Effects of bedding direction on brine imbibition in Lower Shaximiao tight sandstone: An NMR analysis

Abstract

Spontaneous imbibition phenomenon caused by capillarity plays an important role in the production of tight sandstone gas. Understanding the effects of bedding direction on liquid imbibition is important as it can give insights into water block phenomena that can negatively impact gas production. In this paper, four tight sandstone specimens were cored from Sichuan Basin, China with different bedding directions (i.e., the angles between the bedding plane and the horizontal plane of four specimens are 0°, 45°, 60° and 90°, respectively). Nuclear magnetic resonance (NMR) technique was used to monitor the fluid distribution changes during the spontaneous imbibition processes of these specimens. The results show that such tight sandstone exhibited strong hydrophilicity with the small pores dominating throughout the imbibition process. The T 2 spectra for all four rocks specimens not only increased upward but also shifted to larger relaxation times due to brine intake along the minerals surface (further indicating the hydrophilic characteristics of this rock). For such tight sandstone, capillary force and friction resistance between the fluid (brine) and mineral surfaces controlled the imbibition processes. For the 0°, 45° and 60° specimens, “stop illusion” phenomena which the imbibed brine volume had a tiny increase during a long period occurred. However, this was not observed in the 90° specimen (where the bedding direction and fluid flow direction are parallel). This is largely attributed to the significant tortuosity in the flow for the 0°, 45° and 60° specimens compared to the 90° specimen, leading the smallest friction resistance existed in the 90° specimen. Among these four specimens, the 0° specimen had the slowest imbibition rate and lowest imbibition efficiency, while the 90° specimen exhibited the fastest rate and highest efficiency. With the aim of maximizing gas production, this work provides some guidance for the selection of directional fracturing and injection-production methods in the exploitation and production of tight sandstone gas field. • Rock specimens with different bedding directions were used to conduct the spontaneous imbibition experiments monitored by NMR. • Different imbibition behaviors of four specimens were compared and analyzed. • Driving and resistance forces on the fluid during the experiment were summarized and compared. • The “stop illusion” phenomenon was observed and analyzed.