PORE PRESSURE DURING METAMORPHISM OF CARBONATE ROCK: EFFECT OF VOLUMETRIC PROPERTIES OF H2O-CO2 MIXTURES

Abstract

The molar volume of mixtures of CO2 and H2O is a strong function of the fluid composition. Both CO2 and H2O participate in the metamorphism of carbonate rocks, resulting in a change in the fluid composition during reaction. One of the effects of the change in composition is the increase in pore-fluid pressure with only small increases in extent of reaction, ξ. Pressure calculated from the volumetric properties of CO2–H2O mixtures at 400 °C increases greatly with small increases of ξ but drops at greater values because of the increase in pore volume as a result of ΔV solid. The pore pressure increase at small values of ξ, though, readily exceeds the reported tensile strength of carbonate rocks, and the rock cannot sustain significant reaction without fracturing. The result of a small amount of reaction is a fractured rock with increased permeability, which promotes fluid transport.