Fluid Flow Modeling using Geochemistry to Characterize the Songwe Medium Temperature Geothermal System - Tanzania

<p>A geothermal area with only bicarbonate thermal water discharges at medium temperature requires a more integrated analysis than used in classical geochemical exploration. This signature is typical for steam-heated water, which commonly occurs at the margins of a geothermal system. However, these waters can also rise from carbonate rich layers in the central part of the field. Our study shows that fluid flow modeling can identify the exact source, flow pathways and temperatures of reservoir fluids based on water-rock interaction. For the first time, we present a conceptual geothermal fluid flow model based on geochemical data for the Songwe geothermal system in Tanzania.</p><p>Thermal springs discharge along NW-SE fracture zones in two separate areas: the central Songwe graben (Iyola, Main springs, Rambo and Kaguri) and eastern Songwe graben (Ikumbi). The discharge temperatures of springs range between 37 and 85 <sup>o</sup>C with Na-HCO<sub>3</sub> type, and carbonate deposits surrounding most of the springs. We estimated fluid temperatures for a depth of 2.5km by applying K-Mg and Na-K-Ca <sub>(Mg correction) </sub>geothermometers, suggesting that reservoir fluids reach temperatures between 125 and 148 <sup>o</sup>C. We reconstructed reservoir fluid characteristics for that temperatures and propose oversaturated minerals (volcanics, clays, carbonates, apatites, weathered metamophics and hydrothermal minerals) as a model result of interaction between the deep fluids and certain lithologies. Comparison between the modeled oversaturated minerals with minerals in the springs (calcite, aragonite, analcime, muscovite, and smectite) suggests that Kaguri spring water is a result of interaction between deep reservoir fluids with all lithologies, passed on the way to the surface (Metamorphics, Karoo group and Red Sandstone). The fluid signature of Kaguri springs suggest an upflow zone of the geothermal system. Further, our model with oversaturated minerals shows that the thermal water from the reservoir flows laterally along the Red Sandstone layer to the eastern part of study area. It appears as Rambo springs, south of Kaguri springs, and as Main springs and Iyola to the west. The outflow zone might be continuing towards Ikumbi springs, where the fluids also interact with volcanic units. The proposed model shows that carbonate dissolution from the Red sandstone layer is the most common water-rock interaction. The carbonate is embedded in pores and fractures and occurs as matrix in the sandstone. The water-rock interaction is dominated by HCO<sub>3</sub><sup>-</sup> and Na and seen in carbonate depositions at all springs.</p>

Heat Transfer and Fluid Flow Modeling for Supercritical Fluids in Advanced Energy Systems

This chapter aims to model the supercritical fluids thermal hydraulics behaviors including heat transfer, pressure drops, and flow instabilities for the purpose of accurate design and efficient safe operation of advanced energy systems. At first, the convection heat transfer models considering the effect of nonlinear properties and the effect of buoyancy and acceleration have been provided and discussed. Secondly, the hydraulic resistance models for supercritical fluids have been selected and suggested for different conditions. Thirdly, the published models for supercritical flow instabilities based on four different regional partitions are summarized and clarified. At last, two typical case studies have been provided to further intuitively elaborate the thermal hydraulics of supercritical fluids within the advanced energy systems.

VISCOUS FLUID FLOW MODELING WITH THE LATTICE BOLTZMANN METHOD ON GRAPHICS PROCESSORS USING WebGL API

This work is dedicated to the modeling methodology of a viscous fluid flows with the lattice Boltzmann method on graphic processors based on the technology of images rendering in web browsers WebGL. A two-dimensional nine-velocity LBM model (D2Q9) with a collision integral in a Bhatnagar-Gross-Kruk approximation form is shown. The possibilities of calculation acceleration using WebGL technology is described, namely features of using textures to contain values of some physical quantities in numerical algorithms and using fremebuffers to storage the textures, influence of the texture parameters on the numerical algorithms, features of shaders programming. The questions of shader programs using for carrying out stages of physical modeling were considered. The proposed methodology was used to develop an original web program for modeling of classical test problems. Simulations of the Poiseuille flow in a plane channel and the flow around a circular cylinder in a plane channel were performed. The obtained results were compared with the results of calculations performed in the original verified modeling program based on the lattice Boltzmann method and in the Comsol Multiphysics package with the finite element method. Comparisons of the values of the velocity magnitude showed the consistency of the obtained results with the data of other numerical experiments. The analysis of computational speed in comparison with modeling using the optimized algorithm of a method with use of the technology of parallel calculations on CPU OpenMP in the original program is carried out. It is shown that the acceleration of calculations depends on the number of cells of the calculation grid. The results of the fluid flow modeling around a circular cylinder at Re = 1000 are demonstrated, which are obtained 30 times faster than with the calculations obtained with optimized lattice Boltzmann method and OpenMP technology.

Analogy between Thermodynamic Phase Transitions and Creeping Flows in Rectangular Cavities

An analogy is found between the streamline function corresponding to Stokes flows in rectangular cavities and the thermodynamics of phase transitions and critical points. In a rectangular cavity flow, with no-slip boundary conditions at the walls, the corners are fixed points. The corners defined by a stationary and a moving wall, are found to be analogous to a thermodynamic first-order transition point. In contrast, the corners defined by two stationary walls correspond to thermodynamic critical points. Here, flow structures, also known as Moffatt eddies, form and act as stagnation regions where mixing is impeded. A third stationary point occurs in the middle region of the channel and it is analogous to a high temperature thermodynamic fixed point. The numerical results of the fluid flow modeling are correlated with analytical work in the proximity of the fixed points.

Evidence for massive emission of methane from a deep‐water gas field during the Pliocene

Geologic hydrocarbon seepage is considered to be the dominant natural source of atmospheric methane in terrestrial and shallow‐water areas; in deep‐water areas, in contrast, hydrocarbon seepage is expected to have no atmospheric impact because the gas is typically consumed throughout the water column. Here, we present evidence for a sudden expulsion of a reservoir‐size quantity of methane from a deep‐water seep during the Pliocene, resulting from natural reservoir overpressure. Combining three-dimensional seismic data, borehole data and fluid‐flow modeling, we estimate that 18–27 of the 23–31 Tg of methane released at the seafloor could have reached the atmosphere over 39–241 days. This emission is ∼10% and ∼28% of present‐day, annual natural and petroleum‐industry methane emissions, respectively. While no such ultraseepage events have been documented in modern times and their frequency is unknown, seismic data suggest they were not rare in the past and may potentially occur at present in critically pressurized reservoirs. This neglected phenomenon can influence decadal changes in atmospheric methane.