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Carbonated water injection (CWI) is considered to be a win-win association for enhancing oil recovery and reducing environmental footprint. Therefore, the study of CWI potential has gained tremendous attention recently. Unfortunately, the dedicated research so far focus heavily on conventional reservoirs. In order to expand the application scope of CWI, the objective of this study is to carry out a coreflooding investigation of core samples from tight oil reservoirs. The experiments were carefully designed to assess the performance of CWI and to quantify the potential of additional oil recovery and CO2 storage capacity under various experimental conditions. We presented the results of a series of CWI coreflood experiments using the reservoir cores of tight sandstone with live crude oil. It is found that the ultimate oil recovery by CWI is higher than the conventional water injection in both secondary and tertiary recovery mode. The complex mechanisms are also investigated including CO2 content of injected carbonated water (CW), oil swelling and viscosity reduction. It was observed that both secondary and tertiary CWI could recover a significant amount of additional oil compared to conventional water injection. The results revealed that the oil recovery would be lower if CO2 concentration is reduced, but the extent of oil recovery reduction would be much less than the level of reduction in CO2 concentration. Oil swelling as a result of CO2 diffusion into the oil raised the pressure, and the reduction of subsequent oil viscosity were amongst the main mechanisms of oil recovery by CWI, which were obtained by the high pressure coreflood models and PVT studies. Higher oil recovery was obtained with the higher permeability and higher carbonation level. At the end of the injection period, an encouraging amount of dissolved CO2 was stored in the brine and the residual oil of the tight sandstone. The experimental results clearly indicate the potential of CWI for improving oil recovery and CO2 storage capacity in tight oil reservoirs. It is particularly attractive for the implementation of CWI for reservoirs that have limited access to large amounts of CO2. To the best of our knowledge, this is the first time to experimentally investigate CWI potential in tight oil reservoirs.

Carbonated water injection (CWI) is a modified CO2 flooding method for enhanced oil recovery, which takes the both advantages of CO2 flooding and water injection and have attracted much attention recently. The objective of this study is to mimic the dynamic mass transfer process of carbonated water to "live" crude oil through a series of well-designed multi-contact tests. In each test, carbonated water (CW) was brought into contact with live crude oil in a high-temperature and high-pressure PVT cell. Pressure changes during the test were observed and recorded. After equilibrium, all of the transferred carbonated water was taken out of the cell and the swelled oil was proceed to the next contact. The volume of water and liberated gas were measured. The oil swelling factor was also measured, which would verify the existence of moving interface between carbonated water and live crude oil. A total of 12 contact tests were performed. It was observed that the pressure rapidly builds up immediately after the contact of carbonated water and live crude oil in the closed system. For the first contact, equilibrium pressure increased by 6.46MPa and for the last contact equilibrium pressure increased by 2.16Mpa. This result indicates a strong mutual interaction of carbonated water with live crude oil and be beneficial to maintain reservoir pressure. Due to large amount of CO2 transferred from carbonated water to the live crude oil, the swelling effect was quite obvious and a total swelling factor of 1.26 was obtained at the end of the experiments. The volume changes of carbonated water and live crude oil could be good evidence of the existence of moving interfaces during the dynamic mass transfer process. The other enhanced oil recovery mechanism by CWI such as viscosity reduction was also found in the tests. The experimental results clearly indicate that the pressure buildup during the diffusion process originated from the CO2 dissolution from carbonated water and swelling of the oil phase. The partition coefficient is relevant to CO2 solubility in water and live crude oil. The tests vividly replicated the dynamic interactions between live crude oil in a closed system and the flowing carbonated water, in which the oil would be contacted by fresh carbonated water.

This paper proposes a data-driven proxy model to effectively forecast the production of horizontal wells with complex fracture networks in shales. With the multilayer gated recurrent unit (GRU) cell, the proxy model is coupled with newly developed deep learning methods include attention mechanism (Att-GRU), skip connection, and cross-validation to deal with time series analysis (TSA) issue of multivariate operating and physical parameters. In the formulation, the input variables include time, variable bottom hole pressure (BHP), horizontal well length, fracture number, fracture half-length, and fracture conductivity and the output variable refers to the production corresponding to the forecast time. The sample data generated by the boundary element method (BEM) is used in the proxy model learning process. The shuffled cross-validation method is utilized to improve the model accuracy and generalization capability. Results depict that the Att-GRU can accurately forecast the production for shale gas wells with complex fracture networks at a given time and variable BHP while maintaining a high calculation efficiency. The operating and physical parameters analysis indicates that the Att-GRU has learned the underlying physical features of complex fracture networks and variable BHP. Case study from Marcellus shale shows that the proposed Att-GRU is robust in both production forecast and reservoir evaluation, and it is a potential proxy model for transient analysis.

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