Microbial Enhanced Oil Recovery - Models

Models

A plethora of attempts to model MEOR has been published. Until now, it is unclear if theoretical results reflect the scarce published data. Developing mathematical models for MEOR is very challenging since physical, chemical and biological factors need to be considered.

Published MEOR models are composed of transport properties, conservation laws, local equilibrium, breakdown of filtration theory and physical straining. Such models are so far simplistic and they were developed based on:

(A) Fundamental conservation laws, cellular growth, retention kinetics of biomass, and biomass in oil and aqueous phases. The main aim was to predict porosity retention as a function of distance and time.

(B) Filtration model to express bacterial transport as a function of pore size; and relate permeability with the rate of microbial penetration by applying Darcy’s law.

Chemical kinetics is fundamental for coupling bioproduct formation to fluxes of aqueous species and suspended microbes. Fully numerical approaches have also been followed. For instance, coupled nonlinear parabolic differential equations: adding equation for the rate of diffusion of microbes and their capture by porous medium; differential balance equations for nutrient transport, including the effect of adsorption; and the assumption of bacterial growth kinetic based on Monod equation.

Monod equation is indiscriminately used in modelling software, and has a limited behaviour which is inconsistent with the law of mass action that form the basis of kinetic characterization of microbial growth. Application of law of mass action to microbial populations results in the linear logistic equation. And the application of the law of mass action to an enzyme-catalysed process results in the Michaelis-Menten equation, from which Monod is inspired. This makes things difficult for in situ biosurfactant production because controlled experimentation is required to determine specific growth rate and Michaelis-Menten parameters of rate-limiting enzyme reaction.

Modelling of bioclogging is complicated because the production of clogging metabolite is coupled nonlinearly to the growth of microbes and flux of nutrients transported in the fluid.

Published models disregard the ecophysiology of the entire microbial microcosms at oil reservoir conditions. Microorganisms are a kind of catalyst whose activity (physiology) depends on the mutual interplay with other microbes and the environment (ecology). In nature, living and non living elements interact with each other in a complicated network of nutrients and energy. Some microbes produce extracellular polymeric substances and therefore its behaviour in pours media needs to consider both occupation by the EPS and the microbes themselves. Knowledge is lacking in this respect and therefore the aim of maximizing yield and minimizing cost remains unachieved.

Realistic models for MEOR at the conditions of the oil reservoir are missing, and reported parallel-pore models had fundamental deficiencies that were overcome by models considering the clogging of pores by microbes or biofilms, but such models have also the deficiency of being two-dimensional. The utilisation of such models in three dimensional models has not been proven. It is uncertain if they can be incorporated to popular oilfield simulation software. Thus, a field strategy needs a simulator capable of predicting bacterial growth and transport through porous network and in situ production of MEOR agents.