Modelling of the Shear Dilation Based Hydraulic Stimulation in EnhancedGeothermal Systems Considering Fractures in Different Scales

A numerical approach for modelling of shear dilation of existing fractures in hydraulic stimulation of geothermal reservoirs at low elevated pressures is presented. The fractured rock in the reservoir is modelled as a combination of explicitly represented fractures and the rock matrix surrounding these fractures. The efficient modelling of slip-induced permeability enhancement requires coupling of the fluid flow in fractured rock with the mechanical deformation of the rock matrix and the shear dilation of the fractures. For flow simulations, conductive fractures are represented in the domain as high-permeable discontinuities; therefore they dominate the overall flow behaviour. The rock matrix is represented by a low permeability, capturing the effect of small-scale fractures. For the mechanical deformation problem, the rock matrix is assumed to be a linear elastic material, while the fractures in the rock matrix are introduced as internal boundaries. The shear dilation of the fractures is calculated by a joint deformation model (JDM), which connects the shear slip in the fracture surfaces and additional permeability caused by shear displacement. The flow simulations and the mechanical deformation of the rock matrix are both obtained by finite volume discretizations. Several numerical experiments designed by resembling realistic reservoir parameters are conducted to provide better understanding of the shear dilation mechanism. Moreover, fractures present in different scales in a geothermal reservoir. Ignoring the effect of small-scale fractures to the fluid flow in the matrix may result in an overestimate of the permeability enhancement. Hence, the influence of rock matrix permeability on fracture aperture and the overall flow behaviour of the reservoir are examined.

[1]  Eirik Keilegavlen,et al.  A finite-volume discretization for deformation of fractured media , 2018, Computational Geosciences.

[2]  L. Durlofsky,et al.  An Efficient Discrete-Fracture Model Applicable for General-Purpose Reservoir Simulators , 2004 .

[3]  R. J. Pine,et al.  Downward migration of shearing in jointed rock during hydraulic injections , 1984 .

[4]  Christine Ehlig-Economides,et al.  A fully coupled method to model fracture permeability change in naturally fractured reservoirs , 2011 .

[5]  Jonathan Richard Shewchuk,et al.  Delaunay refinement algorithms for triangular mesh generation , 2002, Comput. Geom..

[6]  M. Popescu.  Boundary element methods in solid mechanics , 1985 .

[7]  Kimio Watanabe,et al.  A three‐dimensional stochastic rock mechanics model of engineered geothermal systems in fractured crystalline rock , 2000 .

[8]  H. A. Tchelepi,et al.  Discrete fracture model for coupled flow and geomechanics , 2016, Computational Geosciences.

[9]  H. Takahashi,et al.  Progress toward a stochastic rock mechanics model of engineered geothermal systems , 1996 .

[10]  T. Tamagawa,et al.  Numerical Simulation of Hydraulic Shearing in Fractured Reservoir , 2005 .

[11]  Gudmundur S. Bodvarsson,et al.  Hydraulic conductivity of rock fractures , 1996 .

[12]  Jonny Rutqvist,et al.  Stress-dependent permeability of fractured rock masses: A numerical study , 2004 .

[13]  Matthew G. Knepley,et al.  A domain decomposition approach to implementing fault slip in finite‐element models of quasi‐static and dynamic crustal deformation , 2013, ArXiv.

[14]  Roland N. Horne,et al.  An investigation of stimulation mechanisms in Enhanced Geothermal Systems , 2014 .

[15]  M. C. Fehler,et al.  Hydraulic Fracturing of Jointed Formations , 1986 .

[16]  N. Barton,et al.  The shear strength of rock joints in theory and practice , 1977 .

[17]  Ruben Juanes,et al.  Stability, Accuracy and Efficiency of Sequential Methods for Coupled Flow and Geomechanics , 2009 .

[18]  Stein Krogstad,et al.  Open-source MATLAB implementation of consistent discretisations on complex grids , 2012, Computational Geosciences.

[19]  Nick Barton,et al.  An improved model for hydromechanical coupling during shearing of rock joints , 2001 .

[20]  J. Nordbotten.  Cell‐centered finite volume discretizations for deformable porous media , 2014 .

[21]  Ruben Juanes,et al.  Coupled multiphase flow and poromechanics: A computational model of pore pressure effects on fault slip and earthquake triggering , 2014 .

[22]  Yuanle Ma,et al.  Computational methods for multiphase flows in porous media , 2007, Math. Comput..

[23]  R. Horne,et al.  Investigation of injection-induced seismicity using a coupled fluid flow and rate/state friction model , 2011 .

[24]  Gioia Falcone,et al.  A systematic review of enhanced (or engineered) geothermal systems: past, present and future , 2013, Geothermal Energy.

[25]  J. Nordbotten,et al.  Finite volume methods for elasticity with weak symmetry , 2015, 1512.01042.

[26]  Dominique Bruel,et al.  Using the migration of the induced seismicity as a constraint for fractured Hot Dry Rock reservoir modelling , 2007 .

[27]  N. Barton,et al.  Strength, deformation and conductivity coupling of rock joints , 1985 .

[28]  R. Horne,et al.  An embedded fracture modeling framework for simulation of hydraulic fracturing and shear stimulation , 2016, Computational Geosciences.

[29]  Zhixi Chen,et al.  An experimental investigation of hydraulic behaviour of fractures and joints in granitic rock , 2000 .