Search

Miscible CO2injection appears to be an important enhanced oil recovery technique for improving sweep efficiency and eliminating CO2-oil interfacial tension resulting in up to 10% higher oil recovery compared to immiscible flooding, in addition to the environmental benefits of reducing greenhouse gas emissions through carbon capturing utilising and storage (CCUS). Moreover, this technique could be similarly applicable to natural gas and nitrogen projects to increase oil recovery and to reduce the associated gas flaring. However, miscible displacement may not be achievable for all reservoirs, in particular, reservoirs with high temperature where high injection pressure would be needed to reach miscibility which likely exceeds the formation fracture pressure. Therefore, to further achieve reservoirs’ potential, there is a pressing need to explore a viable means to decrease the miscibility pressure, and thus expand the application envelop of miscible gas injection in reservoirs with high temperatures. more...

Gas flooding is a promising way to enhance oil recovery in unconventional oil reservoirs, but accurate identification of the distributions of movable and residual oil during gas flooding is difficult. In this study, the movable and residual oil distributions of tight conglomerate oil reservoirs during gas flooding are monitored with low-field nuclear magnetic resonance (NMR). The NMR T2spectra are converted into pore throat size distributions using a nonlinear conversion method in conjunction with high-pressure mercury intrusion, and then the lower limit of the pore throat size of movable oil under different pressure differences is determined. In addition, a mathematical model is proposed to predict the lower limit of the pore throat size of movable oil during gas flooding based on the capillary tube model and the fractal characteristics of pore structures in conglomerates. The research studies show that the model prediction results are very close to the results measured from experiments. The lower limit of the pore throat size of movable oil decreases with the increasing pressure difference of gas injection and decreasing core permeability. This study will be beneficial for characterizing residual oil distribution after gas flooding in unconventional oil reservoirs. more...

CO2diffusion in the oil phase in the presence of porous media is of great importance to forecast the performance of enhanced oil recovery and carbon dioxide capture and storage. When CO2is brought into contact with an oil phase, diffusion occurs. The indirect pressure-decay methodology has been applied successfully to determine the mutual diffusion coefficients of supercritical CO2and crude oil at temperature of 65 °C. Theoretically, a one-dimensional double-way mass transfer model incorporating the capillary and adsorption effects of porous media has been constructed to quantify the mutual dynamic diffusion coefficients. The crude oil has been characterized into five pseudo-components for the accurate description of the phase behavior of CO2gas and oil phases. An optimization method along with the PR equation of state is employed to determine each component’s diffusion coefficients in the gas and oil phases for the purpose of minimization of the difference between the experimental data and calculated results. Satisfactory agreements are obtained with a merely 0.97% pressure difference when a variable diffusion coefficient is implemented. The short-time experimental test and a long-time-duration calculation are recommended for further diffusion investigation. The capillary analysis gave a regressed fractal dimension of 2.7049 with the average absolute deviation of 0.602%. The adsorption phenomenon can be greatly influenced by the existence of heavy components. The heavier the components, the greater their impact on adsorption. Furthermore, a faster and enhanced diffusion process is observed at high pressure and temperature. The combination of several complex mechanisms leads to the initial gradual increase of the diffusion coefficient with time, and then, it arrives at a plateau, which is slightly different from the previous diffusion coefficient changing tendency mentioned in the literature. In the presence of a low-permeability formation core, the determined diffusion coefficients of CO2in the gas and liquid phases are increased from 4.38 × 10–9to 6.21 × 10–9and 6.16 × 10–10to 9.21 × 10–10m2/s when the system pressure is decreased from 18.2 to 12.11 MPa, respectively. In addition, the capillary effect can lead to a reduction of the diffusion coefficient in the oil phase but has little effect on the diffusion coefficient in the gas phase. more...

In order to achieve both in-depth profile control and high efficient oil displacement, a novel heterogeneous combination flooding system (HCFS) composed of dispersed particle gel (DPG) and dodecyl dimethyl sulfo-propyl betaine (DDSB) was proposed and prepared. According to the in-depth profile control property and interfacial tension reduction capacity, the compositions of the HCFS were optimized as 20–30 wt % DPG and 0.4–0.5 wt % DDSB. The prepared HCFS exhibited good wettability alteration ability and emulsifying capacity. Results of double-layer heterogeneous core flooding tests showed that the enhanced oil recovery (EOR) result was great in the HCFS flooding stage and the subsequent water flooding stage, which was up to 31%. Moreover, on account of synergistic action of the soft granular profile control agent (DPG) and high efficient oil displacing surfactant (DDSB), the EOR result of HCFS flooding (31.04%) was much better than that of single DPG flooding (21.14%) or DDSB flooding (8.49%). Furthermore, the synergistic EOR mechanism by HCFS treatment was proposed based on the microscopic visualization experiment. In the HCFS flooding stage, DPG particles migrate to the depth and block the dominate channels by accumulating in pore throats; meanwhile, the HCFS efficiently strips the residual oil in high permeability channels. During the subsequent water flooding process, the injected water is diverted to low permeability zones with high oil saturation because of the in-depth profile control effect of DPG. What’s more, the surfactant diffuses to the high oil saturation zones with subsequently injected water, which significantly improved the oil displacement efficiency of subsequent water flooding. Therefore, the HCFS can greatly improve oil recovery by the synergism of increasing swept volume and enhanced oil displacement efficiency. more...