Downhole CO2 in the presence of water forms carbonic acid, which reacts with cement. Some of the final products are water soluble and are leached out. This could cause loss of long term mechanical integrity of the cement sheath, among other issues. Hence, long term prediction of cement sheath carbonation is important to help ensure the cement system designed for CO2 environments creates and maintains zonal isolation throughout the life of the well.
This paper discusses a series of long term tests performed to better understand the effects of CO2 on cement quantitatively. CO2 downhole generally falls within two categories dry CO2, which is a result of carbon capture and sequestration (CCS) activities, and wet CO2, which accounts for naturally formed CO2. Cement samples are tested under downhole conditions of high-pressure/high-temperature (HP/HT) in two different environments supercritical CO2 and carbonic acid, which represent the two different types of CO2 present in the wellbore, respectively. Different types of cement blends have been used in the study to understand the effects of cement composition on corrosion caused by CO2.
Cement samples were periodically withdrawn from the autoclaves to test for transient permeability, compressive strength, Brinell hardness, and chemical composition (X-ray diffraction [XRD], inductively coupled plasma [ICP] tests, pH indicator testing, and thermogravimetric analysis [TGA]). All of the samples were tested for a total of one year. It was observed that chemistry played a very important role in determining how resistant the cement is to attack from CO2.
This study helped provide clear insight into a method that can be used for testing different cement blends for long-term integrity under varying corrosive environments. Knowing the performance of particular cement chemistry under downhole conditions provided considerable benefit in terms of understanding long term integrity of the cement sheath.
