Process for recovering viscous oil from an underground formation containing viscous oil

Oil is recovered from a formation, which contains viscous oil and includes a tar sand deposit, by providing at least one injection bore and at least one production bore arranged at a distance from the latter, which bores extend from the ground surface downwards to the bottom of the formation and, in their lower parts, extend generally horizontally through and in connection with the formation. Solvent is injected into the injection bore and oil-containing fluids are recovered from the production bore until the recovered fluid contains an unfavourable quantity of solvent, preferably at least 90%. After the injection of the solvent has been terminated, both the injection bore and the production bore are isolated, and the formation is subjected to an impregnation period for a predetermined length of time, preferably for a period of between 2 and 20 days per 0.30 m (per foot) of the formation thickness. After the impregnation period, solvent is injected into the production bore and oil-containing fluids are recovered from the injection bore until the fluid obtained contains an unfavourable quantity of solvent, preferably at least 90%. After the injection of solvent into the production bore has been terminated, both bores are isolated and the formation is... to a second impregnation period for a predetermined length of time, preferably... Original abstract incomplete.