特低-超低渗透储层的微观孔隙结构研究

Study on microscopic pore structure of extra/ultra-low permeability reservoirs

莫里青地区的储层微观研究尚不完善,表现在微观孔隙结构特征、孔隙结构差异性成因及储层物性控制因素认识不清。通过综合运用铸体薄片、常规压汞和恒速压汞等技术,结合多元分析方法等数学手段对莫里青断陷双二段储层微观孔隙结构进行精细表征并划分孔隙结构类型,从沉积和成岩作用方面剖析特低-超低渗透储层孔隙结构的复杂性成因,并借鉴多元回归分析思想建立储层渗透率预测方程,筛选储层物性的主控因素。研究结果表明,莫里青地区储层具有特低-超低渗透、孔隙结构复杂、非均质性强的特点。孔隙结构特征总体表现为排驱压力偏低、曲线平台不明显、分选性略差、细歪度为主的特点,可划分为3大类、4小类,即Ⅰ类中孔低渗细喉型、Ⅱ类低孔特低渗微细喉型、Ⅲa类低孔特低渗微喉道和Ⅲb类低孔超低渗微喉道型;原始沉积条件的差异造成了储层微观孔喉特征的初始差异,成岩过程中的压实和晚期碳酸盐胶结作用是导致孔隙结构复杂的另一重要因素。孔喉半径均值、孔隙结构系数、排驱压力3个参数对储层物性具有显著影响,其中孔喉半径均值影响最大。该研究方法与结果对类似的特低-超低渗透砂砾岩储层研究及后期储层分类评价具有一定的指导意义。

The microscopic study of reservoirs in the Moliqing area is still not complete, and the understanding on microscopic pore structure characteristics, reasons for pore structure difference, and reservoir property controlling factors are unclear. By utilizing the thin casting slice, conventional and constant-speed mercury injection techniques combining multivariate analysis methods, the microscopic pore structure of the 2nd Member of Shuangyang formation reservoir in Moliqing fault depression is finely characterized. Based on the sedimentation and diagenesis, the reason for the pore structure complexity of extra/ultra-low permeability reservoirs is analyzed; and the multi-regression analysis is used to establish the prediction equation of reservoir permeability and the main controlling factors of reservoir physical properties are selected. The results show that reservoirs in the Moliqing area have the characteristics of extra/ultra-low permeability, complex pore structure and strong heterogeneity. The pore structure is mainly with the features of low discharge pressure, unobvious curve platform, poor sorting, and fineness, which can be divided into three types: Class Ⅰ medium pore-low permeability-fine throat; Class Ⅱ low pore-extremely low permeability-micro fine throat; Class Ⅲa low pore-extremely low permeability-micro throat, and Class Ⅲb low pore-ultra low permeability-micro throat. The difference of original sedimentary conditions results in initial difference of microscopic pore characteristics, and the compaction during diagenesis and late carbonate cementation are another important factor leading to complex pore structure. The three parameters such as mean radius of the pore throat, pore structure coefficient, and expulsion pressure have significant effects on physical properties of reservoirs, where the effect of mean radius of the pore throat is the most significant. The research method and results can provide guidance to similar extra/ultra-low permeability sandstone reservoirs and subsequent reservoir classification evaluation.