深层页岩气储层孔隙特征研究进展——以四川盆地下古生界海相页岩层系为例

Research progress on the pore characteristics of deep shale gas reservoirs:An example from the Lower Paleozoic marine shale in the Sichuan Basin

孔隙结构对于深层页岩气储层中异常高压的保持乃至页岩气的保存与富集均具有一定的指示作用,是事关深层页岩气能否保存与富集的重要研究内容。为此,通过调研和分析国外深层页岩气储层孔隙特征,对比四川盆地深层、超深层页岩孔隙的最新研究成果,系统分析了深层页岩气储层孔隙的非均质性和连通性,进一步明确了超压对深层页岩油气储层孔隙结构的影响,总结了近年来深层页岩气储层孔隙特征研究成果。研究结果表明:①典型深层、超深层页岩中的微孔段、介孔—大孔段孔隙具有多重分形的特征,多重分形谱相关参数α5--α5+值与多重分形维数相关参数H指数分别对深层页岩储层孔隙的非均质性及连通性具有较好的指示作用;②四川盆地下古生界页岩储层孔隙的连通性、非均质性与埋藏深度不具有明显的相关性,但受页岩总有机碳含量、矿物含量、有机质成熟度等因素的影响则较大;③高上覆地层压力所带来的机械压实作用对超深层页岩的影响显著,但其对深层页岩的孔径、孔隙形态等参数以及介孔体积/微孔体积、介孔比表面积/微孔比表面积等特征比值的影响则较为有限;④页岩层系超压能够在一定程度上抵消上覆地层压力对孔隙(特别是微孔)的机械压实作用,可以延缓甚至改变孔隙度随埋藏深度加深而下降以及孔隙形状系数随埋藏深度加深而减小的趋势,对于页岩气的保存与富集具有积极意义;⑤深层页岩中固体沥青孔隙形状系数与其所处封闭流体系统超压特征具有中度相关性。

Pore structure provides a certain indication on the maintenance of abnormal pressure in deep shale gas reservoirs and the preservation and enrichment of shale gas and is an important research content related to the preservation and enrichment of deep sale gas.This paper systematically analyzed the heterogeneity and connectivity of pores in deep shale gas reservoirs by investigating and analyzing the pore characteristics of deep shale gas reservoirs abroad and comparing the latest research results on the pores in the deep and ultradeep shale of the Sichuan Basin.Then,the influences of overpressure on the pore structures in deep shale reservoirs were clarified further.Finally,the research achievements on the pore characteristics of deep shale gas reservoirs in recent years were summarized.And the following research results were obtained.First,micropores and meso-macropores in typical deep and ultradeep shale have multifractal characteristics.The multifractal spectrum parameterα5-—α5+and the multifractal dimension parameter H index can well indicate pore connectivity and heterogeneity of deep shale reservoirs,respectively.Second,the pore connectivity and heterogeneity of the Lower Paleozoic shale reservoirs in the Sichuan Basin is not in obvious correlation with the burial depth,but is influenced more by total organic carbon(TOC),mineral content and organic maturity of shale.Third,the mechanical compaction caused by high overburden pressure has significant influence on ultradeep shale,but limited influence on pore size and pore morphology of deep shale,as well as characteristic ratios,such as mesopore volume/micropore volume ratio and mesopore specific surface area/micropore specific surface area ratio.Fourth,the overpressure of shale strata can balance out the mechanical compaction of overburden pressure on pores(especially micropores)to a certain extent,and can slow down or even change the decreasing trend of porosity and pore form factor with the increase of burial depth,which is of positive significance to the preservation and enrichment of shale gas.Fifth,the pore form factors of solid bitumen in deep shale are moderately correlated to the overpressure characteristics of the closed fluid system where it is located.