Hailong Chen

To meet the growing energy demand and ensure national energy security, improving the recovery rate of developed oil fields and tapping into their remaining oil potential have become important ways to stabilize crude oil production. Given the constraints posed by the intricate nature of reservoir formation conditions and the properties of crude oil, including high viscosity, significant heterogeneity, and low permeability, certain techniques find it challenging to be effectively utilized. In view of this, this article introduces enhance heavy oil recovery by in-situ generated foamy oil, foam flooding in deep fractured vuggy reservoirs, and a new CO2 responsive fracturing foam fluid, respectively. These results can provide constructive conclusions and suggestions for the study of theories and methods of enhanced oil recovery by gas and foam injection in complex reservoirs. Document Type: Perspective Cited as: Chen, H., Wei, B., Zhou, X., Zhu, J., Xu, Z., Fan, Y. Theory and technology of enhanced oil recovery by gas and foam injection in complex reservoirs. Advances in Geo-Energy Research, 2025, 15(3): 181-184. https://doi.org/10.46690/ager.2025.03.01 References Chen, H., Ji, B., Wei, B., et al. Experimental simulation of enhanced oil recovery on shale rocks using gas injection from material to characterization: Challenges and solutions. Fuel, 2024, 356: 129588. Chen, H., Li, H., Li, Z., et al. Effects of matrix permeability and fracture on production characteristics and residual oil distribution during flue gas flooding in low perme-ability/tight reservoirs. Journal of Petroleum Science and Engineering, 2020a, 195: 107813. Chen, T., Leung, J. Y., Bryan, J. L., et al. Analysis of nonequilibrium foamy oil flow in cyclic solvent injection processes. Journal of Petroleum Science and Engineering, 2020b, 195: 107857. Maini, B. B. Foamy-oil flow. Journal of Petroleum Technology, 2001, 53: 54-64. Tang, Q., Huang, Z., Zheng, C., et al. Switchable surfactant-based CO2-in-water foam stabilized by wormlike micelle. Industrial & Engineering Chemistry Research, 2018, 57(40): 13291-13299. Wang, H., Cai, J., Su, Y., et al. Pore-scale study on shale oil-CO2-water miscibility, competitive adsorption, and multiphase flow behaviors. Langmuir, 2023, 39: 12226-12234. Wanniarachchi, W. A. M., Ranjith, P. G., Perera, M. S. A. Shale gas fracturing using foam-based fracturing fluid: A review. Environmental Earth Sciences, 2017, 76: 91. Wu, R., Wei, B., Li, S., et al. Enhanced oil recovery in complex reservoirs: Challenges and methods. Advances in Geo-Energy Research, 2023, 10(3): 208-212. Xu, Z., Li, Z., Cui, S., et al. Flow characteristics and EOR mechanism of foam flooding in fractured vuggy reservoirs. Journal of Petroleum Science and Engineering, 2022, 211: 110170. Xu, Z., Li, B., Zhao, H., et al. Investigation of the effect of nanoparticle-stabilized foam on EOR: Nitrogen foam and methane foam. ACS Omega, 2020, 5(30): 19092-19103. Yuan, S., Wang, Q., Li, J., et al Technology progress and prospects of enhanced oil recovery by gas injection. Acta Petrolei-Sinica, 2020, 41(12): 1623-1632. Zheng, N., Zhu, J., Yang, Z., et al. Research on the viscoelastic scCO2 foam systems synergistically stabilized by nonionic/zwitterionic mixed surfactants. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2024, 697: 134462. Zhou, L., Wang, S., Zhang, L., et al. Generation and stability of bulk nanobubbles: A review and perspective. Current Opinion in Colloid & Interface Science, 2021, 53: 101439. Zhou, X., Yuan, Q., Rui, Z., et al. Feasibility study of CO2 huff ‘n’ puff process to enhance heavy oil recovery via long core experiments. Applied Energy, 2019, 236: 526-39. Zhou, X., Li, X., Shen, D., et al. CO2 huff-n-puff process to enhance heavy oil recovery and CO2 storage: An integration study. Energy, 2022, 239: 122003.

Fractured reservoirs are an important source of oil and gas energy. After depletion of production, the production capacity of this reservoir decreases rapidly. Effective profile control is needed to improve the sweep efficiency and reservoir heterogeneity. Foam can solve such problems, but its profile control mechanism is not fully understood. Based on this, this paper uses the level set method to study the microscale control mechanism of foam in fractured media. The results show that artificial fractures and high-permeability microfractures are tighter than natural fractures, the Jamin effect of foam is stronger, and the secondary foaming ability is better. Therefore, the plugging ability of foam to natural fractures is far less than that of foam to artificial fractures and high-permeability microfractures. The larger the fracture opening, the larger the foam volume and the smaller the flow rate. As the opening ratio increases gradually, the generated foam flows more to the natural fractures with a large opening, and the effect of foam blocking large fractures becomes worse. The diversion rate curves of different opening ratios show that the foam has a good profile control effect when the opening ratios are 4:1 and 2:1, and even the diversion rate overturns, while the profile control diversion effect is poor when the opening ratio is 10:1, so it cannot play an effective role in profile control. The foam shows the profile control process of preferentially plugging the high-permeability area and allowing more subsequent fluids to enter the low-permeability area. The research reveals the profile control mechanism of foam on fractured reservoirs from the micro level, which is the supplement and verification of relevant macro research and provides a theoretical basis for the efficient development of fractured reservoirs.

Amplitude and phase scintillation respectively refer to the rapid and random intensity and phase fluctuations of Radio Frequency (RF) signals caused by ionospheric irregularities. Both amplitude and phase scintillation occur at low latitudes, while phase scintillation is dominant at high latitudes. Scintillation can severely degrade the positioning performance of Global Navigation Satellite System (GNSS). The Phase Locked Loop (PLL) bandwidth and coherent integration time are important to determine the tracking performance of GNSS receivers. This study investigates the PLL tuning effects on the tracking performance by exploiting a software-defined radio (SDR) BeiDou Navigation Satellite System (BDS) B1C and B2a receiver based on simulated scintillation data. The results show that the configuration of bandwidth has a greater impact on the BDS receiver compared with the integration time under scintillation. The tracking errors caused by phase scintillation are suppressed by the increase of loop bandwidth. On the other hand, the tuning of loop bandwidth has little impact on the carrier to noise density ratio (??/??0 ) level of PLL. Additionally, a Kalman Filter (KF)-based PLL is implemented in the receiver to mitigate scintillation effects. The study is of great importance for better understanding the effects of scintillation with different latitudes on the tracking loop performance of BDS software receiver. It can also contribute to designing PLL adjustment strategies for different scintillations under high and low latitudes to adapt to scintillation.

In situ CO₂foams (ISCF) are studied systematically by combining in situ CO₂gas reactants (carbonate anhydrous, hydrochloric acid) and bio‐based surfactant. Sandpack flooding experiments at 60°C along with PVT experiments were carried out to analyze the oil displacement mechanisms. The results showed that ISCF could increase oil recovery from heterogeneous multilayer formation of permeability ratio over 6, and displacement efficiency increased with the injection volume increased before the injection of 1 PV. The incremental oil recovery by ISCF was much greater than that of conventional foam or in situ CO₂(ISC) without foam under the same injection conditions. The generated CO₂foam could reduce the interfacial tension between displacement phase and displaced phase effectively which contributed to the great increase in capillary number. The CO₂dissolution greatly reduced the viscosity of crude oil, and the highest viscosity reduction rate at 60°C could be as high as 98%. The Ca²⁺concentration of produced liquids analysis revealed the ISCF could distribute intelligently the acid in heterogeneous formations.