Effect of Well Variables on the Performance of Chemical, Jet, and RCT Tubing Cutters

Abstract

An analysis of 72 tubing cuts made during Shell deepwater workovers in the Gulf of Mexico was completed to determine the effect of well variables on the cutting performance of chemical, jet, and radial cutting torch cutters. An initial pre-study analysis indicated that cutting success was only 65%, 50%, and 77% for the chemical, jet, and radial cutting torch cutters, respectively. It was found that some cutting failures could be attributed to specific causes, such as heavy paraffin in the well, improper detonation, low-pressure tool leaks, exceeding tool pressure limits, and improper tool gas ventilation; however, other failures did not have any apparent cause. These remaining cuts were analyzed against specific well variables to look for trends that may indicate the cause of failure. It was determined that chemical cutter success in CaBr2 completions fluids was much lower than all other fluids combined. Specifically, cuts in bromide fluids accounted for 71% of the total number of failed cuts, but only comprised 32% of total attempted cuts. Further investigation indicated that bromide fluids may react more violently with the bromine trifluoride acid than the other types of completion fluids used. From this analysis, it is believed that less acid will be available to sever the pipe wall when cutting in bromide fluids. Additionally, it was found that placing the pipe in tension significantly increased cutting performance. No other significant correlations were found to other well variables. Introduction Choosing the best tool for any particular drilling or workover cutting job is not always trivial. A study of 72 tubing cut attempts by one major operator indicated that cutting success was only 65%, 50%, and 77% for the chemical, jet, and radial cutting torch (RCT) cutters, respectively (Figure 1). There are several potential causes contributing to this larger than expected number of failures. First, the vast array of logistical and mechanical parameters that must be considered often makes cutter selection ambiguous. Despite this fact, service companies generally only rate tools based on pipe size, pressure, temperature, and the maximum and minimum standoff between the tool and the pipe wall. Undoubtedly, satisfying the limits of these rated variables is essential for a successful cut; however, the lower than expected success rates indicates there may be other significant factors. It is apparent that many well variable relationships to cutting performance are extremely complex, forcing decisions to often be made from past history or testing. However, access to specific tool performance data is proprietary or limited, and for this reason it is often necessary to rely upon the service company to choose the most effective tool for conditions. This is complicated by the fact that service companies tend to specialize within one particular tool, potentially limiting the possibility that the best tool will be selected for the job. The goals of this paper are two-fold:to identify other variables that may be important to cutter performance, andto provide a resource of data that may be useful for tool selection.