Study On The Growth And Transport Of The Bacterium Geobacillus Sp. In Simulated Reservoir Conditions

The effective transport and growth of bacteria in reservoir is crucial to the success of microbial enhanced oil recovery (MEOR) technology. In recent years considerable efforts have been put into the effect of microbial metabolic products on physical and chemical properties of oil and water using shake-flask culture experiments, while there's little information about the growth, transport and metabolism of bacteria in reservoir environment.The objectives of this study are to identify whether aerobic bacteria can grow in simulated reservoir conditions or not, and to determine the important factors that control the transport of bacteria through rock porous media. Furthermore, the mechanisms of bacteria retention and permeability reduction are investigated. The main results are summarized as follows:1 One strain of bacterium, identified as Geobacillus sp., was screened and confirmed as a candidate aerobic microorganism for MEOR by laboratory study.The bacterium Geobacillus sp., named G2, was screened from library of strains of Shengli Oilfield. The results of shake-flask experiments showed it exhibited good properties for MEOR: suffering high temperature, growing when different types of crude oil was added as the sole carbon source, reducing the viscosity of crude oil, producing biosurfactants, emulsifying and dispersing crude oil or liquid wax, and so on. Besides, Core flood experiments showed the oil recovery had been increased by 12.9-15.9% of original oil in place (OOIP) after 5 treatments of 50 days by adopting air-assistant technique (liquid/air 1:10 v:v). Blind tube-flood experiments also showed that the oil in the dead area could be effectively drived out by the strain G2. These results indicated that the strain G2 was suited for the theory study of MEOR.2 The optimal medium components and suitable growth conditions for the strain G2 were investigated by using"one-variable-at-a-time"method.The optimal culture conditions of this bacterium were studied by the shake-flask culture experiments. The results showed that the optimal culture conditions were as follows: 3% sucrose, 0.5% NaCl, 0.2% NaNO3, 0.14% KH2PO4, 0.37% Na2HPO4·12H2O, 0.001% FeSO4·7H2O, 0.02% MgSO4·7H2O, 0.001% CaCl2, 0.05% yeast extract, pH 7.0 and at 50-55℃. The ranges of the main factors that affected the growth of the bacterium were also investigated. The results were as follows: the temperature, salinity and pH range for growth of the strain G2 was 35-70℃, 0-10% NaCl and 5.5-9.5 respectively; suitable growth temperature, salinity and pH occurred at 40-60℃, 0.5-8% NaCl 6.0-9.0 respectively. Besides, the influence of high pressure on the growth of the strain G2 was studied by high-pressure culture experiments. The results showed that the strain G2 could grow at 10MPa, but the lag phase of growth became longer. These results further indicated that G2 was the suited experimental object for the theory study of MEOR.3 The growth of the strain G2 was studied in the simulated reservoir conditions when the nutrients and air conditions were adjusted for the better growth of the bacterium.1) Based on the experimental study on abiotic factor oxygen consumption and the estimation of biotic oxygen consumption, the amount of air injection for the growth of G2 in the simulated reservoir conditions of Block Ng3 of Shengli Oilfield was investigated. The results indicated that the amount of oxygen was sufficient for maintaining the growth of microorganisms in the simulated reservoir conditions when the ratio of Vwater to Vair was 1:10 and 1:30 at atmospheric pressure in the case of unsaturated oil and saturated oil respectively. In addition, the amount of nutrients injection was studied by the transport experiments of sucrose and nitrate. The results showed that the concentration of nutrients in the sandpack exports approached the injection concentration nearly when 3-4.5 pore volume (PV) of nutrition was injected. So the amount of nutrients was sufficient for maintaining the growth of the bacterium in the simulated reservoir conditions when 3PV of nutrition was injected. 2) The growth of the strain G2 was studied in the reservoir by simulating the temperature, pressure, porosity, permeability, fluid characteristics and development status of the aimed reservoir. The results showed that the cell density of the bacterium increased from 106 to 108 cells/mL in the simulated reservoir conditions of Block Ng3 when appropriate nutrients and gas were injected. This suggested that the strain G2 was suitable as a candidate aerobic microorganism for implementation of the MEOR technology in this block. Basically, the presence of residual oil had no impact on the growth of this bacteria. But the lag phase of growth was longer than in the shake-flask culture, which suggested that all MEOR studies pertaining to a specific reservoir should be evaluated under the in situ conditions of the reservoir.4 The factors which affect the transport of the strain G2 were investigated systemically in sandpacks and cores. These factors included the linear velocity of injection, temperature, permeability, size and degree of the bacteria aggregation, presence of a residual oil phase and pulse injection. Furthermore, the major mechanisms of bacteria retention were discussed.1) Firstly, the cellular suspension in stationary phase was continuously injected into the core. Subsequently, the cellular suspension was continuously displaced by sterile water. Effluent samples were collected at the export for analyzing the cell density, and the pressure across the core was monitored during the displacement process. The results showed that increasing the fluid flow velocity, temperature, permeability and pulse displacement increased the efficiency of microbial transport, while increasing the size and aggregation of the bacteria decreased the efficiency of transport. Besides, the presence of a residual oil phase increased the bacteria retention and decreased the efficiency of transport. Furthermore, the mechanisms of the bacteria retention during the transport process were studied by the former experiments. The mechanisms of the bacteria retention during the transport of the bacterial suspension included surface adhesion, size exclusion, pore bridging and multi-particle hydrodynamic exclusion. It was likely that all these mechanisms were involved to varying degrees in the transport of the bacterial suspension. When high concentrations (>108 cells/mL) of microorganisms were injected, multiparticle hydrodynamic exclusion and size exclusion of cell aggregates were the most likely operative mechanisms for bacteria retention. Under this condition, the injected bacteria were prone to retain in the entrance of the core, and the rapidly decrease of permeability occurred due to the forming of the external or internal filter cakes.2) The growth and transport experiments showed that diffusive flux had little effect on the transport of microorganisms and their metabolites. This indicated that the main driving force of microbial transport came from hydrodynamic convection during the displacement process.In summary, the results of this study will hopefully provide the technologic support for establishing evaluation methods (especially physical simulation experiment method) for MEOR which adopt air-assistant technique, and will also hopefully provide some data for mathematical model of MEOR. In addition, it will provide the theoretical basis and technologic support for the design of MEOR in the Block Ng3 of Shengli Oilfield.