CPC resists acid corrosion

Operators and service companies cementing wells in zones that produce carbon dioxide (CO₂), wells that are used to sequester CO₂ or wells that have enhanced oil recovery practiced with CO₂ as a drive mechanism face the challenge of cement-sheath destruction from acid corrosion. When CO₂ comes in contact with water, carbonic acid is formed.
Portland cement is subject to corrosion by carbonic acid. Authors have reported reduction in Portland cement-sheath volume and subsequent annular and casing communication of well fluids, hydrocarbons, and CO₂ to the surface and from zone to zone. The causative agent in most cases was acid corrosion in Portland cement sheaths.

Before the introduction of calcium phosphate cement (CPC), Portland cement was modified through the addition of products such as fly ash and/or latex in an attempt to improve Portland cement’s corrosion-resistant properties.

These reduced Portland systems, while adequate when in contact with minimal amounts of CO₂, cannot withstand the corrosive effects of water saturated with CO₂. Figure 1 illustrates this failure by showing weight loss of CPC and modified Portland cement systems at 140°F (60°C) in a solution of carbonic acid and sulfuric acid. This aggressive fluid was used to accelerate the effects of long-term, “real-life” field exposure.

After 2 months of exposure, Portland cement mixed at 16.4 lb/gal lost one-half of its weight; Portland densified to 16.7 lb/gal and containing 2 gal/sk latex, lost 43%; and a reduced Portland cement mixed with fly ash and latex lost 21%. CPC lost 2% after 30 days, when it stabilized and did not lose any more.

Figure 2 shows the effects of carbonic acid on CPC and a reduced Portland cement system consisting of Portland cement blended with 40% silica flour after 53 days at 500°F (260°). CPC experienced no weight loss; the Portland system sample lost 33% of its weight.

Background

In addition to the more or less incidental contact with CO₂, prospects for widespread application of geological sequestration (underground storage) to dispose of CO₂ could expand greatly the requirements of providing long-term sealing to casings and annuli. This disposal may also be used to repressurize depleted natural-gas zones, possibly returning some old wells to production.

Several options exist for CO₂ storage, including depleting and depleted oil and gas fields, deep saline aquifers, the deep ocean, coal seams, and through mineral carbonization. Three underground storage alternatives have been identified:
• Deep, saline, water-bearing formations;
• Depleted oil and gas reservoirs; and
• Unmineable coal seams.
All have demonstrated potential as storage sites.
A major operational challenge is cementation of the casing with a zone sealant that will last essentially forever, so that CO₂ and other reservoir products or stored gas and liquids cannot communicate to the surface or upper zones.

The cement

CPC is a blend of high-alumina cement, phosphate, and fly ash (a byproduct of coal-fired electricity-generating plants). This specially formulated cement was developed jointly by Unocal, Brookhaven National Laboratory, and Halliburton. It has been laboratory tested and proven at temperatures as low as 140°F and as high as 700°F (371°C). Under test conditions that cause Class G, H and latex-containing Portland cements to lose up to one-half their weight, CPC’s properties are only slightly affected or may actually improve. Use of CPC requires no special equipment.

On a central California geothermal job, testing showed that the CPC base slurry (unfoamed) had a compressive strength in excess of 4,000 psi. The same cement foamed with nitrogen to a density of 10.5 lb/gal had a compressive strength of 1,425 psi.

Why store CO₂?

Approximately one-third of all CO₂ emissions due to human activity come from fossil fuels used for generating electricity, with each power plant capable of emitting several million tons of CO₂ annually. A variety of other industrial processes also emit large amounts of CO₂ from each plant, for example oil refineries, cement works, and iron and steel production. These emissions could be reduced substantially, without major changes to the basic process, by capturing and storing the CO₂.

There are many ways in which CO₂ emissions can be reduced, such as increasing the efficiency of power plant or by switching from coal to natural gas. However, most scenarios suggest that these steps alone will not achieve the required reductions in CO₂ emissions. The capture and storage of CO₂ from fossil fuel combustion could play an important part in solving this problem. Widespread use of this technique could be achieved without the need for rapid change in the energy supply infrastructure.

Figure 1. Results of a weight-change test in a 140°F (60°C), acidic CO2 solution are shown. The CPC sample changed less than 2% by weight of cement, while latex and Portland samples lost 21 to 50% of their weight. (All images courtesy of Halliburton)

Underground storage of CO₂ has taken place for many years as a consequence of injecting CO₂ into oil fields to enhance recovery. Now, for the first time, CO₂ is being deliberately stored in a saltwater reservoir, under the North Sea, for climate change reasons. The potential capacity for underground storage is large but not well documented. Other geological storage schemes are under development and plans to monitor them are well advanced.

The deep ocean could be used to store large volumes of CO₂. Indeed, most CO₂ resulting from human activity is eventually absorbed by the oceans. This is considered a longer-term option and will require a much greater understanding of the various processes involved before it can be used.

	Figure 2. The CPC sample on the right was unaffected by exposure to carbonic acid at 500°F (260°C); the Portland sample at left lost 33% of its weight.

For CO₂ storage, the priority is to establish its credibility and acceptability as a safe, reliable, long-term store. Proof that any losses will be insignificant is a major issue for storage. The fact that CO₂ has been naturally stored for geological time-scales enhances the credibility of many of the storage options.