Microscopic characteristics of tight sandstone reservoirs and their effects on the imbibition efficiency of fracturing fluids: A case study of the Linxing area, Ordos Basin

Abstract

The Linxing area within the Ordos Basin exhibits pronounced reservoir heterogeneity and intricate micro-pore structures, rendering it susceptible to water-blocking damage during imbibition extraction. This study delved into the traits of tight sandstone reservoirs in the 8th member of the Shihezi Formation (also referred to as the He 8 Member) in the study area, as well as their effects on fracturing fluid imbibition. Utilizing experimental techniques such as nuclear magnetic resonance (NMR), high-pressure mercury intrusion (HPMI), and gas adsorption, this study elucidated the reservoir characteristics and examined the factors affecting the imbibition through imbibition experiments. The findings reveal that: ① The reservoir, with average porosity of 8.40% and average permeability of 0.642 × 10−3 μm2, consists principally of quartz, feldspar, and lithic fragments, with feldspathic litharenite serving as the primary rock type and illite as the chief clay mineral; ② Nano-scale micro-pores and throats dominate the reservoir, with dissolution pores and intercrystalline pores serving as predominant pore types, exhibiting relatively high pore connectivity; ③ Imbibition efficiency is influenced by petrophysical properties, clay mineral content, and microscopic pore structure. Due to the heterogeneity of the tight sandstone reservoir, microscopic factors have a more significant impact on the imbibition efficiency of fracturing fluids; ④ A comparative analysis shows that average pore size correlates most strongly with imbibition efficiency, followed by petrophysical properties and clay mineral content. In contrast, the pore type has minimal impact. Micropores are vital in the imbibition process, while meso-pores and macro-pores offer primary spaces for imbibition. This study offers theoretical insights and guidance for enhancing the post-fracturing production of tight sandstone reservoirs by examining the effects of these factors on the imbibition efficiency of fracturing fluids in tight sandstones.