CoNiFe ternary hydroxides were synthesized via hydrothermal method and the synthesis conditions (temperature, reaction time and Co/Ni/Fe molar ratio) were investigated. High temperature and long reaction time help to increase the crystallinity of the as-prepared materials yet result in creating the impurities (Co(OH) 2 , Ni(OH) 2 and Fe 3 O 4 ) in addition to layered double hydroxide (LDH). Pure ternary CoNiFe LDH phase is obtained at the optimized condition at 130 8C for 12 h when Co/Ni molar ratios are 3:1 and 1:1. Cyclic voltammetry and galvanostatic charge/discharge tests display the capacitive performances at À20 8C of the ternary LDH material with Co/Ni molar ratio of 3:1 is superior to binary CoFe LDH.

Injecting supercritical CO2 into tight gas reservoirs to displace CH4 is an extremely promising technology of unconventional gas exploitation. However, Darcy's law cannot describe the gas flow due to ultra-low permeability and micro–nano porosity of tight rock. The present work is an analytical approach to investigating the nonlinear seepage characteristics of CH4 displacement by supercritical CO2. Moreover, considering the steady and unsteady state conditions, mathematical models for planar linear flooding, planar radial flooding, and single well and one-well injection/one-well production types are presented, computed, and verified. Their equipotential and streamline charts are first determined by implementing the model. Additionally, by investigating variables such as formation pressure, producing pressure drop, permeability, temperature, well spacing, and mass flow, their contributions to gas production rate are determined. Finally, an approach for improving the gas recovery efficiency is obtained based on the obtained results.

Injecting supercritical CO2 into depleted gas reservoirs enables additional CH4 to be extracted, a process known as CO2 enhanced gas recovery (CO2-EGR). Optimization of the well pattern is another method used to enhance gas reservoir exploitation. The focus of the present work is to address the arrangement of the well pattern when using CO2-EGR. For this purpose, mathematical models with five-spot and seven-spot well patterns are established in steady and unsteady conditions, and their results are validated against previously published models. For the first time, equipotential and streamline charts of the well pattern in CO2-EGR are derived from these models. As a result, the main flow channel of the well pattern is clarified, and the distributions of formation pressure and seepage velocity are determined. Moreover, the relationships between the gas production rate and well pattern parameters such as the producing pressure drop, permeability, formation pressure, temperature, and well spacing are investigated and the factors that influence the recovery ratio are examined. Finally, an optimization strategy for the well pattern parameters in CO2-EGR is proposed to enhance the gas production rate and recovery factor.

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