Solubility product constants for natural dolomite (0–200 °C) through a groundwater-based approach using the USGS produced water database

Abstract

The calculation of a reliable temperature dependent dolomite solubility product constant (K_sp−dol) has been the subject of much research over the last 70 years. This study evaluates log₁₀(^aCa²⁺/^aMg²⁺) values using PHREEQC (Pitzer approach) for a screened subset (n = 11,480) of formation waters in the U.S. Geological Survey National Produced Waters Geochemical Database V2 (PWGD), an extensive inventory of 165,960 formational waters from a range of sedimentary lithologies in North America up to 6.6 km depth (Blondes and others, 2016). Through extensive ground truthing against datasets sourced from Texas Gulf Coast basin and the Mississippi Salt Dome basin we establish both the geochemical data from the PWGD and a new geothermal model of the US that is used to determine temperatures at-formation-depth to be reliable data sources.

The vast majority (90%) of PWGD samples have log₁₀(^aCa²⁺/^aMg²⁺)-temperature values that are interpreted to be indicative of calcite-dolomite equilibrium and buffering by the bulk mineral solubilities. Using statistical models with different parameterisations (different Maier-Kelly formulas, mixed-effects models with various random effects and linear models) log₁₀(^aCa²⁺/^aMg²⁺) values are regressed against the estimated at-formation-depth temperatures to determine K_sp−dol between 0 and 200 °C. This process relies on the well constrained calcite solubility product constant (K_sp−cal).

Local effects that modify log₁₀(^aCa²⁺/^aMg²⁺) values are evaluated through the addition of random effects to the mixed model which both improves the statistical reliability of the K_sp−dol model and enables the determination of K_sp−dol values for local dolomite phases. The nature of these local effects is open to interpretation, but we suggest the primary influence on log₁₀(^aCa²⁺/^aMg²⁺) values is the stoichiometry of the dolomite phase systematically modifying log₁₀(^aCa²⁺/^aMg²⁺) values. We discount the influence on log₁₀(^aCa²⁺/^aMg²⁺) values of dolomite order, the solution ionic strength, equilibration with anhydrite and chlorite group minerals, illitization of smectite and albitization of feldspar.

For the dolomite solubility equation; the mixed-effects model (model J23) chosen as most representative yields a pK_sp−dol (log₁₀K_sp−dol); We determine pK_sp°−dol to be −17.27 ± 0.35 (25 °C, 1 atm) which is close to prior estimates, including the most recent experimental value reported by Bénézeth and others, 2018 (pKs_p°−dol = −17.19 ± 0.3) validating the groundwater regression analysis approach of this study.