Title:

Models of Subsurface Rock-Water Processes Affecting Fluid Flow

Authors:

Nancy Moller and John H. Weare

Key Words:

Enhanced Geothermal Systems (EGS), hydrothermal chemistry, thermodynamics, aluminosilicate minerals, chemical models

Conference:

Stanford Geothermal Workshop

Year:

2005

Session:

HDR/EGS

Language:

English

Paper Number:

Moller

File Size:

160KB

View File:

Abstract:

Successful enhancement of fluid flow in high temperature, low permeability reservoirs (enhanced geothermal systems, EGS) would significantly advance geothermal energy as an economically competitive contributor to the nation's energy supply. However, progress in this approach is limited by the insufficient knowledge about the subsurface chemical processes controlling fluid flow in high temperature geothermal formations.

The objective of our EGS research program is to provide new modeling technologies that accurately characterize and improve the understanding of EGS chemistry. In this program we will o develop highly accurate chemical models of hydrothermal fluids and rock-forming minerals for XTP ranges of interest to EGS; o construct models capable of predicting the effect of subsurface processes such as mixing, dissolution, precipitation and mineral alteration on reservoir rock permeability; o construct models that assess and correctly summarize large amounts of mineral solubility and activity data; o transfer these models via our web site, geotherm.ucsd.edu.

In this presentation, we will review the present state of our GEOFLUIDS and TEQUIL models. The GEOFLUIDS model predicts densities, compositions and phase relations in the system H2O-NaCl-CO2-CH4 for supercritical temperatures and high pressures. TEQUIL predicts solid-liquid-gas equilibria within the H, Na, K, Ca, OH, Cl, SO4, CO3, SiO2, H2O, CO2 system below 250?C. We will discuss our recently initiated EGS program in which the TEQUIL model will be generalized to include the highly important aluminum solution thermodynamics and aluminosilicate minerals.

To construct models that treat properties in composition and temperature regions of interest to the EGS program, high concentration aqueous solutions (I ? 15 m) and temperatures to 250oC (and to 350oC for more limited compositions) will be included in this generalization. Results of initial modeling efforts will be presented describing the hydrolysis of aluminum ions and the solubility of the minerals, gibbsite and boehmite (used to develop the model), as a function of solution concentration and pH.


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