A Depletion Mechanism for the Behavior of Noncondensible Gases at The Geysers

Vapor-dominated geothermal systems, including The Geysers and Larderello, have experienced significant increases in noncondensible gas during their productive histories. A depletion model has been developed to explain these increases. In the model, the low permeability of the reservoir rock matrix initially retards the flow of vapor through the matrix to fractures due to the plugging effect of capillary water. Steam produced early in the life of the field is predominantly from evaporated pore water near fractures and has a low gas concentration. Declining pore pressure in the rock matrix induces boiling of reservoir liquid, resulting in the formation of gas-rich vapor bubbles in larger pores. Gas concentrations of produced steam increase when vaporization of the capillary water in the fine permeable channels of the rock matrix opens pathways for the trapped gas-rich vapor to escape to the reservoir fracture network. Calculations show that steam-water capillary pressures of the matrix at reservoir conditions were great enough to block the flow of the vapor. The Geysers fluids can be modeled as a mixture of vaporized reservoir liquid and gas-rich vapor, in a manner similar to the Y model used in gas geothermometry. Gas dissolved in initial reservoir liquid is derived largely from organic material in the greywacke reservoir rock and reactions with reservoir minerals. Although important reservoir parameters, such as steam-water relative permeability, need to be determined at reservoir conditions, this model holds promise that the noncondensible gas evolution of The Geysers, Larderello and other vapor fields might be quantified and predicted. Introduction Noncondensible gas concentrations in geothermal fields often show disconcerting increases as fields mature. The gas increases have been observed both at Larderello, Italy and at The Geysers, California. Figure 1 shows the history of gas change for Larderello well ALR. Note that the gas/steam ratio in the well was low during initial production, then rose as the steam flow rate declined. The gas/steam ratio peaked at approximately four times the initial concentration at about the time that the steam flow rate decline stabilized at a low rate and the volatile chloride (here measured as HCl in steam condensate) first appeared in steam. The appearance of volatile chloride in steam has been interpreted to indicate severe dry-out of the steam reservoir (Truesdell et al., 1989). These gas increases are not welcome developments. Minor increases above power plant design specifications reduce plant efficiency and increase steam usage. Further increases require retrofit of condenser gas extraction and gas abatement equipment. Gas increases also present a problem for geothermal projects in the emerging greenhouse gas credit market. How does a geothermal developer sell credits for a project when the long-term gas emissions can not be reliably predicted? To date, there has been little scientific consensus on the source of gas in the reservoir and the cause of gas increases. The most commonly held assumption is that gases are stored with water reserves in pore spaces of the reservoir rock, and are produced with the steam as the reservoir is depleted. For example, D’Amore and Truesdell (1985) assumed this process in calculating reservoir liquid saturation using gas geothermometry. Gas increases in the central, low-pressure production areas of the field are attributed to steam migration from gassy field margins (e.g. Truesdell et al., 1993).