Evaluating the Variability of Static Carbon Dioxide Storage Capacity Estimates through Integrated Analysis of Reservoir Structure, Aquifer Performance and Thermodynamic Behaviour: Case Studies from Three Depleted Triassic Gas Fields on the UK Continental Shelf

Abstract

Evaluation of the variability of theoretical and effective CO2 storage capacity estimation within depleted gas reservoirs is dependent on the integrated analysis of reservoir structure, aquifer performance and thermodynamic behaviour.
Four published theoretical CO2 storage capacity methods and one effective method have been used to estimate the capacity and variability of two Triassic depletion drive reservoirs and two Triassic water drive reservoirs located within the UK Southern North Sea and East Irish Sea Basin. Input parameters to the storage capacity equations have shown a degree of natural variability whereas others are more accurately constrained. As such, attempts have been made to more accurately constrain the most variable input parameters.
The geometric, petrophysical and production characteristics of the reservoirs are analysed. Material balance methods are used to assess the reservoir drive mechanism of the reservoirs. If reservoirs are found to experience a water drive, the aquifer strength is estimated. The gas compressibility factor, gas formation volume factor and CO2 density is estimated under initial reservoir temperature conditions using six equations of state for comparison of results. These results are then input to storage capacity equations producing a range of estimates.
The most susceptible parameter to variability was the cumulative volume of water influx to a reservoir, We. Variability was also found to be the result of error in estimation of the original gas in place. As such, the water drive reservoirs made further use of aquifer modelling to achieve more precise estimates of OGIP and We.
The effective capacity coefficients for the various reservoirs have been estimated to assess the proportion of pore space available for CO2 storage. The effective CO2 storage capacity constitutes a fraction of the theoretical CO2 storage capacity which ranges between 0 (no storage possible) and 1 (all theoretically accessible pore volume is occupied by CO2). Overall, it was found that depletion drive reservoirs have the potential to store greater volumes of CO2 than water drive reservoirs whose aquifer waters occupy the newly liberated pore space.