Cement Milling Using Incorrigible Corrosive Brine in an HPHT Environment: A Unique Case Study
Authors: Abdullah Irfan B. , Kumar Mayank D. , Saraf Aman , Khandelwal Nakul , Kaul Karan , Pathak Himanshu 
Published: March 2018

The coiled tubing (CT) industry continues to operate in deeper and higher pressure wells and in more challenging environments, thereby extending the operating envelope for the service worldwide. CT operations for cement milling provides a safer, faster, and more economical solution, but achieving the desired results in a high pressure, high temperature (HPHT) offshore environment using corrosive brine as the milling fluid is challenging.

This paper describes various challenges and customized remedies for a cement milling operation in an HPHT well with high-density brine, which made milling challenging because of limitations on pumping rates, low viscosity, and fluid corrosiveness.

This offshore gas well had a bottomhole static temperature (BHST) of 400 °F and reservoir pressure of 12,000 psi, with a completion of 9-5/8 in. casing from the surface and a 3-1/2-in. liner hanging from 4,606 meter measured depth (mMD). A 1.75-in. outer diameter (OD) CT string was used to accommodate the internal diameter (ID) restriction of the liner. The allowable pumping rate in the 1.75-in. CT was restricted as a result of pipe friction and low viscosity of milling fluid; however, higher annular velocities were needed to lift the cuttings from the wellbore. To overcome this, additional pumping was performed from the annulus of the 9-5/8-in. section. A positive-displacement, metal-on-metal motor was selected over various types of motors. This paper provides details about best practices, including the selection of brine-compatible elastomeric seals, severe corrosion observed on the motorhead assembly (MHA), and redressing of MHA after each run. It also includes details about laboratory tests performed to identify a suitable viscosifier and corrosion inhibitor, as well as the optimum rate of penetration (ROP) and weight on bit (WOB) to avoid large cuttings, surface-fluid handling, and filtering arrangements.

Based on precise tool selection, including power section, bearing section, and corrosive-brine-compatible motor seals, as well as design parameters, such as ROP, annular fluid velocity, particle size, and equivalent circulating density (ECD) under the operating envelope, cement milling of 217 m of cement was completed successfully in an overbalanced condition with no health, safety, and environment (HSE) related issues. Selecting the correct milling motor and mill plays a crucial role in any milling job. Several operational challenges, such as excessive corrosion at a minimum MHA ID, pitting on bottomhole assembly (BHA) components and erosion on ball seats, erosion, and degradation of elastomeric seals at a BHST of 400 °F, were observed. These adverse effects were avoided with engineering controls.

The methods used, tool selection, and customized design helped the operator to find a solution for milling a cement plug in an HPHT well with zinc bromide (ZnBr2) brine, which resulted in reduced rig time and avoided the side tracking of the well. The lessons learned, methods, and best practices described in this paper can be used in a similar application worldwide, which can help to minimize issues related to service quality and HSE.