A novel bound water occurrence model for tight sandstone

Abstract

Understanding the occurrence state and microscopic distribution of bound water, as well as the factors governing bound water saturation and its impact on natural gas flow within nano-scaled pore networks, holds significant importance for reservoir evaluation, productivity prediction, and production. However, the complexity of pore structure poses a challenge in quantitatively characterizing these aspects, and currently, no effective method exists to achieve this characterization. This study takes the Bashijiqike tight sandstone of the Kuqa Depression of the Tarim Basin as an example; by integrating physical property test, XRD, microscopic observation with casting thin section and SEM, HPMI, NMR, and NMR-supervised gas-charging experiment, proposed a novel bound water occurrence model to depict the microscopic distribution of bound water in nano-scaled pore space. Furthermore, the impact of bound water on gas-flowing behaviors was thoroughly examined and the complex relationships between bound water saturation and reservoir macroscopic and microscopic parameters were explored. In addition, based on the grey relational analysis, the critical controlling factor on bound water saturation were screened out.The bound water in the pore space of tight sandstone comprises two components: capillary-bound water within small pores (<rcutoff) and restricted (by pore/throat < rcutoff) large pores, layered water-film within large pores (>rcutoff). The water-film within large pores is non-uniform, non-isothick, and low in content. During gas charing to reservoirs, the water-film thickness decreased gradually as the charging force increased,with a minimum thickness ranging from 0.10 to 3.72 nm. Water-film accounted for 1.75% to 22.96% of the total bound water content, with an average of 7.28%. In contrast to previous studies,we found that water-film had a negligible impact on the minimum pore/throat radius for gas charging, with only an increase of 0.7 nm. However, it significantly altered the physical characteristics, reducing gas permeability from 46.11% to 97.84%, with an average reduction of 78.37%. Furthermore, a water-film effectively eradicated the slip effect, leading to the prevalence of Darcy and diffusion flow as the primary mechanisms for gas seepage. At low pressure, diffusion flow prevails, whereas Darcy flow dominates at high pressure. I/S-mixed layer and illite exert the most significant influence on bound water saturation.