Recently, demands for raw materials like rare earth elements (REEs) have increased considerably due to their high potential applications in modern industry. Additionally, REEs’ similar chemical and physical properties caused their separation to be difficult. Numerous strategies for REEs separation such as precipitation, adsorption and solvent extraction have been applied. However, these strategies have various disadvantages such as low selectivity and purity of desired elements, high cost, vast consumption of chemicals and creation of many pollutions due to remaining large amounts of acidic and alkaline wastes. Membrane separation technology (MST), as an environmentally friendly approach, has recently attracted much attention for the extraction of REEs. The separation of REEs by membranes usually occurs through three mechanisms: (1) complexation of REE ions with extractant that is embedded in the membrane matrix, (2) adsorption of REE ions on the surface created-active sites on the membrane and (3) the rejection of REE ions or REEs complex with organic materials from the membrane. In this review, we investigated the effect of these mechanisms on the selectivity and efficiency of the membrane separation process. Finally, potential directions for future studies were recommended at the end of the review.

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Heterogeneity of the hydrocarbon reservoirs can be defined in pore level or macroscopic scale. Pore level heterogeneity such as pore size and structure controls the quantity of hydrocarbon left (residual saturation) during production, whereas macroscopic heterogeneity determines zones that injected fluid sweeps (Franklin, 1994). During simulation of a reservoir, macroscopic heterogeneity is represented by assigning different rock properties (e.g. permeability and porosity) for different simulation grid blocks. However, due to complex nature of pore entrapment mechanisms, it is only possible to represent pore scale heterogeneity with empirical correlations. Capillary Desaturation Curve (CDC) is a suitable correlation that links residual hydrocarbon saturation to physical properties of a given reservoir in pore scale. Therefore, accurate representation of CDC pattern can be critical for Improved Oil Recovery (IOR) processes evaluation as pore information is used as the basis for residual saturation prediction. Application of CDC data for reservoir simulation and IOR has been reviewed here accordingly.

Water Alternating Gas injection (WAG) or Simultaneous Water and Gas Injection (SWAG) floods have been proposed as very good solution to overcome gravity segregation and better EOR performance in compare to conventional continuous gas injection (CGI). However WAG-Based processes cause some problems associated with increased water saturation including diminished gas injectivity. As an effective alternative for WAG, Gas Assisted Gravity Drainage (GAGD) for conventional reservoirs has been developed (US Patent 2006/0289157) that takes advantage of the natural segregation of gas from liquid hydrocarbon during injection. The GAGD process consists of placing a horizontal producer near the bottom of oil column and injecting gas through existing vertical wells. As the injected gas rises to form a gas zone, oil and water drain down to the horizontal producer. Application of GAGD for IOR in naturally fractured reservoir is discussed here based on some facts and figures.

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