Modelling of multi-lateral well geometries for geothermal applications

Well inflow modelling in different numerical simulation approaches are compared for a multi-lateral well. Specifically radial wells will be investigated, which can be created using Radial Jet Drilling (RJD). In this technique, powerful hydraulic jets are used to create small diameter laterals (25&ndash;50&thinsp;mm) of limited length (up to 100&thinsp;m) from a well. The laterals, also called radials, leave the backbone at a 90° angle. In this study we compare three numerical simulators and a semi-analytical tool for calculating inflow of a radial well. The numerical simulators are FE approaches (CSMP and GOLEM) and an FV approach with explicit well model (Eclipse®). A series of increasingly complex well configurations is simulated, including one with inflow from a fault. Although all simulators generally are reasonably close in terms of the total well flow (deviations &lt;&thinsp;4&thinsp;% for the homogeneous cases), the distribution of the flow over the different parts of the well can vary significantly. Also, the FE approaches are more sensitive to grid size when the flow is dominated by radial flow to the well since they do not include a dedicated well model. In the FE approaches, lower dimensional elements (1-D for the well and 2-D for the faults) were superimposed into a 3-D space. In case the flow is dominated by fracture flow, the results from the FV approach in Eclipse deviates from the FE methods.</p

VESTA - Very-High-Temperature Heat Aquifer Storage

Energy storage is one of the key challenges of the energy transition. Eight international partners from Germany, Switzerland and the USA address this challenge in the joint project VESTA. Goal of VESTA is the generic development and demonstration of high-temperature storage in the underground. Four pilot sites in the DACH region in various geologies and project phases allow feedback loops between generic scientific investigations and application of new geothermal technologies. Specifically, pilot sites that shall 1) demonstrate HT-ATES technology, 2) evaluate technical and non-technical barriers, 3) support development and implementation by providing techniques and optimized component design, and 4) support agencies with scientific and technical knowledge as a basis for advancing regulatory provisions. With this scientific program, VESTA shall form a technical-economic bases for future operational concepts

Porenraumrekonstruktion poröser Medien - Kopplung von Gesteinsstruktur, Gesteinsmechanik und Strömungsverhalten in Abhängigkeit vom Effektivdruck

Zur Untersuchung von hydraulischen Parametern, welche an die Porenraumstruktur und an mechanische Prozesse gekoppelt sind, wurden folgende Laboruntersuchungen an verschiedenen Sandsteinen (Bentheimer, Fontainebleau, Flechtinger) durchgeführt: a) Porenraumstrukturanalyse mittels Quecksilberintrusion und 2D Bildanalyse; b) Skempton Koeffizient B (definiert als Änderung des Porendruckes dPp gegenüber der Änderung des Umschließungsdruckes dPc (B = dPp/dPc)) in Abhängigkeit des Effektivdrucks Peff; c) Permeabilität k in Abhängigkeit des Effektivdrucks Peff an verschiedenen Apparaturen (MTS und Hochdruck-Temperatur-Permeameter HTP). Mittels der Quecksilberintrusion und 2D Bildanalyse wurde neben der Gesamtporosität vor allem auch die Porenform charakterisiert. Dabei wurde im Speziellen auf das Verhältnis zwischen Porenhals- und Porenbauchradius, auf die Zirkularität und auf die Konnektivität der der Poren geachtet. Schon beim Vergleich von zwei Sandsteinen (Bentheimer und Flechtinger) werden signifikante Unterschiede in Porosität und Porenform sichtbar. Der Bentheimer Sandstein mit einer Gesamtporosität von ca. 23% weist gegenüber dem Flechtinger Sandstein (Gesamtporosität von ca. 10%) eine bis zu 5-fach geringere Zirkularität der Poren auf. Auch die Porengrößenverteilung des Bentheimer Sandsteins unterscheidet sich stark von der des Flechtinger Sandsteins. Im Flechtinger Sandstein zeigt sich eine Gleichverteilung der Porengröße von 6.5 bis 200 μm, wohingegen die Porengröße des Bentheimer Sandsteins vor allem im Intervall zwischen 40 und 110 μm liegt. Auch das Verhältnis zwischen dem Porenbauchradius und Porenhalsradius ist für den Flechtinger Sandstein größer. Im Bentheimer Sandstein liegt dieses Verhältnis nahe eins. Bei den mechanisch-hydraulischen Experimenten hat sich gezeigt, dass sowohl der Skempton Koeffizient als auch die Permeabilität eine funktionelle Abhängigkeit zum Effektivdruck haben. Für den Flechtinger Sandstein fällt der Skempton Koeffizient von 0.85 auf 0.65 im Testintervall von 3 bis 26 MPa Effektivdruck. In einem ähnlichen Testintervall (3 bis 30 MPa Effektivdruck) fällt die Permeabilität von 0.56 mD auf 0.42 mD. Beim Bentheimer Sandstein reagiert der Skempton Koeffizient wesentlich sensibler auf eine Effektivdruckänderung und fällt von 0.7 auf 0.4 im Testintervall von 3 bis 29 MPa Effektivdruck. Die Permeabilität bleibt hingegen konstant bei ca. 950 mD über den gesamten getesteten Effektivdruckbereich. Die durch die Experimente gewonnenen Ergebnisse lassen sich nicht vollständig durch eine Strukturverfestigung erklären, sondern sie müssen auch unter dem Aspekt einer heterogenen Porenraumveränderung betrachtet werden. Mittels numerischer Simulation und analytischer Rechnung hat sich gezeigt, dass jede Einzelpore in Abhängigkeit von ihrer Form unterschiedlich auf äußere Druckänderungen regiert. Grundlegend ist zu sagen, je irregulärer die Porenform ist, desto sensibler reagiert die Pore auf Druckschwankungen, d.h. bei gleicher Umgebungsdruckänderung werden höhere Porendrücke induziert. Darüber hinaus hat eine Zunahme der Heterogenität des Porenraumes (Vernetzung von Poren unterschiedlicher Form) auch eine Zunahme der Druckempfindlichkeit zur Folge. Neben diesen poroelastischen Eigenschaften jeder Einzelpore führt auch eine Änderung des Porentypes (von durchströmbarer Pore über Sackgassenpore bis hin zur separierten Pore) in Abhängigkeit vom Effektivdruck zu einer Änderung des Skempton Koeffizienten und der Permeabilität. Mit Hilfe dieses Ansatzes, der Numerik und den analytischen Rechnungen lassen sich die Laborergebnisse qualitativ beschreiben. Eine Effektivdruckerhöhung im Bentheimer Sandstein hat zur Folge, dass sich die Porenform stabilisiert, ohne dass dabei signifikante Fließwege abgetrennt werden. Eine hohe Irregularität (geringe Zirkularität) der Poren führt vor allem bei geringem Effektivdruck zur starken Änderung der Porengeometrie, ausgedrückt durch die starke Änderung des Skempton Koeffizienten. Im Flechtinger Sandstein führt eine Effektivdruckerhöhung daher zu einem Verschluss von Porenhälsen und somit zu einer Veränderung des Porentypes. Diese Typänderung hat zur Folge, dass sich Skempton Koeffizient und Permeabilität gleichermaßen verändern. Mittels numerischer Simulation konnte eine heterogene Porenraumveränderung in 2D nachgewiesen werden. Um die Auswirkungen einer heterogenen Porenraumveränderung auf den hydromechanischen Parameter Skempton Koeffizient und den hydraulischen Parameter Permeabilität ausreichend zu untersuchen, müssen neben den Laborexperimenten, der 2D Modellierung und der Analytik auch noch eine diskrete 3D Modellierung durchgeführt werden. Mittels dieser 3D Modellierung sollen hydraulisch-mechanisch-thermische Wechselwirkungen in porösen Medien, basierend auf ihrer internen Struktur, nachgebildet werden. Teil dieser Arbeit war daher auch die Generierung einer diskreten Porenraumstruktur. Dazu wurde das Sedimentierungstool ”Settle3D” entwickelt, mit dessen Hilfe verschiedene poröse Medien nachgebildet werden können. Das Sedimentierungstool modelliert dabei den Sedimentierungsprozess auf Einzelkornebene unter Verwendung von einfachen physikalischen Gesetzmäßigkeiten wie gradliniger Bewegung und Rotationsbewegung. Mit Hilfe des Sedimentierungstool konnten in dieser Arbeit grundlegende Zusammenhänge für Sedimentgesteine nachgebildet werden.To investigate the hydraulic parameters depending on pore space geometry and mechanical processes, we performed the following laboratory experiments on different sandstones (Bentheimer, Fontainenbleau, Flechtinger): a) pore space geometry analysis by means of mercury injection and 2D image analysis; b) Skempton coefficient B (defined as ratio between pore pressure change and confining pressure change (B = dPp/dPc)) depending on effective pressure Peff change under undrained conditions; c) permeability k depending on effective pressure change at different apparatuses (Mechanical Test System MTS and High-Pressure-Temperature-Permeameter HTP). By means of mercury injection and 2D image analysis total porosity and pore shape geometry were characterized. Pore space geometry was classified by the ratio between pore throat and pore cavity, the circularity and connectivity of the pores. Comparison of two sandstones (Bentheimer and Flechtinger) shows significant differences in porosity and pore space geometry. Total porosity of Bentheimer sandstone is 19.2 (2D image analysis) to 26.4% (mercury injection) which is two times of Flechtinger sandstone (9.5 (mercury injection) to 10.7% (2D image analysis)). With 150 μm pore radius the perimeter length of a pore in Bentheimer sandstone is five times longer than in Flechtinger sandstone. This can be derived from a circularity contrast of 0.1 for Flechtinger and 0.02 for Bentheimer sandstone. Furthermore, the pore size in Flechtinger sandstone is uniformly distributed over the total resolvable range of pore radii (6.5 to 200 μm) but the Bentheimer sandstone shows an accumulation of pore size in a range from 40 to 110 μm. Therefore, the Flechtinger sandstone is more heterogeneous in pore size than the Bentheimer sandstone. Also, the ratio between pore cavity and pore throat in the Flechtinger sandstone is higher than in the Bentheimer sandstone. During the mechanic-hydraulic experiments we determined a functional dependency of the Skempton coefficient and permeability due to effective pressure. For the Flechtinger sandstone the Skempton coefficient started with 0.85 at 3 MPa effective pressure and decreased with increasing effective pressure to a minimum of 0.65 at 26 MPa. At a similar range of effective pressure (3 to 30 MPa) the permeability decreases from 0.56 mD to 0.42 mD. For the Bentheimer sandstone the Skempton coefficient was 0.7 at 3 MPa and decreased with an increasing effective pressure to a minimum value of 0.4 at 29 MPa. Permeability of the Bentheimer sandstone keeps nearly constant over the total range of effective pressure. The results of the laboratory experiment are not only explainable by a stiffening of the grain structure, but also a heterogeneous deformation of the pore space must be considered. By means of numerical simulations and analytical calculations it was shown, that each single pore reacts differently to an exterior pressure change. This diverse behaviour depends on the different pore shape. Basically, we can conclude the more irregular the pore geometry, the more sensitive the pore reacts on a confining pressure change, i.e. at the same confining pressure level the induced pore pressure increases with the irregularity of the pore geometry. Furthermore, an increase in pore space heterogeneity (caused by a connection of pores of different shape) will increase the pressure sensitivity, too. Besides this poroelastic behaviour of each single pore, also a change of the pore type (from catenary over cul-de-sac to closed) depending on an effective pressure change leads to a change in the Skempton coefficient and permeability. By means of this approach, the numerical simulation and the analytical calculation, the laboratory experiments can be explained qualitatively. An increase of effective pressure in the Bentheimer sandstone results in stiffening of the pore space geometry, but a change of pore type and/or a closing of pore throats can be neglected. The high irregularity (small circularity) of the pores leads to maximum change of the pore geometry at small effective pressure levels. Therefore, the maximum change of Skempton coefficient can be observed at small effective pressure rates. In contrast, an increase of effective pressure in the Flechtinger sandstone results in a closing of pore throats and micro cracks and therefore in a change of the pore type ratio. This results in a similar change of Skempton coefficient and permeability. By means of the numerical simulation, a heterogeneous deformation of the pore space geometry in 2D could be demonstrated. In order to show the influence of a heterogeneous pore space deformation on the hydraulic-mechanical parameter Skempton coefficient and hydraulic parameter permeability we have to perform (besides laboratory experiments, 2D modelling and analytical solutions) a discrete 3D modelling as well. Using this discrete 3D model, a coupling of hydraulic, mechanical and thermal properties in porous media depending on the internal pore space structure is possible. Therefore, a principle part of this work was to generate a discrete pore space structure. For this purpose the sedimentary tool ”Settle3D” was developed, which can generate different porous media. By means of the sedimentary tool and simplest physical laws such as translation and rotation we can rebuild the sedimentary process on single grain level. At the current state we can proof basic dependencies for sedimentary rocks

Selected Papers Presented at the 13th EURO-Conference on Rock Physics and Geomechanics—The Guéguen Conference Held on 2–6 September 2019 in Potsdam, Germany

This paper is the editoral of a special issue, that intends to summarize the highlights of the conference programme consisting of 8 keynote lectures, 47 oral presentations and 26 poster presentations given by rock mechanics experts and early career scientists. The programme was divided into the following eight oral presentation sessions: ‘Fluid-driven fractures and seismicity’, ‘Compaction and damage of porous rock I + II’, ‘Laboratory fracture and rock characterization studies’, ‘Poroelasticity and seismicity of reservoir rocks’, ‘Simulation of fractures and faults’, ‘Hydraulic, thermal and mechanical cyclic loading at multiple scales’ and ‘Hydraulic fracturing, hydromechanics and fracture permeability’. The special issue consists of 19 scientific papers which are based on these conference contributions and all of which address different important aspects relevant to the variety of industrial applications outlined above. We want to highlight that rock fracturing and fault activation are two very different processes and that both hard data measured in experiments as well as analytical and numerical models together yield a powerful path to solve industrial and societal challenges associated with the utilization of the subsurface based on scientific evidence.Helmholtz-Zentrum Potsdam Deutsches GeoForschungsZentrum - GFZ (4217

Hydraulic-mechanical properties of microfaults in granitic rock using the Punch-Through Shear test

Fault zones are key features in crystalline geothermal reservoirs or in other subsurface environments due to the fact that they act as main fluid pathways. An adequate experimental description of the evolution of permeability of a realistic microscopic fault zone under in-situ reservoir and fracture parallel flow conditions is required. To address this topic, we demonstrate a novel experimental set up (Punch-Through Shear test) that is able to generate a realistic shear zone (microfault) under in-situ reservoir conditions while simultaneously measuring permeability and dilation. Three samples of intact granite from the Odenwald (Upper Rhine Graben) were placed into a MTS 815 tri-axial compression cell, where a self-designed piston assembly punched down the inner cylinder of the sample creating the desired microfault geometry with a given offset. Permeability was measured and fracture dilation was inferred from an LVDT extensometer chain, as well as the balance of fluid volume flowing in and out of the sample. After fracture generation, the shear displacement was increased to 1.2 mm and pore pressure changes of ± 5 or ± 10 MPa were applied cyclically to simulate injection and production scenarios. Formation of a microfault increased the permeability of the granite rock by 2 to almost 3 orders of magnitude. Further shear displacement led to a small increase in permeability by a factor of 1.1 to 4.0, but permeability was reduced by a factor of 2.5 to 4 within 16 h due to compaction and fault healing. Effective pressure cycling led to reversible permeability changes. CT images showed that the fracture network is rather complex, but depicts all features commonly observed in larger scale fault zones.</p

Permeability Evolution During Shear Zone Initiation in Low-Porosity Rocks

Using an innovative experimental set-up (Punch-Through Shear test), we initiated a shear zone (microfault) in Flechtingen sandstone and Odenwald granite under in situ reservoir conditions while monitoring permeability and fracture dilation evolution. The shear zone, which has a cylindrical geometry, is produced by a self-designed piston assembly that punches down the inner part of the sample. Permeability and fracture dilation were measured for the entire duration of the experiment. After the shear zone generation, the imposed shear displacement was increased to 1.2 mm and pore pressure changes of ±5 or ±10 MPa were applied cyclically to simulate injection and production scenarios. Thin sections and image analysis tools were used to identify microstructural features of the shear zone. The geometry of the shear zone is shown to follow a self-affine scaling invariance, similar to the fracture surface roughness. The permeability evolution related to the onset of the fracture zone is different for both rocks: almost no enhancement for the Flechtingen sandstone and an increase of more than 2 orders of magnitude for the Odenwald granite. Further shear displacement resulted in a slight increase in permeability. A fault compaction is observed after shear relaxation which is associated to a permeability decrease by a factor more than 3. Permeability changes during pressure cycling are reversible when varying the effective pressure. The difference in permeability enhancement between the sandstone and the granite is related to the larger width of the shear zones.ReSalt ProjectH2020 European Research Council http://dx.doi.org/10.13039/100010663Projekt DEA

Hydraulic Diffusivity of a Partially Open Rough Fracture

International audienc

Aluminium release by water-rock interaction during hydraulic tests in a siliciclastic aquifer in Berlin (Germany)

Two hydraulic tests of a Triassic sandstone aquifer were performed to determine hydraulic and geochemical parameters of the reservoir formation. Samples collected during all tests showed neutral and constant pH-value of about 7.5. During the initial step rate test, the aluminium concentration of the water remained below 0.005 mM, but it increased significantly during the production phase of the following single well push-pull test to 4.8 mM. Drill cuttings of the well, collected at reservoir depth (Exter Fm. and the overlaying Tertiary sand), were additionally characterized and used for leaching experiments. These experiments evidenced a strong release of aluminium from the Tertiary, pyrite containing sand, indicating processes of pyrite oxidation being responsible for aluminium mobilization