Combination Package Development of Scale Inhibitors and Hydrogen Sulfide Scavengers for Sour Gas Production in Barnett Shale
Authors: Peng Yang , Shi Liu , Yue Zhiwei , Fan Chunfang 
Published: April 2015

The Barnett Shale, a formation of ~5000 mi2 between the Bend Arch and Fort Worth Basin, was estimated the largest producible onshore reservoir for natural gas production in the United States. The field also features souring production and significant carbonate scale issues (predominated by CaCO3 and FeCO3). As common commercial triazine-based scavengers inevitably elevate the pH of production water, the need for continuous injection of such chemicals often elevates scaling risks to severe levels. For this purpose, combination treatment packages, including both scavengers and custom-tailored scale inhibitors, are preferred in the field. This paper presents the development of test methodology, the combination products, and the scale inhibitor performances of combination product.

This study focuses on circuiting three major individual aspects in the testing system, including (A) a high-calcium brine, (B) scavenger chemistries, and (C) selection of scale inhibitors. During the investigation, B included five scavenger chemistries (two traditional triazine-based and three novel nonamine based), whereas C involved scale inhibitors (including both phosphonates and polymeric types). In a traditional quick testing, scale inhibitor performance is commonly evaluated with or without the occurrence of H2S scavenger. A systematic consideration of the interactions of all the counterparts is necessary for development of a treatment package with optimal field performance. First, the high-Ca brine caused incompatibility issues with all the scavengers (A+B), the extent of which was not pH-dependent, but associated with the chemistries. The incompatibility was significantly mitigated by the addition of inhibitors, indicating potential synergetic effects of use of scale inhibitors in the combined scavenger packages. Second, the stability of inhibitors in the finished scavenger products was another issue (B+C). Most phosphonate inhibitors exhibited significantly better results than the polymeric types. For scaling inhibition performance (A+C with B), most published literature suggests it was driven by the elevated pH from the addition of triazine-based scavengers. However, this mechanism did not fully explain the impact of nontriazine scavengers on scale inhibition. For example, the inhibition efficiencies for calcite were measured differently at the same concentration of SI for two types of nonamine based scavengers, both having pH values of approximately 7. With a series of tests, combination packages were developed for each individual scavenger. With lab testing simulating the application scenarios in the field, all of the combined products demonstrated sufficient scale inhibition results.

The observations presented in this work could contribute the selection of the H2S scavenger product for water stream or full stream applications, particularly if brine compatibility is a primary concern. Further study of the effects of various scale inhibitors on the brine/scavenger compatibility could lead to the development of new combo product to help minimize brine compatibility issues while achieving dual functions of scale control and H2S removal.