Successful Application of Foamed Cement Slurry in an Ultradeepwater Well of 11,900-ft Water Depth (Claiming World Record)

Abstract

Foamed cement was successfully used in the riserless section of an ultradeepwater well located in 11,900ft water depth. Foamed cement was selected to minimize operating costs and provide flexibility to adjust slurry density on a short notice. The seawater column exerted 5,319.1-psi hydrostatic pressure on the annulus. Consequently, nitrogen (N2) density could no longer be neglected. This paper presents simulations performed in preparation for the job, operational considerations, and post-job evaluation. The lead slurry needed a density of 1.25 SG and develops a compressive strength of at least 300 psi within 48 hr. Considering the cost and challenges associated with outsourcing resources under current Covid-19 pandemic restrictions, the foamed cement system was preferred over chemical- or particle- extended cement or blend systems. The N2 ratio for the foamed cement slurry system was 700 scf/bbl. With a base slurry pumping rate of 5 bbl/min, the required N2 pumping rate was 3,500 scf/min, which was greater than the capability of a single N2 pump (3,000-scf/min rate). Because the rig deck space could not accommodate three N2 pumps, one pump would serve as backup; thus, the final plan consisted of using two N2 pumps simultaneously. Two parallel foamed slurry treating lines were rigged up to reduce the fluid velocity in a single line. All laboratory testing was conducted locally. Additives used in the foamed slurry were environmentally friendly. A proprietary process-control system was used during the cementing operation and automatically synchronized the N2 pumps and foam pump rates with the base slurry rate. The cementing crew consisted of 11 individuals, including 2 client representatives. The entire pumping operation was completed in 10 hr. A total base slurry volume of 1016.2 bbl was continuously mixed and pumped at the density of 13.35 lbm/gal (1.60 SG). The resulting foamed slurry volume was 1387.0 bbl with an average foam quality of 27.8% and foamed slurry density of 10.5 lbm/gal (1.26 SG). A total of 119 metric tonne of class G cement and 30,711 L of N2 were consumed during the pumping operation. The lead slurry was followed by 603.9 bbl of 15.86 lbm/gal (1.90 SG) class G cement tail slurry and 349.7 bbl of seawater for displacement. The final surface pressure was 594.6 psi. The lead slurry reached the seabed and the float shoe check was positive. No casing subsidence was observed. Using foamed cement slurry in such extreme conditions demonstrated its robustness and reliability. Through formalized and methodical risk assessment, the team was able to identify and implement mitigating measures that led to an outstanding result. This application also confirmed that N2 density should not be neglected when high-hydrostatic pressure is involved.