Abstract
Iron sulfide scaling can manifest both downhole and topside in sour production scenarios. Topsides development can be mitigated via continuous strategic application of an appropriate sulfide scale inhibitor package, however downhole sulfide scaling presents a more complex challenge. Continuous downhole application of sulfide scale inhibitors via capillary string and valve, or routed to valve via existing gas lift architecture are two common delivery options available to the operator, but are unfortunately both limited to providing scale control at injection valve depth and up-string. More recent options for sulfide scale control from reservoir to wellhead include chemical impregnated proppant for prop-frack and gravel packed wells, however these approaches present a partial solution, and require topping-up of inhibitor chemical active once the emplaced inhibitor becomes exhausted. Scale squeezing is the industry recognized chemical technology for providing proactive scale control from reservoir to wellhead, however for sulfide scale control scenarios this technology option is currently poorly served as the industry lacks effective and robust sulfide squeeze scale inhibitors.
A new squeezable polymeric iron sulfide scale inhibitor has been developed and engineered with optimum desirable scale squeeze characteristics, and then further tailored for application in high temperature, high salinity sour gas ultra-tight sandstone/chalk formations. The new molecule used the basic structure of an existing novel class of sulfide scale inhibitor, however the sequence and nature of functional groups across the polymer backbone were extensively modified and optimized to improve (i) polymer retention and release character for extended squeeze lifetime, (ii) thermal stability, (iii) high calcium brine compatibility and (iv) low formation damage potential. Prior to upscaling for bulk manufacture and subsequent field application, the new polymeric iron sulfide scale inhibitor (FeS-SQSI) was formation damage coreflood tested using target well field conditions and ultra-tight sandstone field core. The results of this remarkable coreflood investigation are presented below, where core permeabilities of sub-0.001 mD were the norm. The flood was performed without interruption or issue, and achieved consistent and continuous formation damage data assessment throughout the 5-week post squeeze shut-in flowback period at very low flowrates.
Sulfide scale inhibitor residuals assay observed consistently elevated concentrations of scale inhibitor across the coreflood flowback ‘plateau phase’ and indicated that the squeeze treatment could generate significant squeeze lifetime, and this is further born out by mass balance performed after deliberate premature termination of the coreflood indicating that in excess of 90% of the injected scale inhibitor remained within the core.
Monitoring showed sandstone core permeability recovering rapidly following start of backflow, with no evidence of any significant formation damage. Post-flood data indicated no loss of core plug integrity, and mineralogical assay confirmed unchanged material composition. The formation damage sandstone coreflood results clearly demonstrated the suitability and robustness of this new iron sulfide scale inhibitor for squeeze application under extremely challenging squeeze application conditions.
