First Compositional Modelling of Primary and Secondary Biodegradation Process in Natural Petroleum Reservoirs
Frank Haeseler1, Françoise Behar1, and Pierre-Yves Chenet2
1Geochemistry Department, Institut Français du Pétrole, 92500 Rueil Malmaison, France
2Beicip Franlab, 92500 Rueil Malmaison, France
Biodegradation is known as one of the main alteration mechanism affecting the composition and quantity of oil that has been generated in a petroleum system. It reduces the API gravity, enhances the acidity and as a consequence, reduces the economic value of the oil produced. To date, basin modelling software tools are not able to predict the biodegradation level and the compositional changes in an oil although some authors proposed degradation kinetics by comparing the residence time of oil with the biodegradation level (Larter et al 2003; Behar et al, 2006; de Barros Penteado et al, 2007). It was mainly a descriptive work aiming at noting the biodegradation level and its consequences in basins for which specific biodegradation kinetics have been proposed.
The aim of the present study is the elaboration of a conceptual model for predicting both the global biodegradation rates and corresponding chemical changes in biodegraded petroleum fluids. First, petroleum fluid is described by chemical classes sensitive to biodegradation: for example the C6-C14 sat, C6-C14 aro, C14+ n+iso alkanes, C14+ cyclo alkanes, C14+ aromatics and NSOs. A representative stoichiometric compound is assigned to each chemical class to which the 4 successive biological reactions are applied :
aerobic conditions: CxHy + (x+1/4y)O2 → xCO2 + 1/2yH2O,
denitrifying conditions: CxHy + (2/3x+1/6y)NO3 → xCO2 + (1/3x+1/12y)N2+ 1/2yH2O,
sulfate reducing conditions: CxHy + (2/5x+1/10y)SO4 → xCO2 + (2/5x+1/10y)H2S + (-2/5x+2/5y)H2O
methanogenic conditions: CxHy + (x-1/4y) H2O (1/2x-1/8y) CO2 + (1/2x+1/8y) CH4.
In this overall schema, there is a dynamic relation between oil and water. Oil is the provider of electron donors and carbon source to the bacteria; water is either the carrier of electron acceptors (oxygen, nitrate or sulphate) or the electron acceptor itself. This overall system is controlled also by temperature and residence time.
Using these equations, it is possible to calculate the consumption of the electron acceptors and to predict the generated amount of the hydrocarbon gas as methane as well as those of CO2 and H2S.
For each model compound, a relative biodegradation rate is assigned which is a combination between the accessibility of the organic molecule to biodegradation and the compositional preference of biodegradation process. Thus, all chemical classes are biodegraded through a parallel schema in which a relative repartition key of the available electron acceptors is the following:
KC14-sat = 34.4%, KC14-aro = 25,8%, KC14+n sat = 17.2%, KC14+iso = 12.0%, KC14+cyclanes = 6.9%, KC14+aro = 3.9%, KC14+nso = 0%.
In petroleum reservoirs, the 4 successive biological processes may occur depending of the available amount of dissolved oxygen, nitrates and sulfates and water. Knowing the relative proportion of the chemical classes of a given non biodegraded oil, estimation of biodegradation losses are done on the corresponding model compounds through the 4 main successive reactions. In terms of mass balance, the amount of dissolved oxygen in water being 10 mg/l, its consumption is expected to be rapid and thus the biodegradation extend very limited. In contrast the methanogenesis process involving water as electron acceptors, should be much more efficient. This model was successfully applied for explaining the natural evolution of the biodegraded oil composition observed in the Potiguar basin (Behar et al, 2006).
When implementing this model into a regional petroleum system, the key point is the dynamic relationship between the two mobile phases: petroleum and water. Indeed, when expelled from the source rock, the petroleum fluid is going through pores previously filled with water. Then, by successive steps of filling and emptying pores in the drains, the petroleum may be finally accumulated into a trap. We defined two types of biodegradation. Primary biodegradation occurs when petroleum goes into a pore previously filled of water. Secondary biodegradation occurs after emptying a pore initially filled with petroleum.
In the first case (Fig.1), when oil invades the pores of a mesh, it pulls out water until the irreducible water saturation (SatIRw) is reached. In other words, the mesh is filled either with oil present at the concentration of 1-SatIRw or with 100% water. Thus, the water associated with oil at the pore level is used for hydrocarbon biodegradation. This means that for a SatIRw of 3%, according to the initial oil type and composition, a maximal degradation of 4.5% to 5% of the oil can be expected. This process may occur as many times as the petroleum fluid is moving through the porous media.
The secondary biodegradation (Fig. 2) occurs on the irreducible residual oil saturation remaining in the porous medium (SatIRo), the rest of the pore volume will be replaced by water at a concentration of (1-SatIRo). This configuration will allow a very efficient biodegradation because the ratio water versus irreductible oil is much greater than 1. In that case, even the most refractory molecules may be altered leading to the formation of tar mats in reservoir sections.
Consequently, the overall formalism of the biodegradation process for a given chemical class depends on whether the rock volume is newly invaded with HC or swept by water with irreducible saturation remaining.
In the newly invaded rock volume V receiving a saturation ΔSatHC,
The mass of component i to be degradated is MBio,i = Satbio. ΔSat.Ф.V. CiBIO. ρi* ISTERIL
In the swept rock volume V with remaining Saturation Satirr,
The mass of component i to be degradated is MBio,i = Satirr. Ф.V. CiBIO. ρi* ISTERIL
The residual concentration of each biodegradable fraction can be described with a kinetic reaction dCires/Cires = k(T).dt, with k(T) maximum at T around 310 K and with more than 99% completion for Δt = 2000 years. At, t= 0, Cires = CiBIO
So far, the different equations describing both primary and secondary biodegradation are written for implementation into a basin simulator.
In the basin simulator, the HC saturation will depend on the volume of the cells. The mass concentration of each HC component is computed from the mass balance of each component.
Since the model steps are usually greater than 2000 years, one may assume as a good approximation that the total mass variation of a biodegradable component i is obtained considering the saturation variation (> or <0) and assuming that the biodegradation of the component is complete in a fraction Sat bio of the newly invaded volume and affect all the residual saturation in the swept volume, as per the equations above. In both situations, the mass of biodegradable components is set to 0 in these volumes, remains identical for the non biodegradable fraction (such as NSO) and increases in CH4 according to the stoechiometry. The final concentration of a component i for a given cell is deduced from the mass of component prior to invasion or water sweeping, the mass prior to biodegradation after invasion or water sweeping, and the mass after biodegradation in the invaded or swept volume. This scheme is validated on a synthetic case study.
The biodegradation can be activated as long as the sterilisation temperature is not reached, and stopped beyond. In the case of a new temperature decrease below such temperature, one may reactivate if needed the reaction.
In conclusion, the proposed conceptual model predicts the parallel chemical changes of petroleum during the 4 successive biological reactions for a given temperature and residence time. The efficiency of biodegradation is directly linked to the available water in contact with oil during the two main phase of filling (primary biodegradation) or emptying (secondary biodegradation) the porous media in which petroleum moves and accumulates. Thus, only low grid refinement model at a regional scale will enables to precisely describes the relative water and oil fluxes through the heterogeneous porous medium.
Larter, S., Wilheims, A., Head, I., Koopmans, M., Aplin, A; di Primio, R., Zwach, C., Erdmann, M., Telnaes, N., 2003. The controls on the composition of biodegraded oils in the deep subsurface – Part 1: biodegradation rates in petroleum reservoirs. Organic Geochemistry 4, 601-613.
Behar, F., de Barros Penteado, H. L., Lorant, F., Budzinski, H., 2006. Organic Geochemistry 37, 1042-1051.
de Barros Penteado, H.L., Behar, F., Lorant, F., Oliveira, D.C., 2007. Organic Geochemistry 38, 1197-1211.
AAPG Search and Discovery Article #90091©2009 AAPG Hedberg Research Conference, May 3-7, 2009 - Napa, California, U.S.A.