Stressed Steel Liner Yields Stronger Casing Repairs

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Abstract A method together with the necessary tools has been developed for expanding a steel liner in a well for the purpose of repairing damaged casing. This repair method has been proven successful by field experience. A glass-fabric mat impregnated with unpolymerized plastic is used between the steel liner and the well casing to fill-in the irregularities and to insure a pressure-tight seal. The plastic carried in the glass fabric is extruded out into the holes in the well casing; thus, a solid plastic plug is formed in the existing holes after the plastic hardens. These plugs are effective in excluding high formation pressure from the liner. The tool which expands the steel liner places it in a prestressed condition so that the liner is under compressive hoop stress while the casing is under tensile hoop stress. Laboratory tests have shown that casing with holes as large as 1 in. in diameter repaired by this method will withstand higher internal and external pressures than specified for the casing by the manufacturer. The failure pressure is limited by the collapse or burst pressure of the steel inner liner where large splits or parted casing are repaired; however, a second liner may be placed inside an earlier-placed liner to obtain additional strength where high pressures may be encountered. Introduction The glass fabric-plastic casing patch, which has been in use for several years, has proved an effective means for repairing holes in pipe; however, the chances of a successful repair decrease rapidly as the hole size in creases above 1 in. Moreover, experience has indicated that a glass fabric-plastic casing patch cannot be used where the pipe (such as well casing) is parted and separated or where there are large splits. These limitations indicated the need for a liner which would be equally effective in repairing both large and small holes in casing as well as large splits or parted casing. Since the strength of steel is considerably greater than the strength of a glass fabric-plastic lining, a means of using a steel liner for subsurface repair of well casing was investigated. The successful use of a steel liner for repairing holes in casing requires fulfillment of the following conditions.The liner as it is run into the well must be substantially smaller in diameter than the bore of the casing.A means must be provided to expand the liner against the casing wall and leave it in place in a condition of maximum hoop compression.The liner must not substantially reduce the size of the wellbore. This paper describes a method of placing a steel liner in well casing so that these conditions are fulfilled, and presents the results of laboratory investigations and subsequent field experience. Description of the Liner and Placement Process A review of field experience with the glass fabric-plastic casing patch showed that the majority of casing repairs were placed in wells cased with 5 1/2-in. casing. On this basis, the initial development of the stressed-steel-liner method was concentrated on this casing size. The initial problem was to provide a steel liner with an outside diameter equal to or slightly greater than the inside diameter of the casing, and still be able to lower it through the casing to the point of well damage. The solution to this problem (which fulfilled one of the previously stated requirements) was found by preforming the liner with longitudinal corrugations, which effectively reduced the outside diameter of the liner. Generally, a reduction of 1/2-in. in outside diameter is sufficient to permit passage of the liner within the casing. Fig. 1 shows a typical section of a liner for installation in 5 1/2-in. casing. Before being corrugated, the liner had a 5.0-in. OD (Fig. 1a); the corrugating operation reduced its outside diameter to 4.5 in. (Fig. 1b). JPT P. 1337^