In preparation for hydraulic fracturing, an operator inspected the 7-in, 32-lbm/ft [178-mm, 47.6-kg/m] casing in five wells and found seven weak spots, which were attributed to drilling operations. Although no leaks were detected, logs indicated that the remaining casing thickness in those spots—which ranged in depth from 2,000 to 8,820 ft [610 to 2,690 m]—would not support the planned fracturing operations.
To restore the internal pressure rating and integrity of the damaged casing, the operator had the choice of running a fracturing string or tieback system or setting casing patches. The objective was to maintain as much of the ID as possible to limit friction losses and thereby enable use of high pump rates per the frac job design. The solution had to fit economic criteria as well as technical requirements.
SLB recommended its 7-in ultraslim prefracturing expandable steel patch, which would leave an ID of 5.69 in [144 mm] after setting. Engineers calculated that each patch, rated to an internal differential pressure of 14,500 psi [100 MPa], would increase the pressure support of the casing by more than 2,500 psi [17 MPa]—adequate to support the planned fracturing operations. Five 13-ft [4-m] patches were selected to cover 1-ft [0.3-m] weak sections and one 36-ft [11-m] patch to cover two weak spots in a single run.
Each tubing-conveyed patch was installed in a day and the wells were pressure-tested to 9,500 psi [66 MPa]. The operator subsequently conducted the high-rate hydraulic fracturing campaign as planned, without incident.
