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Regarding the geological storage of CO 2 in aquifers or in depleted gas fields, the reactivity of the well casing left in the abandoned well is important in order to investigate the potential for leakage. The corrosion behavior of the casing steel tube was investigated in highly pressurized CO 2 environments. The corrosion test of the J55 and N80 steel tube in API specification 5CT was conducted. Those steel tubes have been frequently used as casing tube and are found in abandoned natural gas wells. The following test conditions were achieved with the autoclave apparatus: temperature at 50 to 70 °C, CO 2 pressure at 5 to 18 MPa with/without the impurities of 20 ppm H 2 S and 2000 ppm CO, in/above stagnant 0.5 M NaCl solution of simulated formation water. The test durations were 100, 300 and 1000 hours. After the corrosion test, the specimens were analyzed by Energy dispersive X-ray spectrometry and X-ray diffractometry. The specimens were found to be covered mainly with FeCO 3 , which is considered to be formed from dissolved Fe 2+ and environmental CO 2 . The corrosion rate was evaluated via the weight loss of the specimens after the removal of the surface corrosion products, containing FeCO3. No significant differences were observed between J55 and N80 in terms of FeCO 3 formation and corrosion weight loss. It was found that the weight loss by corrosion, around 100 mg/cm 2 after 100 hours, was remarkably large when the specimen was placed in the solution and was, in contrast, two orders of magnitude smaller when the specimen was above the solution. In the case with the impurities of 20 ppm H2S and 2000 ppm CO, the weight loss in the solution was less relative to the pure CO 2 condition. The weight loss in the solution reached a plateau in initial 100 hours, and was only slightly increased in the 300- and the 1000-hour tests. The ‘plateau’ was considered to be due to the protective effect of the FeCO 3 formed on the specimen. Considering the protective effect of the FeCO 3 , the corrosion of the steel casing over a long period of time is small under the stagnant condition, and would be expected to remain in place with little structural degradation.

An abandoned well, completed without CO2 resistant materials, is thought to be a potential pathway for CO2 leakage to the surface. To examine the chemical impacts of a non-CO2 resistant well, we prepared composite samples consisting of J-55 casing material, Portland cement and sandstone (referred to as “simulated well sample”). Permeability of the simulated well sample decreased during a flow-through experiment using CO2-saturated brine. Precipitation of carbonate was found in both flow-through and batch experiments. The casing material was not degraded by CO2 if bonding between the casing and cement was tight. The sandstone surrounding the cement therefore seemed to act as a buffer medium against CO2 attack.

For a feasible risk assessment of CO2-leakage through wells of the carbon dioxide storage sites at abandoned wells of aquifers and depleted gas fields, the CO2 reactivity and permeability of casing steel, pseudo formation and cement- plug need to be evaluated experimentally as functions of temperature, pressure and formation water chemistry to provide fundamental information for the assessment. Two types of laboratory experiments were conducted. These experiments are shown below: (1) Conventional batch-reaction experiments of cement cores in the system of CO2 and simulated formation water as 0.5 M NaCl, at 50 and 70 C, 5, 8, 18 MPa for 100-1600hr. In this experiment, individual cement cores are allowed to react with wet CO2 and NaCl solution charged in titanium reaction vessels with PTFE separators. (2) CO2-injection reaction involving casing (API Grade J-55)-cement (API class A) and cement (API class A)-shale composites which were saturated with 0.5 M NaCl solution. As a counter experiment for batch experiments, zero CO2 runs for these cements were additionally carried out at 50 and 70 C at 5 MPa for 100 and 400 hr. The CO2-injection runs were carried out at 50 C and 8.5 MPa with a constant differential pressure of 5 kPa. The resultant products were used to analyze alteration depth via micro-focused X-ray computed tomography (-XCT) and electron probe micro analyzer (EPMA). For observation and determination of alteration phases, field mission scanning electron microscopy with energy dispersive spectroscopy (FESEM-EDS) and micro X-ray diffractometry (-XRD) were also performed. The alteration zones identified in the both cement cores of A and G in wet CO2 showed spatial developments of zones appearing in -XCT images as a function of square root of time (t1/2), which can be interpreted as a diffusion-limited reaction. However, in the NaCl solution, these cement cores developed little alteration zones and poorly displayed time-dependency after 100 h.

A hypothetical CO 2 storage site that has some abandoned wells located near the injection well is considered. Generally, cement plugs are set to cover or isolate porous or productive formations and to isolate usable groundwater from hydrocarbons. When CO 2 is injected into a reservoir, supercritical CO 2 or dissolved CO 2 spreads out in the reservoir. This dissolved CO 2 can react chemically with cement seals (plugs and casing cement) in abandoned wells. As a result, minerals in the cement seals (plugs and casing cement) may alter to form other minerals or mineral phases. The first simulations concerning migration of supercritical CO 2 and dissolved CO 2 in a saline aquifer and a depleted gas field reservoir, were carried out using TOUGH2. The following temperatures and depths of reservoir were selected: 50 °C at E.L.-1,000 m, 60 °C at E.L.-1,500 m and 70 °C at E.L.-2,000 m. Secondly, geochemical reactions of cement seals (plugs and casing cement) were simulated. The results of reservoir simulations were used as boundary conditions for the geochemical calculation of cement seals. Geochemical simulation of the reactions yielded the extent (length) of alteration of cement seals (plugs and casing cement) after long time periods; for example, the alteration length of cement seals after 1,000 years was about one meter. This length is short enough so that the usable cement seals (plugs and casing cement) of abandoned well are able to continue to isolate CO 2 in the reservoir from the upper aquifer.