Breakthrough in Milling Technology for Sidetracking Corrosion-Resistant Alloy Casing/Liner Applications with 28% Chrome Content - A Successful Case Study
Authors: Sharma Anurag , Carasco Anant , Palani Vijay , Unnikrishnan Ranjit , Xia Steven , Alsup Shelton , Bandi Mani , Proeschold Detlef , Ragheb Ahmed M. 
Published: November 2018

Corrosion-resistant alloy (CRA) casing, especially high-chrome steel casing, has been widely used for deepwater wells because of the severely corrosive environment. Although sidetracking through standard low-alloy casings or liners is a common practice, milling high-chrome steel casing has proven to be challenging due to its higher hardness and significant work-hardening. Operators around the globe are looking for an effective solution to sidetrack through high-chrome casing.

Sidetracking from CRA casing presents two major challenges: achieving the initial cutout through the casing wall and mitigating the cutter wear while completing the window. Traditionally, the crushed-carbide on the follow mills has proven to be ineffective due to the poor cutting ability, and the high heat from welding on the body makes it prone to twistoff. A pioneering milling solution—an integral bi-mill manufactured from a step-forged body with cylindrical carbide cutters brazed into discrete pockets of the mills to form a robust cutting structure—was designed to solve this challenge. Optimizing the cutter location to enable a uniform distribution of casing volume removal ensures that the workload and cutter wear are distributed equally among cutters, thereby maximizing the performance of each cutter in the mill. The design team optimized the cutter back rake angle to increase cutting aggressiveness to initiate the cutout process. Further, a protective physical vapor deposition multilayer coating was added to increase the hardness and lower the coefficient of friction at the cutting edge, thereby reducing the amount of abrasive wear on the cutters and casing work-hardening.

Full-scale laboratory tests were conducted using two different carbide cutter grades to compare their durability, wear, and impact resistance. During both laboratory tests, the integral bi-mill assembly successfully milled the CRA casing for the next bottom hole assembly (BHA) to pass through. Based on successful laboratory tests, the field run was conducted on 7.00-in., 29-lbm/ft (28% Cr) liner in an offshore well environment in the Arabian Gulf. Two integral bi-mill assemblies were used to successfully mill 10.57 ft of window and drill 14.03 ft of hard limestone formation (20,000 to 25,000psi) with an acceptable mill dull grade. The subsequent 6-in. motor directional BHA passed through the window without any hole-drag.

The breakthrough milling solution incorporating an integral bi-mill with robust cutting structure eliminated the high potential risk of getting a milling tool stuck and avoided subsequent fishing, redrilling, and casing operations that would have resulted in an additional cost of about USD 8 million. The successful milling job brought the new reservoir well into production within a short turnaround time. The success of this technology has opened new avenues for sidetracking applications with CRA casings in the North Sea, Middle East, and Brazil.