INVESTIGATION ON SURFACE AND SUBSURFACE FLUID MIGRATION: ENVIRONMENTAL IMPACT

Transcription

1 Proceedings of the 13 th International Conference on Environmental Science and Technology Athens, Greece, 5-7 September 2013 INVESTIGATION ON SURFACE AND SUBSURFACE FLUID MIGRATION: ENVIRONMENTAL IMPACT MOHAMMED S. BENZAGOUTA 1 1 King Saud University, Petroleum and Natural Gas Eng. Dept PO Box 800 Riyadh KSA, benz@ksu.edu.sa EXTENDED ABSTRACT Natural degradation of crude oil is very slow and might take several years to be degraded. However, its impact on the environment will last as long as neither treatment nor solutions are undertaken. When further migration of crude oil is occurring, it will harm the soil and might contact the subsurface water or groundwater. The most relevant in that respect, concerning impact on environmental sites, would be the type of fluids (Contaminants), mode of fluid transport, type of soil, environmental importance of the concerned sites, and weathering effect. Therefore, the main objectives of this study are investigation and observation of crude oil migration as toxic fluid, its penetration, and weathering effect. Achievement of the abovementioned objectives necessitates the design of an experimental model. The model consists of two separated columns. Those columns were filled with soil and other types of rocks. One setup contains soil in dry condition (dry system), while the other was filled with soil material wet conditions (wet system). Progress of crude oil penetration was recorded versus time by taking soil samples, and water samples were collected from the wet system. This was aimed to report a possible migration of hydrocarbons dissolved in water. Results show that the penetration depth was more performed in wet system during early stages rather than in dry system. However, with time this progress becomes more developed in both systems. It is important to notify that the duration of this progress will last for long time in dry system. The overall penetration at final stage has been found significantly higher in the dry system. In addition, it was observed that during crude oil migration in dry system, chromatographic separation of crude oil components has occurred obviously. The obtained results reveal that immediate treatment action has to be performed in both systems with particular attention to the wet system, owing to its high initial penetration rate. KEY WORDS: crude oil, degradation, toxic, soil contamination, pollution, penetration depth, wettability. 1. INTRODUCTION Crude oil spills and oil fraction are one of the main harmful fluids on earth. Additionally, contamination of aquifers and soil by crude oil or its products can creates a massive damage to principally the fresh water, and therefore the remediation becomes extremely expensive (Delin, et al., 1998). The petroleum exploration, production, transportation and other industrial branches have the potential to affect the environment in different degrees. The toxic fluids contamination occurs in the soil. It will affect the structure at various degrees. The affected portion of soil must be treated to prevent subsequent pollution of surface water and groundwater. Thus, some of oil components or other toxic fluids can carry on migrating for long time before reaching the aquifer and causing pollution. The

2 infiltration of toxic fluids into the subsurface occurs through path channels, cracks and veins. In this study, experimental work is conducted to simulate migration and remediation actions on real models, which consist on dry and wet soil systems taking in consideration regional weather conditions. Fluid penetration depth is determined versus time. 2. ROCK PROPERTIES Rock properties are of importance for fluid circulation within porous media. Porosity and permeability play an important role concerning rock properties. Other factors for rock properties are the hydraulic conductivity which is a main characteristic for soil identification and quality determination. However conductivity and porosity were used sometimes in this investigation interchangeably 3. SOIL CONTAMINATION Contamination is usually by either liquid or solid and is physically or chemically attached to soil particles or trapped in the small space between soil particles. Oil contamination can be either at sea or in soil. Contamination is function of the type of contaminant, its migration and transport type (infiltration). The contamination effect is related to the conductivity, permeability and rock properties. 4. EXPERIMENTAL WORK The importance of this research is an environmental challenge to limit the impact of toxic fluids such as crude oil on soil and subsurface water resources. In addition, the outcome of this study will be applicable for any other toxic fluids, for instance, industrial wastes (liquid), fuel stations, leaks of underground storage tanks, leakages from wellheads and oil reservoirs. The migration of fluid circulation will be a study of a combination between; contact time, weathering effects, rock types and transport phenomena (Fig 1). Figure 1. Depth performance in dry sand (left) and dry carbonate (right) system.

3 Moreover, to achieve the previously mentioned objectives an experimental model has been designed. The experiment setup is composed of two separated columns filled with soil and other types of rocks with known characteristics. The two setups are characterized by a diameter of 29 cm and 5 ports for the purpose of sample collection. The distance between the successive ports is 10 cm. This interval is assessed to detect and determine the penetration depth of the fluid. The columns were filled up with different types of soil samples, which represent the near surface detritus lithological column. The established sequence was from the bottom to the top as follow: gravel and sandy soil in one column, while the other column contains gravel, and fragments of limestone. In addition, weathering conditions (dry and rainfall) were simulated. 5. EXPERIMENTAL PROCEDURE One of the columns was designed to be at dry state, while the other column was allocated to be subjected to rainfall (wet system). The rainfall was simulated through spiral part of pipe including various holes. The role of the holes is to allow water dropping (table water) to the surface of the column. Oil samples were poured periodically in both systems at different times with a constant concentration of cc/cm 2 and have reached a total concentration of cc/cm 2 (degree of contamination). In order to analyze and evaluate the penetration depth of toxic fluid effect, samples were collected from different ports. In addition, it is worth to mention that the used material (soil) is unconsolidated. This was aimed to simulate the surface layers regarding the compaction effect on the conductivity and pore space. Recovered samples were conducted to the microscope analysis to investigate about oil contamination. Penetration depth versus time was also determined. 6. RESULTS AND DISCUSSION Contamination and penetration depth of oil in the column were observed by combination of digital video camera, manual measurements and microscopical analyses. This process was conducted for each collected sample in both wet and dry columns. The duration of this process has lasted for almost seven months, under continuous observation. The infiltration depth was measured based on the contamination of the material in the middle of the column. Thus, the migration depth of the oil contamination on the side was higher than that in the middle due to side flow. 6.1 CASE OF DRY SYSTEM The infiltration depth was observed by collecting samples to identify the progress of contamination. It was found that contamination was depending on the oil concentration in the system. Through the first run (oil concentration cc/cm 2 ), the total amount of oil has been adsorbed owing to its primary contact with the sand at the surface. A real progress of infiltration depth was observed starting only from the second and third run of poured oil sample (Fig 2). Then, change in infiltration depth is occurring progressively versus time as indicated. Observation has shown that infiltration was relatively important in the beginning up to almost 60 days. Afterwards, the infiltration progress is becoming reduced to attaining its highest and noticeable penetration depth of 17 cm after 140 days. Beyond that, infiltration depth progress has become extremely slow (with a contact time of more than 254 days). The reduction of the penetration can be ascribed to parameters such as adsorption, gravity forces and imbibitions. This confirms that the oil contamination can last as long as no treatment has undertaken (Deonarine, et al., 2001).

4 Fig. 2 Infiltration depth versus time for dry sand system Similar trend was also observed in the dry carbonate system, but the progress of infiltration depth was lower than that observed in the dry sand system. Such behaviour was expected, since the carbonate material is known as preferentially oil wet. Thus, the carbonate rock property leads generally to retard the oil migration downwards and then will contribute to deceleration of the downward front movement. In addition, it was observed that chromatographic effect of oil components was occurring specially in the dry sand system. Separation process of crude oil fraction has been observed during the migration through the sand and carbonate deposits. Components in that case were progressing faster with regard to the heavier components. Those latter remained behind the front. This mode of separation might be explained according to the polarity degree of hetero-components found in the oil sample. 6.2 CASE OF WET SYSTEM The material used in the wet system is fine grain sand. In this experiment the procedure consists on spreading water on the material prior and post adding oil to create a wet condition simulating rainfall and stranded oil spills onshore. Runs of oil (similar amount of oil as applied in the dry system) were used alternating with a given amount of water (1 litre each run). Infiltration depth was observed by collecting samples from the set column. At early stage (5 days), we observed that the infiltration front moves faster with regard to the dry systems (Fig. 3). This infiltration was also time related. The trend of the infiltration progress was similar to that found in the dry system; both systems are showing non-linear evolution. However, the progress in the wet system was clearly higher than that achieved in the dry system. Similar case was observed for the carbonate case with the same change as in dry system. It is important to mention that oil infiltration has been found faster in the wet system rather than in the dry system.. Oil tracers in form of fluorescence were also observed from the beginning of the process. This indicates that combined with water some oil components (water soluble components) are present and could penetrate through the lithological system (Hellel, 1998). Consequently and based on the above description, the infiltration speed of the crude oil sample in the dry system was very slow. This is basically due to rock and fluid properties. However, in the wet system, and the presence of water allows the oil to penetrate more into the wet formation with regard to the dry system, owing to the saturation of pore spaces by water.

5 Fig 3 Infiltration depth versus time for dry carbonate system 7 RECOMMENDED SOLUTIONS AND REMEDIATION To tackle similar environmental problem, various treatment processes might be employed and based on the acquired results (Fig 4). In the study case, it is essential to emphasize that infiltration depth of the crude oil in dry system is very slow but it lasts for long time. In addition, the main part of the fluid remains at the surface or near surface. This finding is important in term of remediation, since any treatment procedure to remove the contaminants from similar systems should focus on the surface. On the other hand, the treatment action in the wet system (e.g. shoreline, water wetted system) has to be taken immediately to avoid deeper migration of the toxic fluids. Therefore, specific measurements should be considered, such as injection of slurry or organic materials for example polymer solution, clay to interrupt or to diminish the downwards migration of the fluids. Furthermore and, as it has been stated in the review, remediation can also be operated through the process of heating where hot air or steam is injected below the contaminated zone (Kuo, 1999). This air will heat up the contaminated zone and enhance the release of the contaminants from soil matrix. Another possibility or alternative is to enhance biodegradation, which is called Enhanced Biodegradation Method and it consists on degradation of oily contaminants by incubation of microorganisms (Amro, 2004)

6 Fig 4: Overall obtained evolutions of different models 8 CONCLUSION AND RECOMMANDATIONS It should be emphasized that the ultimate aim of this research is to investigate the frequent crude oil migration into the subsurface leading to contamination. Based on the interpretation of the results, the following conclusions can be drawn: The rate of infiltration was different from one system to another: The dry system is characterized by very low infiltration depth and fluid migration. This is mainly own to availability of pore spaces, which are filled up by the existing single phase (Oil). Penetration performance was clearly higher in the wet system; this has been explained by effect of adsorption and fluid saturation. Different factors are responsible for the control of such penetration; beyond rock and fluid properties, weathering conditions (rainfall) and time have to be considered. The obtained results allow us to take suitable actions for better and efficient site remediation. In dry system, the remediation process should focus on the surface and subaerial system. The remediation in the wet system has to be concentrated on the subsurface. ACKNOWLEDGEMENT The manuscript benefited from financial support from Al-Amoudi research chair in the college of engineering in King Saud University. 9 MAIN CONSULTED REFERENCES Amro, M. M., Factors affecting chemical remediation of oil contaminated waterwetted soil, Chemical Engineering and Technology, Wiley-VCH, 27, No. 8. Deonarine D.J.Jaggernauth, Basdeo Seeram, The Effects of Petroleum Production Operations on the Environment in Trinidad, SPE Paper 71432, presented at the 2001 SPE Annual Technical Conference and Exhibition held in New Orleans, Louisiana, 30 September 3 October Delin, G. N., et.al. Ground Water Contamination by Crude Oil near Bemidji, Minnesota, U. S Geological Survey, USGS fact Sheet , Sept Hellel, D., Estimation of Infiltration Rate in the Vadose Zone: Application of Selected Mathematical Models'', Volume II. Kuo, Jeff, Practical Design Calculations for Groundwater and Soil Remediation, By Lewis Publishers, New York and Washington, D.C. PP (5-6), PP (57-98) and PP ( ),.