GEOCHEMICAL MODELLING AND MONITORING OF CO2 STORAGE AT THE WEYBURN SITE, SASKATCHEWAN, CANADA

The potential for geological storage and sequestration of CO2 is being studied through the IEA Weyburn CO2-Enhanced Oil Recovery (EOR) Monitoring program at the Weyburn Oil Field, Saskatchewan, where 5000 tonne/day of CO2 has been injected since September 2000. This research program is lead by the Petroleum Technology Research Centre (PTRC), in close cooperation with EnCana Resources.

Geochemical monitoring and modeling is an important constituent of the IEA Monitoring program. The goal is to monitor and model the Weyburn CO2 EOR flood, and to use the results to predict the fate of CO2 in the Weyburn reservoir. This will help provide the basis for the evaluation of geological storage and sequestration of CO2.

Prior to the injection of CO2, baseline samples of produced water and gas were taken from approximately 45 wells . After CO2 injection began, these and additional wells were sampled 3 times per year, with a selected subset sampled every three weeks. The fluid analyses include major and trace elements, as well as a number of isotopes.Both temporal and spatial changes in the fluid chemistry and isotope compositions have been observed; with the most significant changes in the HCO3, Ca, Mg concentrations, pH and d13CO2 values. Detailed chemical, isotopic and mineralogical studies of the solid phases are also underway.

Equilibrium geochemical models have been used to “correct” the fluid analyses to reservoir conditions and to calculate scale potential in the production equipment. Mass transfer geochemical models are being used to model the reactions between supercritical CO2, formation fluids and formation mineralogy. The preliminary geochemical modeling and the produced fluid compositions support the following conceptual model: Addition of CO2 results in the dissolution of carbonate minerals and production of bicarbonate. If basic silicate minerals are present with the appropriate reaction rates, they may react with the CO2, buffer the pH, increase the bicarbonate and precipitate carbonate minerals.

An important objective is to verify the geochemical interpretation through co-operation with other modelers. Selected sub-sets of data will be made available to researchers to aid in their studies. To ensure a consistent point of comparison, the modelers will asked to use a common kinetic database for some of their runs.