Abstract:Fast synchrotron-based X-ray microtomography was used to image the injection of super-critical CO 2 under subsurface conditions into a brine-saturated carbonate sample at the pore-scale with a voxel size of 3.64 µm and a temporal resolution of 45 s. Capillary pressure was measured from the images by finding the curvature of terminal menisci of both connected and disconnected CO 2 clusters. We provide an analysis of three individual dynamic drainage events at elevated temperatures and pressures on the tens of s… Show more
“…However, in the case of a drainage event, a ganglion formed by Roof snap-off 34 retains the capillary pressure that is lower than that of the local threshold capillary pressure just before the snap-off event: the reason again is that the trapped phase re-arranges itself in the pore space to minimize the oil pressure. This has also been observed in a recent study on drainage 4 . As the capillary pressure of the connected oil is larger than that of the disconnected phase, with further oil injection it can reconnect.Figure 5Comparison of local capillary pressures before and after snap-off events in drainage and imbibition.…”
Understanding the pore-scale dynamics of two-phase fluid flow in permeable media is important in many processes such as water infiltration in soils, oil recovery, and geo-sequestration of CO2. The two most important processes that compete during the displacement of a non-wetting fluid by a wetting fluid are pore-filling or piston-like displacement and snap-off; this latter process can lead to trapping of the non-wetting phase. We present a three-dimensional dynamic visualization study using fast synchrotron X-ray micro-tomography to provide new insights into these processes by conducting a time-resolved pore-by-pore analysis of the local curvature and capillary pressure. We show that the time-scales of interface movement and brine layer swelling leading to snap-off are several minutes, orders of magnitude slower than observed for Haines jumps in drainage. The local capillary pressure increases rapidly after snap-off as the trapped phase finds a position that is a new local energy minimum. However, the pressure change is less dramatic than that observed during drainage. We also show that the brine-oil interface jumps from pore-to-pore during imbibition at an approximately constant local capillary pressure, with an event size of the order of an average pore size, again much smaller than the large bursts seen during drainage.
“…However, in the case of a drainage event, a ganglion formed by Roof snap-off 34 retains the capillary pressure that is lower than that of the local threshold capillary pressure just before the snap-off event: the reason again is that the trapped phase re-arranges itself in the pore space to minimize the oil pressure. This has also been observed in a recent study on drainage 4 . As the capillary pressure of the connected oil is larger than that of the disconnected phase, with further oil injection it can reconnect.Figure 5Comparison of local capillary pressures before and after snap-off events in drainage and imbibition.…”
Understanding the pore-scale dynamics of two-phase fluid flow in permeable media is important in many processes such as water infiltration in soils, oil recovery, and geo-sequestration of CO2. The two most important processes that compete during the displacement of a non-wetting fluid by a wetting fluid are pore-filling or piston-like displacement and snap-off; this latter process can lead to trapping of the non-wetting phase. We present a three-dimensional dynamic visualization study using fast synchrotron X-ray micro-tomography to provide new insights into these processes by conducting a time-resolved pore-by-pore analysis of the local curvature and capillary pressure. We show that the time-scales of interface movement and brine layer swelling leading to snap-off are several minutes, orders of magnitude slower than observed for Haines jumps in drainage. The local capillary pressure increases rapidly after snap-off as the trapped phase finds a position that is a new local energy minimum. However, the pressure change is less dramatic than that observed during drainage. We also show that the brine-oil interface jumps from pore-to-pore during imbibition at an approximately constant local capillary pressure, with an event size of the order of an average pore size, again much smaller than the large bursts seen during drainage.
“…Fast synchrotron tomography allows the acquisition of images in under 60 s. Many unsteady-state, dynamic, pore-scale processes have been imaged, including Haines jumps (15), capillary pressure changes during reservoir condition CO2-brine drainage (20), ganglion dynamics during the capillary desaturation of oil (21), and reactive transport processes in complex carbonates (22).…”
mentioning
confidence: 99%
“…At high flow rates, the fragmentation of large trapped ganglia has been observed in the pore space of a complex carbonate rock (23). At lower capillary numbers, the trapping of CO2 (20) and oil (21) in disconnected ganglia has been quantified in sandstones. The connection and reconnection of ganglia was observed as a result of the local advance and recession of the wetting phase.…”
The current conceptual picture of steady-state multiphase Darcy flow in porous media is that the fluid phases organize into separate flow pathways with stable interfaces. Here we demonstrate a previously unobserved type of steady-state flow behavior, which we term "dynamic connectivity," using fast pore-scale X-ray imaging. We image the flow of N 2 and brine through a permeable sandstone at subsurface reservoir conditions, and low capillary numbers, and at constant fluid saturation. At any instant, the network of pores filled with the nonwetting phase is not necessarily connected. Flow occurs along pathways that periodically reconnect, like cars controlled by traffic lights. This behavior is consistent with an energy balance, where some of the energy of the injected fluids is sporadically converted to create new interfaces.steady state | pore-scale imaging | immiscible two-phase flow | dynamic connectivity | geologic CO 2 storage T he definition of relative permeability is based on a conceptual model where each phase occupies its own static, connected fraction of the pore space (1-3). Observations of two-phase flow in beadpacks and micromodels at low flow rates show that the wetting-and nonwetting-phase fluids flow through their own network of separate stable pathways. Increasing the saturation of one phase increases the number of channels occupied by one fluid and decreases the number occupied by the other. The nonwetting phase generally occupies the large pores, whereas the wetting phase occupies the small pores and indentations in the surface of the solid (4-7).When the balance of capillary and viscous forces is modified such that viscous forces begin to dominate at the pore scale, the nonwetting phase can be pushed out of the pore space and flow can occur through the advection of disconnected ganglia (8, 9). The onset of ganglion motion is well understood, and this flow behavior has been observed during steady-state flow in model pore spaces (1-3, 7, 10-13).The key observation during slow steady-state flow, typical of most displacements in natural settings, such as during hydrocarbon recovery or CO2 storage, is that the pathway, once established, remains stable (2, 4-7). Flow patterns for different wettabilities have also been quantified in micromodel studies (14). Again, once flow pathways were established, they were not observed to change. However, there are no studies thus far that have investigated this flow behavior in the pore space of natural rocks.Developments in micro X-ray computed tomography (X-ray CT) mean that pore-scale phenomena can now be observed in rocks at reservoir conditions in both laboratory-based scanners and synchrotron beamlines (8,9,(15)(16)(17). This technological advance has allowed, for instance, observations of residual trapping (18) and wetting behavior (19).Fast synchrotron tomography allows the acquisition of images in under 60 s. Many unsteady-state, dynamic, pore-scale processes have been imaged, including Haines jumps (15), capillary pressure changes during reservoir cond...
“…Furthermore, understanding the relations between scales has significant scientific and practical implications. Some studies have focused on finding links between macroscopic processes and pore-scale air-water displacements in simple granular media (DiCarlo et al, 2003;Grapsas and Shokri, 2014), packed glass beads (Moebius and Or, 2014b), or oil-water displacements in cemented porous material Andrew et al, 2015;Oughanem et al, 2015;Bultreys et al, 2015). Scaling can help to determine the soil parameters capable of predicting the long-term fate of pollutants and fluxes of water, heat, solutes, and gases at different scales (Pachepsky et al, 2003).…”
Core Ideas
Large pressure jumps occur in macroporous soils during drainage.
Jumps result from the fast entrance of a fluid phase into macropores.
The entrance occurs after a break in the capillary shield surrounding macropores.
Scaling factors of pressure jump sizes may differ from the Haines jumps.
Discontinuous air–water displacement at the pore scale (from 10−5 to 10−3 m) affects fluid invasion in porous media at the core scale (10−3 to 1 m). Understanding of this effect is essential to upscale flow processes. In this study we used the analysis of pressure jumps to propose an upscaling mechanism. Large pressure jumps occur during drainage in macroporous structured soils; we suggest a hypothesis for their occurrence. Drainage experiments in packed sand and structured soils enclosing large pores showed large jumps (∼5‐hPa peak pressure). Large jumps resulted from a pressure relaxation process that first initiates from pore‐scale air–water displacements and then expands to larger scales. We found that the power‐law exponents for the distribution of the size of large jumps found in structured soil are greater than the typical values reported for Haines jumps in packed granular porous media. The difference in the exponent suggests that the magnitude of displacements occurring in structured soil has different scaling factors than in simple media. A mechanism for this change of scale is proposed on the basis of the large contrast between pore throat and matrix in macroporous soil. The mechanism consists of a fast pressure relaxation in the macropores triggered by a break in the capillary shield at the pore throat. These findings contribute to an explanation of the scaling relations of air–water displacements in complex porous media and unveil links between the soil structure and flow of fluids.
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