Microbial Enhanced Oil Recovery - Advanced Reservoir Simulation

摘要

In this project, a generic model has been set up to include the two main mechanisms in the microbial enhanced oil recovery (MEOR) process; reduction of the interfacial tension (IFT) due to surfactant production, and microscopic fluid diversion as a part of the overall fluid diversion mechanism due to formation of biofilm. The construction of a one-dimensional simulator enables us to investigate how the different mechanisms and the combination of these influence the displacement processes, the saturation profiles and thus the oil recovery curves. The reactive transport model describes convection, bacterial growth, substrate consumption, and surfactant production in one dimension. The system comprises oil, water, bacteria, substrate, and surfactant. There are two flowing phases: Water and oil. We introduce the partition of surfactant between these two phases determined by a partitioning constant. Another effect is attachment of the bacteria to the pore walls and formation of biofilm. It leads to reduction of porosity and, under some assumptions, to increase the fraction of oil in the flow. Surfactant is our key component in order to reduce IFT. The surfactant concentration in the water phase must reach a certain concentration threshold, before it can reduce the interfacial tension and, thus, the residual oil saturation. The relative permeabilities depend on the water phase concentration, so when surfactant is moved into the oil phase, the effect from the surfactant on the oil production is reduced. Therefore, the transfer part of the surfactant to oil phase is equivalent to its “disappearance”. The oil phase captures the surfactant, but it may as well be adsorbed to the pore walls in the oil phase. We have looked into three methods how to translate the IFT reduction into changes of the relative permeabilities. Overall, these methods produce similar results. Separate investigations of the surfactant effect have been performed through exemplifying simulation cases, where no biofilm is formed. The water phase saturation profiles are found to contain a waterfront initially following the saturation profile for pure waterflooding. At the oil mobilization point -- where the surfactant effect starts to take place -- a sufficient surfactant concentration has been built up in order to mobilize the residual oil. A second waterfront is produced, and an oil bank is created. The recovery curve consists of several parts. Initially, the recovery curve follows pure waterflooding recovery until breakthrough of the oil bank. The next part of the recovery curve continues until breakthrough of the second waterfront. The incline is still relatively steep due to a low water cut. In the last part, the curve levels off. Partitioning of surfactant between the oil and water phase is a novel effect in the context of microbial enhanced oil recovery. The partitioning coefficient determines the time lag before the surfactant effect can be seen. The surfactant partitioning does not change final recovery, but a smaller partitioning coefficient gives a larger time lag before the same maximum recovery is reached. However, if too little surfactant stays in the water phase, we cannot obtain the surfactant effect. The final recovery depends on the distance from the inlet to the oil mobilization point. Additionally, it depends on, how much the surfactant-induced IFT reduction lowers the residual oil. The surfactant effect position is sensitive to changes in growth rate, and injection concentrations of bacteria and substrate, which then determine the final recovery. Variations in growth rate and injection concentration also affect the time lag until mobilization of residual oil occurs. Additionally, the final recovery depends on, how much the surfactant-induced interfacial tension reduction lowers the residual oil saturation. The effects of the efficiency of surfactants are also investigated. A super efficient surfactant produces an incremental recovery recovery around 40 % OOIP over that of waterflooding. Application of the less efficient -- and probably more realistic -- surfactant results in an incremental oil recovery of 9 % OOIP, but it is still considered a significant improvement. The bacteria may adhere to the pore walls and form a biofilm phase. The bacteria distribution between the water and biofilm phase is modeled by the Langmuir expression, which depends on the bacteria concentration in the water phase. The surface available for adsorption is scaled by the water saturation, as bacteria only adsorb from the water phase. The biofilm formation implies that the concentration of bacteria near the inlet increases. In combination with surfactant production, the biofilm results in a higher surfactant concentration in the initial part of the reservoir. The oil that is initially bypassed in connection with the surfactant effect, can be recovered as formation of biofilm shortens the distance from the inlet to the point of oil mobilization. The effect of biofilm formation on the displacement profiles and on the recovery is studied in the present work. Formation of biofilm also leads to porosity reduction, which is coupled to modification of permeability. This promotes the fluid diversion mechanism. A contribution to fluid diversion mechanism is microscopic fluid diversion, which is possible to investigate in a one-dimensional system. The relative permeability for water is modified according to our modified version of the Kozeny-Carman equation. Bacteria only influence the water and biofilm phases directly, so the oil phase remains the same. We have assessed the effect from biofilm formation together with microscopic fluid diversion. When sufficient amount of surfactant is produced in the water phase, the effect from surfactant generates a larger contribution to recovery compared to microscopic fluid diversion. To study the MEOR performance in multiple dimensions, the one-dimensional model with the surfactant effect alone has been implemented into existing simulators; a streamline simulator and a finite difference simulator. In the streamline simulator, the effect of gravity is introduced using an operator splitting technique. The gravity effect stabilizes oil displacement causing markedly improvement of the oil recovery, when the oil density becomes relatively low. The general characteristics found for MEOR in one-dimensional simulations are also demonstrated both in two and three dimensions. Overall, this MEOR process conducted in a heterogeneous reservoir also produces more oil compared to waterflooding, when the simulations are run in multiple dimensions. The work presented in this thesis has resulted in two publications so far.