Petroleum Releases to Frozen Active Layers

Migration of petroleum resulting from releases to frozen soils is significantly impacted by ice contained in the soil. At the minimum, ice present as pore ice will act as a solid, changing the pore geometry and thus the capillarity and permeability of the soil. In the extreme, the ground surface will be nearly impermeable, and downward migration will be minimal for the most part. Under these conditions surface flow will dominate, resulting in rapid and extensive spread of contamination upon release, though the higher viscosity at cold temperatures will inhibit lateral movement. In contrast to a release of petroleum to an unfrozen active layer, the increased exposure of the petroleum to the surface elements leads to greater losses of petroleum hydrocarbons by physical weathering (evaporation and photochemical oxidation).

Mackay et al. (1975) and Johnson et al. (1980) both conducted releases of crude oil to frozen ground in mature black spruce forests containing permafrost. Soils at both study sites were predominantly fine grain (silt). Results from sampling events shortly after each release in both studies indicated that overall there was minimal infiltration of the crude oil past the surface moss layer. Mackay et al. (1975) did document that infiltration of the crude oil did occur at spring thaw.

A laboratory study conducted by Barnes and Wolfe (2008) illustrates how pore ice in coarse soil impacts the movement of petroleum as the fluid infiltrates frozen soil. Coarse soils are used extensively in the Arctic for foundations supporting infrastructure necessary for oil production as well as other activities, and are naturally present in Arctic and Antarctic terrain. In this study, petroleum was released to partially water-saturated sand that was frozen to -5°C. Two-dimensional petroleum flow through the frozen sand was approximated by packing moist sand between two vertical sheets of clear Plexiglas secured to a rigid frame and then freezing the entire unit. Once frozen, a volume of colored refined petroleum (JP 2) at a temperature of -5°C was introduced into the column and the progression of the petroleum was documented with time-lapse photography. Results from this study indicate that ice content far less than saturation can greatly affect the movement of petroleum, due to dead-end-pores created by ice forming in relatively smaller pore spaces, thus blocking flow paths. In addition, the formation of preferential flow paths results in deeper penetration of petroleum and unpredictable migration patterns. At the extreme, petroleum infiltration may be limited to the near surface soils due to high ice contents, as others have shown in field tests (Mackay et al. 1975; Johnson et al. 1980; Chuvilin 2001a).

Investigation of petroleum migration in frozen coarse soils in soil flumes can be taken one step further by investigating the infiltration of petroleum into a frozen heterogeneous coarse grain soil (Barnes and Adhikari, unpublished data). For this investigation, a layered soil was created in a soil flume with a layer of fine grain sand (1.3 cm thick) interbedded between coarse grain sand layers. The soil was then thoroughly wetted by introducing water to the top of the flume at timed intervals and allowing the water to drain through the sand layers. The flume was covered (to reduce evaporation) and allowed to drain for a sufficiently long enough time for gravity drainage to end. At this point, water in the pore space is held in the pore space by capillary forces at some residual level. The flume was then insulated on the sides and the bottom and placed in a cold room at -5°C to induce top-down freezing. Once frozen, colored JP2 chilled to -5°C was introduced to the top of the soil layer, and migration of the petroleum through the soil was tracked using time-lapse photography. The test was repeated in layered soil that was prepared in exactly the same manner but left unfrozen. Results from these tests are shown in Fig. 18.2.

The impact the fine grain sand layer has on the movement of petroleum through the frozen soil in comparison to the unfrozen soil is clearly evident in the images shown in Fig. 18.2. The fine sand layer in the frozen soil acts as a barrier to further downward petroleum migration. This result is due to the development of a capillary break between the fine grain sand and the underlying coarse grain sand. As water infiltrates and drains through a layered unsaturated soil, capillary breaks develop at the interface between relatively fine grain soil and underlying coarser grain soil, due to the comparably low relative permeability to water in the coarse grain soil in relation to the overlying fine grain soil. Low relative permeability in these cases is brought about by the comparably lower soil water content in this soil, owing to the larger pore dimensions and thus lower capillary forces in this layer. Once a capillary break develops, the low relative permeability in the underlying coarse soil restricts drainage of water out of the overlying fine grain soil, resulting in high water saturation in the fine grain soil. If a sufficient water saturation exists in the

Fig. 18.2 Migration of petroleum through frozen (images a and b) and unfrozen (images c and d) layered soil. Images a and c were taken 1 h after releasing the petroleum to the soil. Images b and d were taken 1 day after releasing the petroleum to the soil. Image e was taken after the frozen soil from images a and b was thawed from the top down fine grain soil prior to freezing (at least 91.7%), the pore space will be filled with ice once frozen, creating a barrier to infiltration of any liquids such as inadvertently spilled petroleum. During top-down thawing of the frozen sand in this investigation, the petroleum drained and redistributed as the thawing front advanced downward (Fig. 18.2). Some interference with the sides of the flumes was encountered, so the image has been trimmed to show the central portion of the redistributed plume. One will also note the extensive distribution of petroleum throughout the entire thickness of coarse sand above the thin layer of fine sand, which most likely developed through capillary movement of the petroleum.

In layered soil, the development of capillary breaks in frozen soil (ice-rich capillary breaks) as shown in Fig. 18.2 results in substantial increase in lateral petroleum movement upon the release of petroleum, in comparison to unfrozen soils and in comparison to frozen non-layered (homogeneous) soil (Barnes and Wolfe 2008). Others have noted the preferential lateral movement of petroleum in frozen soil Mackay et al. (1975). Damian Gore personal communications) the development of ice-rich capillary breaks may in part be the reason for these occurrences.

Ice has a substantial impact on petroleum distribution in frozen soils. Present in soil pores, ice impacts flow paths taken by infiltrating petroleum, resulting in extensive lateral distribution and possibly deeper penetration into the subsurface as petroleum seeks preferential preferential paths with relatively low ice contents. In layered frozen soils, complex distributions of petroleum will develop as infiltrating petroleum encounters soil layers saturated with pore ice. Segregated ice (ice lenses) formed in fine grain soils will also impact the flow paths taken by infiltrating petroleum by creating impermeable barriers to flow. During thawing, petroleum released to a frozen soil will redistribute as the properties of the porous media change and water is added through thawing ice contained in the soil and from infiltration from thawing snow and ice on the ground surface. In fine soils containing segregated ice, petroleum movement may be enhanced as the ice melts and petroleum flows through the relatively higher permeable soils where the segregated ice existed.

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