Experimental Investigation of Spontaneous Imbibition in a Tight Reservoir with Nuclear Magnetic Resonance Testing

Abstract

Hydraulic fracturing is the significant technology for exploiting tight resources. Spontaneous water imbibition is an important mechanism governing the process of hydraulic fracturing, and the water imbibition from the fracture into the matrix is an essential factor that affects the reservoir production performance. In this study, imbibition experiments and nuclear magnetic resonance (NMR) testing were combined to analyze fluid flow tight core samples in a pore-scale level. The imbibition experiments were categorized into two systems, the gas/water/rock system and the oil/water/rock system. The NMR measurements were performed at different times for these two systems. The relationship between T 2 relaxation time, pore radius, and pore types was established. Theoretical models describing water imbibition into porous media were used to facilitate the interpretation of the experimental results. The results demonstrate that the volume of imbibed water is large during the early imbibition period and that the imbibition recovery increases rapidly as time proceeds. The volume of imbibed water reaches a constant level at the end of the experiment. The volume of imbibed water in the oil/water/rock experiment is less than that in the gas/water/rock experiment; however, the experiment shows an inverse relation for the duration of the imbibition. For the gas/water/rock system, the water is originally imbibed into micropores and small mesopores present in the natural core samples. There are four types of T 2 distributions related to the oil/water/rock imbibition process. Finally, the experimental results indicate the effect of boundary conditions, wettability, temperature, and oil viscosity on water imbibition. The oil recovery due to water imbibition for the oil/water/rock is mainly controlled by the capillary force, gravity force, and characteristic length of the core sample. Water-wet conditions are more preferable for spontaneous imbibition. Through a detailed study of imbibition experiment and NMR testing, significant insight is provided into the fluid flow in the tight porous media.

Recent advances in spontaneous imbibition with different boundary conditions

“…Spontaneous imbibition is a very important mechanism of oil recovery from naturally fractured reservoirs and unconventional reservoirs (Lai et al, 2016;Teklu et al, 2017). When water is injected into reservoirs, it preferentially goes through the fracture system.…”

Nuclear Magnetic Resonance Measurement of Oil and Water Distributions in Spontaneous Imbibition Process in Tight Oil Reservoirs

“…Combining C and T 2 spectrum, the pore-radius distribution, shown in Figure 3, is obtained. The relationship between T 2 , pore radius, and pore type is shown in Table 3 following the methods of Lai et al [36] and Liu et al [42].…”

“…Zhou et al [35] studied the influencing factors of SI, including rock size, rock properties, fluid properties, wettability, initial oil saturation, and boundary condition, by combining NMR and conventional SI experiments. Lai et al [36] demonstrated that the water is first imbibed into micropores and small mesopores, and variations in the T 2 spectrum are principally reflected in the T 2 stage when the relaxation time is ≥10 ms. Wang et al [37] reported oil contribution from micropores up to 53.34%, and that permeability of 0.1 mD is a critical point at which the oil contribution of mesopores surpasses that of micropores.…”

Permeability regain and aqueous phase migration during hydraulic fracturing shut-ins

“…Scholars also tried to obtain the relationship of pore radius distribution from transverse relaxation time (T 2 ) and from conventional mercury injection method considering these two methods can describe pore structures 28,29 . Meanwhile, assuming pore spaces can be simplified to be columnar or spherical, numerous studies gave a linear relation between T 2 and pore radius 30–32 . Based on these technologies, scholars begin to use this technique to monitor water profile during imbibition in tight sandstone, shale or other unconventional rock samples 33 .…”