Pre- and Post-Development Influences on Fieldwide Geysers NCG Concentrations

Abstract

Pre-development Geysers noncondensible gas (NCG) concentrations map as a low NCG trough adjacent to the Big Sulphur Creek Fault, which, in the southern part of the steamfield, is the southwest margin of the reservoir. Along its entire southwest margin, which includes the Mercuryville Fault in the northern part of the steamfield, the reservoir truncates abruptly, with a rapid decrease in heat flow to the southwest of it. The low NCG trough coincides closely with the shallowest, (and highest elevation) steam and the axis and peak of the 1.1-1.2 Ma intrusive (the “felsite”) that intruded the Jurassic age (Franciscan) sedimentary rocks which form the bulk of the reservoir. We consider these features to be genetically related. It is generally agreed that The Geysers evolved from an early, liquid dominated reservoir to the current vapor dominated reservoir. This evolution involved a massive groundwater invasion of the reservoir from southeast to northwest, followed by surface venting and a boildown to form the vapor dominated reservoir. We propose that the coincidence in the southeast Geysers of the lowest initial NCG concentrations with the structurally highest felsite and steam makes this the likely entry point for groundwater invasion/displacement of the original reservoir liquid. Higher initial NCG concentrations concentrically outward from the trough (except to the southwest) reflect lesser degrees of flushing and venting. Post-development, NCG concentrations are primarily determined by two opposing processes. Absent injection, NCG in the steam reservoir increases in concentration as the underpressured reservoir (relative to hydrostatic) draws a flux of NCG from lateral and/or deeper metamorphic or magmatic sources. Injection-derived steam is very low in NCG and tends to dilute or displace insitu steam. In the Units 13/16 area of the southeast Geysers, NCG concentrations, which had been rising steadily, were initially lowered as a result of massive injection of reclaimed water. After a six year hiatus, however, NCG concentrations are again rising, suggesting that boiling efficiency of injectate has diminished. Lower injection rates have been imposed in this area to regain a better balance between heat loss from boiling and heat replenishment through heat flow. Massive injection of reclaimed water began in the north Geysers in late 2003. NCG concentrations have been lowered by 25% and are currently level. In the north Geysers a high temperature reservoir (HTR; up to 350°C) is known to underlie the “normal” (240°C) reservoir. While injection-induced microearthquakes (MEQs) are common throughout The Geysers, deep (>2.7 km) MEQs are far more abundant in the north Geysers. The occurrence within the HTR of abundant MEQs may indicate deep permeability and optimal conditions for boiling injectate to mine heat while enhancing reservoir pressure and suppressing NCG.