MERITS OF PRESSURE AND GEOCHEMICAL DATA AS INDICATORS OF CO2/BRINE LEAKAGE INTO A HETEROGENEOUS, SEDIMENTARY AQUIFER

Many different monitoring techniques have been evaluated both synthetically and from the field to better understand the spatial and temporal movement of CO2 and pressure plumes within and above a CO2 storage reservoir. The distance between the leak source and the nearest monitoring well or sensor location may deem certain monitoring techniques as an unviable detection method. It is important to determine the efficacy of the different possible monitoring techniques with uncertain leak locations to design optimal monitoring protocols to ensure non-endangerment to drinking waters.

The purpose of this study is to evaluate and compare the reliability of above-zone pressure and groundwater solution chemistry monitoring as leak diagnostics. We generate synthetic pressure, total dissolved solids (TDS) and pH data via many subsurface realizations and leak simulations that are based on a specific part of the High Plains Aquifer, with an average well density of 1 per km2 (Carroll et al., 2014). We use a simulation approach to explore how the following principal uncertainties influence the three signals to correctly diagnose a CO2/brine leak:

• The heterogeneity of the aquifer flow properties. This study uses two-facies heterogeneity models (sand/shale) to evaluate the effects of heterogeneity on spatial and temporal scales of the signals.

• The distance between the source leak and the monitoring well. It is possible that unknown abandoned wells or faults could serve as random leak locations unknown a priori (Gal et al., 2013).

• The leakage rates of CO2 and brine. The magnitude and duration of the leak will be one of the main determiners of the extent of the three signals. We explore a range of CO2 injection periods and wellbore permeability (Wainwright et al., 2013).

Detection likelihoods are calculated to describe how frequently pressure, TDS, and pH signals will coincide with a leak for observations made at different distances and times from the initiation of the CO2/brine leakage. The pressure signal gives a more spatially extensive signal than either TDS or pH. When considering the samples that only experience the highest leakage volumes, there is a 50% likelihood of detecting a pressure change 400 m away at times ≥30 years as seem. However, the TDS and pH detection likelihoods are <20% at 100 m distance for times ≥30 years.