Characteristics and Influencing Factors for Forced Imbibition in Tight Sandstone Based on Low-Field Nuclear Magnetic Resonance Measurements

Abstract

Spontaneous imbibition (SI) generally occurs under forced pressure (the difference between hydraulic fluid pressure and original pore pressure) during shut-in time. However, the experimental study of SI is commonly performed at atmospheric pressure and the effect of the forced pressure is often neglected. How the forced pressure influences the SI behaviors under different factors is still not clear. In this paper, low-field nuclear magnetic resonance (LF-NMR) was adopted to study the mechanism of SI in the tight sandstone rock sample under forced pressure (forced imbibition, FI). The effects of boundary conditions, initial water saturation, bedding plane (BP) direction, and fluid salinity on oil recovery were also systematically investigated. Results showed that the ultimate oil recovery (UOR) varied from different boundary conditions. An inverse correlation also exists between the water uptake and salinity. As osmotic pressure exists, more water was imbibed into core samples with the decrease of KCl salinity. The rock sample with perpendicular BP direction has a higher UOR than that with parallel BP direction. Moreover, the initial water saturation has a great effect on UOR. Higher water saturation would result in a lower UOR. This study aims to provide some insights into understanding the mechanism of FI in UOR enhancement.

Recent advances in spontaneous imbibition with different boundary conditions

“…However, according to the pattern of spontaneous imbibition, all boundary conditions can be divided into two categories, i.e., all open faces covered by water ( Fig. 1a) (Xu et al, 2018) and part of open faces covered by water ( Fig. 1b) (Harimi et al, 2019;Zhang et al, 2019).…”

Imbibition and Oil Recovery Mechanism of Fracturing Fluids in Tight Sandstone Reservoirs

“… 23 , 24 Adding surfactants to the fracturing fluid to change the wettability and reduce the interfacial tension can effectively improve the oil recovery after fracturing reformation. 25 − 30 Based on the mass method experiment, it was found that the degree of salinity is negatively correlated with spontaneous imbibition recovery; 31 , 32 fractures can effectively expand the imbibition area of the dense matrix in contact with water, thereby increasing the recovery and imbibition rate. 33 Guo and Ren noted that there is a certain optimal interfacial tension that reduces the amount of bypassed oil in the formation, resulting in the highest imbibition recovery, rather than a lower interfacial tension corresponding to the higher oil recovery.…”

“…The pore throat radius of tight oil reservoirs is on the nanometer scale and the thickness of the adsorption layer is on the micron scale. − Pores above the submicron scale play a leading role in the process of oil displacement in tight reservoirs owing to the thickness of the adsorption layer, and nano-submicron pores contribute less to imbibition recovery . In cores with different pore structure combinations, the ratio of medium to large pore throats determines the effect of spontaneous imbibition and oil displacement; residual oil is mainly retained in the microporous throats. , Cheng, Lai et al investigated the influence of gravity and pore throat structure on the imbibition effect during the imbibition process and noted that the nanopore throat is the main imbibition point. , In previous studies, scholars have mainly used the mass and volume methods to conduct imbibition experiments, exploring the factors that affect imbibition and displacement; however, these experiments were mostly based on formation water. , Adding surfactants to the fracturing fluid to change the wettability and reduce the interfacial tension can effectively improve the oil recovery after fracturing reformation. − Based on the mass method experiment, it was found that the degree of salinity is negatively correlated with spontaneous imbibition recovery; , fractures can effectively expand the imbibition area of the dense matrix in contact with water, thereby increasing the recovery and imbibition rate . Guo and Ren noted that there is a certain optimal interfacial tension that reduces the amount of bypassed oil in the formation, resulting in the highest imbibition recovery, rather than a lower interfacial tension corresponding to the higher oil recovery. , Jing et al studied the influence of matrix permeability, temperature, pressure, pore size, and fracture density on the degree of dynamic imbibition production; a normalized model of the dimensionless imbibition recovery degree is obtained using dimensionless experimental parameters .…”

The Characteristics of Oil Migration due to Water Imbibition in Tight Oil Reservoirs

“…As a key unconventional energy, tight oil has received increased attention by many scholars and has gradually become another research topic following shale gas [1]. The commercial exploitation of tight oil has greatly affected the energy structure of the world, especially North America [2,3].…”

“…After hydraulic fracturing stimulation, shut in operations for a period are beneficial to the production increase of tight oil. Extended shut in time can promote fracturing fluid imbibition into reservoirs to displace crude oil and has been proven to be effective for enhanced oil recovery [1].…”