Modeling Adiabatic Boiling in the Biliran Geothermal Wells Using CHIM-XPT (2016)
John Paul A. Mendoza1*, Maria Ines Rosana Balangue-Tarriela1, and Mark H. Reed2
1National Institute of Geological Sciences,
University of the Philippines Diliman, Quezon City 1101 Philippines
2Department of Earth Sciences, University of Oregon, Eugene, OR 97403 USA
Boiling is a common process in geothermal wells where the primary water quickly ascends to the surface and liquid water is converted to steam due to depressurization and cools downs (i.e., no heat exchange with surrounding rocks). An assumption in the present study is that there is no heat exchange between wall rock and boiling water; thus, the process is isenthalpic. This study presents the results of changes in the chemical composition of fluid from a geothermal system as it ascends to the surface together with the description of minerals precipitating out of the solution at certain temperature conditions. The results of the study will contribute significantly to the assessment of scaling potentials in a geothermal field. Using FORTRAN Programs SOLVEQ and CHIM-XPT, adiabatic boiling was simulated for the normal enthalpy wells of Biliran geothermal field. Results of theoretical geothermometry for the wells are consistent with the reported chemical geothermometers. Aside from a steam phase dominated by water vapor and CO2, Well BN-1 formed chlorite, calcite (up to 170 °C) and talc in the initial boiling model. Well BN-2 precipitated mostly talc and calcite almost all throughout its ascent. The occurrence of calcite calculated from the model is consistent with the abundance of calcite scales and veins in BN-1 while BN-2 is dominated by aragonite. Minor differences in the mineralogy of the wells is mainly due to the significant difference in the fluid and gas chemistry amongst wells in the field. The partitioning of CO2 into a gas phase drives the increase in pH for both wells. Both the formation of the gas phase and the fractionated minerals reflect changes occurring in the total concentration of the aqueous phase wherein species fractionated into the gas or solid phase decrease in the total aqueous concentration.
As the hot geothermal fluids buoyantly rise through a fracture or an exploration or production well, the initial aqueous phase boils as a consequence of decreasing pressure. In liquid-dominated fields, this results in the separation of a gas (steam) phase from the liquid phase, which affects the pH of the rising fluids. Non-volatile components (e.g., silica) remain in the liquid phase while volatile components such as CO2, N2, and other reduced gases (H2, CH4, and H2S) enter the steam phase (Henley 1984, Reed 1992). Temperature of the initial liquid phase consequently decreases as fluids ascend because it takes energy to convert liquid water to steam (Reed 1992, Seward 2014). At the same time, vein and scale minerals may precipitate (e.g., calcite and sulfides) and are eventually removed (fractionated) as they form, thus making the system partially open (Reed 1992). The precipitation of these minerals may pose problems (scale and blockage of formation permeability) to steam production; thus, it is important to recognize which particular minerals would form. This study in particular aims to determine the changes in the composition (chemistry, pH, etc.) of a particular well as fluid ascends to the surface, and to predict which minerals would precipitate out of the solution at certain temperature conditions. However, the study is limited to observing normal enthalpy neutral-Cl wells due to more complex corrections needed in reconstructing excess enthalpy wells (Arnorsson et al. 2007). . . . read more
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