Challenges with Mineral dissolution

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⁺ + CO₃^2-,

until the product of the Ca²⁺ and CO₃^2- activity (the effective concentration) equals the solubility product (K_sp):

K_sp = (CO₃^2-) (Ca²⁺),

In this equation, (CO₃^2-) and (Ca²⁺) denote the activities of the two ions, which are related to their concentrations through their activity coefficients, g_CO3^2- and g_Ca²⁺.

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 CO₃^2- 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.