Quantifying reactive chemistry along an injected CO2 flow path at the field scale using a Monte Carlo simulation approach

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A Markov Chain Monte Carlo reactive transport modeling approach was applied to the problem of constraining the distribution of key reactive mineral phases along a flow path between a CO2 injector well and a monitor well at the Weyburn–Midale field in Saskatchewan. The methodology entails postulating a spatially correlated mineral distribution consisting of calcite, dolomite, anhydrite, and K-feldspar, characterized by specified volume fractions and intrinsic dissolution rates, in contact with an ambient brine composition along a one-dimensional flow path. Thousands of forward reactive transport simulations, conducted with PHREEQC, were run for the column, with simulated changes in brine chemistry compared with both synthetic test problem data as well as field data. We used a composite likelihood function to compare modeled and measured pH and concentrations of Ca2+, Mg2+, and Si. New realizations were proposed by replacing a small, contiguous section of the column with a new distribution of minerals that maintained spatial correlation with the remainder of the column. Proposed realizations were accepted/rejected using the Metropolis–Hastings acceptance criteria. If a proposal was accepted, the modified mineral distribution served as the basis for a new distribution, otherwise the modification was rejected as a non-improvement. Application of the inverse modeling approach to a synthetic problem demonstrated nearly complete recovery of a specified initial mineral distribution (i.e., “synthetic truth”) along the flow path, provided that “measurements” (synthetic data) were available across the entire column. Partial recovery of the synthetic truth was still achievable as the amount of data available for inversion was reduced to a single “measurement” collected at the column endpoint, mimicking the typical situation in the field. When applied to real brine chemistry data from a 1-km×1-km test area at the Weyburn–Midale reservoir, the inversion approach identified variations in the amounts of dolomite and calcite available for reaction along the flow path, given simplifying assumptions concerning permeability, pressure gradient, mineral specific surface, and the extent of mixing in the formation. These variations are qualitatively consistent with known compositional variability in mineralogy between the CO2 injector and the monitoring well location.