The removal of mineral scale in the oil field is typically perceived as a somewhat nonscientific discipline. Simply pumping down a few drums of acid, letting it soak or circulate, and bringing the well back online is considered the norm. This could not be further from the truth. There is as much science in dissolution of mineral scale as in any other production chemistry discipline. Consider two examples of complexity that exist in this subdiscipline of flow assurance: first, the complex placement challenges that exist in downhole-tubing-scale removal and understanding the mechanical complexity involved in spotting dissolver in a vertical system so that it remains in place to perform the job. The second example is concerned with iron-sulfide dissolution, where many different polymorphs exist and all behave in ways very different than hydrochloric acid at different pressures and temperatures.
The research that exists in the literature shows the science of removal chemistries and techniques very well. The use of acid, in particular hydrochloric acid, is a very-well-established field methodology. The challenges, however, that can exist with using acid, and their mitigation, are the subject of a renewed research focus. Two areas in particular involve the corrosivity of mineral acid and the secondary deposition of unwanted scales that can occur on flowback. These two examples alone show how use of acid can create a problem, possibly larger than the one it solves.
Some of the most interesting research is in the area of nonacid dissolvers; the intent of this chemistry is to remove the mentioned issues of corrosivity and secondary deposition. The target is to develop a dissolver system without the drawbacks of mineral or organic acid but with the same dissolution performance. Previous research has focused on chelating agents but has shown that only a certain performance can be achieved. However, in recent literature, some significant steps have been made with the discovery of synergistic chelating agents and formulations that are now approximating the performance of classic acid systems.
Researchers and readers are encouraged to read the recommended-additional-reading papers and take the time to delve into the intellectual-property landscape, which contains a plethora of information on the development and evolution of chemistry for scale removal.
Recommended Additional Reading
SPE 172794 A Systematic Approach To Dissolve Iron Sulfide Scales by Hicham El Hajj, Halliburton, et al.
IPTC 18139 Challenge and Successful Application for Scale Removal in Oil Field, Egypt: Field Study by E. Hamdy, Sapesco, et al.
IPTC 17303 Horizontal-Well Productivity Restoration—Successful Stimulation Application in a Large Onshore Field by Shobhit Tiwari, Cairn India, et al.
SPE 171766 Challenge of Acidizing Horizontal Wells in Tight Carbonate Reservoirs—Weak-Acid and Nonacid Alternatives by Sameer Punnapala, Abu Dhabi Company for Onshore Oil Operations, et al.
Jonathan Wylde, SPE, Head of Global Innovation, Clariant Oil Services
01 September 2015
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