After Cuttings Re-Injection (CRI) wells are drilled and cased over the targeted zone, the next step in the process towards finalizing the well is usually perforating the casing. Perforating guns can be deployed with the rig on location or rigless after the production string has been run in the well. Operations with the rig usually offer the benefit of being able to use larger guns with improved formation penetration and larger perforation tunnel size. The extra penetration helps bypass the damaged zone around the wellbore to get into the better permeability of the undamaged formation rock for improved injectivity. The larger perforation tunnel allows more contact with the formation and increased flow area, factors which also help improve injectivity. Rigless perforating generally requires smaller guns which can pass through the tubing, and therefore offer less formation penetration and smaller perforation tunnels. However, one of the major benefits of the rigless operation is the rig time savings, which can often be substantial.
Very good injectivity is a must for CRI wells to manage the large amount of cuttings that will be injected. Large diameter holes in the casing are necessary along with a high shot density for more holes, and 7″ perforation guns were required for the 9-5/8″ casing to achieve these characteristics for injection purposes. When large perforating guns are used, the tubing-conveyed perforating (TCP) technique is usually prefered over cable-conveyed guns because of possible cable damage from shock when the guns are detonated.
The TCP technique often requires more rig time however and usually results in more overall cost than for a wireline operation with a small number of gun runs. Another consideration for the CRI wells is that they are drilled by a new rig with a relatively inexperienced rig crew. Considering rig performance, using wireline for the perforating operation could therefore be more effective in reducing operating time. However, a number of factors have to be taken into account when preparing for perforating using longer, large diameter gun strings on cable, including gun string weight, shock on cable head when the guns are detonated and cable weak points. The shock forces for this case were modeled with special software and the necessary high-strength cable, shock absorbers and weak points were procured for the operation. Rig up limitations also had to be considered to manage the length and weight of the heavy string and a special sequence was prepared and implemented using an on-site crane.
The job was managed safely and efficiently in the single planned run, marking the first time this type of operation has been done in the country and this paper summarizes the process and the efficiency results.