38. What are wellbores and how do you maintain them?
Wellbores are the holes drilled deep into rock formations like oil and gasto both inject materials like CO2 and bring to the surface materials like oil. Wellbores are the routes through which oil moves upwards or CO2 and other substances move downwards. They have cement and metal casings all the way down, especially nearer to the surface, in order to stabilise the wellbore, to provide pathways for fluid movement and the installation and retraction of instrumentation, to protect drinking water, and for other functions.
Wells are constructed to very high standards and can be effectively monitored. For example, pressure and temperatures can be monitored both inside the well casing and between the casing and the surrounding rock (in the annulus). Fluid samples can be obtained from within the reservoir as a further check against unexpected behaviour, and specialised monitoring tools can be lowered inside the well casing to measure the condition of the well to full depth. If imperfections are discovered in the casing or the annulus (which at most depths is filled with cement), standard procedures from the oil and gas industry can be used to repair (remediate) these imperfections. Figure 27 is an illustration of an injection well for a carbon storage project that shows some of the monitoring equipment that can be included.
Figure 27. Some wellbores in CCS projects include various listening and measuring devices, such as this wellbore in a new Saskatchewanproject called Aquistore. In this case the monitoring equipment in the wellbore helps to ensure that no CO2 escapes to the surface. (Picture courtesy of PTRC's Aquistore Project)
The operators of the Weyburn and Midale oilfields have developed world leading expertise on the safe operation of wellbores where CO2 injection is utilised at an industrial scale. Research in the WMP has demonstrated the safe, long term integrity of wells that are carefully constructed and monitored.