Influence of Freezing and Thawing Cycles on Petroleum Distribution

As is known, the freezing-thawing processes are attended by structure-forming processes which result in changes in soil properties, which in turns influence petroleum redistribution in the soil and its transformation, fractionating and formation of organic-mineral composition. Results from the experimental investigations of Chuvilin et al. (2001a, b) showed cryogenic expulsion of petroleum from freezing to thawing zone in several different freezing soils (Fig. 18.3). Barnes et al. (2004) showed with a mass balance that the primary mode of downward petroleum migration in a freezing soil is through ice formation in the pore space, resulting in displacement of petroleum out of the pore as the void is filled with ice. The resulting crystallization pressure is usually enough for petroleum displacement, due to nonpolar nature of the liquid leading to only slight connectivity with mineral particles.

Petroleum distribution in the pore space, composition of the petroleum, initial content in soils, and freezing speed all influence the efficiency of cryogenic expulsion; for example, in sandy soils the amount of petroleum expulsion into underlying unfrozen soil is more than in clay soils. A coefficient of oil expulsion can be used to quantify the efficiency of cryogenic expulsion. This coefficient is equal to the ratio of displaced petroleum to the initial petroleum content. The experimental developed relation of the coefficient of oil expulsion from freezing rate is shown in Fig. 18.4.

Soil Pore Freezing Thawing
Fig. 18.3 Pattern of the water content (w) and petroleum content (Z) with height of soil freezing at -7°C. a, b Sand (initial water and petroleum content 16% and 5% respectively). c, d Clay (initial water and petroleum content 43% and 5.4% respectively)
Fig. 18.4 Influence of the freezing rate on the coefficient of oil expulsion (Kn) in sand samples (initial water and oil content 16% and 5%, respectively)

The displacement of petroleum from the frozen soil to the unfrozen soil in the freezing soil sample shown in Fig. 18.4 was determined to be 70% from initial petroleum content under the favorable conditions of the test. In part, we can assume that the cryogenic expulsion is related to the petroleum "cryogenic metamorphiza-tion" — the separation of the more mobile petroleum hydrocarbon components from the petroleum. These hydrocarbons then migrate ahead of the freezing front. This process is poorly studied. One can suppose that naphthenes will be more mobile. Naphthenes are saturated hydrocarbons which don't display the associative properties under temperature reduction. In nature, the cryogenic expulsion may be the significant factor contributing to the petroleum's mobile formations and further dissipation. This process could have predominant influence in the active layer drained soils, where petroleum hydrocarbons partition into infiltrating water and migrate downward further into the soil horizon.

Laboratory studies of microstructure of freezing oil polluted sediments by White and Willams (1999) and White and Coutard (1999) have shown that their microstructure in frozen soil containing petroleum differs from frozen soil without petroleum under the same conditions. Soil structure change with addition of petroleum depends on the petroleum concentration in the soil. Relatively small concentrations of petroleum (below 200 ppm) promote the aggregation of particles and an increase in sediment porosity, resulting in an increase in hydraulic conductivity. Relatively high content of petroleum, on the contrary, prevents soil particle adhesion, resulting in sediment consolidation and an associated decrease in porosity and hydraulic conductivity. A four-fold increase in hydraulic conductivity (2.9 x 10-4-9.8 x 10-4 cm s-1) relative to uncontaminated material was observed where petroleum hydrocarbon concentrations were 50 and 200 ppm TPH (total petroleum hydrocarbons), in a silt subjected to four freeze-thaw cycles. When TPH values approached 1,000 ppm, hydraulic conductivity decreased from 2.9 x 10-4 cm s-1 (uncontaminated silt) to between 5.3 x 10-5 and 8.5 x 10-5 cm s-1.

Grechischev et al. (2001a, b) investigated the influence of petroleum on the formation of segregated ice in fine grain soils. These researchers found that formation of ice lenses depends on composition and properties of the petroleum (crude oil in these studies) contained in the soil. Crude oil with relatively high hardening temperature (above 0°C) was found to reduce the ice segregation and cryogenic heaving of sediments. The influence of low-temperature crude oils is the opposite. Samples containing crude oil were characterized by the magnitude of the resulting cryogenic heaving. For crude oil with low hardening temperature (about -20°C), the value of ice segregation and cryogenic heaving was measured to be almost two times larger than for soils containing no crude oil (Grechischev et al. 2001a, b).

Recently, Haghighi and Ghoshai (2007) have used X-ray computed tomography (CT) to image petroleum (gasoline in this study) in freezing and thawing soils. The use of non-invasive imaging techniques allowed visualization and quantification of petroleum mobilization and displacement, and changes in petroleum blob morphology (volume, specific surface area and fractal dimension) in soil during freezing and thawing conditions. These researchers observed significant mobilization of petroleum from middle sections of the column towards the column end during freezing. Petroleum volumes changed by up to 150% in certain regions of the column. Porosity distribution in the column changed with freezing, but porosity changes were reversible on thawing. The mean volume of the petroleum blobs increased significantly after freeze-thaw at the two column ends where petroleum migrated, and the blobs over the entire column became more spherical in shape with freeze-thaw. This research confirms redistribution of petroleum and its complicated transformation at freezing and thawing.

0 0

Post a comment