Effect of CO 2 on the buoyancy of geothermal fluids

The presence of large amounts of CO 2 in geothermal fluids is shown to generally enhance convective instability. The buoyancy of water‐CO 2 solutions can be dominated by the large volume changes that occur when CO 2 enters or leaves solution. The transfer of water between the liquid and vapor phases which accompanies the movement of CO 2 into or out of solution also contributes to changes in the fluid volume and resultant buoyancy. The effective thermal expansivity α eff in the CO 2 rich fluid of the Broadlands geothermal region of New Zealand exceeds the coefficient of thermal expansion of ordinary water α ell for depths are large as a 2 km; α eff is more than an order of magnitude larger than α ell at depths less than 500 m.