Coiled Tubing Underbalance Cleanout with Multiphase Flowmeter: Optimization Workflow and Lessons from the North Sea

Abstract

Coiled tubing (CT) cleanout interventions are complex operations that have significant potential for increased efficiency and reduced operational risks with the use of a multiphase flowmeter. Fluid return data provide valuable information that can revolutionize conventional operating processes. However, until now, there have been no practical recommendations available that summarize the experience, decision-making workflow, and guidelines when utilizing multiphase flowmeters during CT cleanout operations. Challenges with CT cleanouts are typically addressed during planning and design stages. Steps for risk control are planned as part of the execution program, whose level of optimization depends heavily on real-time data from downhole and surface measurements. Surface flowmeter data are generally only used for monitoring rather than to make informed decisions for operational optimization and de-risking. In this case study, a practically proven workflow utilized multiphase flowmeter data to deliver accurate measurements of oil, water, gas, and solids returns at the surface during a CT cleanout to safeguard operations and enable cleanout strategy adjustments in real time. In the Norwegian continental shelf (NCS), depleted wells require CT cleanout operations to be in underbalance with the use of nitrified fluids. Interventions are planned to allow natural production from the well to assist solids transport to surface during the underbalanced CT cleanout, hence reducing the amount of base oil and nitrogen required. Uncertainties in reservoir pressures make estimation of production rate during the cleanout difficult to achieve. A dual-energy gamma-ray multiphase flowmeter eliminates those uncertainties by giving a quantitative measurement of the three phase production rates in real time, allowing the CT operator to control and optimize nitrified fluids use by monitoring the pumping rates versus the flowback rates. The optimization allows an estimated reduction of nitrified fluids up to 20% due to the capabilities of quantifying hydrocarbons flow to assist the cleanout. Measurements of solids also helped to avoid unnecessary long CT wiper trips to the heel of the well that can take from 6 to 8 hours. By breaking down the cleanout interval into sections, the flowmeter helps to identify early signs of leakoff and prevent the excessive solids bedding in the wellbore that puts the CT at a greater risk of getting stuck. This study summarizes the results and lessons learned from an application of the dual-energy gamma-ray multiphase flowmeter during CT intervention work performed in the NCS. Furthermore, the study provides practical recommendations supported by a series of case studies and multiple field examples where real-time measurements from the flowmeter were used to build a dynamic workflow with the objective of increasing efficiency and safeguarding the CT intervention.