Flow behavior of N2 huff and puff process for enhanced oil recovery in tight oil reservoirs
In the present work, the potential of N2 huff and puff process to enhance the recovery of tight oil reservoir was evaluated. N2 huff and puff experiments were performed in micromodels and cores to investigate the flow behaviors of different cycles. The results showed that, in the first cycle, N2 was dispersed in the oil, forming the foamy oil flow. In the second cycle, the dispersed gas bubbles gradually coalesced into the continuous gas phase. In the third cycle, N2 was produced in the form of continuous gas phase. The results from the coreflood tests showed that, the primary recovery was only 5.32%, while the recoveries for the three N2 huff and puff cycles were 15.1%, 8.53% and 3.22%, respectively.The recovery and the pressure gradient in the first cycle were high. With the increase of huff and puff cycles, and the oil recovery and the pressure gradient rapidly decreased. The oil recovery of N2 huff and puff has been found to increase as the N2 injection pressure and the soaking time increased. These results showed that, the properly designed and controlled N2 huff and puff process can lead to enhanced recovery of tight oil reservoirs.
Experimental Study on Physical Simulation of Large-Scale Interfracture Injection and Production with Different Injection Media in Tight Oil Reservoirs

 Given the difficulty of water injection and effective displacement system establishment in Changqing oilfield, this research carried out the interfracture injection and production experiment of a large outcrop model with two types of injection fluid medium, natural gas and surfactant, and studied the interfracture injection and production law of different displacing medium and the principal contradiction existing in the different displacing medium. The results show that natural gas drive in tight oil reservoir is beneficial to reduce the Jamin resistance and form a rapid oil production. Still, the gas drive process is easy to develop gas breakthrough, leading to a decrease in gas utilization ratio, and its oil displacement efficiency is 17.25%. Additionally, due to the strong adsorption of surfactant, the seepage capacity of a porous medium in the process of surfactant-oil displacement is reduced, and blockage is formed at the injection end, resulting in the poor oil displacement effect of surfactant. However, compared with natural gas injection, the surfactant has higher oil displacement efficiency, up to 22.05%. Therefore, for tight oil reservoirs, rational utilization of the advantages of different mediums and controlling their disadvantages are essential for the development of such reservoirs.
Modeling diffusion and gas–oil mass transfer in fractured reservoirs
Abstract Diffusion may play a key role in a number of oil recovery processes such as heavy oil and naturally fractured reservoirs. In fractured media, several laboratory experiments and numerical studies demonstrated that CO 2 injection can improve recovery. Molecular diffusion, gravity drainage, and oil swelling are the main contributing mechanisms. Proper modeling of diffusion at the reservoir PVT and geological conditions is not a trivial task. The challenge is in computing the diffusion coefficients for the non-ideal multicomponent mixtures in the oil and gas phases, and in physically accurate representation of the diffusion driving forces. One common approach in most numerical models is to use the classical Fick's law which simplifies the multicomponent diffusion fluxes by only considering the main-diffusion (diagonal) terms and neglecting the cross-diffusion (off-diagonal) terms in the diffusion matrix. The diffusion fluxes are assumed independent and the diffusion driving force of each component is proportional to the component self concentration gradient. In this work, we demonstrate analytically and numerically that this simplified approach may not honor the total flux balance and, in some applications, fails to capture the right direction of the diffusion flux. We propose an alternative model based on the generalized Fick's law. The proposed model can be seen equivalent to the Maxwell–Stefan model. The diffusion fluxes take into account the species interactions and the diffusion coefficients are dependent on temperature, pressure, and composition. We also tackle another problem related to cross-phase mass transfer of components in the gas and oil phases. This mechanism may occur in fractured media when fractures saturated with gas get in contact with under-saturated oil in the surrounding matrix. Intra-phase gas (gas-in-gas) and oil (oil-in-oil) diffusions cannot be initiated due to phase discontinuities between the fracture and the rock matrix. Some approaches in the literature that allow for direct gas-to-oil diffusion may not have a sound basis. We propose an accurate and numerically efficient model to describe the gas–oil transfer mechanism. The proposed model, based on irreversible thermodynamics, uses the chemical potential gradient as the driving force. The gas–oil transfer coefficients are functions of the gas and oil diffusion coefficients. In this model, the component fluxes are assumed continuous across the gas–oil contact and the interface fluid is in thermodynamic equilibrium. Several numerical and experimental cases are provided to validate the proposed model.