Zhao Xinli, Yang Zhengming, Zhou Sibin, Luo Yutian, Liu Xuewei, Zhang Yapu, Lin Wei, Xiao Qianhua, Niu Zhongkun
College of Engineering Science, University of Chinese Academy of Sciences, Beijing 100049, China.
Institute of Porous Flow & Fluid Mechanics, Chinese Academy of Sciences, Langfang 065007, China.
ACS Omega. 2021 Sep 16;6(38):24672-24682. doi: 10.1021/acsomega.1c03382. eCollection 2021 Sep 28.
The key to the efficient development of a tight reservoir is its accurate evaluation. In this study, the pore throat structure characteristics of sandstone samples in the study block were analyzed by high-pressure mercury injection technology. According to the characteristics of the capillary pressure curve, the sandstone samples in the study block were divided into three types: the first type has a reservoir permeability greater than 0.7 mD and a core mercury injection saturation of 96% with a good reservoir quality; the second type has a reservoir permeability ranging from 0.4 to 0.7 mD and a core mercury injection saturation of 80% with a moderate reservoir quality; and the third type has a reservoir permeability between 0.1 and 0.4 mD and a core mercury injection saturation of 50% with a poor reservoir quality. Also, high-resolution synchrotron radiation imaging and scanning electron microscopy were used to observe the pore throat structure, connectivity, and microscopic heterogeneity of sandstone samples, showing an increasing level of pore disconnection, serious microscopic heterogeneity, and poor reservoir performance as reservoir permeability declines. As mineral composition tests show, the lithology of the tight sandstone in the target block is mainly medium-grained and fine-grained feldspar lithic sandstone and the longitudinal heterogeneity of lithology and mineral components of tight sandstone is relatively weak at above the centimeter level. Besides, based on the high-pressure mercury injection test data, fractal theory is applied to calculate the fractal dimensions of the three types of reservoirs. The result shows a gradual increase in fractal dimensions with the decrease of reservoir quality, in which the closer the fractal dimension is to 3, the more serious the microscopic heterogeneity is, and the stronger the roughness of the pore surface is. As a result, the more heterogeneous the tight reservoir gets, the more likely the injected fluid is to flow along the developed and connected pore regions.
致密油藏高效开发的关键在于其准确评价。本研究采用高压压汞技术分析了研究区块砂岩样品的孔喉结构特征。根据毛管压力曲线特征，将研究区块砂岩样品分为三类：第一类储层渗透率大于0.7mD，岩心压汞饱和度为96%，储层质量良好；第二类储层渗透率为0.4～0.7mD，岩心压汞饱和度为80%，储层质量中等；第三类储层渗透率为0.1～0.4mD，岩心压汞饱和度为50%，储层质量较差。此外，利用高分辨率同步辐射成像和扫描电子显微镜观察了砂岩样品的孔喉结构、连通性和微观非均质性，结果表明随着储层渗透率降低，孔隙连通性变差、微观非均质性严重、储层性能变差。矿物成分测试表明，目标区块致密砂岩岩性主要为中细粒长石岩屑砂岩，致密砂岩岩性和矿物组分的纵向非均质性在厘米级以上相对较弱。此外，基于高压压汞测试数据，应用分形理论计算了三类储层的分形维数。结果表明，分形维数随储层质量降低而逐渐增大，其中分形维数越接近3，微观非均质性越严重，孔隙表面粗糙度越强。因此，致密油藏非均质性越强，注入流体越容易沿发育连通的孔隙区域流动。
