The Effects of Nonuniform Wettability and Heterogeneity on Induction Time and Onset of Spontaneous Imbibition

Abstract

Summary Spontaneous imbibition is a capillary-dominated displacement process in which a nonwetting fluid is displaced from a porous medium by the inflow of a more-wetting fluid. Decades of core-scale experiments have concluded that spontaneous imbibition occurs by a uniformly shaped saturation front with a rate that scales with the square root of time. The imbibition rate during early stages of spontaneous imbibition (the onset period) has been reported to deviate from the square-root-of-time behavior, although its effect on the imbibition process is not well-understood. Controlled-imbibition tests, presented in this paper, demonstrate that restricted wetting-phase flow during the onset period gives irregular saturation fronts and deviation from the square-root-of-time behavior. The deviation was caused by local variation in porosity and permeability or by a nonuniform wettability distribution, and was directly visualized or imaged by positron-emission tomography (PET). Without knowledge of local flow patterns, the development of irregular saturation fronts cannot be observed; hence, the effect cannot be accounted for, and the development of spontaneous imbibition might be erroneously interpreted as a core-scale wettability effect. Restricted wetting-phase flow at the inlet affects Darcy-scale wettability measurements, scaling, and modeling; our observations underline the need for a homogeneous wettability preference through the porous medium when performing laboratory spontaneous-imbibition measurements.

Simulation of Counter-Current Spontaneous Imbibition Based on Momentum Equations with Viscous Coupling, Brinkman Terms and Compressible Fluids

“…For moderate values of o a delay (very low imbibition rates) is seen at early times and then followed by linear recovery against square root of time. That can explain the observed induction times sometimes reported experimentally before theoretical imbibition behavior is established (Akin et al 2000;Tang and Firoozabadi 2000;Zhou et al 2000;Føyen et al 2019). We see especially that the recovery lines of o = 10 3 and 10 4 m 2 /s are parallel to the one without viscous terms.…”

“…Spontaneous imbibition has been studied during the induction time with in-situ images (Føyen et al 2019). They indicate a period where local fronts form and develop before merging to more symmetrical profiles.…”

“…11 Simulation of the experimental counter-current imbibition data from Zhou et al (2000) where two cores were aged in crude oil with different aging time (indicated). First the slopes of the linear data were matched assuming zero Brinkman terms (full lines), then the original data were matched by including nonzero Brinkman terms to capture the induction time oil-wet state or low initial water saturation (Tang and Firoozabadi 2000;Zhou et al 2000), non-uniform wetting or interaction with epoxy used to seal core surfaces (Føyen et al 2019); conditions that complicate the development of a continuous water film. As there are many reservoirs with mixed-or oil-wet state and low water saturation, this mechanism can increase the imbibition time.…”

“…For the second question we note that counter-current spontaneous imbibition in 1D linear systems theoretically should follow a square root of time profile (McWhorter and Sunada 1990). Several imbibition experiments, however, show that the expected flow regime is not established until after a certain delay time (Akin et al 2000;Føyen et al 2019). Unconventional porous media can also deviate from the square root of time profile and be better described by other exponents.…”

Experimental Investigation on the Effect of Pore Size on Spontaneous Imbibition Recovery in Oil-Wet Reservoirs

“…Spontaneous imbibition is a capillary-dominated displacement process in which a nonwetting fluid is displaced from a porous medium by the inflow of a more-wetting fluid [1]. As the primary source of energy in unconventional reservoir, spontaneous imbibition has made significant contributions to the recovery of matrix area and fracture area during the process of fracturing fluid reverse drainage and well soaking, and thus, it is regarded as an important tool to EOR [2][3][4].…”