致密砂岩储集层微观孔喉结构及其分形特征——以西加拿大盆地A区块Upper Montney段为例

为表征致密砂岩储集层微观孔喉结构及其分形特征,利用铸体薄片、扫描电镜、高压压汞等测试方法,对西加拿大盆地A区块下三叠统Upper Montney段致密砂岩样品进行了研究。结果表明,研究区储集层主要孔隙类型包括溶蚀孔、原生剩余粒间孔和晶间微孔,见少量微裂缝;储集层孔喉半径多小于0.30 μm,分布曲线呈多峰形态,有效储集空间主要由亚微米级和纳米级孔喉组成;研究区致密砂岩储集层可划分为Ⅰ类、Ⅱ类和Ⅲ类,其对应的孔喉总分形维数分别为2.31、2.46和2.63,Ⅰ类储集层的物性较好,非均质性相对弱;不同大小孔喉的分形特征有差异,通常亚微米级孔喉的分形维数较纳米级孔喉大,即亚微米级孔喉非均质性更强。孔喉分形维数与其结构有关,不同孔喉的发育造成了致密砂岩储集层非均质性各异。

Pore Throat Structure and Fractal Characteristics of Tight Sandstone Reservoirs: A Case Study of Upper Montney Formation in Block A in Western Canada Sedimentary Basin

In order to further characterize the structure and fractal characteristics of different pore throats in tight sandstone reservoirs, the tight sandstone samples from Upper Montney formation in Block A of western Canada sedimentary basin are studied by using cast thin section, SEM and high pressure mercury injection analyses. The results show that the pore types of the reservoir in the study area mainly include dissolution pore, primary residual intergranular pore and intercrystal micropore, and a small number of microfractures are noted. Most of the pore throat radius of the reservoir is less than 0.30 μm, showing a curve with multiple peaks, and the effective storage space is mainly composed of nano- and submicron- pores. The tight sandstones in the study area can be divided into Type Ⅰ, Type Ⅱ and Type Ⅲ with the corresponding average overall fractal dimensions of 2.31, 2.46 and 2.63, respectively. The Type I samples have relatively good physical properties and relatively weak heterogeneity. The fractal characteristics of pore throats with different sizes are different. The fractal dimension of the submicron pore throats is higher than that of nano pore throats, indicating a stronger heterogeneity of the submicron pore throats. The fractal dimension of pore throats is related to pore throat structure, and the development of different pore throats in tight sandstone reservoirs results in different heterogeneity of pore throats.