A Kinetic Model of CO2 Generation and Mineral and Isotopic Alteration During Steamflooding

Abstract

Summary CO2 is commonly produced during steamflooding. In higher-temperature steamfloods the amount of CO2 produced is such that the only viable source is carbonate minerals. We show by reference to CO2 concentrations in natural geothermal systems that in most common rocks and sediments, CO2 is generated naturally under hydrothermal (~572°F) conditions by the dissolution of carbonate minerals. CO2 solubility is a strong positive function of temperature and, at 572°F, is more than sufficient to account for high CO2 concentrations like those encountered at Chevron's Buena Vista (BV) Hills steamflood pilot test. A linear kinetic model is developed to describe chemical equilibration and CO2 generation in the hot parts of the reservoir. The model also describes CO2 removal through carbonate precipitation in the cooler parts of the rock formation where the fluid moves ahead of the thermal front. Changes in carbon isotope concentration are included in the model. The small difference between the 13C in the produced gas and the source carbonates at BV Hills suggests that little CO2 is precipitated as carbonate within that reservoir, a prediction that can be tested by poststeamflood coring.