四川盆地龙马溪组页岩孔隙度实验方法分析

Study on Porosity Measurement Determination Methods of a Shale Reservoir in the Longmaxi Formation,Sichuan Basin

孔隙度是页岩气藏勘探、评价的关键参数,快速、准确测定页岩孔隙度对于储量计算至关重要。目前有氦气法、液体饱和法、图像分析法应用于页岩孔隙度测定,但方法间、实验室间比对效果欠佳。本文选取69件四川盆地下志留统龙马溪组页岩样品,应用X射线衍射、氩离子抛光-扫描电镜、压汞-氮气吸附等分析测试技术研究样品特征,开展方法间的孔隙度测定比对实验,分析各方法的适用性。结果表明:①龙马溪组页岩黏土含量高、有机质孔隙多、孔喉细小、渗流能力差,导致了外来液体和气体难以快速进入孔隙中,对岩心图像资料的分辨率有很高要求;②液体饱和法(酒精法)使用20MPa压力饱和样品24h,导致了岩心损伤,测得的孔隙度偏离真实值;图像分析法因忽略了矿物基质中的微孔(孔径<2nm)和介孔(孔径<50nm)等因素使孔隙度测定结果不可靠;③氦气法孔隙度测定结果与烘干温度、稳定时间密切相关,温度越高则测得的孔隙度越大,稳定时间过短会使孔隙度结果偏小。在60℃下烘干岩心24h以上至恒重状态后,通过设置合理的稳定时间,氦气法可获得准确的孔隙度结果,适当提高注入压力有助于加快实验速率。

BACKGROUND:Porosity is a key parameter for exploration and evaluation of shale gas reservoirs.Rapid and accurate determination of shale porosity is crucial for reserve calculation.At present,there are helium gas method,liquid saturation method and image analysis method applied to shale porosity determination,but the comparison between methods and laboratories is lacking.OBJECTIVES:To evaluate the applicability of different methods for determining shale porosity.METHODS:The characteristics of 69 shale samples from the Longmaxi Formation were studied by analyzing and testing techniques such as X-ray diffraction,argon ion polishing-scanning electron microscopy,and mercury injection-nitrogen adsorption method.Comparison experiments for porosity measurement were conducted among helium method,liquid saturation method and image analysis method.RESULTS:The Longmaxi Formation shale had high clay content,many organic pores,small pore throats,and poor seepage capacity,which made it difficult for foreign liquids and gases to quickly enter the pores,which required high resolution of core image data.The liquid saturation method(alcohol method)used 20MPa pressure to saturate the sample for 24 hours,resulting in core damage,and the measured porosity deviated from the true value.The image analysis method ignored the micropores(pore size<2nm)and mesopores(pore size<50nm)in the mineral matrix and other factors,making the porosity measurement result unreliable.The porosity measurement result of the helium method was closely related to the drying temperature and stabilization time.The higher the temperature,the larger the measured porosity,and if the stabilization time is too short,the porosity result will be smaller than the real values.CONCLUSIONS:After drying the core at 60℃for more than 24 hours to a constant weight,by setting a reasonable stabilization time,accurate porosity results can be obtained by the helium method.Properly increasing the injection pressure helps to speed up the experimentation rate.