Simultaneous Control of Subsurface Scale, Corrosion, and H2S Using a Single Capillary String: A Real-World Chemical Application in the Permian Basin
Authors: Gonser Michael , Allan Bridgette , Wylde Jonathan 
Published: September 2022

Abstract
This paper details the laboratory development and subsequent field trial of a Downhole, Non-Triazine H2S Scavenger (DNTS) that was co-injected via a single capillary string (cap-string) with the incumbent scale/corrosion inhibitor (CI/SI) combination product. The target sour producer wells were configured with single cap-strings, and it was considered critical that H2S suppression was moved upstream downhole in high-risk sour producers to improve overall control of H2S in those wells and across the topsides process. The dual injection approach was preferred to ‘1-can’ injection (all production chemicals formulated into one fluid package) as it allowed targeting and more effective use of chemicals deployed for control of corrosion, scale and H2S within the receiving wells and for the topside process.
Product evolution followed classic laboratory development routes, utilizing industry standard performance techniques for scale, corrosion, and H2S scavenger assessment. Particular emphasis was placed on inter-chemical and ‘multiple chemical active’-material compatibilities for downhole co-injection and return to topsides with production.
Three gas-lifted wells currently supported by CI/SI dosing via cap-string were selected for field trial based on their H2S levels. A three-phase approach was followed, with Phase I providing initial proof-of-concept validation of the dual application technique in Well A; and in Phase II the technique was rolled out to Wells B and C to demonstrate the techniques versatility when applied to adjacent wells that had different CI/SI and scavenger requirements. Phase III provided the ultimate test when the three trial wells were dosed simultaneously and the impact on topsides process H2S control was assessed with reference to meeting the all-important facility prescribed daily operational allowed H2S target.
The laboratory testing phase confirmed that the products (DNTS and CI/SI) could be safely deployed downhole together via the same injection string, and chemical-chemical and chemical-materials compatibility issues were determined to be negligible. Field trialing showed scale and corrosion inhibition to be unaffected by the dual injection (via routine residuals monitoring), and scavenger performance metrics (as sampled at the wellhead) during steady-state production and treatment indicated impressive scavenging performance. Injection of DNTS downhole resulted in rapid impact on topsides H2S, and wellhead H2S samples indicated almost two orders of magnitude of improvement in H2S control with reduction from 1000 ppm to &lt;10 ppm achieved. The co-injection of DNTS and CI/SI assured management of scale, corrosion and H2S within the target wells from point of injection up to surface, and then onto process. Consequently, topside MEA-triazine dosing was reduced, reflecting the lower H2S levels observed, and overall, the dual downhole deployment clearly improved downhole and topside control of H2S. The reduced requirement for MEA-triazine promised significant operational cost-savings besides chalking up a sizeable Environmental, Social, and Governance (ESG) CO2 intensity ‘win’ due to massively reducing the incumbent (MEA-triazine) chemical usage.
The co-injection of scale, corrosion, and H2S treatments via single capillary met H2S production specifications for three different sour gas-lift wells and at process export, while improving asset integrity within subsurface equipment, improved safer operating conditions for personnel topsides, reduced corrosive liquid handling issues, and greatly increased the value of the export sales gas via H2S specification management.