Experimental Study On Migration Of Oil Contaminants In Permafrost Regions

Large area of permafrost and seasonally frozen ground are distributed in China. As aspecial soil, frozen soil is highly sensitive to temperature whose properties are veryinstable. With the development of national economy and the need of the energy, the oilpipeline construction gradually extends to cold regions. The China-Russia Crude OilPipeline (CRCOP) currently built on permafrost and seasonal frozen ground areas inNortheast China, which has been under operation, is just an important project for energysupply. During the processes of oil exploration, refining, storage, transportation and use,crude oil and its products will disseminate or leak in surface or underground. Thephysicochemical, thermal and mechanical properties of rock and soil mass contaminatedby petroleum change significantly, thereby, which will cause a series of seriousgeotechnical and environmental issues. Investigating the migrating process of oilcontaminants in permafrost can help to solve the geotechnical and environmental issuesduring the periods of oil pipeline construction and operation. It can also expand thefurther understanding of formation mechanism of frost hazards, permafrost degenerationand environmental degradation in permafrost regions.A series of experiments were conducted in laboratory to simulate and study themigrating process of oil contaminants in soil-water system of the permafrost regionsbased on the specificity of the permafrost regions and the CRCOP. These experimentsinclude the simulation of migrating process and the influences of the water-eluviation,the temperature gradient and the freeze-thaw cycles on migrating process, thesimulation of the gravity and capillarity as the driving forces, the changing mechanismof thermal conductivity and freezing temperature of polluted soils. In addition, scanningelectron microscope (SEM) was used to visually analyze the microstructure of thecontaminated soils.The main conclusions are made as follows:1. The approximate migrating process of oil contaminants in permafrost regions hasbeen investigated. The leaked oil contaminants infiltrate into the active layer under thegravity in permafrost regions. Some of them are absorbed by soil particles; Some ofthem continue to downward infiltrate with the pore water; At last, most of themaggregate near the freezing-thawing surface as a result of the hindrance of frozen soillayer. Dissolved oil-phase can infiltrate with the water-phase. During the migratingprocess of petroleum hydrocarbons, the migration rate of alkane is the fastest, then, followed by nonhydrocarbon, aromatic, and asphaltene. Under the effect ofwater-eluviation, the larger carbon number of alkane, the faster migration rate. Themigration ability of asphaltene with branched chain is stronger than the n-alkanes withthe similar carbon number.2. The water-eluviation, temperature gradient and freeze-thaw cycles in permafrostregions can significantly influence the migrating process. There is a close relationshipamong the temperature, water and oil. The influence mechanisms of the temperaturegradient and freeze-thaw cycles are essentially alike. The temperature gradient caninfluence the physical properties of oil contaminants, the adsorption effect of soilparticles on oil contaminants, the distribution and state of water phase in soil sample,and then influence the migrating process. The effect of freeze-thaw cycles contains twoaspects: one is frost action and the other is thawing action. The effect of frost action issimilar to the one of temperature gradient. Under the effect of thawing action, the oilviscosity will be decreased, while the adsorption capacity of soil particles will also bedecreased. Besides, the ice will melt and infiltrate due to gravity. The thawing actioncan weaken the effects accumulated by frost action.3. The driving force of migrating process of oil in permafrost regions mainlyincludes gravity, capillarity, temperature gradient, water-eluviation and concentrationgradient. Under the effect of gravity and capillarity, the distribution laws of oil phaseand water phase in soil are just contrary. The migration of oil phase can displace waterphase and then lead to redistribution. The interactional mechanism between oil phaseand water phase is cognized. Water phase already existed in the soil can hinder themigration of oil phase. There are two reasons. One is that capillary water can hinder themigration of oil phase in capillary tubes; the other is that the hydrated film around soilparticles can decrease the capillarity exerted on oil phase. The state of water phase insoil column can influence the migration of oil phase. Oil phase already existed in thesoil can hinder the redistribution of water phase too. Oil phase and water phase coexistin the pores of soil matrix, and the redistribution of oil or water need to break theoriginal oil-water equilibrium state.4. The thermal conductivity of soils polluted by petroleum in soil-water-oil systemdepends on the position, state, relative content and thermal conductivity of each phase.The thermal conductivity of polluted soils increases with increasing water content.When water content is the same, the values of the thermal conductivity of soil mass atthe negative temperatures are bigger than the ones at the positive temperatures. It firstlyincreased, then decreased, and finally increased again with progressively increasing oil concentration when the dry density and water content were kept constant. The values ofthermal conductivity are the biggest when the wet phase is saturated. The thermalconductivity of the polluted soils mainly depends on the state of water phase, which islittle influenced by the state of oil phase. The thermal conductivity of polluted soilsincreases with increasing saturation of wet phase. When wet phase content is the same,the interaction between water and oil phases can weaken the influence of the other partyon thermal conductivity. When the soil mass is close to saturated state of wet phase, theinteraction between water and oil phases will decrease. With an increase in oil content,the freezing temperature will decrease too.5. The results of SEM test show that the pore spaces of soil matrix of polluted soilsincrease, and the soil particles are extruded into aggregate. Scanned images show thatmost of the morphology of soil skeleton is clot. Soil particles densely arrange and fewerpores occur. Oil contaminants entered into soil coexist with water and gas phases in thepores of soil matrix. Under the influence of external actions, oil phase encapsulates andextrudes soil phase, thus soil particles obviously gather into cluster whose pores areexpanded.