Displays

Deep hydrothermal systems in fractured media are a potential source of geothermal energy. A key problem prevails as a consequence of utilisation that the geochemical system is perturbed and scaling may build up over time. Barite stands out as one of the most ubiquitous scaling agents in deep geothermal systems. It causes irreversible efficiency loss and may be responsible for geothermal power plants to become non-profitable. Due to complex parameter interplay and underlying uncertainties, it is imperative to utilise numerical simulations to investigate temporal and spatial precipitation effects.
In this work, the impact on fracture permeability in the near field of the injection well is assessed. A one-dimensional reactive transport model is set up with heterogeneous nucleation and crystal growth kinetics. In line with potential target hydrothermal systems in the North German Basin, the following parameters are considered in a sensitivity analysis: injection temperature (50 to 70 °C), pore pressure (10 to 50 MPa), fracture aperture (10^-4 to 10^-2 m), flow velocity (10^-3 to 10⁰ m s^-1), molar volume (50.3 to 55.6 cm³ mol^-1), contact angle for heterogeneous nucleation (0° to 180°), interfacial tension (0.07 to 0.134 J m^-2), salinity (0.1 to 1.5 mol kgw^-1 NaCl), pH (5 to 7), and supersaturation (1 to 30).
Nucleation and consequently crystal growth can only begin if the threshold supersaturation is exceeded. Therefore, contact angle and interfacial tension are the most sensitive in terms of precipitation kinetics. If nucleation has occurred, crystal growth becomes the dominant process, which is mainly controlled by fracture aperture. Results show that fracture sealing can happen within months (33 days) and the affected range can be in the order of tens of metres (10 m). Predicting the threshold supersaturation is a crucial point in this context, as it essentially determines if barite precipitation becomes relevant.
The uncertainty of parameters influencing nucleation at in-situ conditions is high, emphasising the need to investigate these in more detail. The presented models suggest that barite scaling must be recognised as a serious threat if the supersaturation threshold is exceeded, in which case, larger fracture apertures could help to minimise kinetic rates.

How to cite: Tranter, M., DeLucia, M., and Kühn, M.: Barite scaling in fractures simulated with nucleation and crystal growth kinetics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-417, https://doi.org/10.5194/egusphere-egu2020-417, 2020.

Enhanced Geothermal Systems (EGS) are widely used in the development and application of geothermal energy. They usually consist of two parallel deep boreholes, where cold water is injected into one borehole and abstracted at the second one after being heated when passing through the fractured network system. Recently, simple analytical solutions have been proposed to estimate the water pressure at the output. Nevertheless, these methods do not take into account the influences of the coupled thermal and mechanical processes. In this research study we build a fully coupled Thermal – Hydro-mechanical model (THM model) to simulate the processes of heat extraction, deformation and water flow in the nearby fractured rock formations. The influences of single thermal – hydraulic and mechanical – hydraulic effects were compared with the fully coupled and decoupled results, showing that temperature influences mostly the water pressure in the second borehole, compared with temperature. The mechanical effect alone has little influences on the water pressure. A sensitive analysis was also conducted to study which parameters affect the simulation results the most. It was shown that the initial permeability and temperature are playing the main roles in this simulation.

How to cite: Zhou, D., Tatomir, A., and Sauter, M.: Study of heat extraction and flow process by fully coupled thermal- hydro- mechanical model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5998, https://doi.org/10.5194/egusphere-egu2020-5998, 2020.

Energy transition requires the use of low-carbon energies such as geothermal energy for the production of electricity or heat. Geothermal exploitation has a number of preferential targets, including fault zones in the context of graben. High temperatures can, indeed, be observed where fluids rise through fault zones, but geothermal processes are complex to understand and to model in such a 3D tectonic context. For instance, seismic observations and then observations at well-scale show structures on different spatial scales that can overlap and interconnect. These structures then present strong heterogeneities in physical properties (e.g. fault core or damage zones). In addition, this knowledge evolves over time, from the exploration to drilling and exploitation phases. One of the challenges of numerical modelling is to represent this complexity while being readily upgradeable in the light of exploration. We are developing an adaptive approach using the CIMLIB/EXALIB library. Geometrical complexity and physical properties are defined by distance functions (level set functions) to geologic objects that are inherited from a geologic modelling software. Coupled fluid flow and heat transport processes are then modelled in 3D with adaptive meshing. The mesh can, indeed, be adapted according to static criteria such as geometry or dynamic criteria such as physical processes. This approach will be illustrated by examples derived from an ongoing GIS-Geodenergies project in the Upper Rhine Graben.

How to cite: Dentzer, J., Hachem, E., Goblet, P., Stab, O., and Bruel, D.: Geothermal modelling in fault zones with the CIMLIB/EXALIB library, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10065, https://doi.org/10.5194/egusphere-egu2020-10065, 2020.

In the framework of the GEMEX project (cooperation between Europe and Mexico for geothermal development), a dense network of 45 stations was deployed in 2017-2018 in the Los Humeros caldera, Mexico.

Thanks to this network an intense local seismic activity has been recorded in the geothermal field and surroundings, from which it has been possible to identify high-frequency Volcano-Tectonic events (VT, >10 Hz) and Long-Period events (LP, 1-8 Hz). The former set of events is mainly associated with the local tectonics and power plant activities; while the latter has been generally recorded after strong earthquakes (Mw>7) occurred in Mexico.

Consequently, we adapted and applied two tomographic techniques to generate highly resolved seismic models; 1) the Enhanced Seismic Tomography (EST) method using the travel times of local seismic events. The method incorporates the Double Difference tomography and the post-processing Weighted Average Method to generate Vp and Vs models, and 2) the surface wave tomography method based on ambient noise analysis. In this case, we generated 3D anisotropic models of phase and group velocities of the Rayleigh and Love waves from Green functions retrieved by cross-correlation of the continuous records.

Thanks to the severe pre-processing of the whole seismic database that allowed to correct several errors on the data, and to the methods applied, we were able to obtain reliable and highly resolved models with both techniques.

Finally, the two sets of events (VT and LP) have been relocated using the 3D seismic velocity models of the region in order to better characterize the structure of the geothermal field and identify regions where the fluids could have a role on the triggering of the LP seismic activity observed.

This work is performed in the framework of the Mexican European consortium GeMex (Cooperation in Geothermal energy research Europe-Mexico, PT5.2 N: 267084 funded by CONACyT-SENER: S0019, 2015-04, and Horizon 2020, grant agreement No. 727550).

How to cite: Calò, M., Figureoa Soto, A., Cruz-Hernandez, S., Granados Chavarría, I., De la Rosa, B., Angulo Carrilo, J., Toledo, T., Gaucher, E., and Jousset, P.: Seismic models of the Los Humeros caldera (Mexico) using the GEMEX project data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11017, https://doi.org/10.5194/egusphere-egu2020-11017, 2020.

In the framework of the international collaboration between Mexico and Europe for the development of geothermal energy (GEMex consortium), a seismic network of 45 seismic stations (25 broad-band and 20 short-period) was installed around the super-hot geothermal system of Los Humeros (Mexico) for more than one year. Los Humeros power plant is nested inside a quaternary caldera located in the eastern part of the Trans-Mexican Volcanic Belt that crosses the whole country from the Pacific coast to the Gulf of Mexico.

Among the several targets of the data collected by this network, an important task is to produce a seismic image of the caldera and of the geothermal reservoir. Here we present the 3D anisotropic shear wave velocity models retrieved by the seismic ambient noise tomography.

Thanks to the severe pre-processing of the whole seismic database we were able to obtain reliable and highly resolved models.

To carry out the model we applied a rigorous data quality assessment consisting in: 1) correction of the orientation of the sensors using the polarization of surface waves associated with tele-seismic and regional earthquakes, 2) assessment of the synchronization of the stations and correction of the times using daily cross-correlations functions, 3) finally to asses the quality of the stacked cross-correlations, knowed as Green’s functions (GF), we analyzed the noise sources directivity, inter-station distance and level of emergence of surface waves depending on the type of sensor used.

The processing allowed to pick clearly about 600 dispersion curves per velocity type (group and phase of R and L waves), using the NDCP code (Noisy Dispersion Curve Picking), that allows to display and select dispersion patterns both in time and frequency domain, for both causal and anti-causal part of the GF.

2D tomography maps were calculated from 0.5 to 9 s for each type of velocity. Depth inversion for the whole velocities types was carried out using surf96, allowing reconstructing the 3D anisotropic structure of the caldera for the first time.

The resulting models provides a larger view of the caldera and its anisotropic patterns down to 10 km depth. In these models, we were able to define the depth of the caldera rim, some important features of the internal part of the caldera and a low velocity body that could be associated with the hot sources feeding the reservoir. Our model are in strong agreement with those retrieved applying other geophysical methodologies (e.g. magnetotelluric, passive travel-time tomography, gravimetric, etc.).

This work is performed in the framework of the Mexican European consortium GeMex (Cooperation in Geothermal energy research Europe-Mexico, PT5.2 N: 267084 funded by CONACyT-SENER : S0019, 2015-04, and Horizon 2020, grant agreement No. 727550).

How to cite: Granados Chavarria, I., Calò, M., Figueroa Soto, Á., and Jousset, P.: 3D Anisotropic velocity model of the Los Humeros geothermal field, Mexico, using seismic ambient noise tomography., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12544, https://doi.org/10.5194/egusphere-egu2020-12544, 2020.

Characterization of fractured rocks is a key challenge for optimizing heat harvesting in geothermal systems. The use of heat as a tracer, facilitated by the development of such advanced techniques as active line source (ALS) borehole heating and the distributed temperature sensing (DTS), shows the great potential for characterizing fractured rocks. However, there is so far a limited number of theoretical and numerical studies on how these tests could be used for estimating both fracture-network and rock-matrix properties.

We use deep neural networks to describe heat tracer test data and demonstrate how the cumulative density function (CDF) or probability density function (PDF) of the heat tracer test data can be deployed in the inversion mode, i.e., to infer the fracture parameters with. Our approach utilizes the methods of distributions, developed previously to estimate the CDF of solute concentration described by a reactive transport model with uncertain parameters and inputs. The method is applied to analyze several synthetic heat tracer test datasets obtained from a particle-based forward model of transport processes in heterogeneous fractured rocks. The study considers alternative representations of fracture networks with a large range of variation of the fracture network properties, as well as several experimental conditions (e.g., ambient/forced thermal and hydraulic conditions, pulse/continuous changes in temperature). This allows us to characterize the system by combining the information from several thermal tests.

How to cite: Roubinet, D., Zhou, Z., and Tartakovsky, D.: Numerical strategies for characterizing fractured rock from heat tracer experiments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13699, https://doi.org/10.5194/egusphere-egu2020-13699, 2020.

Abstract: Hydraulic fracturing process is invisible in geological media and is difficult to be observed. The acoustic emissions (AEs) or microseismic (MS) technologies are useful approaches to estimate the hydraulic fracturing process. However, the AEs or MS do not provide the geometry of hydraulic fractures directly, but only provides the coordinates of AEs/MS sources. Traditional analytical and statistical approaches of reproducing fracture geometry using AEs/MS sources are relatively empirical. In this work, we monitored the AEs induced by hydraulic fracturing experimentally. Then, data-driven simulation based inverse analysis approach is developed to estimate the fracture geometry according to AEs sources. The difference between hydraulic fractures and AEs sources is defined as objective function. Then, the in-situ stresses are found using the inverse analysis based on data-driven simulation. As shown in Fig. 2, the geometry of hydraulic fractures is reproduced using data-driven simulation and AEs technology.

Keywords: hydraulic fracturing, inverse analysis, data-driven simulation, acoustic emissions technology, AiFrac

How to cite: Dai, H. and Tang, X.: Inverse analysis of hydraulic fracture geometry based on data-driven simulation and acoustic emissions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18177, https://doi.org/10.5194/egusphere-egu2020-18177, 2020.

Quartz dissolution and precipitation is an important pore reducing process in geothermal reservoirs. We present a single-phase reactive flow model coupled with hydrodynamic flow and heat transfer components and implement it into COMSOL Multiphysics. The model includes diffusion and advection, and analytical equations are used to describe quartz kinetics and equilibrium concentrations with respect to the silicate phases. The numerical model can a priori be used to analyze the evolution of the porosity/permeability, and hence the productivity of the reservoir induced by heat extraction in geothermal reservoirs. A geothermal reservoir is modeled with realistic time steps, where its geometry is represented as a porous medium block in which chemical reactions occur between the pore fluid and the rock matrix. Future developments include adding a fracture and fracture networks to the system and analyzing the changes in effective stresses in the presence of reactive flow. Economic reservoir development requires a combined analysis of the thermo-hydro-mechanical and chemical processes, and precipitation processes may be important in post-seismic fluid flow processes.

How to cite: Gisler, B. and A. Miller, S.: Reactive Flow Model for Porosity Reduction by Quartz Dissolution/Precipitation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20114, https://doi.org/10.5194/egusphere-egu2020-20114, 2020.

Enhanced geothermal system (EGS) is an important way of geothermal development, which takes advantage of the fractures serving as the channels of working fluid flow and heat transfer. But constrained by the geometries of hydraulic fractures formed through conventional fracturing technologies, the heat transfer areas are limited. Radial borehole fracturing combines hydraulic fracturing and radial boreholes which extend to the formation radially from 10 to 100 meters and have diameters of 20 to 50 millimeters. This paper aims to investigate whether radial borehole fracturing can increase the fracture areas in EGS system comparing with perforation fracturing. 
Nine cubic concretes (300*300*300mm) were cast after mixing sand, cement and water. Six of them contained radial boreholes and three had perforations. All cubic concretes were heated to 200℃ and fractured by a tri-axial fracturing test system with injection rates of 30ml/min and horizontal principal stress differences being 6 MPa. Then the fractures were scanned and the fracture areas were calculated. Three different angles between radial boreholes/perforations and maximum horizontal stress (0°, 45°, 90°) and two quantities of radial boreholes (2, 4) were studied. 
Experimental results show that radial borehole fracturing creates greater fracture areas then that of perforation fracturing if the orientations of radial boreholes and perforations do not consist with the direction of maximum horizontal stress. Because the fractures turn to the direction of maximum horizontal stress more quickly for perforation fracturing when perforations and radial boreholes have identical angles, namely radial boreholes guide the fractures better as they extend into the concretes. Besides, concretes with 4 radial boreholes have smoother fractures than concretes with 2 radial boreholes. In addition, the breakdown pressure of radial borehole fracturing is lower and increasing the quantities of radial boreholes reduces the breakdown pressure. 
This experimental investigation reveals that radial borehole fracturing can form larger fracture areas than perforations fracturing, which promotes the efficiency of heat extraction in EGS system.

How to cite: Tian, S., Guo, Z., Sun, Y., Wang, Q., Liu, Q., Sheng, M., and Shen, Z.: Feasibility of Radial Borehole Fracturing in Geothermal Exploitation: an Experimental Study, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20902, https://doi.org/10.5194/egusphere-egu2020-20902, 2020.

The dry hot rock (DHR) is a widely distributed renewable and clean energy. Cryogenic fracturing, such as liquid nitrogen fracturing technology, in DHR not only avoids the consuming of water, but also enhances the fracturing with the rock damage induced by thermal stress. During fracturing, cryogenic fluid (extremely low temperature) is utilized to trigger sharp a thermal gradient and fracturing surrounding boreholes, which generates fracture networks and increase the permeability of DHR. In this work, the TOUGH-FEMM simulator, which links the TOUGH2 thermal-hydraulic simulator and a mechanical simulator based on hybrid the finite-element meshfree method (FEMM), is developed to model three-dimensional cracking induced by cryogenic injection. The results of the numerical simulations agree with the experimental results showing that the fracture network is generated and connected to the borehole. An increased connectivity between a production borehole and the fracture network can significantly enhance fluid and hydro carbon production.

How to cite: Zhao, H. and Liu, Z.: Simulating cryogenic fracturing process with TOUGH-FEMM, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21313, https://doi.org/10.5194/egusphere-egu2020-21313, 2020.