The controllability of mining is a key factor affecting the commercial application of methane hydrates, and the addition of chemical additives can significantly accelerate the mining process. However, the effect of additive concentration on hydrate decomposition is not yet well understood. In this study, we systematically investigate the effect of ethanol concentration on the decomposition of methane hydrate under varying thermodynamic conditions using molecular dynamics (MD) simulations. To quantitatively characterize the decomposition process and mechanism of methane hydrates, the combination of angular order parameter (AOP), radial distribution function (RDF), mean square displacement (MSD), diffusion coefficients and system energy was for the first time used. The results showed that the addition of ethanol contributed to the formation of methane bubbles and accelerated the decomposition of hydrates. The mass transfer effect of ethanol molecules and the reconstruction of the hydrogen bond network of water molecules determined the stability of hydrates. From 0 to 40 mol% ethanol concentration, the hydrate decomposition increased with increasing the concentration of ethanol. Both increasing the temperature and decreasing the pressure are beneficial to the decomposition of the hydrate system. These results provide the selection of optimal ethanol concentration for the decomposition of methane hydrate and reveal its decomposition mechanism, and shed important light for the controllable production of gas hydrates.

With the increasing demand for efficient extraction of residual oil, enhanced oil recovery (EOR) offers prospects for producing more reservoirs’ original oil in place. As one of the most promising methods, chemical EOR (cEOR) is the process of injecting chemicals (polymers, alkalis, and surfactants) into reservoirs. However, the main issue that influences the recovery efficiency in surfactant flooding of cEOR is surfactant losses through adsorption to the reservoir rocks. This review focuses on the key issue of surfactant adsorption in cEOR and addresses major concerns regarding surfactant adsorption processes. We first describe the adsorption behavior of surfactants with particular emphasis on adsorption mechanisms, isotherms, kinetics, thermodynamics, and adsorption structures. Factors that affect surfactant adsorption such as surfactant characteristics, solution chemistry, rock mineralogy, and temperature were discussed systematically. To minimize surfactant adsorption, the chemical additives of alkalis, polymers, nanoparticles, co-solvents, and ionic liquids are highlighted as well as implementing with salinity gradient and low salinity water flooding strategies. Finally, current trends and future challenges related to the harsh conditions in surfactant based EOR are outlined. It is expected to provide solid knowledge to understand surfactant adsorption involved in cEOR and contribute to improved flooding strategies with reduced surfactant loss.