CHALLENGES with Mineral dissolution

General comments

During geothermal production, ions and molecules are sometimes removed from – or added to – the geothermal brine, for example to prevent scaling and corrosion or to enable CO2 sequestration. Such changes in water composition may promote dissolution of minerals in the reservoir after injection. If rock-cementing minerals dissolve, it may compromise the mechanical strength of the reservoir rock. Thus, care should be taken when selecting and designing means for preventing scaling and corrosion to avoid the introduction of other problems in the reservoir.

Mineral dissolution

Subsurface solutions are often saturated or nearly saturated with respect to the minerals in the reservoir rock due to the long time span that subsurface solutions have reacted with the reservoir rock. For example, calcite quickly equilibrates with the formation water via the reaction:

CaCO3 = Ca2+ + CO32-,

until the product of the Ca2+ and CO32- activity (the effective concentration) equals the solubility product (Ksp):

Ksp = (CO32-) (Ca2+),

In this equation, (CO32-) and (Ca2+) denote the activities of the two ions, which are related to their concentrations through their activity coefficients, gCO32- and gCa2+.

Changes to the solution composition during geothermal production may disturb this equilibrium between the pristine formation water and the reservoir minerals. Thus, removal of Ca or changes in the solution chemistry that affect the carbonate speciation and results in lowering of the CO32- activity (e.g., pH decreases), would cause the ion activity product to decrease below that dictated by the solubility, initiating dissolution of calcite (e.g., Holmslykke et al., submitted). Such changes could occur because of CO2 addition, because of Ca removal to avoid CaCO3 scaling, or because of ion exchange intended to remove corrosive Pb2+ or Cu2+, also removes Ca2+.

Above, we have used calcite as an example. However, the removal of ions to a state below saturation may cause dissolution of several minerals in the reservoir after injection. Similarly, addition of CO2 to the injected formation water may lower the pH and shift the original equilibrium, so that many minerals become sub-saturated with a risk of mineral dissolution (Kampman et al., 2014).

Mineral dissolution may be beneficial for geothermal exploitation as it should increase porosity and permeability. However, if the dissolving minerals are rock-cementing minerals, the mechanical strength of the geological formation may be challenged (Figure 1) with the risk of decreasing e.g., the borehole stability.

The effects of cation removal on the reservoir properties are studied in the PERFORM project.


Figure 1: Schematic illustration of dissolution of the rock cementing minerals. Left: pristine state. Right: After dissolution of the rock cementing minerals.

References

Holmslykke H. D., Kjøller C., Fabricius I. L. Seasonal heat storage in geothermal sandstone reservoirs: Effect on reservoir quality by injection of Ca-depleted formation water in the Lower Triassic Bunter Sandstone Formation. Submitted to Geothermics.

Kampman, N., Bickle M., Wigley, M., Dubacq, B., 2014. Fluid flow and CO2-fluid-mineral interactions during CO2-storage in sedimentary basins. Chemical Geology 369,22-50.