Correlations between steam saturation, fluid composition and well decline in vapor-dominated reservoirs

A large body of field data from Larderello and other vapor-dominated geothermal reservoirs shows striking temporal correlations between (1) decline of well flow rate, (2) gas/steam ratio, (3) chloride concentration, (4) degree of superheat and (5) vapor fraction. The latter is inferred from concentrations of non-condensible gases in samples of well fluid, using chemical phase equilibrium principles. Observed temporal changes in the vapor fraction can be interpreted in terms of a “multiple source” model, as suggested by D'Amore and Truesdell (1979, Models for steam chemistry at Larderello and The Geysers. Proc. 5th Workshop Geothermal Reserv. Engng, Stamford, California, pp. 283–297). This provides clues to the dynamics of reservoir depletion and to the evaluation of well productivity and longevity.     

On production behavior of enhanced geothermal systems with CO2 as working fluid

Numerical simulation is used to evaluate the mass flow and heat extraction rates from enhanced geothermal injection–production systems that are operated using either CO₂ or water as heat transmission fluid. For a model system patterned after the European hot dry rock experiment at Soultz, we find significantly greater heat extraction rates for CO₂ as compared to water. The strong dependence of CO₂ mobility (=density/viscosity) upon temperature and pressure may lead to unusual production behavior, where heat extraction rates can actually increase for a time, even as the reservoir is subject to thermal depletion. We present the first ever, three-dimensional simulations of CO₂ injection–production systems. These show strong effects of gravity on the mass flow and heat extraction due to the large contrast of CO₂ density between cold injection and hot production conditions. The tendency for preferential flow of cold, dense CO₂ along the reservoir bottom can lead to premature thermal breakthrough. The problem can be avoided by producing from only a limited depth interval at the top of the reservoir.     

State of the art of geothermal reservoir simulation

Computer modeling of geothermal systems has become a mature technology with application to more than 100 fields world-wide. Large complex three-dimensional models having computational meshes with more than 4000 blocks are now used routinely. Researchers continue to carry out fundamental research on modeling techniques and physical processes in geothermal systems. The new advances are adopted quickly by the geothermal industry and have also found application in related areas such as nuclear waste storage, environmental remediation and studies of the vadose (unsaturated) zone. The current state-of-practice, recent advances and emerging trends in geothermal reservoir simulation are reviewed.     

Reinjection studies of vapor-dominated systems

Reinjection of produced brines is currently being considered for Larderello and other vapor-dominated geothermal fields as a potential means for safe disposal and enhanced energy recovery. In this context is it necessary to develop a detailed assessment of the impact of injection on reservoir performance, in particular on power output and reservoir longevity.As a step towards such an assessment, the present work explores the effects of cold water injection into idealized model reservoirs using numerical simulation techniques. The rock matrix parameters and thermodynamic conditions employed are representative of the Larderello steam fields.One-dimensional radial flow near an injection well is modeled to study in detail the propagation of hydrodynamic and temperature fronts. Simulated results are subjected to single-phase pressure transient analysis to examine the applicability of this technique for determination of formation parameters.Gravity effects for injection into a thick two-phase reservoir are studied within a vertical two-dimensional mesh. Comparisons are made between shallow and deep injection.Mixed production/injection schemes are investigated for a five-spot geometry using an areal two-dimensional mesh. It is found that production pressures and power output change little due to injection, whereas longevity of the field can be substantially increased.A one-dimensional vertical column representing a cross-section of Larderello is used to study the effects of injection at different depths in the more depleted zones of this reservoir.     

An algorithm for computing smoothing splines in tension

A characterization of smoothing splines is derived which leads to procedures using either locally defined bases or small support bases. Difficulties in trying to compute the latter for splines in tension are discussed. A smoothing algorithm which avoids these difficulties by using locally defined bases is presented.     

Generating orally active galanin analogues with analgesic activities

The endogenous neuropeptide galanin has anticonvulsant and analgesic properties mediated by galanin receptors expressed in the central and peripheral nervous systems. Our previous work showed that by combining truncation of the galanin peptide with N- and C-terminal modifications afforded analogues that suppress seizures or pain upon intraperitoneal (i.p.) administration. To generate orally active galanin analogues, the previously reported lead compound Gal-B2 (NAX 5055) was redesigned by 1) central truncation, (2) introduction of D-amino acids, and 3) addition of backbone spacers. Analogue D-Gal(7-Ahp)-B2, containing 7-aminoheptanoic acid as a backbone spacer and an oligo-D-lysine motif at the C terminus, exhibits anticonvulsant and analgesic activity post-i.p. administration. Oral administration of D-Gal(7-Ahp)-B2 demonstrates analgesic activity with decreases in both acute and inflammatory pain in the mouse formalin model of pain at doses as low as 8 mg kg(-1) .     

Enhanced geothermal systems (EGS) using CO2 as working fluid—A novel approach for generating renewable energy with simultaneous sequestration of carbon

Responding to the need to reduce atmospheric emissions of carbon dioxide, Brown [Brown, D., 2000. A Hot Dry Rock geothermal energy concept utilizing supercritical CO₂ instead of water. In: Proceedings of the Twenty-Fifth Workshop on Geothermal Reservoir Engineering, Stanford University, pp. 233–238] proposed a novel enhanced geothermal systems (EGS) concept that would use carbon dioxide (CO₂) instead of water as heat transmission fluid, and would achieve geologic sequestration of CO₂ as an ancillary benefit. Following up on his suggestion, we have evaluated thermophysical properties and performed numerical simulations to explore the fluid dynamics and heat transfer issues in an engineered geothermal reservoir that would be operated with CO₂. We find that CO₂ is superior to water in its ability to mine heat from hot fractured rock. Carbon dioxide also offers certain advantages with respect to wellbore hydraulics, in that its larger compressibility and expansivity as compared to water would increase buoyancy forces and would reduce the parasitic power consumption of the fluid circulation system. While the thermal and hydraulic aspects of a CO₂-EGS system look promising, major uncertainties remain with regard to chemical interactions between fluids and rocks. An EGS system running on CO₂ has sufficiently attractive features to warrant further investigation.     

T2SOLV: an enhanced package of solvers for the TOUGH2 family of reservoir simulation codes

T2SOLV is an enhanced package of matrix solvers for the TOUGH2 family of codes. T2SOLV includes all the Preconditioned Conjugate Gradient (PCG) solvers used in T2CG1, the current solver package, as well as LUBAND, a new direct solver and DLUSTB, a PCG solver based which implements the BiCGSTAB(m) method. LUBAND, the new direct solver, is faster, more reliable and capable of solving problems orders of magnitude larger than the MA28 routine which it replaces. A significant capability of T2VOC is two types of matrix preprocessing, which make possible the solution of a class of challenging numerical problems (previously tractable only with direct solvers) using the PCG routines. For regular grids, the size of problems tractable with direct solvers is doubled by implementing a D4 alternative diagonal gridblock ordering option.     

Analysis of the stanford geothermal reservoir model experiments using the LBL reservoir simulator

This paper describes the results of an analysis of data obtained from a series of heat-sweep experiments performed in the Stanford Geothermal Reservoir Model using the Lawrence Berkeley Laboratory reservoir simulator. The physical reservoir model is an experimental system consisting of a pressure vessel which contains a granite rock matrix with production and recharge capabilities to simulate the heat-sweep process in a fractured hydrothermal reservoir under liquid-phase conditions.Arrangements were made with the Lawrence Berkeley Laboratory to test their geothermal reservoir simulator on the physical model data. The objectives were to provide insight into the detailed physical processes occurring in the relatively complex physical system and to provide feedback to LBL on the capability and possible improvements to the LBL reservoir simulator to model a complex physical system.The overall conclusion of this work is that the LBL simulator does an excellent job of predicting the physical processes in the Stanford Geothermal Reservoir Model experiments for extreme thermal gradient conditions and for a system with very complex boundary conditions. The analysis demonstrates the importance of specifying relevant parameters accurately to provide adequate modeling for the important physical processes.     

An analytical solution for heat transfer at a boiling front moving through a porous medium

A problem of current interest in geothermal energy extraction is the injection of cold water into a porous medium containing superheated vapor. Such injection will cause a boiling front to move away from the injection point. When flow is approximated as being one-dimensional radial, it can be shown from similarity variable concepts that temperature, pressure, and boiling rate at the front are constant, independent of time. From heat and mass balance considerations an analytical solution is obtained for front temperature and evaporation rate. Comparison with detailed numerical simulations of the injection process shows excellent agreement.