Downhole pressure and chemical monitoring for CO2 and brine leak detection in aquifers above a CO2 storage reservoir

We assess the effectiveness of downhole monitoring of pressure and chemistry for detecting CO2 and brine leakage into underground sources of drinking water (USDW) overlying a geologic CO2 storage (GCS) reservoir. This assessment uses synthetic data sets generated in an uncertainty quantification (UQ) model framework. This framework combines the results of three model types: (1) a model of a hypothetical, 50-yr, 5-MT/yr GCS operation in the Vedder Fm. at the Kimberlina site in the southern San Joaquin Basin in California, USA, (2) models of CO2/brine leakage in legacy wells into overlying aquifers, and (3) models of CO2/brine plume migration in those aquifers. Leakage into overlying aquifers causes changes in pressure, CO2 saturation, and total dissolved solids (TDS), the latter being related to the solubility and speciation of CO2. This study captures the influence of leakage depth along the legacy well, regional groundwater flow, and aquifer heterogeneity on leak detection using downhole pressure and TDS monitoring. We consider CO2/brine leakage for legacy wells located at various distances from the CO2 injector at the Kimberlina site. Because gaseous CO2 is much less dense than supercritical CO2, the depth where leakage occurs along the wellbore is a key factor affecting the CO2, TDS, and pressure plumes. Supercritical CO2 that migrates upwards to depths less than 800 m undergoes a phase change to gaseous CO2, with the large volumetric expansion causing a large rise in pressure and a large spread in the TDS and pressure plumes. Because the pressure and TDS plumes migrate with the CO2 plume, they are strongly affected by regional groundwater flow and aquifer heterogeneity. We evaluated the likelihood of leak detection over a wide range of CO2/brine leakage-mass magnitude for downhole monitoring at various sensor depths, as well as for locations upgradient, downgradient, and laterally from the leaking well. Leak-detection effectiveness of downhole monitoring improves as the volumes of the CO2 and TDS plumes increase with cumulative CO2/brine leakage mass, groundwater gradient, and aquifer permeability. The effectiveness of pressure monitoring also increases strongly with CO2 leakage rate and can be useful for early detection of large leaks. Because TDS and pressure plumes migrate with the regional groundwater flow, downgradient monitoring wells are more effective than lateral or upgradient wells. Pressure monitoring is found to be more effective than TDS monitoring for shallow depths, while TDS monitoring is more effective at depths greater than about 600 m for this site. The addition of two monitoring depths to a single-depth monitoring well greatly improves leak detection. Monitoring three (shallow, medium, and deep) depths in the same well is found to be equally effective for pressure and TDS for the Kimberlina site. However, these comparisons are affected by the choice of detectable TDS and pressure thresholds, which depend on the level of unfilterable background noise for a specific site. For GCS sites like the Kimberlina site, with a thick sequence of overlying aquifers, unbroken by confining, low-permeability aquitard layers, an effective use of an individual monitoring well would be to monitor pressure at a shallow (< 600 m) depth and to monitor TDS at a medium-depth (600-900 m) interval and for at least one deep (> 900 m) interval. Because pressure and TDS changes evolve so gradually, continuous monitoring is not necessary.