[1] We report the results of an experimental investigation into the multiphase flow properties of CO 2 and water in four distinct sandstone rocks: a Berea sandstone and three reservoir rocks from formations into which CO 2 injection is either currently taking place or is planned. Drainage relative permeability and residual gas saturations were measured at 50 C and 9 MPa pore pressure using the steady state method in a horizontal core flooding apparatus with fluid distributions observed using x-ray computed tomography. Absolute permeability, capillary pressure curves, and petrological studies were performed on each sample. Relative permeability in the four samples is consistent with general characteristics of drainage in strongly water-wet rocks. Measurements in the Berea sample are also consistent with past measurements in Berea sandstones using both CO 2 /brine and oil/water fluid systems. Maximum observed saturations and permeabilities are limited by the capillary pressure that can be achieved in the experiment and do not represent endpoint values. It is likely that maximum saturations observed in other studies are limited in the same way and there is no indication that low endpoint relative permeabilities are a characteristic of the CO 2 /water system. Residual trapping in three of the rocks is consistent with trapping in strongly water-wet systems, and the results from the Berea sample are again consistent with observations in past studies. This confirms that residual trapping can play a major role in the immobilization of CO 2 injected into the subsurface. In the Mt. Simon sandstone, a nonmonotonic relationship between initial and residual CO 2 saturations is indicative of a rock that is mixed or intermediate wet, and further investigations should be performed to establish the wetting properties of illite-rich rocks. The combined results suggest that the petrophysical properties of the multiphase flow of CO 2 /water through siliciclastic rocks is for the most part typical of a strongly water-wet system and that analog fluids and conditions may be used to characterize these properties. Further investigation is required to identify the wetting properties of illite-rich rocks during imbibition processes.

No abstract

Dissolution of CO 2 into brine is an important and favorable trapping mechanism for geologic storage of CO 2 . There are scenarios, however, where dissolved CO 2 may migrate out of the storage reservoir. Under these conditions, CO 2 will exsolve from solution during depressurization of the brine, leading to the formation of separate phase CO 2 . For example, a CO 2 sequestration system with a brine-permeable caprock may be favored to allow for pressure relief in the sequestration reservoir. In this case, CO 2 -rich brine may be transported upwards along a pressure gradient caused by CO 2 injection. Here we conduct an experimental study of CO 2 exsolution to observe the behavior of exsolved gas under a wide range of depressurization. Exsolution experiments in highly permeable Berea sandstones and low permeability Mount Simon sandstones are presented. Using X-ray CT scanning, the evolution of gas phase CO 2 and its spatial distribution is observed. In addition, we measure relative permeability for exsolved CO 2 and water in sandstone rocks based on mass balances and continuous observation of the pressure drop across the core from 12.41 to 2.76 MPa. The results show that the minimum CO 2 saturation at which the exsolved CO 2 phase mobilization occurs is from 11.7 to 15.5%. Exsolved CO 2 is distributed uniformly in homogeneous rock samples with no statistical correlation between porosity and CO 2 saturation observed. No gravitational redistribution of exsolved CO 2 was observed after depressurization, even in the high permeability core. Significant differences exist between the exsolved CO 2 and water relative permeabilities, compared to relative permeabilities derived from steady-state drainage relative permeability measurements in the same cores. Specifically, very low CO 2 and water relative permeabilities are measured in the exsolution experiments, even when the CO 2 saturation is as high as 40%. The large relative permeability reduction in both the water and CO 2 phases is hypothesized to result from the presence of disconnected gas bubbles in 123 460 L. Zuo et al. this two-phase flow system. This feature is also thought to be favorable for storage security after CO 2 injection.

This paper demonstrates that the nature and extent of residual CO 2 trapping depend on the process by which the CO 2 phase is introduced into the rock. We compare residual trapping of CO 2 in Berea Sandstone by imbibing water into a core containing either exsolved CO 2 or CO 2 introduced by drainage. X-ray computed tomography measurements are used to map the spatial distribution of CO 2 preimbibition and postimbibition. Unlike during drainage where the CO 2 distribution is strongly influenced by the heterogeneity of the rock, the distribution of exsolved CO 2 is comparatively uniform. Postimbibition, the CO 2 distribution retained the essential features for both the exsolved and drainage cases, but twice as much residual trapping is observed for exsolved CO 2 even with similar preimbibition gas saturations. Residually trapped exsolved gas also disproportionately reduced water relative permeability. Development of process-dependent parameterization will help better manage subsurface flow processes and unlock benefits from gas exsolution.