Performance of colloidal silica grout at elevated temperatures and pressures for cement fracture sealing at depth

Abstract

Hydrocarbon well decommissioning requires the long-term sealing of abandoned wells. Current plug and abandonment (P&A) operations are not always able to address all potential fluid migration pathways, resulting in the possible upwards migration of hydrocarbons from formations penetrated by the wellbore. The development of innovative materials to improve well sealing remains a major challenge. This paper presents a proof of concept for the use of colloidal silica (CS)-based grout to improve the sealing performance of P&A operations. CS is a non-toxic suspension of silica nanoparticles (<100 nm) undergoing gelation upon destabilisation. Due to its excellent penetrability and controllable gel time, CS has the potential for repairing fine-aperture cracks within the cement sheath, at the cement/casing interface, or within a cement plug, where the penetration of cementitious grouts is restricted due to their relatively large particle size. In this study, the suitability of CS grout for deployment up to 1500 m depth was successfully demonstrated. Firstly, a range of CS grout mixes were investigated to test the feasibility of grout emplacement considering a timescale of 2 h for pumping operations from the surface to depth. Secondly, to investigate the sealing performance, the CS grout was injected into fractured cement cores (0.2 and 0.5 mm fracture aperture) and exposed to pressure and temperature conditions simulating downhole scenarios up to 1500 m depth (based on gradients for North Sea, UK). Fracture permeability upon water injection was assessed pre- and post-treatment. This work found that permeability values after treatment were reduced by three orders of magnitude, thus confirming the potential of CS grout for repairing fine-aperture cracks.