Innovative Testing of Mills and Wellhead Freeze Implemented on Offshore High-Rate 10,000 psi Gas Well

Abstract

A stuck 9⅝-in. 10,000 psi Inconel 925 back pressure valve (BPV) required a specialized testing apparatus to validate the mill design. The 18¾-in. Unihead and compromised BPV mandated an ice plug freeze capable of withstanding a 10,000 psi test as a well control barrier on the high-pressure sour gas well. The development, testing process and field results to mill the BPV and apply the ice plug freeze will be presented.

Mill machine lathe tests were conducted to achieve preliminary cutting structure and the mill profile. The test apparatus simulated milling an 8.635-in. Inconel 925 BPV stuck in a 9⅝-in. Inconel 725 tubing hanger. A test stand with 26-in. outside diameter (OD) casing and 9⅝-in. inner casing were constructed and double wrapped with 500 ft. of ¾-in. helical coil to simulate the freezing of an 18¾-in. Unihead, with 10,000 psi pressure integrity inside the 9⅝-in. tubing.

The rectangular inserts selected during the milling machine tests using one mill design for hanger and BPV created excessive vibrations, high torque, chipped inserts and slow milling. The first test revealed the inability of one mill to cut two materials and profiles in addition to limitations of available milling parameters. The cutting matrix was changed and two distinct mills were designed and specialized for each profile. The test fixture was also modified to enable a broader range of milling parameters and centralization in a robust testing fixture. The two mills successfully milled the required profile in the second test and the optimal milling parameters were obtained.

The ice plug test was performed on a vertical stand of 26-in. OD casing wrapped with helical coil and 9⅝-in. OD inner casing. The annulus and tubing were filled with water and liquid nitrogen free flowed from tanks through a two inch hose. The mass was cooled to minus 70°F with an ambient temperature of 86°F and repeatedly pressure tested to 2,000 psi and 10,000 psi. The test provided positive proof of the ice plug's integrity as a well control barrier for the large diameter application.

The innovative testing for the mill and freeze applications provided needed information for success in the field. A BPV of this size, material and design had never been milled before. This was the first time a freeze of this size and pressure rating had been placed and tested to 10,000 psi. The project highlighted the value of off-site, pre-job validation testing opposed to the on-site trial and error approach, which can prove costly and ineffective.