Recent Developments in Mechanical Descaling Operations: A Case Study from Saudi Arabia

Abstract

Scale removal in sour gas wells has been one of most challenging operations in Saudi Arabia during the last decade. Operators face particularly demanding downhole environments with temperatures above 300 °F, significant concentrations of H2S and CO2, presence of complex mixed-scale deposits with limited dissolution, and risk of H2S release and corrosion with chemical removal methods. These conditions have led to mechanical removal methods using coiled tubing (CT) as the preferred technique for well descaling operations. During the last 5 yr, CT descaling operations have consisted of temporary formation isolation via bullhead using CaCO3 chips as a bridging agent, mechanical scale removal with CT milling and high-pressure rotary jetting tools, and abrasive perforation or matrix stimulation to enhance well productivity. Mechanical alternatives for isolation with bridge plugs are not feasible due to the presence of FeS scale in the wellbore. Wellbore configurations also require pump rates above 2.0 bbl/min to ensure suitable cleanout and transportation of solids to surface. The recent incorporation of CT equipped with a rugged version of fiber optic telemetry and a downhole measurements package into the descaling operations workflow has enabled pumping rates above 2.0 bbl/min. Key bottomhole data has also been obtained, leading to a better understanding of the scale removal progress, optimization of downhole tool operation; reduced well intervention risk, and enhancement of overall job efficiency. During the CT mechanical descaling stage, differential pressure across a high-pressure rotary jetting tool was kept at its optimum range of operation while maintaining slight overbalance conditions to minimize the risk of gas influx and avoid loss of circulation while removing the formation isolation. Throughout the abrasive perforating stage, real-time depth correlation saved one CT run (i.e., ±24 hr), and higher pump rates significantly reduced operation time and maximized shot size and penetration. This paper discusses an enhanced workflow for CT descaling operations where the implementation of the real-time downhole measurements package with an enhanced working envelope resulted in a significant increase in operational efficiency, reduced risk, and optimized job performance.