页岩储层孔隙度测定方法优选与推荐

Selection and recommendation of shale reservoir porosity measurement methods

中国页岩气资源丰富,勘探开发潜力大,其中孔隙度是页岩储层评价和页岩气储量计算的关键参数。页岩主要发育微纳米孔隙,孔隙结构复杂,渗透率低,对其孔隙度进行精确测试较常规储层难度更大。为了解决页岩柱塞样氦气法在不同实验条件下所测孔隙度差异较大,且不同实验方法所测页岩孔隙度可比性不明确的问题,此次在详细总结了页岩孔隙度测定各种方法优缺点的基础上,选取四川盆地南部地区五峰组-龙马溪组页岩作为研究对象,利用改进的实验设备开展了样品烘干温度、抽真空时间、氦气饱和平衡判定条件和氦气平衡压力等实验和对比分析,明确了不同测定方法之间的差异,形成了页岩气储层孔隙度测定推荐方法。研究结果表明:①页岩柱塞样氦气法实验条件宜采用线切割方法制备柱塞样,样品在105℃烘干时间不少于24 h,将样品室抽真空至0.1 Pa,氦气饱和平衡判定条件采用30 min压力变化小于0.21 kPa,氦气注入压力设置为1.378 MPa;②改进后的页岩柱塞样氦气法较常规柱塞样氦气法所测孔隙度明显增大,平均增大0.79%;③平行样品颗粒样氦气法大于柱塞样氦气法和液体饱和法所测孔隙度,反映出页岩中存在少部分不连通孔隙;④柱塞样氦气法和液体饱和法所测孔隙度总体差异不大,在孔隙度较小时前者略大。结论认为,改进后的“抽真空+严格平衡条件”柱塞样氦气法所测页岩有效孔隙度可靠性高,推荐优先采用此方法进行页岩孔隙度测定,孔隙度的准确测定有助于页岩气有利区优选和页岩气储量计算。

There are abundant shale gas resources with great exploration and development potential in China.Porosity is a key parameter of shale reservoir evaluation and shale gas reserve calculation.Shale is characterized by developed micro and nano pores,complex pore structures and low permeability,which makes its accurate porosity measurement more difficult than that of conventional reservoirs.Shale porosity measured by the plunger helium method under different experimental conditions is more different,and shale porosity measured by different experimental methods is not comparable clearly.To solve these problems,this paper summarizes the advantages and disadvantages of different shale porosity measurement methods in detail.Then,the Wufeng Formation–Longmaxi Formation shale in the southern Sichuan Basin is selected as the research object to test sample drying temperature,evacuation time,helium saturation equilibrium discrimination condition and helium equilibrium pressure by using the modified experiment devices for comparative analysis.The differences between different measurement methods are clarified,and the recommended method for shale gas reservoir porosity measurement is proposed.And the following research results are obtained.First,the suitable experimental conditions of the shale plunger helium method include preparing plunger samples by linear cutting method,drying samples for more than 24 h at 105℃,evacuating sample chamber to 0.1 Pa,taking pressure change less than 0.21 kPa in 30 min as helium saturation equilibrium discrimination condition,and setting helium injection pressure at 1.378 MPa.Second,compared with the conventional plunger helium method,the porosity measured by the improved shale plunger helium method is significantly increased,with an average increase of 0.79%.Third,the porosity measured by the parallel sample particle helium method is higher than that measured by the plunger helium method and the liquid saturation method,which indicates that there are a few unconnected pores in the shale.Fourth,there is no significant difference overall between the porosity measured by the helium method and the one measured by the liquid saturation method,and the former is slightly higher when the porosity is lower.In conclusion,the improved plunger helium method with“evacuation+strict equilibrium condition”has high reliability in effective shale porosity measurement,so this method is recommended firstly for shale porosity measurement.The accurate porosity measurement is conducive to the selection of favorable shale gas areas and the calculation of shale gas reserves.