Cement Additives in Drilling

Special Cement Systems

Introduction

Although not additives in the strict sense, there are a number of special cement systems that are distinct from the Portland cement-plus-additives systems we have described: Special cement systems include:

• Latex systems
• Salt systems
• Blast furnace slag (BFS) systems
• Epoxy systems
• Multi-particle cement systems

Latex Systems

Latex systems are used to control problems with gas migration behind pipe. Latex-modified cement systems include a suspension of very small spherical polymer particles stabilized with surfactants. This polymer emulsion usually contains about 50% solids. Latex cements exhibit normal color and consistency, but after curing the hydrated cement particles are connected by a film of latex particles.

The addition of latex provides a number of benefits including:

• Improved slurry pumpability
• Decreased slurry fluid loss
• Decreased permeability of the set cement
• Increased tensile strength
• Reduced shrinkage
• Increased elasticity
• Improved bonding between cement/steel and cement/formation
• Less shattering when perforated
• Increased resistance to contamination by well fluids
• Improved durability

Polyvinyl acetate latex systems are mixed at a concentration of 2.5% to 25% BWOC, and can be used up to temperatures of 122 °F. Styrene butadiene latexes work equally well but are effective up to temperatures as high as 350 °F.

Salt Systems

Cement systems that contain significant quantities of sodium chloride (NaCl) or potassium chloride (KCl) are sometimes referred to as “salt cement.” Salt is found in cement systems for three reasons:

• Offshore, it is present in the mix water.
• It is added to achieve certain properties.
• During cementing of a wellbore across a massive salt formation or fresh-water-sensitive formation, high salt content slurries will prevent dissolution or clay swelling.

Seawater will reduce thickening time, increase fluid loss rate, increase compressive strength at low temperatures, and depress the effect of bentonite as an extender. Addition of salt to cement can create complex interactions and therefore behave as an extender or accelerator, depending on concentration and conditions.

Salt-saturate cements are useful when completing wells across producing formations that contain montmorillonite, illite, and chlorite—clays that swell on contact with fresh water, reducing permeability. Salt concentrations as low as 10% BWOW can be sufficient to prevent damage.

Drilling through massive salt evaporate layers is a challenge because the salt dissolves, changing the cement slurry properties, and deforms plastically, encroaching into the wellbore. Advanced, proprietary additive combinations have been developed that permit cement systems containing anywhere from 5% to 37% NaCl BWOW. There is no official industry consensus regarding slurry design for salt-zone completions, but some experts believe that most salt zones are cemented with systems containing between 8% and 18% NaCl BWOW.

Blast Furnace Slag Systems

Blast furnace slag (BFS) is a byproduct of steelmaking. When finely ground, it contains more than 95% oxides of calcium, silicon, aluminum, and magnesium. Unlike Portland cement, BFS requires an activator other than water to set. These include caustic soda (NaOH), soda ash (Na₂CO₃), sodium silicate, sodium sulfate, calcium sulfate, lime, or mixtures of these compounds. (Portland cement itself is an activator.)

Compared to Portland cements, BFS cements exhibit better sulfate resistance, slower diffusion of chloride and alkali ions through the set cement matrix, and lower set cement permeability. They are used in situations requiring salt-saturated cements, carbon dioxide resistant cements, and certain squeeze cementing applications.

Epoxy Systems

Wells that require cements that are particularly resistant to chemical attack (such as chemical waste disposal wells, or carbon dioxide enhanced oil recovery wells), are candidates for epoxy-based organic polymer systems. Such systems are sometimes called “synthetic cements.” Epoxy cements are prepared by mixing an epoxy resin with a hardening agent and a solid filler. The compressive strengths range from 8,000 to 10,000 psi, compared to less than 5,000 psi for conventional cements.

Multi-Particle Cement Systems

Multi-particle cement uses particles of various sizes to create a suspension of cement. The smallest particles – typically silica – are so small they do not settle in water. The intermediate particles are made of cement, and the largest is a fine sand.

The particle size is arranged such that the smallest particles fit within the spaces between the intermediate particles, which, in turn, fit within the spaces between the largest particles. The three different-sized particles fill over 80% of the space of the cement, which means that far less water is required to make the system flow. The slurry is also less susceptible to water contamination, and can easily incorporate more exotic particles that effect various unique properties to the cement.