**4. Foam rheology**

 Determining foam rheology is complex and it is considered difficult to predict the behavior of foam flow [20]. The performance of foam-based fracturing job is highly dependent on the rheology of the foam under downhole conditions and the efficiency of fracturing job depends on non-Newtonian behavior of foam [41, 42]. Foams are considered versatile, complex, and unique due to their high viscosity and low density characteristics [43]. Foam apparent viscosity is determined by accounting for the contribution of foam film thickness, bubble deformation, and the expansion of foam interface due to surface tension gradient [44]. The apparent viscosity of foam is strongly dependent on various process variables such as foam quality, shear rate, temperature, pressure, surfactant concentration, and salinity [8, 41, 42]. The effects of these parameters are discussed below.
