Permeability Study of API Class G and B Cements Considering Seawater and WBM Contamination

Abstract

In preparation for decommissioning work in the Southern North Sea sector, the permeability of historically used cement systems was investigated to help determine their effectiveness as well barriers. Because the placement of cement typically involves some degree of contamination with other wellbore fluids, determining the influence of seawater and water-based drilling fluid (WBM) contamination on the permeability of the set cement was of particular interest. This study investigates the historically used API Class G cement at 16 lbm/gal and Class B cement at 15.8 lbm/gal using contemporary material samples. Cores were prepared using neat cement, cement contaminated with up to 50 vol% seawater, and cement contaminated with up to 50 vol% WBM. The permeability to nitrogen flow of the set cores was measured using a Hassler sleeve. To further investigate the structural changes causing the higher permeability observed in the contaminated samples, the structure of the cores was imaged using X-ray computer tomography (CT) scans. All Class B mixtures within the investigated contamination range, all Class G mixtures with seawater contamination, and Class G mixtures up to 30 vol% WBM contamination produced set cores after curing. The neat Class G cores showed approximately twice the permeability of the Class B cement cores, with the contaminated slurries of both cement types showing much higher permeability—approximately one to two orders of magnitude greater. Further analysis using the CT scans revealed irregularities in the structure density in the contaminated samples, which might explain the increased permeability; however, further work is necessary to fully understand the mechanism causing this change. Understanding the potential effects of contamination on permeability and the structure could be useful for gaining insight into barrier performance of the cement sheath in circumstances where contamination is suspected.

Integrity Experiments for Geological Carbon Storage (GCS) in Depleted Hydrocarbon Reservoirs: Wellbore Components under Cyclic CO2 Injection Conditions

“…However, the literature reports a sparse permeability range for class G cement. Nevertheless, it is commonly accepted that a typical value for low permeability in cement is 1 × 10 −18 m 2 or even less, as suggested by studies conducted by Reinicke et al [29], Müller-Hoeppe et al [59], Czaikowski et al [60], Teodoriu et al [61], and Le-Minous et al [62]. Some researchers have reported permeabilities reaching up to 1 × 10 −17 m 2 for a standard class G cement, as indicated by studies conducted by Goode [63], Nelson and Guillot [64], and Lund et al [65].…”