Cutting Through the Heat:Coiled Tubing Milling in Geothermal Extremes

Abstract

Geothermal wells in the Philippines present intervention challenges due to high temperatures and large-diameter wellbores. Unlike conventional oil and gas wells, geothermal environments often feature extended open-hole sections and experience persistent circulation losses. These conditions complicate intervention and limit the effectiveness of traditional cleanout methods. As rig availability remains limited, coiled tubing (CT) has emerged as a viable rig-less solution. Recent field experience in CT-based milling has prompted the development of tailored best practices for scale and debris removal. Traditional oilfield milling relies on surface circulation or acid dissolution, which are less effective in geothermal wells due to scale composition, large wellbore geometry, and fluid losses. Nitrified high-viscosity fluids failed to cure losses or lift debris effectively. A staged milling method was developed, starting with an injectivity check through a pilot hole. If injectivity is present, dual injection-though the CT and annulus-supports motor operation while pushing debris into loss zones. If injectivity is absent, the operation shifts to conventional circulation. The approach emphasizes adaptive execution, managing debris in-well while minimizing stuck pipe risk under partial or no-return conditions. Quenched milling operations were supported with backside pumping between 4 and 10 bpm. Wells with partial injectivity responded well to dual injection, enabling steady milling despite the absence of returns. In non-injective wells, returns enabled conventional debris recovery. Bit selection had a significant impact on performance. Early operations used PDC-style cutter mills for durability against unknown scale types, but these exhibited slower cutting and frequent motor stalls. Subsequent use of carbide junk mills enabled more aggressive scale removal, improved consistency, and fewer stalls-likely due to better interaction with calcite-rich or brittle scale. Across five wells, over 2500 meters of scale were successfully milled using this dual-injection method. The ability to switch strategies-both in circulation and tooling- proved essential in adapting to varied well responses and scale severity, reinforcing CT’s expanding role in geothermal interventions. Milling with significant CT-to casing standoff induces vibration that risks metal fatigue-exacerbated by stall torque and tool impact. Evaluating vibration-dampening tools or stabilizers is critical to reducing fatigue-related failure. This becomes even more relevant in future live-well milling, where gas-based pumping will amplify vibrations through increased velocity and pressure changes.