METHOD OF WELL STAGE CEMENTING UNDER CONDITIONS OF ABNORMALLY LOW FORMATION PRESSURES IN LOST CIRCULATION ZONE Russian patent published in 2002 - IPC

FIELD: construction of oil and gas wells. SUBSTANCE: method includes installation of stage cementing collar above roof of lost-circulation formation; injection of spacer fluid, portion of nonaerated cement slurry; forcing of cement slurry by displacing fluid; pumping of low-density fluid to annular space during forcing of cement slurry; mounting of cementing head with lower and upper separating plugs and locking member after installation of stage cementing collar. After injection of spacer fluid, lower part of casing is cemented, above lost circulation zone, by lowering of lower separating plug. Cement slurry is forced by displacing fluid with upper separating plug. After that, locking member is dropped with subsequent washing out of excessive cement slurry. After setting of cement slurry, upper part of casing is cemented through stage cementing collar. Portion of nonaerated cement slurry is injected in amount ensuring filling of annular space in interval from foot of lost circulation formation to stage cementing collar. Subsequent additional portion of cement slurry in amount ensuring filling of annular space from shoe of casing to foot of lost circulation formation is aerated during injection by increasing aeration degree within rage calculated by formulas. Density of aerated cement slurry in well, kg/cu.m is determined by formula. Low-density fluid is used in the form of two-phase foam with aeration degree calculated by formula. Two-phase foam density, kg/cu.m is determined by formula. Consumption of two-phase foam is determined from inequality. Nonaerated cement slurry is used in the form of the following composition, wt.p.p: oil-well portland cement, 100; superplasticizer, 0.9-1.5; polyvinyl alcohol, 0.5-1.0; sulfonol, 0.4-1.0; water, 35. Two-phase foam is used in the form of foaming fluid of the following composition, wt. p. p: aqueous solution of calcium chloride, ρ-1120 kg/cu.m, 100; commercial lignosulfonate, 0.7-1.0. EFFECT: higher efficiency of cementing process. 3 cl