Cementing Solutions for Corrosive Well Environments

Abstract

Main factors which are responsible for the corrosion of the cement sheath in wells are determined based on lab tests and field analyses from a literature survey. A description of the chemistry, mineralogy, physical properties of API well cements and their mechanisms of corrosion in the presence of aggressive formation and injection fluids (such as magnesia or sulfate containing brines and CO 2 ) are given. API cement hydration mainly produces Calcium-Silicate-Hydrate (C-S-H) phases, which are responsible for the strength, and portlandite "Ca(OH) 2 " which is basically a weak point within the cement matrix. Increasing permeability and portlandite content reduce strength and chemical resistance of set cement towards corrosive media. The addition of pozzolanic materials eliminates portlandite and allows lowering the water content in the cement system. Both effects can reduce the permeability and improve the mechanical properties of set cement. Cement specimens were prepared and exposed to CO 2 loaded water at 300 °F and 3,000 psi for 6 months. Mechanical properties tests, microscopy, and quantitative CaCO 3 analyses revealed significantly less corrosion and negative impacts for an API cement-pozzolan blend compared to a conventional API cement design at same density. Practical and economical concepts for improvements of the cement sheath with respect to cement slurry design, cementing process and the impact of factors such as temperature or cement admixtures are presented to mitigate cement corrosion. IntroductionWell cementing is a fundamental and essential part during the drilling of a well. The primary goals of the annular cement sheath in wells are zonal isolation, supporting the casings and protecting them from corrosion. The cement sheath must not only withstand downhole stresses induced by pressure and temperature fluctuations, but also corrosive attacks from aggressive formation and injection fluids (such as CO 2 flooding for enhanced oil recovery or "CO 2 capture and storage"). Cement sheath failures leading to loss of zonal isolation are the biggest concerns since they affect the wellbore integrity and so the life of the well with economical consequences: decline in production rates, loss of production time because of remedial cementing, and in worse case, even complete well failure/collapsing so that well abandonment is the last resort. Therefore it's imperative to design cement systems that counteract all negative impacts on the sheath integrity during the life of a well, to ensure maximum durability. Synthetic and epoxy resin-based binders provide high chemical resistance and good mechanical properties in the well (Cole 1979). Nevertheless, their extremely high costs limit their use to special applications. Calcium aluminate or phosphate based cements also proved to be more highly corrosion resistant than Portland cements (Sugama 2006). But these cements are sensitive towards contaminations with Portland cements and so must be handled separately requiring advanced planning which results in expensive log...

Wellbore cement degradation in contact zone with formation rock

“…Some researchers indicate to the necessity of improving cement bonding to the formation rock (Brandl et al 2011). Newhall (2006 reported that a preflush with a sodium silicate will improve cement bond.…”