One of the reasons for cementing casing is to isolate zones. After evaluating available data, there are two target zones (upper and lower zones), and the upper zone is concluded as a water zone. It is important to isolate both zones and then perforate the lower zone. We run a cement job to put cement between both zones to prevent interflow. Why don’t we put a seal between them instead of cement? With that seal between them, we can then pre-perforate the lower casing. We eliminate the cementing unit and shape charges.
In Figure 1, a downhole drilling tools for zonal isolation is run to complete the well. Many downhole tools companies can help to install tubular steel and expandable tubular products. Tubular products comprise solid expanded tubular, pre-perforated expanded tubular and unexpanded tubular with external seal. The pre-perforated liner is placed in the oil zone. After the assembly is on depth, the expanding cone is pressurized to move it upward and expand unexpanded tubular with an external seal. The seal is then in-contact with the formation and preventing future water encroach (Figure 2). Pressuring the expanding cone continues on the second and the third unexpanded tubular with an external seal (Figure 3). The second external seal will prevent water flow into the oil zone and the third seal will prevent water production through the annulus of zonal isolation assembly. Please note that the burst strength of the assembly must be strong enough to hold the expansion pressure.
This completion technique is not suitable for unconsolidated formation since during the life of the well, the seal will leak. The technique is also not suitable for formation with a high concentration of sulfate which will rapidly corrode the assembly. An unexpanded casing is run instead and zonal isolation cement sheet is put between casing and formation to prevent direct contact of acid with casing.
Expandable tubular and external seal are available in the market now. Example materials are chromium steel tubular, fiberglass tubular for expandable tubular and rubber, epoxy for external seal.
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