NMR Investigation of Brine Imbibition Dynamics in Pores of Tight Sandstones under Different Boundary Conditions

Abstract

This study explores imbibition dynamics and flowback of fracturing fluids in Penglaizhen sandstones by imbibition experiment and nuclear magnetic resonance analysis. A gas–water system was considered and samples were subjected to different boundary conditions and fully immersed in the wetting phase for imbibition. A close range of the pore radius was observed between 0.1 nm and 9.3 μm for all samples. Basic features of the imbibition phases were represented by the imbibition profile, and imbibition parameters were quantified in terms of gas recovery, imbibition capacity, imbibition index and pore connectivity, diffusion ability, and imbibition potential under the respective boundary conditions. The study reveals that capillary imbibition (mainly in all-face-open (AFO) and two-end-closed (TEC)) and diffusion imbibition (mainly in two-end-open (TEO) and one-end-open (OEO)) are the major mechanisms by which fracture fluids flow into the formation matrix. There is high gas recovery in all samples (>79%), which can be attributed to the high water-wet nature of the samples, while the residual gas saturation is very low with the highest value in OEO at 0.17 and the lowest in AFO at 0.01. The strong capillary force in AFO and TEC boundary conditions is the key factor for higher gas recovery than TEO and OEO samples. However, the low diffusion ability of AFO and TEC samples depict a low gas displacement effect. On the other hand, TEO and OEO boundary conditions show an increased gas displacement effect through increased diffusion ability. The imbibition index (which represents pore connection) and imbibition potential from the highest to the lowest also correspond to the diffusion ability from the highest to the lowest.

Nuclear Magnetic Resonance Investigation of Forced Imbibitions in Longmaxi Shales: Consideration of Different Boundary Conditions

“…The forced imbibition dynamics of shale with different boundaries in this study are quite different from the spontaneous imbibition dynamics under ASO, TSO, and OSO boundaries obtained by Umeobi et al . and Yang et al Umeobi et al found that the shale spontaneous imbibition flow in all boundary conditions are contributions of both the capillary pressure and diffusion. Imbibition in AFO is predominantly by the capillary pressure with negligible diffusion, but imbibitions of TSO and OSO are predominantly by diffusion imbibition.…”

“…32,33 Moreover, the values of imbibition potential are such that TSO > ASO > HSO > OSO, illustrating the different contributions made by late imbibition stages to the whole forced imbibition process. However, these findings are contrary to the previous studies, 31,32,59 which can be attributed to the intrinsic differences of imbibition mechanisms between spontaneous imbibition and forced imbibitions coupled with the boundaries. 4.5.…”

“…The diffusion ability index represents the contribution of the water diffusion process during the late imbibition stage when the forced imbibition reached equilibrium under each boundary condition. Higher diffusion ability also depicts a greater gas displacement effect and enhances gas production . As is depicted in Table , the diffusion ability is 0.1248, 0.0468, 0.0018, and 0.0560 for ASO, TSO, OSO, and OEO shales, respectively.…”

“…Higher diffusion ability also depicts a greater gas displacement effect and enhances gas production. 31 As is depicted in Table 2, the diffusion ability is 0.1248, 0.0468, 0.0018, and 0.0560 for ASO, TSO, OSO, and OEO shales, respectively. Therefore, the gas displacement effect in the late forced imbibition stage will be the highest in ASO imbibition, followed by HSO and TSO, and the lowest under the OSO case.…”

“…Kim and Kovscek adopted CT scanning technology to quantificationally investigate the fluid distribution and saturation curves of spontaneous imbibition processes under ASO, TSO, and OSO conditions. Umeobi et al compared imbibition dynamics and flowback of fracturing fluids in sandstones by NMR imbibition experiments with AFO, TEC, and two-end-open (TEO) boundary conditions. Through the NMR imbibition experiments on fractured shales under three boundaries, Yang et al found that boundary conditions mainly affect the imbibition by changing the movement relationship and contact area between the core and water.…”