The utilization of biopolymers as an alternative to synthetic polymers in chemical flooding for enhanced oil recovery (EOR) has gained importance due to their ability to withstand harsh reservoir conditions and environmental friendliness. Understanding biopolymer behavior under such conditions is critical to determine if they exhibit consistent behavior or vary from one to another. This study focuses on evaluating the rheological properties and core flooding outcomes of three specific biopolymers, namely hydroxyethyl cellulose (HEC), xanthan gum, and guar, under reservoir conditions of 212°F, the salinity of 135,000, and pressure of 2200 psi where the previous works lacked to examine the behavior of these biopolymers under such combined conditions. Findings indicate the non-uniform behavior of biopolymer rheological properties under these conditions, highlighting the critical need to evaluate them before using them in the EOR process. At the reservoir conditions, increasing pressure resulted in decreased viscosity for xanthan gum but increased viscosity for guar. The viscosity of HEC initially decreased with increasing pressure but then showed an increase. Also, all biopolymers displayed shear thinning and weak gel behavior (storage modulus/loss modulus > 0.2) under reservoir conditions. Core flooding experiments using actual Bahariya formation cores showed xanthan, guar, and HEC recovered 22%, 8.9%, and 1.8% of the residual oil saturation which is equivalent to 6%, 2.7%, and 0.6% of original oil in place respectively. This suggests xanthan gum exhibits superior rheological properties and oil recovery effectiveness among the tested biopolymers under harsh reservoir conditions.