Experimental Investigation of Methods for Measuring the Permeability of Dry and Water-Saturated Ultra-Deep Tight Sandstones

ABSTRACT Ultra-deep tight sandstone reservoirs have extremely low permeability and porosity due to extremely deep burial depth, and extremely high in-situ stress. Therefore, hydraulic fracturing is widely used to create diversion fractures to improve oil well productivity. Incompatible fracturing fluids can severely damage the permeability and fracture surface of the tight sandstone matrix. Rigorous evaluation of formation sensitivity and fracturing fluid compatibility is critical before hydraulic fracturing operations. There are three types of permeability experimental evaluation methods: steady-state permeability method, pressure pulse decay method, and pressure transmission method. In this study, a series of comparative experiments were designed to accurately evaluate the sensitivity and compatibility of injected fluids in ultra-deep tight sandstones. CT scanning and 3D reconstruction techniques were used to obtain digital data on pore space and pore-throat connections of downhole cores. Through statistical regression analysis, a modified correlation between the pressure pulse decay method and the pressure transmission method is proposed. The proposed experimental and regression methods can also be used to validate permeability measurements in other types of reservoirs. INTRODUCTION With the development of oil exploration, people pay more and more attention on deep and ultra-deep strata. Since the 21st century, China has made a series of breakthroughs in deep oil and gas exploration, especially in the central and western basins (Li et al., 2023). Unlike conventional reservoirs, ultra-deep tight sandstone reservoirs are characterized by large buried depth, complex structure, tight matrix, and strong heterogeneity. Rocks with extremely low permeability are more affected by stress than those with high permeability (Jones., 1997). Due to the extremely low permeability and porosity (Li et al., 2019; Li et al., 2020), hydraulic fracturing is widely used to create diversion fractures to increase well productivity of tight reservoirs (Li et al., 2018). The fracturing fluid used in hydraulic fracturing can significantly alter some formation properties. Due to the high temperature and pressure, tight reservoirs are susceptible to stress, flow rate, and output, especially in the case of deep and ultra-deep tight sandstonereservoirs (Gilicz., 1991). Therefore, it is critical to evaluate formation sensitivity and fracturing fluid compatibility before hydraulic fracturing (Zhang et al., 2019).