Sandbed consolidation with mineral precipitation

A new method has been developed to prevent sand reentrainment during oh pro duction from unconsolidated or poorly consolidated reservoir formations. Co nsolidation of the zformation around the well is achieved through in situ p recipitation of a sparingly soluble salt, namely, calcium phosphate. Contro l of the depth of salt formation is achieved by alternating injection, mixi ng, and reaction of two aqueous solutions of calcium chloride and potassium phosphate, Calcium phosphate crystals precipitate and grow on the grain su rfaces, forming sufficiently uniform coatings. The formation of relatively uniform coatings on the grains causes an acceptably small decrease of the p ermeability, which is a feature of primary importance for oil production. T he grains are gradually "cemented" with bridges of calcium phosphate crysta llites and form a consolidated and still porous structure. As a result, the rate of hydrocarbon production for the problematic reservoir can be increa sed considerably without undesirable reentrainment of sand. The proposed me thod for consolidation has been successfully tested in sandbeds, Several se ries of experiments have been carried out under diverse conditions to estab lish the optimum parameter values for the implementation of this method. A set of optimum conditions at 25 degreesC were determined and these conditio ns gave satisfactory consolidation with permeability loss of ca.60% of the initial value. The conditions of precipitation were chosen so that the prec ipitated phase was octacalcium phosphate [Ca4H(PO4)(3).2.5H(2)O], along wit h its byproduct hydroxyapatite [Ca-5(PO4)(3)OH]. Experiments were also carr ied out at 70 degreesC and have shown that it is feasible to consolidate lo ose sandpacks at oil reservoir conditions, (C) 2000 Academic Press.