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In a previous study we used a the grain based modeling approach (GBM) to carry out a sensitivity analysis of mineral size and random distribution of minerals and particles and their influence on elastic properties, strength and fracture types in Aue Granite. In the present study, the same calibrated model is used to further evaluate the influence of these parameters as well as the particle, parallel-bond, and smooth joints properties of the model on the resulting fracture patterns. It was found that complex crack networks always develop in heterogeneous GBMs, and crack network complexity increases with increasing heterogeneity. While each of the particles, parallel bond, and smooth joint properties govern the development of a specific crack type, no obvious relations between these properties and the total number of cracks and the resulting crack network complexity were found. [ABSTRACT FROM AUTHOR]

Abstract: This research addresses the thermomechanical response of a saturated sandstone in a geothermal reservoir. Several experiments were performed using a triaxial testing system to investigate thermal effects on poroelastic parameters of Flechtinger sandstone, an outcropping equivalent of the (Rotliegend) reservoir rock. In drained experiments, confining pressure was cycled, pore pressure was maintained constant and temperature was increased step-wise. The temperature dependence of the drained bulk modulus differed at low and high effective pressures. The unloading drained bulk modulus increased from 10.25GPa to 11.74GPa with increasing temperature from 30°C to 120°C at high stresses and decreased from 3.39 to 3.05GPa at low stresses. The Biot coefficient decreased with increasing effective pressure and temperature. The inelastic behavior of the rock, the interrelation between thermal expansion coefficient and bulk modulus, and the path dependence of heat transfer processes govern the temperature effect on granular rock and changes in pore geometry. [Copyright &y& Elsevier]

Abstract Enhanced geothermal systems (EGS) are engineered reservoirs developed to extract heat from low permeability and low porosity geological formations. Cyclic hydraulic stimulation treatments can be used in some cases to create hydraulic fractures and gain access to the target formation fluids, drain the geothermal fluid and increase the overall productivity of the reservoir. During these operations, successive cycles of injection at high flow rates are conducted to decrease the effective minimum principal stress to the tensile strength of the material, developing a mode I fracture. Opening of the newly developed fractures induces additional deformation of the reservoir rocks whose impact on the far field reservoir hydraulics has not been addressed so far for real case applications. These rates of compressive deformation can be significantly high in the context of cyclic stimulation treatments and can lead to a poroelastic pore pressure increase which spans quasi-instantaneously at greater distances than diffusive processes. In this study, such a poroelastic response resulting from cyclic deformation during hydraulic stimulation treatment of a well is investigated using hydromechanical coupling between pore pressure variations and rock deformation. The effects of this poroelastic response on the hydromechanical state of a reservoir is illustrated with field measurements of a cyclic hydraulic stimulation treatment conducted at the Groß Schönebeck geothermal research site in August 2007. This study demonstrates that the pore pressure increase monitored in a neighbor well located approximately 475 m away from the stimulated well at reservoir depth is controlled by diffusive processes responsible for the long-term increase of pore pressure, but also by poroelastic effects responsible for the quasi-instantaneous local peaks in pore pressure. The results from this applied study helped to quantify the relevance of the poroelastic behavior of stimulated reservoir rocks in a way than can improve current understanding of such hydraulic stimulation settings. [ABSTRACT FROM AUTHOR]

The pressure dependent fracture permeability of four granitic rock samples was determined in a triaxial test cell under hydrostatic loading conditions at a temperature of 30 °C. Tensile fractures were generated in cylindrical samples by Brazilian tests. Confining pressures were cycled twice between 2 and 50 MPa. Permeability and strain responses were determined for two samples with aligned fracture surfaces and for two samples with a shear displacement of 1 mm. Fracture apertures were also determined optically before and after testing and the fracture surfaces were scanned. Permeability of the intact granite matrix is below 1e−18 m 2 and sample permeabilities of fractured samples range between 1e−17 m 2 and 1e−12 m 2 depending on displacement and confining pressure. Fracture permeabilities of the aligned samples range from 1e−12 m 2 to 2e−10 m 2 and samples with displaced fracture surfaces have fracture permeabilities between 1e−11 m 2 and 2e−9 m 2 . Without shear displacement, fracture permeability is reduced with time, even at low constant confining pressures of 2 MPa, while fracture permeability is constant at the same conditions for the displaced samples. With increasing confining pressure, permeability reduction is significantly less for the displaced samples compared to the aligned samples. Fracture permeabilities of displaced fractures in granites are similar to fracture permeabilities in sedimentary rocks using proppants. The results indicate that self-propped displaced fractures in granites may allow sufficient fluid flow for reservoir engineering purposes such as enhanced geothermal system development, without the use of propping agents. [ABSTRACT FROM AUTHOR]

Abstract: Experimental measurements to derive the drained bulk C and the solid bulk C s u compressibility, the poroelastic Biot coefficient , Skempton coefficient B as well as porosity were performed under drained, undrained and unjacketed pore pressure conditions. The experiments were conducted for two different kinds of sandstones, Bentheimer and Flechtinger. During all experiments, the confining hydrostatic pressure was continuously increased up to 85MPa. To measure the undrained pore pressure response within the interior of the sample, a novel fibre optic sensors technique was applied. Subsequently, we compared the direct measurement of , B dir and to indirect calculated values using C and C s u . For low effective pressure, it showed that all parameters display a non-linear decrease with increasing effective pressure. At high effective pressure the decrease of all poroelastic coefficients is almost linear with increasing effective pressure. In accordance with Terzaghi's effective stress concept, the comparison between direct and indirect methods showed a good to excellent agreement for all calculated parameters. Differences between the direct and indirect methods were used to quantify drained and unjacketed pore compressibilities, as well as jacket effects during the experiment. [Copyright &y& Elsevier]

Cyclic hydraulic fracturing, which employs cyclic injection with alternating high and low injection rates or pressurization, is suggested to assist in reducing induced seismicity. Our laboratory work demonstrates the impact of cyclic hydraulic fracturing on tensile-dominated fracturing of intact Pocheon granite core specimens. Cyclic injection of water reduces the breakdown pressure by up to ∼20% and the maximum acoustic emission amplitude is reduced by ∼14 dB on average. We observe hydraulic fractures in granite specimens through using computed tomography. Cyclic hydraulic fracturing creates complex fractures with more branches and smaller apertures compared with those created by continuous injection. The average injectivity of the fractured specimens by cyclic injection is smaller than the injectivity measured on specimens fractured by continuous injection. [ABSTRACT FROM AUTHOR]