Occurrence of Vaterite in CO₂ Well Bore Gas-Rich Environments: A New Process of Carbonate Formations

J. R. Boles
University of California, Santa Barbara, CA, [email protected]

Well scales can be detrimental to production of geothermal and hydrocarbon fluids. Scaling is also relevant to understanding the diagenetic effects resulting from fluid mixing and pressure drops during faulting. Vaterite scale (metastable hexagonal CaCO₃), a rarely reported mineral in geologic settings, occurs in well tubing from several southern California oilfields. The mineral forms in gas-rich well bore environments at depths ranging from 1.4 to 1.8 km. Some of the vaterite is extremely coarse grained (up to 300um crystals), much coarser than the micron-size crystals reported in the literature. Compared with calcite in adjacent parts of the tubing string, vaterite has extremely light carbon and oxygen isotopic composition that suggests it forms from a CO₂ rich and aqueous fluid depleted part of the system. Oxygen isotopic values are from 7 to 13 per mil more negative than calcite scales and carbon values are 25 to 30 per mil more negative than calcite scales in the same field, suggesting that the oxygen within the vaterite has come from oxygen in the CO₂ gas rather than from dissolved carbonate ion in equilibrium with an aqueous fluid. Presumably, the Ca₂₊ ion is provided by a fluid film on the tubing string. Calcite scales in the same field typically have positive carbon values and relatively positive oxygen values as would be expected from rapid crystallization of calcite due to CO₂ degassing from modified marine pore water. The vaterite appears to have crystallized by a fundamentally different process than the calcite scale and is interpreted to form as CO₂ gas interacts with small amounts of fluid on the tubing string. These occurrences suggest that vaterite may be much more common than presently known and that it may be expected to form in CO₂ rich subsurface environments, such as those associated with CO₂ sequestration.

AAPG Search and Discovery Article #90088©2009 Pacific Section Meeting, Ventura, California, May 3-5, 2009