Method of cleaning permeable formations

933,905. Wells; soil stabilization. SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ N.V. June 5, 1962 [June 7, 1961], No. 21738/62. Classes 68 (1) and 85. In the recovery of fluid from a formation penetrated by an injection or producing well the following treatment is repeated as necessary: (1) Pumping a predetermined volume of a selected liquid down the well into contact with the formation; (2) Injecting the selected liquid into a portion of the formation; and (3) pumping a predetermined volume of a formationplugging liquid down the well after the selected liquid to seal the most permeable zone of the formation. The plugging liquid becomes a freeflowing liquid after a predetermined time in the well so that repeated treatments increase the permeability over the whole depth of the formation. Slugs of selected liquid separated by slugs of plugging liquid are pumped down the well into an interval of a formation at a pressure less than the fracturing pressure until the selected liquid has been forced into all permeable parts of the interval. In a deep well the slugs may be removed by back-flushing and replaced by slugs of different volumes. The selected liquid may be acid, alkaline for dissolving mud, or an organic solvent for dissolving ashphalt or paraffin. The plugging liquid may be a liquid hydrocarbon in which a mixture of fatty acids have been dissolved and converted to soaps. In a well which had been fractured and acidized the selected liquid was acid and contained walnut shells as a propping agent. The invention may be used to permit the injection of an oil into a formation, the oil being one which may be thermally coked to consolidate the formation.