Abstract
Gas injection into a naturally fractured oil reservoir keeps the reservoir pressure and increments the initial recovery from the reservoir. The main aim of this work was to develop a numerical model to calculate the mass transfer (molecular diffusion and convection) between a gas injected in the fracture and residual fluid (gas and oil) in a matrix block. The dual continuum model is applied to describe flow behaviour and fluid recovery in porous media. Finally, the model is validated by comparing the outcomes with the results of two experimental works available in the literature. The mathematical model results are in agreement with the laboratory data including recovery of each component, saturation profile, and the pressure gradient between matrix and fracture. Modeling results show that after 25 days of N2 injection, the lighter and heavier components (C1 and C5) are recovered about 51% and 39%, respectively. These amounts for CO2 injection are 49% and 27%. It is found that the convection mechanism has a great effect on preventing the pressure drop of the reservoir during injection operations. In the nitrogen injection, without considering the convection, after 30 days, the matrix pressure reaches 1320 Psi from 1479 Psi but after 30 days, considering the convection, the pressure reaches 1473 Psi from 1479 Psi.
Highlights
The major parameter for production from heavy oil reservoirs is high oil viscosity [1,2,3]
The studies reveal that the diffusion of CO2 into the core accounts oil to swell and subsequently reduce the viscosity which results in a higher gravity drainage rate [19, 20]
The presence of dual porosity including porous media and fracture networks with different physical properties causes the structure of fractured reservoirs much more complex than conventional non-fractured reservoirs so that conventional methods which have been used for studying the performance of the reservoirs cannot present appropriate results to examine the behavior of reservoirs [10, 14, 25]
Summary
The major parameter for production from heavy oil reservoirs is high oil viscosity [1,2,3]. The simulation outcomes have revealed that constant diffusion coefficients may not predict an acceptable result during gas flooding and oil recovery might be overestimated or underestimated It has been observed by Darvish et al [33] for a higher porosity case that, the viscosity reduces later and the effect of the molecular diffusion on oil recovery will increase if the porosity of the matrix is big. Diffusion could be an important recovery mechanism in naturally fractured reservoirs while there are very few attempts to model diffusion and convection between a flowing gas through the fracture and oil and gas in the matrix. The dual continuum model is the model used for gas injection into naturally fractured reservoirs in this paper. This model will be examined in the following and the proposed model will be validated with experimental data available in the literature
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