Petroleum Releases to Unfrozen Active Layers

In permafrost-affected regions the thickness of the active layer will be minimal — centimeters to a few meters, depending upon local conditions. The active layer begins to thaw during the spring snowmelt and continues to thicken until reaching maximum thickness in late August or September (Hinzman et al. 2005). As the active layer thaws a layer of water-saturated soil develops, which may be as thick as the entire thawed thickness. Thus, the downward flow of petroleum will be impeded due to low relative permeability to petroleum as a consequence of high soil-water saturation. With downward flow impeded, an increased flow takes place through the near surface layer of partially decayed vegetation that is typically present in many arctic ecosystems or bare ground where vegetation is not predominant. Results from field studies conducted by Mackay et al. (1974a, b, 1975) as well as Johnson et al. (1980), in which petroleum was released to unfrozen soil underlain by permafrost, illustrate how high water contents in poorly drained soils impede downward migration of released petroleum. This flow pattern leads to relatively large aerial distributions of petroleum, tempered by entrapment of the petroleum onto organic matter present in the uppermost layer of soil. However, even under these conditions petroleum does move downward through underlying mineral soil. In areas of large accumulations of petroleum, soil water will be displaced and petroleum will progress into lower mineral soils. Furthermore, over time, the petroleum may migrate deeper into the soil horizon as the active layer freezes and thaws.

In contrast, a study conducted by Mackay et al. (1975) where petroleum was released to unfrozen unsaturated (relatively low soil water contents) soils in a tundra environment resulted in infiltration of petroleum to the top of the frost line or to the water table where present. The petroleum then flowed downgradient (down slope) through a relatively thin horizontal layer of very permeable soils directly above the frost line. Using fundamental principles, the theoretical distribution of petroleum in active layer soils can be investigated.

The thin nature of the active layer and the saturated soil contained within will influence the distribution of petroleum throughout the active layer, and may allow for petroleum to be distributed as a free-phase liquid throughout the entire saturated zone. Recognizing the complex nature of characterizing the water-saturated zone contained in the active layer, due in part to the constant change taking place as thawing and refreezing occurs, the fundamental characteristics of how petroleum may distribute following a release can still be examined. Farr et al. (1990) described the distribution of free-phase LNAPL, such as petroleum, in porous media under hydrostatics considering a deep ground-water aquifer, and developed the mathematical relationships for LNAPL saturation as a function of depth from ground surface. These relationships can be re-derived to take into account the thin saturated zone typically found in a thawed or thawing active layer. As in Farr et al. (1990), total liquid saturation (water and petroleum; ST) as a function of capillary pressure between air and petroleum (Pcao) is as follows:

where S is water saturation, S is the petroleum saturation, S is residual saturation w 7 o * ' r

(assumed to be the same for both liquids), X is the pore size distribution coefficient, and Pdao is the displacement pressure between air and petroleum. Similarly water saturation (Sw) as a function of capillary pressure between petroleum and water (Pcow) can be described as follows:

where Pdow is displacement pressure between petroleum and water. Capillary pressures as a function of elevation from the frozen soil layer (z) between each fluid are as follows:

In (3) and (4) po is density of the released petroleum, pw is water density, g is the gravitational constant, b is the thickness of the saturated zone prior to the petroleum release, and To is the thickness of petroleum that would be found in a monitoring screened through the entire saturated thickness. For these calculations, an assumption is made that the thickness of the water-saturated zone stays constant.

To investigate the influence the water-saturated thickness has on petroleum saturation, consider the fluid properties and soil properties for a sandy loam shown in

Table 18.1. Assume that the thickness of petroleum that would be measured in a monitoring well installed in the impacted area is 0.7 m for this example. Again acknowledging our assumptions of hydrostatic conditions and no hysteresis, (1)-(4) can be used to estimate petroleum saturation as a function of depth for different values of saturated zone thickness prior to the petroleum release. Results from these calculations are shown in Fig. 18.1.

Table 18.1 Soil and fluid properties for the example provided in the text (soil properties are from Rawls et al. 1982)

Soil property Value

Table 18.1 Soil and fluid properties for the example provided in the text (soil properties are from Rawls et al. 1982)

Soil property Value

Porosity (F)

0.437

Residual saturation (Sr)

0.08

Pore size distribution (l)

0.553

Air-petroleum displacement pressure (Pdao)

758 kg m-1 s2

Petroleum-water displacement pressure (Pdow)

330 kg m-1 s2

Fluid property

Water density (pw)

1,000 kg m-3

Petroleum density (po)

740 kg m-3

Saturated Soil Density

Petroleum Saturation

Petroleum Saturation

Fig. 18.1 Petroleum saturation with elevation from the top of permafrost. The petroleum saturation curve on the far left corresponds to a thick water-saturated zone where the top of the frozen layer does not interfere with the migration of the petroleum. Curves to the right of the bounding curve on the far left correspond to water saturated zone thickness prior to release of the petroleum to the active layer of 0.4, 0.3, 0.2, and 0.1 m respectively

The first notable result in Fig. 18.1 is the increase in the maximum value for petroleum saturation as the thickness of the saturated zone prior to the release of petroleum decreases. As shown in Fig. 18.1, the petroleum saturations between the top of the frozen soil layer and the elevation at which the maximum value of saturation is reached also increase, as the thickness of the saturated zone prior to release decreases. In addition, at the relatively thinner water-saturated thickness, the saturation of petroleum near the surface of the soil is greater in comparison to saturation values calculated for relatively deeper water saturation zone thicknesses.

While fluctuating water surface elevations, freeze and thaw cycling, and soil heterogeneity will most likely greatly affect the distribution of petroleum in the soil, these results indicate that petroleum release in active layers with shallow saturated zone thicknesses results in comparably greater initial mobility and, thus, a potentially wider lateral distribution of petroleum. This conclusion can be drawn due to the direct correlation between a fluid saturation and the relative permeability of porous media to that fluid. Petroleum as free product will also be distributed throughout the saturated soil thickness, leading to a widespread dissolved phase plume emanating from the source and subsequently little dilution of the dissolved phase plume. In addition, the volume of petroleum contained in a subsurface with a shallow saturated zone will most likely be greater than what would be predicted from models developed by Farr et al. (1990), Lenhard and Parker (1990), and Charbeneau et al. (1999).

0 0

Post a comment