Well-Intervention Challenges To Service Wells That Can Be Drilled
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 100172, "Well-Intervention Challenges To Service Wells That 'Can Be Drilled'," by C.G. Blount, SPE, and M.B. Mooney, SPE, ConocoPhillips; F.R. Behenna, SPE, CTES LP; and R.K. Stephens, SPE, and R.D. Smith, BP, prepared for the 2006 SPE/ICoTA Coiled Tubing and Well Intervention Conference and Exhibition, The Woodlands, Texas, 4-5 April.
Drilling rigs continually are setting new records in measured-depth (MD) vs. true-vertical-depth (TVD) ratios, horizontal-section lengths, and delivering operationally complex completions. Common well-intervention technologies may have difficulty fulfilling the intervention needs of the wells that can be and are being drilled and completed. The full-length paper presents an overview of available well-intervention technology, outlines limitations of each technology, and offers case histories from challenging interventions performed in Alaska.

Introduction
Advances in drilling technology have enabled operators to drill complex wells ranging from long step-out wells to multilateral completions with horizontal laterals that may extend more than 7,000 ft. These advances in drilling technology have allowed operators to reach distant offshore targets from onshore facilities such as those used in Wytch Farm and Sakhalin Island and to drain large areas from small, environmentally conscious pads such as those in Alpine, West Sak, and other Alaska North Slope fields.
The engineering and new technology used in drilling and completing these complex wells are impressive. However, when the rig moves off, the production department is left with wells that cannot be serviced with "conventional" service tools and techniques. Shortcomings in servicing these unconventional wells have driven numerous intervention-technology advances including pumpdown equipment, electric-line tractors, extended-reach tools and techniques for coiled-tubing (CT) applications, CT tractors, and use of hydraulic workover (HWO) units for accessing the wellbores.
Even with the ability to access these complex wells, performing routine jobs such as setting plugs, shifting sleeves, production logging, profile modifications, and fill cleanouts can become orders of magnitude more difficult and costly to perform than similar services in conventional wells.

Conveyance Methods
Slickline. For slickline working in straight holes, depth limitations vary depending on numerous parameters including material, fluid in the hole, tool weight, and the work to be performed. Modeling of slickline operations in a vertical well suggests that the depth limit is approximately 33,500 to 35,400 ft with a light 100-lbm tool string and limiting hanging weight to 50% of breaking strength. There was no provision for performing work at this depth. Modeling included slickline sizes ranging from 0.092-to 0.125-in. diameter, with the larger diameter achieving only slightly greater depths than the smaller-diameter lines. Use of a 0.25 friction factor suggests that slickline could reach approximately 50,000 ft in a well with a constant 60° inclination. Factors such as material properties, amount of wellbore debris present, and wellbore tortuosity will affect these depths dramatically. Experience in highly deviated wells also indicates that the friction factor most likely will be higher than 0.25.