Supporting Information for the manuscript “Micro-CT characterization of wellbore cement degradation in SO4

2--bearing brine under geological CO2 storage environment” by Manguang Gan, Liwei Zhang, Xiuxiu Miao, Yan Wang, Xiaojuan Fu, Mingxing Bai, and Xiaochun Li

1. CT characterization of the reaction between wellbore cement and pH=2.5 sulfuric acid solution (cured in 1wt% NaCl solution with no presence of SO4

2-)One end of the cement sample(~ 5mm) which cured at the pressure of 17MPa

was immersed in a solution of pH=2.5 sulfuric acid, then the structural changes of cement before and after the reaction for 3.5d and 7d were compared by micro-CT scanner. The results were shown in Figure S1, and the red in the figure indicated dissolution, while the blue indicated precipitation (the same below). The dissolved area was mainly concentrated in the tip, and a very small amount of precipitation occurs below the dissolved area. When the cement sample reacted with sulfuric acid solution for 7d, the outer surface of the cement became darker than the case after 3.5d reaction, indicating that the dissolution after 7d reaction was greater than the case after 3.5d reaction. The surface of cement contacting sulfuric acid was dissolved and a dissolved shell was formed surrounding the cement after 7d reaction. It can be noted that the calcium sulfate precipitation after 3.5d reaction dissolved after reacting for 7d.

Figure S1: CT-observed structural changes before and after the reaction between cement and pH=2.5 sulfuric acid solution: (top) reaction for 3.5d, (bottom) reaction for 7d. From left to right:

before reaction, after reaction, the CT number difference before and after reaction

In addition, the experiment between another group of cement samples which under the same curing condition and pH=2.5 sulfuric acid solution was conducted for 14d, and structural changes before and after the reaction were shown in Figure S2. The dissolved amount of cement reacting with sulfuric acid solution for 14d was greater than that for 7d, therefore the reaction region had formed a more thicker layer of dissolved shell which surrounded the cement. A small amount of precipitation was produced at the end of the reaction area, which was expected to be calcium sulfate precipitation. Figures S1 and Figure S2 show that calcium sulfate had an alternating precipitation-dissolution-precipitation behaviour.

Figure S2: CT-observed structural changes after reaction between cement and pH=2.5 sulfuric acid solution for 14d, from left to right: before reaction, after reaction, the CT number

difference before and after reaction

A section of the reaction area in cement sample was selected, and the pore structure images of the section after reacting with the sulfuric acid solution for 3.5 and 7 days were shown in Figure S3. After the reaction between cement and pH=2.5 sulfuric acid solution for 3.5d, a layer of dissolved layer has been formed on the exterior of the cement, while a very small amount of calcium sulfate precipitation has been produced in the penetrating hole. As the reaction progressed, the thickness of the dissolved shell greatly increased, and a very small amount of precipitation has been produced within the dissolved layer in the inner part of the cement.

Figure S3: The CT number difference diagram of a cross section in the reaction area between cement and pH=2.5 sulfuric acid before and after reaction. From left to right: reaction for 3.5 d

and reaction for 7 d.

2. CT characterization of the reaction between wellbore cement and pH=3 sulfuric acid solution (cured in 1wt% NaCl solution with no presence of SO4

2-)The acidification reaction between the cement sample and pH=3 sulfuric acid

solution was conducted, and the structural changes of the cement after the reaction for

3.5d and 7d were shown in Figure S4. After the reaction with sulfuric acid solution for 3.5d, the cement mainly dissolved in the area in contact with sulfuric acid, while a very small amount of precipitation was produced at the end of the reaction area. As the reaction progressed, the amount of dissolution became more after the reaction between cement and sulfuric acid for 7d, compared with the case of 3.5d. Unlike the case with pH=2.5 sulfuric acid solution, calcium sulfate precipitation still existed after the reaction for 7d.

Figure S4: CT-observed structural changes after the reaction between cement and pH=3 sulfuric acid solution: (top) reaction for 3.5d, (bottom) reaction for 7d; From left to right: before

reaction, after reaction, the CT number difference before and after reaction

Besides, the experiment of another cement sample reacting with pH=3 sulfuric acid solution was conducted for 14d, and the structural changes after the reaction were shown in Figure S5. The amount of cement dissolution after reaction between wellbore cement and sulfuric acid solution for 14d was larger than that for 7d, and a dissolved shell was formed on the exterior of the cement to surround the cement. There was a very small amount of precipitation at the interface between the cement immersed end and the non-immersed end.

Figure S5: CT-observed structural changes after reaction between cement and pH=3 sulfuric acid solution for 14d, from left to right: before reaction, after reaction, the CT number difference

before and after reaction

A section of the reaction area in cement sample was selected, and the pore structure images of the section after reacting with the sulfuric acid solution for 3.5 and 7 days were shown in Figure S6. After 3.5 days of reaction between cement and sulfuric acid, a thin layer of dissolved layer has been formed on the exterior of the cement, while a very small amount of calcium sulfate precipitation has been produced in the penetrating hole. The thickness of the dissolved layer and the amount of precipitation produced in the penetrating hole increased as the reaction progressed, and a small amount of precipitation was observed in the interior of the cement. Therefore, the amount of cement dissolved increased with the increase of reaction time. Comparing Figure S3 with Figure S6, it can be seen that the amount of dissolution after the reaction between cement and pH=2.5 solution was significantly higher than that after the reaction with pH=3 solution, which comes to a conclusion that the acid-induced corrosion of cement by lower pH solution was more significant.

Figure S6: The difference diagram of a cross section reaction in the reaction area between cement and pH=3 sulfuric acid before and after reaction, from left to right: reaction for 3.5 d, reaction for

7 d.

3.Comparison between acidification and carbonation results after 14d reaction (cured in 1wt% NaCl solution with no presence of SO4

2-)In order to study the contribution of carbonation and acidification to cement

corrosion, the acidification-only results of the reaction between cement and pH=2.5 sulfuric acid solution (Figure S7) and the carbonation results of the reaction (no presence of SO4

2-) between cement and CO2 (Figure S8) were compared. After the same reaction time (14d), the main differences between the acidification-only results and the carbonation results are as follows: (1) The amount of dissolution after acidification was larger than that of carbonation, and a thicker dissolution layer on the exterior of the wellbore cement was formed after acidification, compared with the case of carbonation. Besides, a layer of carbonate was formed inside the dissolved layer for the case of carbonation, which can effectively hinder the reaction between wellbore cement and CO2. Since the exterior of the sample does not form a carbonate layer for the acidification-only case, the acid can further migrate into the cement and corrode the interior of the cement, which lead to a larger amount of cement dissolution and a thicker dissolved shell formation outside the cement, compared with the case of carbonation. (2) The amount of carbonate precipitation due to carbonation was larger than the amount of CaSO4 precipitation due to reaction with sulfuric acid in the penetrating hole, indicating that carbonation contributes more to the blocking of the penetrating hole than reaction with sulfuric acid. (3) Cracks were formed after reacting with sulfuric acid (acidification only), while that phenomenon did not occur after reacting with CO2 (acidification+ carbonation). The cause of the crack formation may be that the exterior dissolution lead to loosening and weakening of cement after the acidification reaction, while the volume expanded during the internal cement hydration process and thus produced extrusion, causing formation of cracks under the condition that the external constraint decreases due to loosening and weakening of cement.

Figure S7: CT number difference diagram of cement sample before and after reaction between cement and pH=2.5 sulfuric acid solution for 14d

Figure S8: CT number difference diagram of cement sample before and after reaction between cement and CO2-saturated brine for 14 d (Modified from Wang et al.[1])

Reference[1] Y. Wang, L. Zhang, X. Miao, N. Wei, and X. Li, CO2–Induced Structure Alteration of Oil Well Cement: a Micro CT Study, 14th Greenhouse Gas Control Technologies Conference Melbourne, 2018, pp. 21-26.