Drilling and cementing naturally fractured carbonate reservoirs can be challenging and cause serious well-control issues because of the high probability of heavy or total losses. The Kashagan field located in the Caspian Sea area is characterized by a carbonate reservoir with high pressure, high contents of H2S and CO2, and presence of natural karsts/fractures.
Because of well-control issues resulting from heavy losses while entering the reservoir section, the closed-hole circulation drilling (CHCD) technique is often used to mitigate the problem. As a standard configuration, the reservoir section is cased with a 5 1/2-in. liner, cemented, and then perforated, but the heavy losses sometimes limit the casing and cementing zones. The alternative completion architecture is to deploy a 4 1/2-in. slotted liner with a preinstalled plug, properly tested either at surface or at bottomhole, to act as a second barrier while running the liner using the mud-cap technique and also while running in hole with the upper completion.
Different types of plugs were investigated and evaluated, considering the minimum rating requirements (6,500 psi as maximum differential pressure across the plug), the ease of installation, and testing and retrieving procedures. The main criteria for the selection of the best solution were simplicity and reliability.
Among all the possible options, three different plugs were selected and deemed fit-for-purpose for the completion design. Among these options, the simplest solution was deemed to be a chemical cement plug for ease of installation and the removing procedures. To confirm reliability in terms of load capability, different laboratory tests were carried out to determine the mechanical properties of the plug and the removal of the plug on demand.
The plugs were placed in a blank joint of a 4 1/2-in. slotted liner and laboratory tests conducted to test the integrity of the plug. The lab results were successful and subsequently three chemical cement plugs have been run in the field. Subsequent well operations such as running the completion assembly and displacing the balanced fluid to the lighter packer fluid have been carried out without any problem.
Laboratory tests, field installation, and results are presented here and discussed. This should help the industry solve challenges in running slotted liners in difficult well conditions.