Abstract:A coupled thermo-mechanical model has been developed to assess permeability changes in the vicinity of an underground coal gasification (UCG) reactor resulting from excavation and thermo-mechanical effects. Thereto, we consider a stepwise UCG reactor excavation based on a pre-defined coal consumption rate and dynamic thermal boundary conditions. Simulation results demonstrate that thermo-mechanical rock behavior is mainly driven by the thermal expansion coefficient, thermal conductivity, tensile strength and elastic modulus of the surrounding rock. A comparison between temperature-dependent and temperature-independent parameters applied in the simulations indicates notable variations in the distribution of total displacements in the UCG reactor vicinity related to thermal stress, but only negligible differences in permeability changes. Hence, temperature-dependent thermo-mechanical parameters have to be considered in the assessment of near-field UCG impacts only, while far-field models can achieve a higher computational efficiency by using temperature-independent thermo-mechanical parameters. Considering the findings of the present study in the large-scale assessment of potential environmental impacts of underground coal gasification, representative coupled simulations based on complex 3D large-scale models become computationally feasible.

Underground coal gasification (UCG) has the potential to increase worldwide coal reserves by utilization of coal deposits not mineable by conventional methods. This involves combusting coal in situ to produce a synthesis gas, applicable for electricity generation and chemical feedstock production. Three-dimensional (3D) thermo-mechanical models already significantly contribute to UCG design by process optimization and mitigation of the environmental footprint. We developed the first 3D UCG model based on real structural geological data to investigate the impacts of using isothermal and non-isothermal simulations, two different pillar widths and four varying regional stress regimes on the spatial changes in temperature and permeability, ground surface subsidence and fault reactivation. Our simulation results demonstrate that non-isothermal processes have to be considered in these assessments due to thermally-induced stresses. Furthermore, we demonstrate that permeability increase is limited to the close reactor vicinity, although the presence of previously undetected faults can introduce formation of hydraulic short circuits between single UCG channels over large distances. This requires particular consideration of potentially present sub-seismic faults in the exploration and site selection stages, since the required pillar widths may be easily underestimated in presence of faults with different orientations with respect to the regional stress regime.

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Harrar, J.E., Lawrence Livermore Natl. Laboratory Locke, F.E., Lawrence Livermore Natl. Laboratory Otto Jr., C.H., Lawrence Livermore Natl. Laboratory Lorensen, L.E., Lawrence Livermore Natl. Laboratory Monaco, S.B., Lawrence Livermore Natl. Laboratory Frey, W.P., Lawrence Livermore Natl. Laboratory Abstract A pilot-size brine handling system was operated from Magmamax Well 1 in southern California to study the characteristics of siliceous scale deposition and to evaluate the possibility of treating the brine with chemical additives to control scaling. The rates of formation, chemical constitution, and morphology of the scales were examined as functions of temperature, brine salinity, substrate material, and antiscalant additive activity. Potential antiscalant compounds were screened using a silica-precipitation inhibition test at 90 deg. C. The most active classes of compounds were those containing polymeric chains of oxyethylene and polymeric nitrogen compounds that are cationic in character. The best single compound was Corcat P-18 TM (Cordova Chemical Co. polyethylene imine, molecular weight 1,800). This compound had no effect on the scale formed at 220 deg. C but it reduced the rates of scaling at 125 and 90 deg. C by factors of 4 and 18, respectively, and it also functioned as a corrosion inhibitor. The best additive formulation for the brines of the Salton Sea Geothermal field (SSGF) appears to be a mixture of an organic silica-precipitation inhibitor, a small amount of hydrochloric acid, and a phosphonate crystalline deposit inhibitor. Introduction Interest in utilizing the geothermal resources of the Imperial Valley in California for the generation of electricity has accelerated rapidly in recent years. One resource in particular, the SSGF, is attractive because of its high temperature and size. Recent estimates of its potential for electrical power generation range between 1,300 and 8,700 MW per year (over a 20-year period). The fluid of this resource, however, is a highly corrosive, high-salinity brine containing several constituents that form deposits of scale on power plant components as the brine is cooled. Economical utilization of the SSGF will require techniques for limiting scaling and corrosion to acceptable levels. Scale deposition control at SSGF is particularly difficult because the scale that forms in the portions of the brine handling equipment operating at low pressures and temperatures (100 to 150 deg. C) is predominantly silica and it deposits at rates approaching 0.2 in./D. (Energy extraction systems in which the brine is flashed and injected at high temperature mitigate this problem, but considerable energy is discarded.) Chemical treatment scheme to retard the low temperature scale have been considered, but until recently there have been no systematic investigations of this approach. In 1976, Owen and coworkers demonstrated effective control of the siliceous scales by acidification of the brine with hydrochloric acid, and this technique has been verified in New Zealand by Rothbaum et al. However, for SSGF brines, acidification has several disadvantages:because concentrations >300 ppm of HCl are required, chemical costs are high;the pH of the brine must be lowered from 6 to 3 for complete scale control, and this sharply increases corrosion rates, andacidification tends to interfere with effluent brine treatment Processes involving sludge-bed reactor clarification. Other methods of scale control such as seeding with a silica sludge and the use of scale adhesion inhibitors also have been examined briefly. In this paper we present the results of tests of organic chemical agents for silica scale control in hypersaline geothermal brines. Prior to this work, virtually no knowledge existed on the types of compounds that would interact with silica under the severe geothermal conditions of high temperature, high ionic strength, and high fluid shear rates. Accordingly, to screen a large number of substances rather rapidly, we designed a small-scale flash system as a brine treatment test apparatus and operated it from SSGF Magmamax Well 1 and Woolsey Well 1. SPEJ P. 17^