In certain cases, multiple production casing sizes are used across formations of the same pressure regime especially in vertical or slightly deviated wells with long pay sections. The normal perforation practice would be to use a smaller gun size to perforate the smaller lower casing section before making another perforating run with bigger gun size to perforate the upper casing. Such perforating technique not only adds cost and time to the completion program but also reduces the efficiency of the perforation tunnels due to damage caused by the completion fluids in the wellbore between the two perforation runs. In addition, underbalanced perforation can only be performed on the first perforating run. Isolation of the first zone would be required to create under-balance for the second run, thus adding cost and complexity to the operations. Furthermore, multiple perforation runs in a well tends to damage the existing perforations due to pressure surges created in the subsequent runs. These pressure surges generally pack wellbore solids into the existing perforation tunnels.
This paper describes in greater details the innovative technique of perforating with multiple gun sizes across multiple casing sizes in a single run. It discusses the engineering design and considerations involved which allows the application of this new technique safely without imposing additional risks to the perforating operation. This technique provides the operators with a viable option that is more economical and efficient compared to conventional practices of perforating across different sizes of casing using multiple runs.
The key areas of concern when designing the perforating technique with two guns sizes are the combined detonation pressures and the shock waves effects from both gun sizes on the perforation string. Both of these factors were carefully calculated for the specific environment in which the guns to be run. From these engineering calculations, a suitable gun string can be design to stand the explosive effect resulted during perforation. Based on the detonation pressure, suitable gun adaptor is designed using appropriate alloy to stand the ballistic effect during the perforation. This gun adaptor allows instantaneous ballistic transfer between the two gun sizes. The use of reactive liner perforating shaped charges also reduces the damage on the perforation tunnels as well as eliminating the limitation due to wellbore and reservoir conditions.
This innovative perforating technique of running multiple gun sizes in a single run was applied successfully in a gas well in Pakistan. The pay zone was completed with 7 in. liner and 5 in. liner. The operator was initially considering either to perforate the well using wireline conveyed perforation or using shoot and pull technique with tubing conveyed perforation with one gun sizes at a time. However, both methods involve long and costly operations. Considering all the above challenges, the new perforating technique was recommended. The perforating string consisted of 82 m of 3-1/8 in. and 91 m of 4-1/2 in. guns which were run in tandem. The overall operation only took place in 24 hours which saved the cost and rig time for the operation. The spent guns were retrieved in good condition with all charges fired. The post production result indicated excellent well productivity with total calculated skin of negative 3.
This paper makes three major technical contributions to knowledge of the petroleum industry. Firstly, it documents successful application of this technique to connect multiple gun sizes in a single run. Secondly, it describes the design considerations involved as well as the calculation required to safely design the multiple gun sizes perforation string. Thirdly, it identifies a technique of perforating multiple casing sizes without exposing the earlier perforation to risks of formation damage by shooting the entire interval in one go without having to use the smaller sized gun in larger casing size.
This novel technique ensures that the objectives of the perforation were safely met. The well production target was achieved and a new technique was proven to perforate two different casing sizes in a single run with the largest possible gun size used in each casing.