Experimental Study of Displacement Mechanisms in Microbial Improved Oil Recovery Processes
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The Microbial Improved Oil Recovery (MIOR) project has been defined at the Norwegian University of Science and Technology (NTNU). It is a cooperative project between the Department of Petroleum Engineering and Applied Geophysics and the Department of Biotechnology both at NTNU. The project involves experimental investigations of the MIOR process. The bacterium Rhodococcus sp.094, extensively studied at NTNU, is used to test its effect on oil recovery. The growth of bacteria is evaluated in sandstone core plugs saturated with the oil and water. Detailed studies are performed on variations in concentrations of the bacteria, effective surfactants and other microbiological products and preliminary results are published. The purpose of this thesis is to describe the principles of the MIOR method. This is fundamental research involving investigations of the potential mechanisms of MIOR in petroleum engineering. In a MIOR process, the oil industry aims to increase the population of bacteria with beneficial properties for mobilizing additional oil in the reservoir. Determining which mechanisms allows bacteria to increase oil recovery and finding how these mechanisms occur and interact is the key to understanding, predicting and planning MIOR projects successfully. Investigations of the displacement mechanisms of MIOR are based on the analysis of experimental results performed with two variants of Rhodococcus sp. 094 at laboratory temperature. Surfactant producing bacteria and nonsurfactant producing bacteria are examined. The experimental data are obtained from experiments on core samples, measurements of parameters such as the interfacial tension, contact angle and wettability index and investigations of flow visualization using glass micromodels. A number of core flooding experiments with the bacteria are carried out to test the effectiveness of bacteria to reduce oil saturation in core plugs with different initial wetting properties. Initial water wet and neutral (chemically treated) wet Berea sandstone samples are served to represent the different wettability states. Core floods are performed with brines and/or bacterial solutions with various salinities to examine the effect of salinity on the MIOR process. A modified pendant drop method is used to measure the interfacial tension of the oil/water/bacteria system and the contact angle in the oil/water/bacteria/quartz system. Wettability indices are determined by using the Amott-Harvey method. Wettability alteration is analyzed by two methods; the contact angle values and wettability indices. The reduction of the interfacial tension and changes in wettability are determined. The visualization of two phase flow is performed on water wet and oil wet micromodels and the reduction in the residual oil saturation and congestion of bioproducts in the pore channels are evaluated. The effect of interfacial tension reduction and wettability changes is visualized by the micromodel experiments. A pore network model is tested to simulate the two phase flow obtained experimentally in the micromodels. However, numerical modeling of the MIOR process is outside the scope of this thesis and is recommended for further work. The experimental results make it possible to propose a theory about how these bacteria improve oil recovery. The theory suggests which type of bacteria is most active and thereby gives the interfacial tension reduction, the wettability alteration and favorable changes in the flow pattern.