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Abstract Hot Dry Rock (HDR) is considered as one of the most promising energy to increase the supply of local and renewable energy to alleviate energy crisis for sustainable development. Fractures and fracture networks of HDR are the dominate channel for working fluid to transfer mass and heat, so a detailed description of fracture is critical to numerical simulation. The coupled THM governing equations are formulated as the mathematical model, based on the local thermal non-equilibrium theory. A core-scale 3D numerical model with a single idealized rough fracture is created. The distributions of pressure, temperature, stress and deformation were discussed. Sensitivity analysis was conducted to evaluate the influence of fracture roughness and aperture on the outlet temperature of the model. [ABSTRACT FROM AUTHOR]

The current study presents a numerical modeling approach for three-dimensional simulation of hydro-shearing in jointed rocks for the generation of a man-made, multi-fracture heat exchanger in Hot Dry Rock (HDR) Geothermal reservoir. Various literatures have suggested the presence of in-situ fractured rock mass even in massive granite formations. Thereby, 3D numerical modelling is essential, since the prediction of fracture growth in 3D is key to investigation of different fracture designs and furthermore various operational parameters in order to optimize the heat exchanger design and the resulting energy production. The numerical approach incorporates the Dynardo's approach of homogenized continuum method to simulate the hydro-shearing process in jointed rocks unlike vast majority of commercial and scientific approaches which use the discrete modelling technique. The main motivation of the continuum approach is the numerical efficiency of the 3D coupled hydraulic-mechanical simulations of hydraulic fracturing process in comparison to other alternatives. The input parameters of the numerical model from the best available well log and reservoir data are calibrated from diagnostics measurements such as Micro-seismic events, Bottom Hole Pressure, etc to assign the correct level of forecast quality to the important mechanisms of hydraulic fracturing. The numerical procedure is applied to a prospective Granite reservoir in Thüringen, Germany within the purview of a German joint research project – optiRiss. The integrated approach involves ANSYS as a pre-processor and solver, Dynardo's fracturing simulator on top of ANSYS, Tamino – a post-processing tool and optiSLang – an environment for optimization and uncertainty quantification. [ABSTRACT FROM AUTHOR]

We propose an inversion method of the direct wave amplitudes P, SV, and SH to determine the general moment tensor (MT) and estimate the associated uncertainties. This method is a generalization of the one we developed to determine the double-couple (DC) or fault-plane solution from sparse observations in reservoir contexts (Godano et al., 2009). Like the previous one, it is based on a simulated annealing inversion algorithm. First, we test the reliability of this new method on synthetic data. The inversion allows retrieval of the expected moment tensor, but the analysis of the uncertainty associated with the solution shows that the resolution of the inversion is sensitive not only to the direct wave amplitudes but also to uncertainties in the velocity model and the event location. Next, we test the method on four induced microearthquakes recorded by the Soultz-sous-For6ts hot-dry-rock (HDR) reservoir network. The inversion correctly converges for three events, and the obtained MTs display a dominant DC part (70 to 90%). The uncertainty associated with the non-DC part is relatively high. This prevents us from determining if the small non-DC part is significant or an artifact related to noise in the data and/or uncertainties in the velocity model and at the event location. [ABSTRACT FROM AUTHOR]

Mechanisms of earthquakes and induced seismic events are frequently found using the moment tensor description rather than the traditional double couple (DC), as it allows for nonshear source phenomena. However, non-DC source components are sensitive to event mislocations and inexact velocity models of the crust. Inaccuracies such as these can generate spurious non-DC components in the mechanism, which should be taken into account during the interpretation. We perform a synthetic case study simulating seismic observations at Soultz-sous-Forêts, Alsace, hot dry rock (HDR) site. Synthetic P and S amplitudes for several shear-tensile source models are inverted, assuming hypocenter mislocation and velocity structure mismodeling in several types of station coverage. Satisfactory reconstruction of the source mechanism is achieved, except for rather extreme model simplification and extremely poor station coverage. Thus, our results suggest that non-DC mechanisms can be successfully resolved in local studies with reasonable station configuration, when errors in event location and velocity profile are realistic. [ABSTRACT FROM AUTHOR]

Abstract: This paper reports the results of laboratory and field experiments of CO2 sequestration into the Ogachi hot dry rock(HDR; the temperature is 200 degree C) site, where a part of CO2 will be expected to be fixed as carbonates by interaction with rocks (Georeactor; Ca extraction from rocks and carbonate fixation). In 2007, CO2 dissolved water (river water with dry ice) was directly injected into OGC-2 (from September 2nd to 9th) and Run #2(from September 11th to 16th). Several tracers were also injected at the same time. Water samples are collected at the depth of ca. 800 m by a sampler (500 ml in volume) and monitored for their chemical and isotopic compositions. During the Run #2 experiment, river water was injected into OGC-1 at 2 days after injection of CO2 water into OGC-2. During the field experiments, dissolution or precipitation rates of calcite were determined by using a technique of ”in site analyses”. Calcite crystals covered with Ti rod or Au film is hold in a crystal cell and set in a crystal sonde. The crystal sonde is then put into OGC-2 and water samples at the certain depth is introduced into the sonde. After 1 hour, the sonde is recovered and the calcite crystal is observed by a newly developed phase shift interferometer to analyze the dissolution or precipitation rates of calcite from the reservoir fluids. The “in situ analyses” show that calcite precipitation was observed within 2 day after the injection. This supports the view that most of CO2 injected might be fixed as carbonate. [Copyright &y& Elsevier]

Multi-stage stimulation using alternative injection has been successfully applied in low mobility hydrocarbon production. However, fracture initiation and growth induced by different injection schemes have been inadequately studied for hot dry rock (HDR) geothermal reservoirs. Here, the impact of injection schemes on hydraulic fracture (HF) propagation regimes was determined with PFC2D software. The results show that the propagation of natural fractures (NFs) created by cyclic injection are dominated by the mode of shear activation and direct penetration. However, cyclic injection with frequent starting and stopping can produce non-uniform stress and fatigue, resulting in crack initiation and more branched fractures growth. The stepped injection can activate NFs effectively, and the HF propagation are featured by a style of turning at the tip of NFs. However, the stepped injection often produces a single main fracture with few branches. Different injection methods can lead to different propagation regimes and ultimately result in variation of the fracture network. A numerical model of the FORGE site that contains relevant geological structure and a fracture network was established with 3DEC software, and the impact of NFs on the HF network formation was investigated systematically. Compared with cyclic injection, the fracture network formed by stepped injection is more susceptible to the distribution of the NFs. The value of stepped injection is about 1.31 times in surface area and 1.17 times in aperture than the cyclic injection. Cyclic injection is conducive to creating fractures in the matrix, while stepped injection is more inclined to activate the preexisting NFs. The method presented here can be adopted to other geologic settings to optimize the fracture growth regime and provide a scientific basis for Enhanced Geothermal System (EGS) multi-stage fracturing design. • The relationships between injection methods and fracture propagation were inferred. • The impact of natural fractures on fracture network formation were investigated. • The multi-stage, variable injection fracturing method for HDR was proposed. [ABSTRACT FROM AUTHOR]