Questions concerning acid inhibitor effectiveness at high velocities and in the oxygen rich environments common to Coiled Tubing (CT) applications prompted laboratory testing of several acid inhibitors at various concentrations in common acid stimulation fluids.
Following the laboratory testing, special CT nozzles were manufactured to position standard corrosion coupons in the direct path of the stimulation fluid flow as it exited the CT during acid treatments. The data gathered from the corrosion coupons were used to validate the laboratory tests.
As a result of these studies, unnecessarily high acid inhibitor volumes and undesirable or even potentially hazardous field practices were discontinued. The information learned from the studies was incorporated into field practices, which resulted in safer, lower risk operations, reduced the chance of formation damage, and lowered the overall operating costs for acid stimulation treatments.
This paper documents the testing and results of the lab and field studies. In addition, the nozzles described in this paper provide an easily implemented and inexpensive quality control tool for CT acid work in virtually any area CT stimulations are performed.
At Prudhoe Bay, Alaska as in other areas, CT is commonly used for spotting acid stimulation treatments. A recurrent area for concern during these treatments is the effectiveness of acid inhibitors in the high velocity domain common to CT stimulation procedures. Another concern is the low volume to surface area common to the 1-1/2" OD by 0.095" wall CT used at the time of this study and the potential loss of inhibitor concentration in the leading edge of the treatment fluid as it is pumped though the CT. The effect of repeated cycles of acid and oxygen environments was also considered as a possible corrosion source.
Common practices of years past included introduction of neat inhibitors, many containing arsenic compounds, just prior to pumping an acid treatment down the CT. Acid inhibitors have also been shown to adhere to certain clays1.
Therefore, the inhibitors used in the past were not only difficult to handle due to relatively high toxicity, but the highly concentrated slug of inhibitor pre-flush was also potentially damaging to the formation.
Even after these early slug treatment days, some vendors were recommending up to 200% more inhibitor to protect the CT than other vendors, and it was not uncommon to even double the higher recommendations for a safety margin in the unknown realm of high velocity treatments and subsequent effects on CT.
The excessively high concentrations of corrosion inhibitor were also found to interfere with the performance of surfactants, non emulsion agents, and mutual solvents requiring higher concentrations of these additives as well. In addition to the cost of the additives, there were also concerns that the resultant stimulation fluids might adversely affect the wettability of the formation1.
During the literature search prior to designing tests, it became apparent that there were no standard API or NACE standards for acid corrosion inhibitor testing. (It should be noted that there still is no industry standard for acid corrosion inhibitor testing in the petroleum industry.)
Literature pointed out that acid type and concentration, temperature, test pressure, time, agitation, chemical additives, acid-volume per steel-area ratio, and type and condition of steel used would all affect corrosion rates2. Further complicating the issue was the increasing "acceptable rate of corrosion" as temperatures increase. It seemed apparent that the "acceptable corrosion rate" values increased due to decreasing inhibitor effectiveness at higher temperatures, and not changes in required performance to meet a given goal.