Improving Frac-Plug Milling and Flowback Efficiency Through Multiphase Metering Application in Deep and Tight Carbonate Horizontal Gas Wells

Abstract

Multi-stage frac completions are frequently used to increase well productivity and meet the ever-growing energy demand. Commonly, each stage is isolated with a plug and these plugs need to be milled when the well is ready to be flowed back. Milling and flowback operations require extreme vigilance and close monitoring. This paper explains the use of a multiphase flowmeter to optimize milling and flowback operations and save water in deep, tight carbonate horizontal gas wells. Frac plug milling operations are best performed in balanced well conditions. Underbalance conditions cause unwanted gas flow with solids at surface, which will cause equipment damage, if not maanged well. Overbalance conditions result in excessive precious underground water loss and hinder wellbore clean-out. A multiphase flow meter was used at the wellhead to monitor fluid return rate during milling. By integrating coiled tubing pump rate, depth and pressure measurements with the multiphase meter readings, downhole well conditions were assessed and the milling parameters were optimized. Additinoally, flowback conditions were also measured and analyzed in real-time to optimize choke strategy to maximize water recovery and minimize flowback duration. Multiphase meters were used during milling and flowback operations for several horizontal gas wells with multistage frac completions. During milling, both water and Nitrogen were pumped into the wellbore to lighten the hydrostatic head and avoid fluid losses while circulating for wellbore clean-out. The operation efficiency was assessed based on the return rate of individual fluids at surface. Fluid losses into the formation and reservoir influx were quantified. The multiphase meter measurements allowed the operators to adjust choke size and wellhead pressure to maintain balanced conditions, and control the water-nitrogen pump rates. Accurately measuring oil, water and gas flow rates with high resolution also helped with determining the choke bean-up strategy to maximize frac fluid recovery for increased fracture conductivity, while minimizing the flowback duration. Monitoring the ratios of produced fluid rates helped evaluate the wellbore clean-up performance and take necessary action to increase clean-up efficiency. Changing well productivity over time was also quantified in real-time, which allowed to optimize the flowback end time with maximum achievable well productivity, without waiting to recover all the frac fluid. The data helped quantify the wellbore productivity with respect to the frac fluid recovery. The practices explained in this paper can improve frac plug milling and flowback operations. By closely monitoring surface returns through the multiphase meter, it will preserve valuable underground water resources, maximize fracture conductivity and wellbore productivity especially in sub-hydrostatic reservoirs, which are challenging to mill.