Prediction of buried calcite dissolution in the Ordovician carbonate reservoir of the Tahe Oilfield, NW china: Evidence from formation water

The Lower–Middle Ordovician reservoir of the Tahe Oilfield is dominated by limestones with reservoir spaces formed by the generation of dissolution pores, meaning that buried karst formation can be evaluated by studying water–rock reactions between groundwater and calcite. The hydrogeological information preserved in this reservoir indicates that the Ordovician groundwater were high-salinity and high-closure, characteristics that are of significance to water–calcite reactions. Theoretical chemical thermodynamics combined with equilibrium calcite solution ionization allowed us to establish a dissolution–precipitation evaluation model for calcite, with the theoretical activity of Ca2+ in solution (aCaeq2+) controlled by temperature, pressure, [ΣCO2] − [Ca2+] and solution pH, and with the actual activity of Ca2+ in solution (aCa2+) being controlled by the concentration of various ions in solution. Ionization reaction directions are controlled by ΔG   values; these values can be calculated using aCaeq2+ and aCa2+. Here, ground water data were collected from 34 wells that intercepted Ordovician sediments within the Tahe Oilfield, and calcite ΔG values were calculated for these wells. These data indicate that the groundwater in this oilfield favours the dissolution of limestone, with limestones in the west and south of the Tahe Oilfield being more susceptible to dissolution, consistent with observations within the oilfield. The methods employed during this study can also be used to quantitatively assess susceptibility to burial dissolution and to support reservoir evaluation.