Carbon dioxide corrosion studies of oil-well portland cements were initiated using a new microsample technique to determine the effect of carbonic acid on portland cement slurry formulations and to develop a high carbonic acid corrosion-resistant cement for carbon dioxide Enhanced Oil Recovery applications.
Earlier results from studies using two-inch API cement cubes showed carbonic acid had essentially no effect on cement after relative short test periods at elevated temperatures. Similar results with two-inch API cement cubes also were reported in recent literature. Because carbonic acid corrosion in cements was difficult to observe and measure using two-inch API cement cubes, a new microsample technique was developed. Use of this new technique, which represents an accelerated testing method, showed oil-well cements undergo a rapid deterioration in a wet carbon dioxide environment. Similar tests with two-inch cubes showed essentially no cement deterioration under the same conditions.
Experimental details of this new microsample technique are discussed. Data relating cement strength loss and carbon dioxide penetration depths to cement type and slurry formulation are reviewed. included in the discussion is a new cementing formulation which shows significant promise as a high carbon dioxide- resistant, oil-well cement.
Carbon dioxide Enhanced Oil Recovery applications seen a surge of activity in the last several years. Of the 40 projects that are estimated to be underway, 20 are in Texas and 7 are in the Gulf Coast region. while the remaining are distributed throughout the midwest and western states and Canada.
Although the carbon dioxide Enhanced Oil Recovery process and carbon dioxide corrosion in oil and gas production are well documented in the literature,1 very little published information is available on carbonic acid corrosion in oil-well cements.2 However, the corrosion of cement structures by the leaching action of the carbon-dioxide-laden waters is well documented in the literature.3 It is well known that carbon-dioxide-laden water can reduce hydrated portland cement to a soft amorphous silica gel.4
The basic chemistry describing this process is as follows.
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3-
CaOH2 + H + HCO3 → CaCO3
C-S-H + H + HCO3 + CaCO3 + amorphous silica
In Step (l), approximately 1% of the dissolved carbon dioxide reacts with water to form carbonic acid.5 As the carbon-dioxide-laden water diffuses into the cement matrix, the dissociated acid is free to react with the calcium hydroxide, which makes up 20% of the cement composition, and the hydrated calcium silicates (Steps (2) and (3), respectively). If the reaction would stop after the initial carbonation of the calcium hydroxide, the cementitious calcium carbonate formed would cause an increase in compressive strength. Although a strength increase is observed, as more carbon-dioxide-laden water invades the matrix, several new equilibria are established.
(4) CO2 - H2O + CaCO3 ↔Ca(HCO3)2
(5) Ca(HCO3)2 + Ca(OH) 2 ↔ 2CaCO3 + H2O
In the presence of excess carbon dioxide (Step 4), the calcium carbonate is converted to water-soluble calcium bicarbonate.