Why cement flexibility is critical to well integrity

Cement is critical to well integrity. It provides hydraulic isolation, preventing fluid flow between producing zones, groundwater aquifers, and the surface. But the cement sheath is not always able to deliver an acceptable long-term solution for today's demanding drilling environment.

Changes in downhole conditions with pressure and temperature fluctuations impose stresses on the cement sheath. Consequently, shrinking and debonding of the cement sheath creates very small micro-annuli allowing fluid migration. Besides these external forces that cause cement sheath damage, evaluations made at nanoscale level show that chemical bonds between cement components are relatively brittle.

The mechanical properties of well cements have become a topic of considerable interest lately. The drilling engineers and cementers have progressively moved away from the sole measurement of compressive strength in press machines and ultrasonic analyzers to a more in-depth study of properties such as flexural strength, rupture elongation percentage, Young’s modulus, and poisons ratio.

During the life of a well, the set cement can fail because of shear and compressional stresses. There are several stress conditions associated with cement sheath-failures:

• Radial loads caused by the expansion of the tubulars due to high fluid pressures inside the casing during testing, perforating and hydraulic fracturing, or expansion caused by temperature changes during the fluid production stage,
• Stress conditions, resulting from exceedingly high pressures that occur inside the cement sheath because of thermal expansion of the interstitial fluid.
• Conditions that involve tectonic movement of the formation.

When such stresses are exerted on set cement in the wellbore, the set cement can fail in the form of radial circumferential cracking of the cement matrix or by a breakdown of the cement/casing or cement/formation bonds. Such failures compromise zonal isolation and can lead to severe well problems.

The mechanisms leading to the failure of the cement sheath mentioned are directly linked to the tensile strength of the set cement and is attenuated when the ratio of tensile strength to Young’s modulus is increased. The larger this ratio (i.e., cement with relatively high tensile strength and low Young’s modulus), the more resistant the cement is to failure. A solid material will deform when a load is applied to it. A stiff material needs more force to deform compared to a soft material, implying that it would have a larger Young's modulus. Materials with very high Young's modulus can be approximated as rigid.

On the other hand, tensile strength, also known as flexural strength, modulus of rupture, bend strength, or transverse rupture strength is a material property, defined as the stress in a material just before it yields in a flexure test. So, if it is measured as the force required to deform the material, then the more force the material withstands, the more flexible it is regarded.

This all means that in the pursuit of a more flexible cement engineers look for high tensile strengths and low Young's modulus or an increased ratio among them.

Some operators and service companies additionally look into the “percentage of elongation prior rupture” as a way to report the tensile strength. This is simply a measurement of length change (as a percentage of original length) before the material break.

Experimental methods that measure the percentage of rupture elongation always report forces exerted to create the rupture. Thus the increase of the ratio between tensile strength and Young's modulus remain as the mechanism for the well cementing industry as they look for highly resilient and flexible well-cement compositions to withstand the stresses outlined above.

Read more: Cement slurry design for oil well applications: Standards you need to know

Ways to improve Cement Matrix Flexibility

Cements with improved flexibility have been elaborated to reduce the potential for stress-induced cement-sheath cracking which may lead to long-term gas migration.

Flexible particles (such as rubber), elastomeric polymers, thermoplastics, or latexes has been used in existing cement systems. Together with foamed cement, they all have shown to improve flexibility.

Here are some of the methods used to reach the goal of improving flexibility in an approach to improve zonal isolation:

a. Slurry density reduction

Conventional slurry density reduction, using extenders that accommodate additional water (e.g., sodium silicate and bentonite), increases the flexibility of the set cement. Although the flexibility is increased, the effects on compressive strength and permeability are detrimental. Foam cement's ability to improve flexibility is also linked to its reduced density and increased porosity.

b. Elastomeric composites / Flexible particles

Adding fillers, such as carbon black, calcium carbonate or silica, can adjust the elasticity of the set cement. Also, improvements have been made with rubber particles or thermoplastics (like polyamide, polypropylene, and polyethylene), or polymers (like styrene-divinylbenzene or styrene-butadiene).

Once used, the resulting set cements not only exhibit improved elasticity but also, in some cases, expansion properties. An additional value of these particles is that having specific gravities between 0.9 and 1.2, they can also reduce the cement system density.

c. Fibers

Fibers or ribbons added to a cement matrix also improves flexural strength. For this purpose, nylon fibers have been used for many years. This technique has its background in the addition of metallic micro ribbons to improve impact resistance and toughness in kickoff plugs and multilateral junctions.

Read also: The challenge with cementing.

WHAT IS THE WAY FORWARD?

Cement is currently used in wells as the prime material for zonal isolation, but given its operational limitations, alternative materials, which offer significant advantages over cement, are being proposed and developed by the industry.

However - when compared with cement that has been used for hundred years or so - uncertainty with regards to the long-term integrity of the alternative materials acts as a disincentive for their use.

From the list of materials that the UKOG guideline for plug and abandonment materials covers, one that stands up when the deciding factor on the selection evolves around a material's flexibility is the Thermo-setting polymers (Group C).

Set Solid-Free resins have at the same time higher compressive strengths, lower Young's modulus and higher tensile strengths.

When compared to cement, the Thermo-setting slurries (after going in its solid state) offers lower permeability; superior adhesion and less shrinkage; and resistance to many caustic and corrosive chemicals (i.e., CO2, H2S, hydrocarbons, brine) at high pressures and temperature conditions. But more importantly for what bother us today: enhanced flexibility and toughness after setting.

Figure 1. Comparison of the mechanical properties of a set resin (mixed at 8.9 ppg) versus a set 15.8 ppg class G Portland cement slurry.

As you can see in figure 1, the commercially available polyester resin known as ThermaSet®, has the capability of producing a final solid material that offers a superior strength to cement. It is also far more flexible and tolerates more than four time the loads before yielding.

Please refer to the Wellcem website for information about ThermaSet®,

So, what the future holds? Well, it may all come down to, as it was put by Luis Navas a cementing specialist working with ADNOC in UAE: “we (cementing engineers) will soon all become resin engineers instead”.

Read more: Resin curing process

If resin increases availability and its production costs are optimized to make it accessible even during cost-control scenarios typical of industry down-cycles, then resin may become the norm for primary and remedial zonal isolation.