Safe Storage of CO2
Understanding the structure of a typical injection well is crucial to comprehending the concerns associated with the safe storage of CO2 underground, which is a primary issue with Carbon Capture and Storage (CCS). Many individuals fear the potential for inadvertent leakage of stored CO2. To address these concerns adequately, it is essential to be familiar with the construction and composition of an average injection well.
The process of drilling a well involves the use of casing, which is a steel pipe inserted into the wellbore to prevent cave-ins and seal off formations. The casing is joined together using threaded connections and is cemented in place by a cement crew. Different grades of casing are used based on the well's construction requirements. The well typically has multiple strings of casing, including conductor casing, surface casing, intermediate casing, and production casing.
The conductor casing is the first string of casing and is placed in a large hole at the surface. The surface casing provides a foundation for the well structure and supports the blowout preventer (BOP) stack. Intermediate casing seals off troublesome formations and is often the longest section of casing. Production casing isolates the producing zones and protects them.
Drilling progresses from larger hole sizes to smaller ones, and each casing string is selected to fit inside the previous one. Liners may also be used, which are shortened strings of casing that do not extend to the surface. Liners save money by hanging from previously run casing or liner strings.
Primary cementing is the process of cementing the casing into place. Its main purposes are to isolate oil, gas, and water zones from the wellbore and bond the casing to the hole wall. After drilling to the casing point, the crew pulls the string out of the hole, and the production casing is set and cemented. Perforations are made in the casing to allow oil and gas to flow into the well during production.
The following youtube video provides the overview of the drilling and completion process.
Despite the implementation of proper cementing to seal each casing, there remains a possibility of developing leakage pathways that could lead to the contamination of water reservoirs or the release of injected CO2 into the atmosphere. Various factors contribute to these potential leakage pathways, as depicted in the accompanying illustration. Challenges such as casing corrosion, inadequate cementing, deterioration of cement caused by the low pH of carbonic acid, cracks in the cement, preexisting fault lines, or the formation of new ones due to pressure changes induced by reservoir fluid injections, all have the potential to result in the leakage of injected CO2.
EPA regulates the construction, operation, permitting, and closure of injection wells used to place fluids underground for storage or disposal through underground injection control (UIC) program. UIC regulations mandate the consideration of a variety of measures to assure that injection activities will not endanger underground sources of drinking water (USDWs). Here's a breakdown of the different classes of wells regulated under the UIC program:
Class VI wells are designed for the geologic sequestration of carbon dioxide (CO2) to mitigate climate change. These wells are used to inject and store CO2 deep underground in suitable geologic formations, such as depleted oil and gas reservoirs or saline aquifers. Class VI wells are specifically regulated to ensure the safe storage of CO2 and protection of underground sources of drinking water.
Before constructing a Class VI well for carbon dioxide (CO2) injection and storage, a comprehensive site characterization process is imperative. This process aims to assess the suitability of the site for CO2 injection and storage, ensuring its geological stability and absence of faults, fractures, or other factors that could lead to CO2 leakage from the formation. In essence, the chosen formation should act as a secure "tomb" for the stored CO2.
Can you write a blogpost on Class VI wells?