adverse influences on their reservoir productivity evaluation and prediction. In order to provide reliable technical support for the efficient natural gas development in the late stage of Dongsheng Gasfield, this paper firstly summarized the logging response characteristics of low-resistivity gas layers in this area. Then, based on genetic mechanism analysis, conventional logging identification methods of low-resistivity gas layers were investigated and some new methods were put forward, including the log overlapping method and the four porosity difference/ratio method. And the following research results were obtained. Apart from the distribution form of shale, the cationic additional conductivity of clay mineral and the invasion of drilling fluid, the genesis of low-resistivity gas layers is influenced mainly by the following two types of factors. One is low-resistivity gas layers caused by high porosity, high permeability and high movable water saturation. The other is the product of high bound water (capillary water) saturation caused by complex pore structures. In other words, bound water is mainly capillary water, which forms a good conductive network, resulting in the occurrence of low resistivity characteristics in the reservoir. In conclusion, when the proposed conventional logging method is used to identify low-resistivity gas layers, the identification effect is better with an interpretation coincidence rate as high as 83%.