Surveillance and optimization of unpiggable lines through real time modeling

Several oil and gas production facilities handle fluids with some wax content and Wachs et al. (1) have estimated that about 20% of the world petroleum reserves are waxy oils. As precipitation of wax is temperature driven, one mitigation is to not allow the crude to achieve temperatures below the wax appearance temperature (WAT) through the use of thick insulation to maintain heat from the reservoir. The most common approach for managing deposited wax is pigging to physically scrape and remove the wax. Due to cost reductions in design or operational challenges (e.g., decommissioned and or plugged flow lines) an increase in unpiggable flow lines is occurring. This challenge is exacerbated in aging fields where reservoir temperatures and rates have declined resulting in more of the flowline operating below the WAT. For these cases, robust surveillance approaches are needed to ensure the wax within the flowline is managed and that alternative methods, like chemical solutions, are optimized. One of the methodologies for assessing wax build-up is through the use of hydraulic models for understanding the effective flowline diameter as a function of time and relating that to wax deposit build-up. In this work, a comprehensive evaluation using field instrumentation and hydraulic modelling tools to measure wax buildup in a shallow water, single line tie-back system is presented. The methodology was then used to optimize well cycling frequency and chemical usage resulting in significant OPEX reduction.