Inhibiting breakthrough of driving fluid via a permeable geological layer into an oil production well

A method for selectively reducing the permeability of one or more relatively permeable geological layers of an oil-bearing formation, to inhibit breakthrough of driving fluid from a driving fluid injection well via at least one of said layers into an oil production well, which method comprises the steps of:

• injecting a driving fluid comprising a first compound into the formation via the injection well wherein the first compound comprises an alkaline material;
• detecting the first compound in well fluid of the oil production well;
• upon detection, injecting a second compound, the second compound comprising iron ions, into the formation via the oil production well, to react with the first compound to form an insoluble salt in order to provide a flow restriction generated by a third compound which comprises a reaction product of the first and second compounds in at least one relatively permeable geological layer through which breakthrough of the driving fluid into the oil production well has occurred.