Drainage type oil and heavy-oil displacement in circular capillary tubes: Two- and three-phase flow characteristics and residual oil saturation development in the form of film at different temperatures

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It is still uncertain to what extent pore scale mechanisms, such as the counter and co-current nature of multiphase flow, the trapping mechanisms, the distribution of phases, and heat transfer mechanisms affect the process of isothermal and non-isothermal gravity drainage dominated oil and heavy-oil recovery. This type of processes is encountered during gas injection into oil reservoirs for enhanced oil recovery under isothermal conditions. Steam injection in thick reservoirs where gravity displacement is an effective mechanism and steam assisted gravity drainage (SAGD) are well-known examples of a non-isothermal gravity dominated heavy-oil recovery applications. It is commonly observed that field scale applications of the latter yield less recovery than estimated. One may also encounter this type process in the removal of any crude oil contamination in shallow zones where steam injection is used for cleaning. All these require in-depth analysis of the problem at the pore scale to account for the residual oil saturation (Sor) in the swept zone. In this paper, we used a single capillary tube (radius<0.03 cm) to mimic an elementary volume in the swept area during gravity dominated displacement applications and studied the flow characteristics of two and three phase flow with emphasis on film development. We carried out two-phase (air–oil) and three phase (air–oil–initial water saturation) flow displacements in a capillary tube under different temperature conditions, varying the air injection rate and the capillary properties. Detailed visualization experiments were carried out to analyze (1) the effects of heavy oil viscosity, wettability and spreading coefficient on displacements at different temperature conditions, (2) the interplay among capillary, gravity and viscous (air injection rates) forces and wettability using different capillary sizes (pore size), and (3) the residual oil saturation in the form of film development and phase distribution in the capillaries (mainly the thicknesses of the wetting and non-wetting phases). The experimental observations suggest that for heavy oil there is a threshold capillary number around 1.0E−2 over which the oil recovery (and therefore the residual oil saturation) is very sensitive to the capillary number, i.e., the injection rate, interfacial tension, wettability and temperature. At lower capillary numbers (typical range for oil reservoirs) temperature and hence viscosity do not have a significant influence in the residual oil saturation of the processed and crude oils; for horizontal displacement the residual oil saturation is a function of the capillary forces and for gravity drainage experiments it depends of the competition between capillary and gravity forces (Bond number).