Optimization of Single-Trip Milling Using Large-Diameter Coiled Tubing
Authors: Wilson Adam 
Published: January 2017

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 179090, “Optimization of Single-Trip Milling Using 2-in. Coiled Tubing,” by Elizabeth Snyder, SPE, and Justin Noland, SPE, Sanjel, prepared for the 2016 SPE/ICoTA Coiled Tubing and Well Intervention Conference and Exhibition, Houston, 22–23 March. The paper has not been peer reviewed.
Larger-diameter coiled tubing (CT) recently has been used to perform millouts because of its improved setdown force and increased annular velocities (AVs) for cleanout purposes. Service companies and operators have reduced the number of wiper trips when using larger-diameter CT, to save time and money. Milling efficiency using 2-in. CT can be dramatically improved by maintaining proper fluid rheology throughout the operation. By doing so, 2-in. CT has been used to perform single-trip millouts, reducing operational time by 40%.
Fluids
The main objective of CT operations in horizontal wells is to clean the lateral completely of any debris without compromising the well’s integrity. The first and most important component of single-trip-millout operations is correct fluid rheology. This system comprises the AV, Reynolds number (RE), and fluid viscosity. With correct use of chemicals and proper AVs, the fluid system allows sand and debris to travel out of the wellbore.
The RE determines the flow regime as laminar, transitional, or turbulent. Turbulent flow is characterized by swirling of the water (i.e., presence of eddies), which agitates the settling bed, enabling sand and debris to flow out of the lateral and, in turn, out of the well. RE can be broken into three components: fluid velocity, hydraulic diameter (flow area between casing and coil), and kinematic viscosity (funnel viscosity). In this case, the hydraulic diameter is predetermined by the 2-in. CT working in 7- to 4.5-in. cased wells, leaving the velocity and viscosity dependent on each operation.
When pumping any treatment fluid through CT, a fluid friction reducer (FR) must be pumped continuously to reduce friction pressure generated from pumping fluid through a restricted area. The FR decreases the circulating pressure, enabling higher pump rates to be achieved. A rheology-control unit uses electric variable-frequency-drive pumps to add FR and other chemicals to the fluid and to provide the ability to change dosage on the fly. These pumps are accurate down to thousandths of a gallon, enabling higher chemical usage in smaller doses. The chemicals are injected directly into the flow, then inline mixers enable shearing of the chemical to attain consistent mixing. Direct injection and inline mixers eliminate the unnecessary waste and oversaturation of chemicals seen when using conventional mixing tubs. This optimized fluid increases the longevity of the recirculated water while maintaining a high RE, in turn decreasing the cost to the operator.
Another common practice of conventional millouts was to send multiple linear-gel sweeps throughout the lateral. The objective was to increase the viscosity of the fluid to lift sand and debris out of the well. However, it has been discovered that the more-viscous-gel sweeps lead to laminar flow, which is not conducive to moving sand and debris in the lateral. The laminar fluid flows over debris, enabling it to settle and stay in the lateral section of the well. Gel sweeps have viscosities of 20 cp or greater, compromising the RE down to below 20,000. Because the viscosity is high and the RE is too low to transport sand and debris, gel sweeps were limited during optimized single-trip millouts.