A Demonstration Of Acid Corrosion Of Coiled Tubing Tools

15th Mar 2021

Acid stimulation through Coiled Tubing

In many regions, and particularly the Middle East acid stimulation of reservoirs through Coiled Tubing (CT) is a common operation. In a significant number of these operations the Bottom Hole Assembly (BHA) has a degree of sophistication, for example to enable CT access to lateral or horizontal wellbores.
To mitigate the impact of corrosion on the CT and wellbore tubulars it is standard practice to use corrosion inhibitors and flush fluids to condition the steel tubulars both before and after pumping the acid. Protecting wellbore tubulars and prolonging the life of the well and completion is the primary concern, extending CT life is also an important consideration due to the cost of replacement. Widely available inhibitors and flush fluids make it possible to conduct acid stimulation through CT with minimal damage to the CT or wellbore tubulars.

 

CT BHA Corrosion

CT BHA’s are typically manufactured from steel which is chemically alike the CT and wellbore tubulars, but a common observation is that the CT BHA’s are suffering from a higher degree of corrosion. Improved operation economics can be achieved, and risk of tool failure reduced by addressing this concern.

Tools that have been used for acid stimulation operations are often observed to have no evidence of corrosion on the outside of the tool but internal components, piston areas, seal grooves and threads often appear to suffer increased corrosion.

 

Picture1

Example of component with outside showing little corrosion but
the connecting thread has significant corrosion.

 

One explanation is that the flush fluids pumped after the acid clean the CT ID, BHA ID, BHA OD and wellbore tubulars adequately to prevent corrosion. However, the flush is not sufficient to neutralize or remove all the acid from internal chambers of the tools.

Picture2

Acid corrosion demonstration

To demonstrate the effect of small amounts of acid being left inside BHA internal components for a prolonged period a simple experiment was conducted. Two identical CT BHA components were submerged in 10% HCL for 24 hours to represent the through tubing acid stimulation operations. Both components were then removed from the acid. One component was rinsed in fresh water and cleaned with a common oil. The other component was simply removed from the acid and left to air dry. Both components were left side by side for 72hrs and the condition of them visually monitored. The experiment therefore represents two operational scenarios, one where when the BHA is removed from the well and disassembled and cleaned. The second, where the BHA is removed from the well and transported to a workshop for disassembly and cleaning 72hrs after the BHA is retrieved from the well. 

0 hour:

Two identical components submerged in 10% HCL

Picture3

 

24 hours:

Picture4

Components submerged in acid for 24hrs

 

24 hours:

Picture5

- Left hand component removed from acid and placed in tray

- Right hand component cleaned in water then oil and placed in tray

- No significant corrosion on either component

 

48 hours:

Picture6

Component with no cleaning has begun to corrode

 

72 hours:

Picture7

Component with no cleaning has significant corrosion

 

96 hrs:

Picture8

Component with no cleaning corroded beyond use

Conclusion

After being submerged in acid for 24 hours the two components were in good condition and perfectly acceptable for further use. The component that was cleaned and oiled showed no deterioration. The component left with small amounts of acid on it corroded badly and is not suitable for further use.
 

As a representation of a through tubing acid stimulation, the experiment supports the hypothesis that corrosion to the BHA is not occurring during the operation. Corrosion occurs in the time between the tools being removed from the well and the tools being completely disassembled and cleaned. This corrosion can be partially mitigated by submerging the tools in neutralizing solution as soon as they are retrieved from the well.