Characterisation of Contaminated Soil by Crude Oil Associated with Produced Water
Figure 8: Profile of Spread Crude Oil Contaminants at the Case Study Area for the Initial State (A) and for Residual Contaminants After One Year of Biodegradation (B)
A
Maximum: 5.00E4 x104
Minimum: 0
5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
describes advection and dispersion of a sorbing, volatilising, diffusion, adsorption and decaying solute in variably saturated soil:
∂t
∂ (θc) + (ρbcp ∂
∂t
) + (av
∂t
∂
cG
) = .[−θDLG
c + uc] = RL + RP + Sc
(3)
where θ is the volume fraction of fluid within the soil, c is the dissolved concentration (kg/m3), ρb is the bulk density (kg/m3), cb is the mass of adsorbed contaminant per dry unit weight of solid (mg/kg), av is the air volume fraction, cG is the solute concentration in air (kg/m3), DLG is the combination of hydrodynamic dispersion tensor for water and diffusion in air (m2/day), u is the vector of directional velocities (m/d), RL denotes reactions in water (kg/m3/day), RP equals reactions involving solutes attached to soil particles (kg/m3/day) and Sc is the quantity of solute added per unit volume of porous medium per unit time (kg/m3/day). The effects of chemical reactions on solute transport are generally incorporated in the advection, diffusion–dispersion, volatilisation and adsorption equation through additional terms. A chemical reaction such as Equation 4 should be considered:
aA + bB rR + sS
(4)
where a, b, r and s are the valences for ions. A general kinetic rate law for species A can be expressed as:
∂cA ∂t = −λcn1
A BR S cm2
cn2
+ γcm1
(5)
where cA, cB, cR and cS are concentrations of reactant species A and B and resultant species R and S, respectively; λ and γ are the
rate constants for the forward and reverse reactions, respectively, n1, n2 and m1, m2 are empirical coefficients. The sum of n1 and n2 defines the order of the forward reaction, while the sum of m1 and m2 defines the order of the reverse reaction. Equation 5 expresses the rate of change in species A as the sum of the rates at which it is being used in the forward reaction and generated in the reverse reaction.
1. Whittle KJ, Hardy R, Mackie PR, Philos Trans R Soc Lond B Biol Sci, 1982;297:193–218.
2. Stanley RG, United States Government Printing Office, 1995. 3. Reed M, Johnsen S, Plenum Press, 1995. 4. Christopher L, John Wiley & Sons, 1994.
5. Edwards WC, Vet Clin North Am Food Anim Pract,
1989;5(2):363–74.
6. Edwards WC, Gregory DG, Vet Hum Toxicol, 1991;33(5):502–4.
7. Lee R, Seright R, Hightower M, et al., Ground Water Protection Council Produced Water Conference, Colorado Springs, Co, 16–17 October 2002.
EXPLORATION & PRODUCTION – VOLUME 8 ISSUE 1
B
Maximum: 5.00E4 x104
Minimum: 0
5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
The validated CFD model has been applied to the case study under discussion. Boundary conditions are specified from the experimental tested points as well as boundaries for various mass and scalar equations inside the computational domains. The obtained results from the experimental tests of TPH values and the rate of its degradation have been considered as boundary conditions in CFD analysis to predict and investigate the spreading of crude oil inside the soil around the pit and to simulate the biodegradation of crude oil contaminants. The governing equations were discretised using a finite volume method and solved using the commercial CFD code COMSOL Multiphysics 3.2. Stringent numerical tests were performed to ensure that the solutions were independent of the grid size. The coupled set of equations was solved iteratively, and the solution was considered to be convergent when the relative error in each field between two consecutive iterations was <1.0x10-6
. The calculations presented here have all been obtained on a Pentium IV PC (3 GHz, 2GB RAM) using the Windows XP operating system. The results of the CFD model are presented in Figure 8 for the profile of the spreading crude contaminants and the residual contaminants after one year of biodegradation. The results show that the degradation at the spots of high concentration of contaminants was limited, while wide degradation of contaminants was observed at the spots of low concentration of the pollutant.
Conclusion
The main points to draw from this research work are: the oil- contaminated soil is one of the apparent consequences of the unmanaged produced water disposing into surface pits; natural attenuation cannot be the proper way to remediate contaminated soil; the bioremediation technology is the most active method of contaminated soil remediation; the biodegradation process is affected by many parameters, the most crucial one being the concentration of the contaminants; a good understanding of the relationship between the efficiency of the biodegradation process and the concentration of the contaminants is needed for successful application of the remediation plan; and the deployment of CFD achieved valuable results, where the number of samples, quantity of chemicals, time and other efforts have been reserved, in addition to the accurate results gained. n
8. Riser-Roberts E, Boca Raton, FL: Lewis Publishers, 1998. 9. Reddy KR, Admas JF, Richardson C, Pract Periodical of Haz,
Toxic, and Radioactive Waste Mgmt, 1999;3(2):61–8.
10. RAAG, Memorial University of Newfoundland, St John’s, NF, Canada, 2000.
11. Jah UJJ, Antaib SP, Int Biodeterior Biodegradation, 2003;51:93–9.
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