Nagasree Garapati Publications Content

Publications

REFEREED PUBLICATIONS IN JOURNALS

4.  N. Garapati, J.B. Randolph, M.O. Saar Brine displacement by CO2, energy extraction rates, and lifespan of a CO2-limited CO2-Plume Geothermal (CPG) system with a horizontal production well, Geothermics, 55, pp. 182-194, 2015. Abstract
Several studies suggest that CO2-based geothermal energy systems may be operated economically when added to ongoing geologic CO2 sequestration. Alternatively, we demonstrate here that CO2-Plume Geothermal (CPG) systems may be operated long-term with a finite amount of CO2. We analyze the performance of such CO2-limited CPG systems as a function of various geologic and operational parameters. We find that the amount of CO2 required increases with reservoir depth, permeability, and well spacing and decreases with larger geothermal gradients. Furthermore, the onset of reservoir heat depletion decreases for increasing geothermal gradients and for both particularly shallow and deep reservoirs.
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3.  A.J. Luhmann, X.-Z. Kong, B.M. Tutolo, N. Garapati, B.C. Bagley, M.O. Saar, W.E. Seyfried Jr. Experimental dissolution of dolomite by CO2-charged brine at 100oC and 150 bar: Evolution of porosity, permeability, and reactive surface area, Chemical Geology, 380, pp. 145-160, 2014. Abstract
Hydrothermal flow experiments of single-pass injection of CO2-charged brine were conducted on nine dolomite cores to examine fluid–rock reactions in dolomite reservoirs under geologic carbon sequestration conditions. Post-experimental X-ray computed tomography (XRCT) analysis illustrates a range of dissolution patterns, and significant increases in core bulk permeability were measured as the dolomite dissolved. Outflow fluids were below dolomite saturation, and cation concentrations decreased with time due to reductions in reactive surface area with reaction progress. To determine changes in reactive surface area, we employ a power-law relationship between reactive surface area and porosity (Luquot and Gouze, 2009). The exponent in this relationship is interpreted to be a geometrical parameter that controls the degree of surface area change per change in core porosity. Combined with XRCT reconstructions of dissolution patterns, we demonstrate that this exponent is inversely related to both the flow path diameter and tortuosity of the dissolution channel. Even though XRCT reconstructions illustrate dissolution at selected regions within each core, relatively high Ba and Mn recoveries in fluid samples suggest that dissolution occurred along the core’s entire length and width. Analysis of porosity–permeability data indicates an increase in the rate of permeability enhancement per increase in porosity with reaction progress as dissolution channels lengthen along the core. Finally, we incorporate the surface area–porosity model of Luquot and Gouze (2009) with our experimentally fit parameters into TOUGHREACT to simulate experimental observations.
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2.  N. Garapati, J.B. Randolph, J.L. Valencia Jr., M.O. Saar CO2-Plume Geothermal (CPG) Heat Extraction in Multi-layered Geologic Reservoirs, Energy Procedia, 63, pp. 7631-7643, 2014. Abstract
CO2-Plume Geothermal (CPG) technology involves injecting CO2 into natural, highly permeable geologic units to extract energy. The subsurface CO2 absorbs heat from the reservoir, buoyantly rises to the surface, and drives a power generation system. The CO2 is then cooled and reinjected underground. Here, we analyze the effects of multi-layered geologic reservoirs on CPG system performance by examining the CO2 mass fraction in the produced fluid, pore-fluid pressure buildup during operation, and heat energy extraction rates. The produced CO2 mass fraction depends on the stratigraphic positions of highly permeable layers which also affect the pore-fluid pressure drop across the reservoir.
/ Download
1.  N. Garapati, B.J. Anderson Statistical Thermodynamics Model and Empirical Correlations for Predicting Mixed Hydrate Phase Equilibria, Fluid Phase Equilibria, 373, pp. 20-28, 2014. Abstract
Natural gas hydrate deposits contain CH4 along with other hydrocarbon gases like C2H6, C3H8 and non-hydrocarbon gases like CO2 and H2S. If CH4 stored in natural gas hydrates can be recovered, the hydrates would potentially become a cleaner energy resource for the future producing less CO2 when combusted than does coal. The production of CH4 from natural gas hydrate reservoirs has been predicted by reservoir simulators that implement phase equilibrium data in order to predict various production scenarios. In this paper two methods are discussed for calculating the phase equilibria of mixed hydrates. In the first method, the phase equilibrium is predicted using a ‘cell potential’ code, which is based on van der Waals and Platteeuw statistical mechanics, along with variable reference parameters to account for lattice distortion, and with temperature-dependent Langmuir constants proposed by Bazant and Trout. The method is validated by reproducing the existing phase equilibrium data of simple and mixed hydrates and the structural transitions that are known to occur, without the use of any fitting parameters. A computationally-simple method is to use empirical correlations of gas hydrate dissociation pressure with respect to temperature and gas-phase composition as they are easy to implement into the simulators. The parameters for the empirical expression were determined for the CH4–C2H6 mixed hydrate system by non-linear regression analysis of available experimental data and data obtained from the first method.
/ Download

PROCEEDINGS REFEREED

4.  N. Garapati, J. Randolph, S. Finsterle, M.O. Saar Simulating Reinjection of Produced Fluids Into the Reservoir, Proceedings of 41st Workshop on Geothermal Reservoir Engineering, 2016. no_download
3.  N. Garapati, J.B. Randolph, J.L. Valencia Jr., M.O. Saar Design of CO2-Plume Geothermal (CPG) subsurface system for various geologic parameters, Proceedings of the Fifth International Conference on Coupled Thermo-Hydro-Mechanical-Chemical (THMC) Processes in Geosystems: Petroleum and Geothermal Reservoir Geomechanics and Energy Resource Extraction, 2015. no_download
2.  N. Garapati, J.B. Randolph, M.O. Saar Superheating Low-Temperature Geothermal Resources to Boost Electricity Production, Proceedings of the 40th Workshop on Geothermal Reservoir Engineering 2015, 2, pp. 1210-1221, 2015. no_download
1.  M.O. Saar, Th. Buscheck, P. Jenny, N. Garapati, J.B. Randolph, D. Karvounis, M. Chen, Y. Sun, J.M. Bielicki Numerical Study of Multi-Fluid and Multi-Level Geothermal System Performance, Proceedings World Geothermal Congress 2015, 2015. no_download

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REFEREED PUBLICATIONS IN JOURNALS

4.  N. Garapati, J.B. Randolph, M.O. Saar Brine displacement by CO2, energy extraction rates, and lifespan of a CO2-limited CO2-Plume Geothermal (CPG) system with a horizontal production well, Geothermics, 55, pp. 182-194, 2015. Abstract
Several studies suggest that CO2-based geothermal energy systems may be operated economically when added to ongoing geologic CO2 sequestration. Alternatively, we demonstrate here that CO2-Plume Geothermal (CPG) systems may be operated long-term with a finite amount of CO2. We analyze the performance of such CO2-limited CPG systems as a function of various geologic and operational parameters. We find that the amount of CO2 required increases with reservoir depth, permeability, and well spacing and decreases with larger geothermal gradients. Furthermore, the onset of reservoir heat depletion decreases for increasing geothermal gradients and for both particularly shallow and deep reservoirs.
/ Download
3.  A.J. Luhmann, X.-Z. Kong, B.M. Tutolo, N. Garapati, B.C. Bagley, M.O. Saar, W.E. Seyfried Jr. Experimental dissolution of dolomite by CO2-charged brine at 100oC and 150 bar: Evolution of porosity, permeability, and reactive surface area, Chemical Geology, 380, pp. 145-160, 2014. Abstract
Hydrothermal flow experiments of single-pass injection of CO2-charged brine were conducted on nine dolomite cores to examine fluid–rock reactions in dolomite reservoirs under geologic carbon sequestration conditions. Post-experimental X-ray computed tomography (XRCT) analysis illustrates a range of dissolution patterns, and significant increases in core bulk permeability were measured as the dolomite dissolved. Outflow fluids were below dolomite saturation, and cation concentrations decreased with time due to reductions in reactive surface area with reaction progress. To determine changes in reactive surface area, we employ a power-law relationship between reactive surface area and porosity (Luquot and Gouze, 2009). The exponent in this relationship is interpreted to be a geometrical parameter that controls the degree of surface area change per change in core porosity. Combined with XRCT reconstructions of dissolution patterns, we demonstrate that this exponent is inversely related to both the flow path diameter and tortuosity of the dissolution channel. Even though XRCT reconstructions illustrate dissolution at selected regions within each core, relatively high Ba and Mn recoveries in fluid samples suggest that dissolution occurred along the core’s entire length and width. Analysis of porosity–permeability data indicates an increase in the rate of permeability enhancement per increase in porosity with reaction progress as dissolution channels lengthen along the core. Finally, we incorporate the surface area–porosity model of Luquot and Gouze (2009) with our experimentally fit parameters into TOUGHREACT to simulate experimental observations.
/ Download
2.  N. Garapati, J.B. Randolph, J.L. Valencia Jr., M.O. Saar CO2-Plume Geothermal (CPG) Heat Extraction in Multi-layered Geologic Reservoirs, Energy Procedia, 63, pp. 7631-7643, 2014. Abstract
CO2-Plume Geothermal (CPG) technology involves injecting CO2 into natural, highly permeable geologic units to extract energy. The subsurface CO2 absorbs heat from the reservoir, buoyantly rises to the surface, and drives a power generation system. The CO2 is then cooled and reinjected underground. Here, we analyze the effects of multi-layered geologic reservoirs on CPG system performance by examining the CO2 mass fraction in the produced fluid, pore-fluid pressure buildup during operation, and heat energy extraction rates. The produced CO2 mass fraction depends on the stratigraphic positions of highly permeable layers which also affect the pore-fluid pressure drop across the reservoir.
/ Download
1.  N. Garapati, B.J. Anderson Statistical Thermodynamics Model and Empirical Correlations for Predicting Mixed Hydrate Phase Equilibria, Fluid Phase Equilibria, 373, pp. 20-28, 2014. Abstract
Natural gas hydrate deposits contain CH4 along with other hydrocarbon gases like C2H6, C3H8 and non-hydrocarbon gases like CO2 and H2S. If CH4 stored in natural gas hydrates can be recovered, the hydrates would potentially become a cleaner energy resource for the future producing less CO2 when combusted than does coal. The production of CH4 from natural gas hydrate reservoirs has been predicted by reservoir simulators that implement phase equilibrium data in order to predict various production scenarios. In this paper two methods are discussed for calculating the phase equilibria of mixed hydrates. In the first method, the phase equilibrium is predicted using a ‘cell potential’ code, which is based on van der Waals and Platteeuw statistical mechanics, along with variable reference parameters to account for lattice distortion, and with temperature-dependent Langmuir constants proposed by Bazant and Trout. The method is validated by reproducing the existing phase equilibrium data of simple and mixed hydrates and the structural transitions that are known to occur, without the use of any fitting parameters. A computationally-simple method is to use empirical correlations of gas hydrate dissociation pressure with respect to temperature and gas-phase composition as they are easy to implement into the simulators. The parameters for the empirical expression were determined for the CH4–C2H6 mixed hydrate system by non-linear regression analysis of available experimental data and data obtained from the first method.
/ Download

PROCEEDINGS REFEREED

4.  N. Garapati, J. Randolph, S. Finsterle, M.O. Saar Simulating Reinjection of Produced Fluids Into the Reservoir, Proceedings of 41st Workshop on Geothermal Reservoir Engineering, 2016. no_download
3.  N. Garapati, J.B. Randolph, J.L. Valencia Jr., M.O. Saar Design of CO2-Plume Geothermal (CPG) subsurface system for various geologic parameters, Proceedings of the Fifth International Conference on Coupled Thermo-Hydro-Mechanical-Chemical (THMC) Processes in Geosystems: Petroleum and Geothermal Reservoir Geomechanics and Energy Resource Extraction, 2015. no_download
2.  N. Garapati, J.B. Randolph, M.O. Saar Superheating Low-Temperature Geothermal Resources to Boost Electricity Production, Proceedings of the 40th Workshop on Geothermal Reservoir Engineering 2015, 2, pp. 1210-1221, 2015. no_download
1.  M.O. Saar, Th. Buscheck, P. Jenny, N. Garapati, J.B. Randolph, D. Karvounis, M. Chen, Y. Sun, J.M. Bielicki Numerical Study of Multi-Fluid and Multi-Level Geothermal System Performance, Proceedings World Geothermal Congress 2015, 2015. no_download