Asphaltene deposition during miscible and immiscible gas injection in tight heavy oil reservoir: Implications for formation damage

Abstract

Gas injection is frequently used to displace the trapped oil. However, this may lead to deposition of asphaltene, which can profoundly block the pores and throats of the porous media, resulting in reduced porosity and permeability. The rock lithology, composition of the injected fluids, pressure, and temperature are among the parameters that may influence asphaltene deposition. This study investigated the asphaltene precipitation/deposition while injecting rich hydrocarbon gas into a heavy oil reservoir. The experiments were conducted under two conditions: miscible and immiscible gas injections. Two rich hydrocarbon gases with different compositions were used, and experiments were conducted under reservoir conditions. Experiments were performed in carbonate cores previously saturated with heavy live oil. The extent of formation damage was assessed by measuring asphaltene deposition during the flooding of miscible and immiscible rich hydrocarbon gases into the core plugs. The outlet oil samples from the core flooding were analyzed using molecular weight measurements, UV spectroscopy, and the IP143 method. During the miscible flooding process, where rich hydrocarbon gas was injected into live oil at 210 °F, the molecular weight of the outlet oil samples decreased by 53%. The porosity and permeability of the carbonate core in miscible flooding at 210 °F, after washing with cyclohexane, showed a reduction of 13.6% in porosity and 42.8% in permeability. In the course of immiscible flooding with rich hydrocarbon gas, the outlet oil at 104 °F decreased by 48.3% in molecular weight, 35.5% in absorption coefficient, and 30.7% in asphaltene content. The porosity and permeability of the carbonate core in immiscible flooding decreased by 11.8% and 43.25%, respectively. The results showed that both miscible and immiscible injection processes reduce the porosity and permeability of carbonate cores. The damage was particularly severe during the miscible flooding.