Solvent injection recovery processes were introduced as a more energy-efficient and environmentally friendly alternative to Steam injection processes. However, BTX chemicals (Benzene, Toluene, and Xylene), commonly used for crude oil recovery due to their strong solvency and low asphaltene precipitation, are acutely toxic and harmful to the environment. These chemicals are easily soluble in water causing groundwater contamination. This paper evaluates the recovery efficiency of two green solvents, Limonene, and beta-pinene, on two samples of Californian heavy oil (C1 has an 874.8 cP viscosity and C2 has 178500 cP viscosity).
On both C1 and C2, 5 core flood experiments were conducted, in total 10 experiments were run. CO2, limonene, and Beta-pinene were tested as solvents on both oils. Limonene and beta-pinene were both chosen due to their ready availability in the State of California. Both these solvents are plant-derived, non-toxic, and biodegradable. They also have much higher flash points than BTX solvents allowing for safer handling. They have been either injected as sole solvents or co-injected with CO2 during the experiments. Limonene and beta-pinene were injected at 2 mL/min while CO2 was injected at 2000 ml/min with a back pressure of 45-55 psi. Core packs were prepared by filling the pore space of Ottawa sand with 60% PV oil samples and 40% PV water by volume. Produced oil and water samples were collected every 20 min during the experiments. Thermogravimetric analyses (TGA/DSC) were conducted on these samples to identify oil, water, and solvent percentages.
Because CO2 is insoluble in these types of high viscosity crude oils, CO2 flooding resulted in immiscibility with almost no oil production. Since both limonene and beta-pinene are aromatic solvents, by sole limonene or beta-pinene injection miscible flooding was achieved. Limonene achieved 35 and 23 vol. % oil recovery from a total of 60% oil for C1 and C2 respectively while Pinene achieved 31 and 27 vol. %. Co-injections of green solvents with CO2 are expected to yield higher recovery due to the presence of two active drive mechanisms namely miscible and immiscible. Co-injection of limonene and CO2 provided the greatest recovery with 45 vol. %, however, recovery efficiencies of pinene and CO2 had comparable recoveries with that of pinene possibly due to phase trapping. Produced samples analysis showed that oil percentages in produced samples were higher for Limonene than Pinene.
Our results indicated that limonene and beta-pinene are very promising solvents for heavy oil recovery. Because these solvents are citrus-based, they are both easy to handle and non-toxic. Hence, we believe that our study can be a breakthrough for many heavy oil and bitumen reservoirs all around the world.