The Source of Noncondensible Gas Increases at The Geysers Field, California

摘要

This paper presents a model for mid-life gas increases at The Geysers based upon the reservoir cap being the source of the additional gas. An earlier model for gas increases (Powell and Li, 2003) does not account for the quantity of gas produced by the field. In addition, changes in argon concentration during these increases show that the gas responsible for the increased gas concentrations comes from a source different from local reservoir. Vertical gradients in gas concentration suggest that the top of the reservoir and reservoir cap are the likely sources for the high gas component. A model for the chemistry of the vertical gas gradient proposes that since hydrogen and hydrogen sulfide concentrations are controlled by temperature, their concentration gradients are invariant with depth. Carbon dioxide, methane, nitrogen and ammonia accumulate at the top of the reservoir, with some fraction of ammonia descending with steam condensate due to its relatively high solubility in condensate. Deep steam chemistry rich in hydrogen and hydrogen sulfide, as predicted by this model, is similar to the chemistry of downhole steam samples collected from Geysers well DX-42 in 1999. A mixing model of shallow and deep steam components makes a reasonable match with the gas chemistry of the gas increase in Geysers well GDCF 63A-29, and a calculation of gas contained in the local reservoir and cap can account for the gas produced by the well to date, but just barely. The major unknown in this work remains the amount of gas held in the resevoir cap rock.