High pressure mercury injection and scanning electron microscopy applied to characterize micro-and nano-scale pore throats in tight sandstone reservoirs: A case study of the fourth member of Shahejie Formation in Yi176 block, Zhanhua Sag, Bohai Bay Basin

Thirty-two samples of tight sandstone from the fourth member of Shahejie Formation in Yi176 block, Zhanhua Sag, Bohai Bay Basin were selected for high pressure mercury injection and overpressure pore infiltration experiments. The microscopic pore throat structure and permeability of tight sandstone reservoirs, especially the flow characteristics of fluid in nano-and micro-scale throats, were discussed along with their microscopic features observed with environmental scanning electron microscopy (SEM). Experiments showed that the discharge pressure in the study area has a wide distribution. According to the distribution of pore throats and relative permeability, the reservoirs in this area were divided into 4 major types and 6 sub-categories. dv/d(logd) reflects the distribution of pore throats at all levels. Combined with the identification of the relative sizes of pore throats by environmental SEM, the pore throat system in the study area was divided into 4 categories:nano scale, micro-nano scale, nano-micro scale and micro scale, while throat was classified into nano and micro types. According to the correlation analysis, it was found that the nano-scale throats controlled permeability, while the micro throats contributed more to porosity. High pressure mercury injection not only characterizes the structural characteristics of conventional pore throats, but also reflects the size of the micro-and nano-pore throats and the fluid flow characteristics in them, which allows further detailed evaluation of reservoirs.