Geochemical Model and Report

The Primary Store Geochemical Model and Report explains the methodology used to investigate possible geochemical interactions between the pore fluids and the surrounding rock caused by injecting CO₂ into the saline pore fluids of the Bunter Formation. The geochemical methodology outlined in this report is well explained and is consistent with industry workflow used to examine aqueous geochemical processes.

It is tempting to assume that all geochemical interaction can be modelled anywhere in the system at anytime i.e. linking all the chemical interactions with the static and dynamic models to produce an integrated flow and chemical interaction model. However, this is not the case, since the data needed to do this is not available in the detail required. This would demand an extremely large data set that would include not only the thermodynamic properties of all potential interactions but also a set of kinetic data at different temperatures and pressures for a very saline aqueous solution. The data set would also have to include chemical properties of the minerals in the Bunter sandstone. This complete data set is not yet available. What can be examined are specific geochemical interactions that are known from experience to be important in similar reservoirs. These specific interactions can be modelled on a smaller scale. This is the approach used in this report and is standard practice.

There are many geochemical modeling software packages available to look at different aspects of the geochemistry. Some models focus on the stability of certain minerals as the chemistry of the water is changed indicating which minerals may dissolve or precipitate. Other models examine the change of chemistry combined with flow but usually in a limited area and with limited chemical interactions. Both types of models were combined in this study. PHREEQC (ph redox equilibrium) is freeware originating from the USGS, it is essentially an equilibrium saturation model. It can be used to establish what relevant minerals will be saturated or undersaturated with respect to the saline brine. This will indicate if a mineral has the potential to precipitate or dissolve. It is important to distinguish ‘potentially precipitate’ from ‘will precipitate’ and at what rate. The latter involve kinetic data that are not part of equilibrium thermodynamic modelling. PHREEQC will take into account the solubility of CO₂ into the brine and the changes in chemistry this will cause. It also has functionality that will estimate the effect of the very high salinity on the equilibria involved. Geochemist’s Workbench (GWB) is commercial software provide by Computer Modelling Group. GWB is fundamentally similar to PHREEQC in that it calculates thermodynamic equilibria but it has increased functionality when it comes to integrating the input and outputs with other geochemical tools and visualizing the results. Both models require a database of empirically measured geochemical/thermodynamic values. In the workflow, measures were taken to compare the results of both PHREEQC and GWB to ensure consistency. There are numerous problems associated with geochemical modelling of very high salinity fluids and these are discussed within the report and the methodology used is reasonable.

GEM is available from Computer Modelling Group and is a coupled fluid and reactive transport simulator. This involves a similar geochemical database to PHREEQC and GWB but can be used to investigate reactions coupled with fluid flow. This was used specifically to look at the problem of halite precipitation close to the injector well as CO₂ is injected into the well bore.

All geochemical models need a good understanding of the initial geochemistry of the rock and the initial geochemistry of the aqueous fluid within the pore space. The report presents full geochemical analysis of five brine samples taken from the 42/25d-3 well. These samples were taken as part of the previous White Rose project specifically for the investigation of chemical interaction when injection CO₂. The analyses appear to be of high quality with many more species analysed than is normal for a routine water sample for standard oil and gas activity. However, no detail on sampling procedure or analytical procedure is given. The samples indicate a pore water of very high salinity of approximately 250,000 mg/Kg Total Dissolved Solids (TDS).

Mineralogical analysis is presented from XRD analysis of core samples taken from the Bunter Sandstone and the overlying Rot Halite and Rot Clay. The Bunter sandstone is a relatively ‘clean’ sandstone and consists primarily of Quartz (56-75%) and Feldspar (10-15%) with minor amounts of illite-mica, chlorite, calcite, dolomite, halite and anhydrite.

The focus of the geochemical modeling concentrates on some specific questions which are considered the main geochemical problems caused by injection CO₂ into the system.

1. Will there be interactions between the CO₂ saturated brine and the Bunter Sandstone that will cause mineral precipitation or dissolution?
2. Will there be clay/halite interaction with the CO₂ saturated brine which will cause dissolution and therefore degradation of the top seal, the Rot Halite and Rot Clay?
3. Will there be chemical interaction immediately adjacent to the injection wells that will cause problems with injectability?
4. If pressure management (dewatering of the aquifer) is used to increase the storage capacity of the Endurance structure what would be the chemistry of the produced water?

The modelling presented in this report outlines the methodology and results associated with these questions. As CO₂ dissolves in water the pH decreases due to dissociation of the CO₂ to produce a weak acid. However, because the mineral assemblage contains some carbonate minerals (dolomite and calcite) these serve to buffer the pH and therefore no drastic change in pH would be expected. To buffer the pH, small amounts of calcite may dissolve, and small amounts of dolomite may precipitate. Even if the solution was not buffered, the drop in pH would have very little effect on the stability of the silicate mineral including the most abundant mineral, quartz. There is no indication quartz would precipitate or dissolve. The mineral stability calculated for CO₂ saturated brine also suggests that even if illite were added to the system, a constituent of the Rot Clay, it would be unaffected by increasing CO₂ levels. The most significant change is with respect to halite stability which becomes undersaturated and hence has the potential to dissolve. The seal immediately above the Bunter sandstone is the Rot Clay but if formation water breached this layer (possibly through fractures) the brine would be undersaturated with respect to halite may cause some dissolution of the Rot Halite. However, if the brine remains undisturbed, equilibrium would be established between the halite and the pore fluids resulting in a thin layer of saturated brine just below the Rot Halite. This layer of saturated pore fluid should prevent large-scale dissolution of halite. As a result of these findings, it has been suggested that the CO₂ should be injected in the lower part of the formation so as not to cause excessive fluid flow in the upper part of the formation. Also, supercritical phase CO₂ is less dense than water and as more is injected a layer of dense phase CO₂ will occur at the top of the reservoir underneath the Rot Formation. This will act as a barrier to halite dissolution as the dense phase CO₂ will contain very little water and be unable to cause halite dissolution.

What happens close to the well bore when CO₂ is injected is counterintuitive to the undersaturation with respect to halite discussed above. As CO₂ in the dense phase essentially dehydrates the surrounding rock, water dissolves into the CO₂ removing the water from the pore space around the well bore. This halo around the well bore is a function of rate of CO₂ injection as well as chemical reaction. This dehydration around the well bore has two consequences. It means that there is little chance the cement around the well bore will be dissolved by relatively low acidic formation water as the water has been removed by the flooding CO₂. This is positive as it maintains cement integrity around the well bore. It is also possible that this dehydration will cause halite precipitation directly around the well bore reducing the permeability and therefore the injection rate of CO₂. As most of the water is removed by the CO₂ the remaining water becomes increasingly saturated with respect to halite, as the Na⁺ and Cl^– are not removed at the same rate as the water. Depending on the rate of CO₂ injection capillary pressure may be enough to add more saline water to the well bore area, which is further evaporated increasing the chance of extensive halite precipitation. This halite precipitation has been known to occur in other saline sequestration projects such as the Quest facility in Alberta, Canada. To study this phenomenon a coupled reactive transport geochemical model was built in GEM software. The results from the model indicate that halite precipitation will occur, but it is dependent on the rate of CO₂ injection. At high rates (50mmscf/d) porosity reduction by halite precipitation was minimal as the amount of CO₂ pushed the precipitation front away from the immediate well bore area. However, at lower rates (10mmscf/d) significant halite precipitation was modelled to occur close to the well bore. This information could be used to establish an injection strategy. To remediate halite precipitation, fresh water ‘washing’ of the near well bore area is a proven method to dissolve any precipitated halite and re-establish near well bore permeability.

The subject of dewatering the aquifer has been considered to increase the storage capacity of the Endurance structure. This could be done by drilling new dewatering wells and/or there is the possibility that the structure could naturally dewater due to the outcropping of the Bunter sandstone at the sea floor. The outcrop is beyond a deeper structural spill point that would keep the more buoyant CO₂ within the structure. It is assumed, but not explicitly stated, that the saline aquifer brine would be disposed of in the surrounding seawater. Preliminary work has been done to investigate if any of the dissolved chemical species in the brine would be harmful to marine life. There appear to be some heavy metal aqueous species that are considered toxic but they are in very low concentrations.  If this dewatering were an option, more detailed work would be required to identify the species and their concentrations and compliance with all environmental regulations.