T.J. Heimovaara

Biofilm development and the dynamics of preferential flow paths in porous media

A two-dimensional pore-scale numerical model was developed to evaluate the dynamics of preferential flow paths in porous media caused by bioclogging. The liquid flow and solute transport through the pore network were coupled with a biofilm model including biomass attachment, growth, decay, lysis, and detachment. Blocking of all but one flow path was obtained under constant liquid inlet flow rate and biomass detachment caused by shear forces only. The stable flow path formed when biofilm detachment balances growth, even with biomass weakened by decay. However, shear forces combined with biomass lysis upon starvation could produce an intermittently shifting location of flow channels. Dynamic flow pathways may also occur when combined liquid shear and pressure forces act on the biofilm. In spite of repeated clogging and unclogging of interconnected pore spaces, the average permeability reached a quasi-constant value. Oscillations in the medium permeability were more pronounced for weaker biofilms.

Formation Damage and Impact on Gas Flow Caused by Biofilms Growing Within Proppant Packing Used in Hydraulic Fracturing

Formation damage as a result of hydraulic fracturing of unconventional gas reservoirs is known to occur by many speculated processes such as: filter cakes on fracture faces, matrix swelling, cleat plugging, gel damage and water blocking. In low permeability matrices, capillary forces can also prevent effective dewatering and result in water blocking of gas flow. Another type of formation damage that may be qualitatively understood but not quantified is the impact of biofilms. This paper combines two micro-scale modeling techniques to evaluate and predict the effects of biofilms on proppant packed fractures in unconventional gas reservoirs. Both a two phase flow model for gas and liquid and a modern cellular automaton biofilm model were combined to simulate the impact on gas flow rates in biofouled propped fractures. Initial simulations of just two phase flow without biofilms but varied proppant surface wettabilities, indicated that hydrophobic proppant surfaces provide better dewatering than hydrophilic surfaces. Gas flow rates dropped in half when biofilms were added to the model at a pore volume of approximately 10%. In addition, further modeling indicated even the same biofilm volume but different distribution within grains and pore-throats can impact gas flow rates as much as 10%. It is hoped that this work will help hydraulic fracturing engineers improve their fracture designs and subsequent treatments to maximize gas flow rates from their assets.

Theoretical analysis of municipal solid waste treatment by leachate recirculation under anaerobic and aerobic conditions

Long-term emissions of Municipal Solid Waste (MSW) landfills are a burden for future generations because of the required long-term aftercare. To shorten aftercare, treatment methods have to be developed that reduce long-term emissions. A treatment method that reduces emissions at a lysimeter scale is re-circulation of leachate. However, its effectiveness at the field scale still needs to be demonstrated. Field scale design can be improved by theoretical understanding of the processes that control the effectiveness of leachate recirculation treatment. In this study, the simplest and most fundamental sets of processes are distilled that describe the emission data measured during aerobic and anaerobic leachate recirculation in lysimeters. A toolbox is used to select essential processes with objective performance criteria produced by Bayesian statistical analysis. The controlling processes indicate that treatment efficiency is mostly affected by how homogeneously important reactants are spread through the MSW during treatment. A more homogeneous spread of i.e. oxygen or methanogens increases the total amount of carbon degraded. Biodegradable carbon removal is highest under aerobic conditions, however, the hydrolysis rate constant is lower which indicates that hydrolysis is not enhanced intrinsically in aerobic conditions. Controlling processes also indicate that nitrogen removal via sequential nitrification and denitrification is plausible under aerobic conditions as long as sufficient biodegradable carbon is present in the MSW. Major removal pathways for carbon and nitrogen are indicated which are important for monitoring treatment effectiveness at a field scale. Optimization strategies for field scale application of treatments are discussed.

A toolbox to find the best mechanistic model to predict the behavior of environmental systems

Reliable prediction of the long-term behavior of environmental systems such as Municipal Solid Waste (MSW) landfills is challenging. While many driving forces influence this behavior, characterization of them is limited by measurement techniques. Therefore, a model structure for reliable prediction needs to optimally combine all measured information with suitable mechanistic information from literature. How to get such an optimal model structure? This study presents a toolbox to find and build the model structure that describes an environmental system as close as possible. The toolbox combines environmental frameworks to include all suitable mechanistic information; it fully couples kinetic and equilibrium reactions and contains multiple resources to obtain biogeochemical parameters. Several possible optimal model structures are quickly built and evaluated with objective statistical performance criteria obtained via Bayesian inference. By applying the novel methodology, we select the best model structure for anaerobic digestion of MSW in full scale landfills. Display Omitted Finding the best mechanistic model to predict the behavior of environmental systems.The toolbox allows to quickly assess multiple biogeochemical reaction networks.Coupled kinetic & equilibrium processes and mechanistic parameter resources.Networks are evaluated with objective statistical criteria given measured data.The toolbox enabled to find the optimal network for anaerobic digestion of MSW.