Probability assessment of pressure and geochemical monitoring for CO2 leakage detection in the above zones at geological carbon sequestration sites

Abstract:

Above-zone monitoring of pressure has been proposed as one of means to quality a geological carbon sequestration site by detection of leakage. Advancements in chemical monitoring, such as fiber optic sensors, will make it possible to monitor geochemical chemical parameters needed for CO₂ leakage detection in the same zones. The main objective of this study is to assess pressure and geochemical monitoring for CO₂ leakage detection with Montecarlo simulations at a hypothetic site where CO₂ is potentially leaked into an above zone through a flawed well.

Configuration of the above-zone in the hypothetic site was taken from a real site where CO₂ has been injected into the deep reservoir since 2008 because over three years of pressure data in the above zone has been recorded at this site. Because geochemical data in the above zone at this site is limited, we also extracted some geochemical data from the TWDB database. Threshold values (or background noises) were estimated (or characterized) from the pressure and geochemical data for identifying CO₂ leakage signals in the above zone. In the model set-up, pressure, dissolved CO₂, pH and salinity in the above zone caused by CO₂ leakage from a flawed well were simulated with analytical approaches, depending on the parameters, such as CO₂ leakage rate, permeability, porosity, thickness of the monitoring interval, distance of the monitoring well to the leakage location. More than one million set of the parameters, simulated with the Monte-Carlo method, were used to model responses of pressure, dissolved CO₂, pH, and salinity which were further compared to the threshold values to estimate probability of CO₂ leakage detection using the pressure and the geochemical parameters.

Our study shows that pressure in the morning well has much shorter time to respond to the CO₂ leakage from the flawed well than geochemical parameters. However, if the monitoring well was in a favorable location, the geochemical monitoring may have better responses to small leakage than pressure monitoring. This study suggests that a combining use of pressure and geochemical monitoring in the above zone may likely increase probability of CO₂ leakage detection.