One of the reason for cementing casing is to isolate zones. After evaluating data available on the table, there are two target zones and upper zone is concluded as water zone. It is important to isolate both zones and then perforate lower zone. We run 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 cementing unit and shape charges.
In Figure 1, a zonal isolation assembly is run to complete the well. The assembly comprises solid expanded tubular, pre-perforated expanded tubular and unexpanded tubular with external seal. The pre-perforated liner is placed on oil zone. After the assembly is on depth, the expanding cone is pressurized to move it upward and expand unexpanded tubular with external seal. The seal is then in-contact with formation and preventing future water encroach (Figure 2). Pressuring the expanding cone continues on the second and the third unexpanded tubular with external seal (Figure 3). The second external seal will prevent water flow into 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 high concentration of sulphate which will rapidly corrode the assembly. An unexpanded casing is run instead and cement sheet is put between casing and formation to prevent direct contact of acid with casing.
Expandable tubular and external seal are available in market now. Example materials are chromium steel tubular, fiberglass tubular for expandable tubular and rubber, epoxy for external seal.