In the United States carbon capture and storage as a means of emissions mitigation is moving towards commercial implementation. In anticipation of large numbers of CCS sites coming online it is important to understand wells as potential leakage pathways and identify existing technologies that can measure and monitor the integrity of wells that are exposed to CO2. Well integrity is important because wells and the annuli and pathways that may exist within them can act as leakage pathways for CO2 back to the surface or as conduits for leakage between formations. Studies related to the integrity of wells have been conducted on well cements created in both the laboratory and in the field. This paper will discuss recent laboratory and field work, the differences between the results of the two types of experiments, and how the results of the two are not contradictory but are due to differing conditions between the lab and the field. This paper will also discuss how the results of field measurements can be used to identify the types and combinations of tools that may be used to detect wellbore integrity problems in future.
In the United States carbon capture and storage (CCS) as a means of emissions mitigation is moving towards commercial implementation. In the next several years the seven DOE partnerships may be starting large-scale injection projects in addition to other government and commercial CCS projects are also expected to come online. In anticipation of large numbers of CCS sites coming online it is important to understand how CO2 may leak out of the storage reservoir. Well integrity is important because wells and the annuli and pathways that may exist within them can act as leakage pathways for CO2 back to the surface or as conduits for leakage between formations.
Oil and gas wells are typically a series of nested casings and well cement. Figure 1 shows a schematic of a "typical" well showing the nested nature of the casings and cement. Figure 2 shows a schematic of the leakage pathways that can exist within a well. Because there are many different possible leakage pathways within a well it is necessary to examine the condition of the casing and the cement and identify any annuli or defects that exist within the well. It is possible that CO2 can move through the interfaces between the casing and the cement or the cement and formation, through the cement matrix, through defects in the cement, or through the defects in the casing and then up the wellbore. Watson and Bachu 1 have statistically assessed the leakage potential for wells using a population of 316,500 abandoned wells in Alberta, Canada. The researchers looked at surface casing vent flows and gas migration from the well. They found that 4.6% of the wells had some sort of leakage. Further analysis showed that the major factors related to leakage were the geographic area of the well, The deviation of the well, the type of well (abandoned open hole or abandoned cased hole), the abandonment method, oil price and regulatory environment, and uncemented casing/hole annulus. Of the leaks that occurred 81% of them were above the top of the cement which indicates that the cement is a factor in preventing leaks and providing zonal isolation.
This paper discusses work related to the integrity or wells and well cement exposed to CO2. It describes a degradation mechanism for well cement exposed to CO2 and discusses laboratory research on the effect of carbonic acid or CO2 on well cement, field research on the effect of CO2 on wells and well cement, and how the results of the field experiments can be used to identify the types and combinations of tools that may be used to detect wellbore integrity problems in future.