scholarly journals Geomechanics and Fluid Flow in Geothermal Systems

Geofluids ◽  
2020 ◽  
Vol 2020 ◽  
pp. 1-3
Author(s):  
Víctor Vilarrasa ◽  
Roman Y. Makhnenko ◽  
Francesco Parisio
Keyword(s):  
Fluid Flow ◽  
Geothermal Systems

Modeling fluid flow through complex fracture network in geological media by using hierarchical hydraulic properties

2020 ◽  
Author(s):  
Kyung Won Chang ◽  
Gungor Beskardes ◽  
Chester Weiss
Keyword(s):  
Fluid Flow ◽  
Surrounding Medium ◽  
Computational Cost ◽  
Energy Resource ◽  
Fracture Network ◽  
Fractured Media ◽  
Enhanced Geothermal Systems ◽  
Geothermal Systems ◽  
Hydraulic Stimulation ◽  
Complex Fracture

<p>Hydraulic stimulation is the process of initiating fractures in a target reservoir for subsurface energy resource management with applications in unconventional oil/gas and enhanced geothermal systems. The fracture characteristics (i.e., number, size and orientation with respect to the wellbore) determines the modified permeability field of the host rock and thus, numerical simulations of flow in fractured media are essential for estimating the anticipated change in reservoir productivity. However, numerical modeling of fluid flow in highly fractured media is challenging due to the explosive computational cost imposed by the explicit discretization of fractures at multiple length scales. A common strategy for mitigating this extreme cost is to crudely simplify the geometry of fracture network, thereby neglecting the important contributions made by all elements of the complex fracture system.</p><p>The proposed “Hierarchical Finite Element Method” (Hi-FEM; Weiss, Geophysics, 2017) reduces the comparatively insignificant dimensions of planar- and curvilinear-like features by translating them into integrated hydraulic conductivities, thus enabling cost-effective simulations with requisite solutions at material discontinuities without defining ad-hoc, heuristic, or empirically-estimated boundary conditions between fractures and the surrounding formation. By representing geometrical and geostatistical features of a given fracture network through the Hi-FEM computational framework, geometrically- and geomechanically-dependent fluid flow properly can now be modeled economically both within fractures as well as the surrounding medium, with a natural “physics-informed” coupling between the two.</p><p>SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.</p>

scholarly journals An investigation into the controls on fracture tortuosity in rock sequences and the impact on fluid flow in the upper crust

2020 ◽  
Author(s):  
Nathaniel Forbes Inskip ◽  
Tomos Phillips ◽  
Kevin Bisdom ◽  
Georgy Borisochev ◽  
Andreas Busch ◽  
...  
Keyword(s):  
Fluid Flow ◽  
Large Scale ◽  
Small Scale ◽  
Fine Scale ◽  
Geothermal Systems ◽  
Parallel Plates ◽  
Core Flooding ◽  
Fracture Surfaces ◽  
Fracture Orientation ◽  
The Impact

<p>Fractures are ubiquitous in geological sequences, and play an important role in the movement of fluids in the earth’s crust, particularly in fields such as hydrogeology, petroleum geology and volcanology. When predicting or analysing fluid flow, fractures are often simplified as a set of smooth parallel plates. In reality, they exhibit tortuosity on a number of scales: Fine-scale tortuosity, or roughness, is the product of the small-scale (µm – mm) irregularities in the fracture surface, whereas large-scale (> mm) tortuosity occurs as a result of anisotropy and heterogeneity within the host formation that leads to the formation of irregularities in the fracture surfaces. It is important to consider such tortuosity when analysing processes that rely on the movement (or hindrance) of fluids flowing through fractures in the subsurface. Such processes include fluid injection into granitic plutons for the extraction of heat in Engineered Geothermal Systems, or the injection of CO<sub>2</sub> into reservoirs overlain by fine-grained mudrocks acting as seals in Carbon Capture and Storage projects.</p><p>Although it is generally assumed that tortuosity is controlled by factors such as grain size, mineralogy and fracture mode, a systematic study of how these factors quantitatively affect tortuosity is currently lacking. Furthermore, in anisotropic rocks the fracture orientation with respect to any inherent anisotropy is also likely to affect tortuosity.</p><p>In order to address this gap, we have induced fractures in a selection of different rock types (mudrocks, sandstones and carbonates) using the Brazil disk method, and imaged the fracture surfaces using both a digital optical microscope and X-ray Computed Tomography. Using these methods we are able to characterise both the fine-scale (roughness) and large-scale tortuosity. In order to understand the effect of fracture orientation on tortuosity we have also analysed fractures induced at different angles to bedding in samples of a highly anisotropic mudrock taken from South Wales, UK. Results indicate that fine-scale tortuosity is highly dependent on the fracture orientation with regards to the bedding plane, with fractures normal to bedding being rougher than those induced parallel to bedding. Finally, in order to measure the effect of tortuosity on fluid flow, we have carried out a series of core flooding experiments on a subset of fractured samples showing that fracture transmissivity decreases with increasing tortuosity.</p>

Geothermal manifestations in the Tyrrhenian area: the role of faults in channelling superhot geothermal fluids

2021 ◽  
Author(s):  
Martina Zucchi
Keyword(s):  
Fluid Flow ◽  
Fault Zones ◽  
Ore Deposits ◽  
Contact Metamorphism ◽  
Normal Faults ◽  
Geothermal System ◽  
Low Permeability ◽  
Geothermal Systems ◽  
Rock Interaction ◽  
Geothermal Fluids

<div> <p><span>Extensional tectonics and related magmatism affecting continental crust can favour the development of geothermal systems. Granitoids intruded in the upper crust represent the main expression of magmatism; they are strictly controlled by brittle structures during their emplacement and exhumation. The cooling of the magmatic bodies produce a thermal perturbation in the hosting rocks resulting in thermo-metamorphic aureoles of several meter thick, usually characterised by valuable ore deposits. After the emplacement and during the cooling stage such granitoids can promote the geothermal fluids circulation mainly through the fault zones. In case of favourable geological and structural conditions, geothermal fluids can be stored in geological traps (reservoirs), generally represented by rock volumes with sufficient permeability for storing a significant amount of fluid. Traps are confined, at the top, by rocks characterised by low, or very low permeability, referred to as the cap rocks of a geothermal system. Several studies are addressed to the study of fluid migration through the permeable rock volumes, whereas few papers are dealing with fluid flow and fluid-rock interaction within the cap rocks. </span></p> </div><div> <p><span>In this presentation, an example of fault-controlled geothermal fluid within low permeability rocks is presented. The study area is located in the south-eastern side of Elba Island (Tuscan Archipelago, Italy), where a succession made up of shale, marl and limestone (Argille a Palombini Fm, early Cretaceous) was affected by contact metamorphism related to the Porto Azzurro monzogranite, which produced different mineral assemblages, depending on the involved lithotypes. These metamorphic rocks were dissected by high-angle normal faults that channelled superhot geothermal fluids. Fluid inclusions analyses on hydrothermal quartz and calcite suggest that at least three paleo-geothermal fluids permeated through the fault zones, at a maximum P of about 0.8 kbar. The results reveal how brittle deformation induces fluid flow in rocks characterised by very low permeability and allow the characterisation of the paleo-geothermal fluids in terms of salinity and P-T trapping conditions. </span></p> </div>

Anisotropic permeability and fluid dispersion in pervasively fractured lavas, Rotokawa Geothermal System, New Zealand.

2021 ◽  
Author(s):  
Warwick Kissling ◽  
Cecile Massiot
Keyword(s):  
Fluid Flow ◽  
New Zealand ◽  
Geothermal Field ◽  
Geothermal System ◽  
Geothermal Systems ◽  
Fracture Density ◽  
Tracer Transport ◽  
Fracture Models ◽  
Flow Through ◽  
Permeability Anisotropy

<p>Geothermal provides nearly 20% of New Zealand’s electricity as well as increasing opportunities for direct use. In New Zealand’s ~20 high temperature geothermal systems, fluids flow dominantly through fractured rocks with low matrix permeability. It is important to understand the nature of these fracture systems, and how fluids flow through them, so that the geothermal systems may be more efficiently and sustainably used. Here we present fluid flow calculations in several distinct discrete fracture models, each of which is broadly consistent with the fracture density and high dip magnitude angle distributions directly observed in borehole image logs at the Rotokawa Geothermal Field (>300°C, 175 MWe installed capacity). This reservoir is hosted in fractured andesites. In general, fractures are steeply dipping, and the reservoir is known to be compartmentalized.</p><p>Our new code describes fluid flow through large numbers (e.g., thousands) of stochastic fracture networks to provide statistical distributions of permeability, permeability anisotropy and fluid dispersion at reservoir scale (e.g., 1 km<sup>2</sup>). Calculations can be based on both the cubic flow law for smooth-walled fractures and the Forchheimer flow model, which includes an additional term to describe the nonlinear drag (i.e. friction) in real fractures caused by surface roughness of the fracture walls.</p><p>Models with fracture density consistent with borehole observations show pervasive connectivity at reservoir scales, with fluid flow (hence permeability) and tracer transport predominantly along the mean fracture orientation. As the fracture density is varied, we find a linear relationship between permeability which holds above a well-defined percolation threshold. Permeability anisotropy is in general high (~10 to 15), because of the steeply dipping fractures. As fracture density decreases, mean anisotropy decreases while its variability increases. Significant dispersion of fluid occurs as it is transported through the reservoir. These fracture models will inform more traditional continuum models of fractured geothermal reservoirs hosted in volcanic rocks, to provide a better description of fluid flow within reservoirs and aid the responsible and sustainable use of that resource in the future.</p>

Faults controlling geothermal fluid flow in a karst geothermal system (Western Alpine Molasse Basin, France and Switzerland)

2020 ◽  
Author(s):  
Giovanni Luca Cardello ◽  
Michel Meyer
Keyword(s):  
Fluid Flow ◽  
Fracture Network ◽  
Carbonate Reservoir ◽  
Thrust Belt ◽  
Geothermal Systems ◽  
High Angle ◽  
Near Surface ◽  
Geothermal Fluids ◽  
Fold And Thrust Belt ◽  
Controlled Porosity

<p>Karst geothermal systems fluid flow is dominated by structurally controlled porosity, which constrains the paths of aquifer recharge and the upwell of geothermal fluids. In fold-and-thrust belt settings associated with continental collision, geothermal fields occur within basins generally interested by low-enthalpy geothermal systems. Despite that, the deeper and warmer levels of multiply stratified aquifers within the detached sedimentary covers are vertically connected to shallower depths by high-angle faults, thus making of them interesting targets for exploration.</p><p>In the frame of the geothermal exploration steered by the Geneva Canton, this work aims at determining how fracture connectivity, orientation and permeability anisotropy has implications on fluid flow within high-angle faults. Recent software development (e.g., FracPaQ) allows to quantify such interconnection providing insights into spatial variation of multiscale fault-controlled porosity in order to have dynamic feedbacks between fluid flow, permeability rise/fall. We use the inner Jura fold-and-thrust belt and the other carbonate relieves surrounding Geneva as an outcrop analogue for the deeper carbonate reservoir, lying at depth beneath the siliciclastic Molasse deposits. Hereby, we present new structural and morphostructural lineament maps and scan box analyses from outcrops that provide a multiscale analysis on fracturing across the study area. The sampling sites are representative of fractured fold hinges constituted of Mesozoic carbonates crossed by high-angle faults.</p><p>The map analysis show that the late Oligocene-early Miocene growing carbonate anticlines are shaped by a series of fore- and back-thrusts resulting in salient-and-recess curvy thrusts accommodating different amount of shortening across high-angle tear-faults. With the support of high-resolution LIDAR images, we observe that at the large scale (e.g., five kilometers), as fault zone broadens across transfer zones, the background fracture network is more intense at the salient flanks. Major faults occur as segmented, thus not providing near-surface structure capable of giving any earthquake significantly larger than the already measured ones (e.g., M<sub>L</sub> 5.3, Epagny earthquake 1996). Our preliminary results identify the W- and the NNW- striking systems strike-slip faults as the preferred patterns of fluid flow. Cross-cutting relationships vary with their position into the bended belt, thus making them suitable to be multiply reactivated during the Jura arc indentation. At the outcrop scale, the most mature fault zones associated with larger displacement are characterized by high fracture intensity and connectivity. Field evidences show that NNW- and W/NW- striking systems are vein-rich whereas N- and NE-striking systems are accompanied by open fracture sets although they may work with opposite fluid-flow vertical directivity. Mechanical and regional chronological development of the fracture network is also discussed as related to the regional fault evolution.</p>

Thermoporoelastic Analysis of Artificially Fractured Geothermal Reservoirs: A Multiphysics Problem

2020 ◽  
Vol 142 (8) ◽  
Author(s):  
Arash Dahi Taleghani ◽  
Milad Ahmadi
Keyword(s):  
Fluid Flow ◽  
Heat Generation ◽  
Induced Seismicity ◽  
Closed Loop ◽  
Thermal Power ◽  
Surface Subsidence ◽  
Electricity Production ◽  
Working Fluid ◽  
Hydraulic Fractures ◽  
Geothermal Systems

Abstract Geothermal systems are identified as either open-loop system (OLGS) or closed-loop systems (CLGS). In OLGS, fluid is produced from the subsurface, while there might be a concurrent fluid injection into the reservoir. The loss of working fluid, surface subsidence, formation compaction, and induced seismicity are major challenges in OLGS. To address the indicated challenges, closed-loop geothermal systems can be considered as an alternative option. In this method, a working fluid with low-boiling point is circulated through the coaxial sealed pipes to harvest heat from the formation of rock and fluid. Induced seismicity is essentially caused by the drastic quick changes in pore pressure. Thereafter, seismic risk assessment is expected for any new geothermal technology before starting the field implementation phase. To improve the heat recovery from closed-loop wells, we suggest highly conductive hydraulic fractures for CLGS to improve the heat generation rate. In conventional hydraulic fracturing treatments, fractures facilitate fluid flow; however, in the proposed configuration, induced fractures enhance heat flux into the wellbore. Considering the multiphysics nature of CLGS, a comprehensive analysis of this problem requires simultaneous modeling of fluid flow, energy transfer (heat), and rock deformation. A thermoporoelastic model is developed in finite element methods to simulate this problem. The numerical results suggest that fractures significantly improve thermal power and cumulatively produced heat in CLGS. The thermal conductivity of the proppants is the key parameter enhancing heat generation. The level of surface subsidence in the proposed technique is negligible due to the lack of geofluid production from the reservoir. Significant numbers of abandoned oil or gas wells exist around the globe which can be converted into the geothermal wells to produce electricity. This study shows the feasibility of electricity production from CLGS with minimum environmental hazards.

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