Hydraulic fracturing in unconventional shale gas reservoirs Dr.-Ing. Johannes Will, Dynardo GmbH, Weimar, Germany
Founded: 2001 (Will, Bucher, CADFEM International) More than 35 employees, offices at Weimar and Vienna Leading technology companies Daimler, Bosch, Eon, Nokia, Siemens, BMW are supported by us Software Development Dynardo is your engineering specialist for CAE-based sensitivity analysis, optimization, robustness evaluation and robust design optimization. CAE-Consulting Our expertise: Mechanical engineering Civil engineering & Geomechanics Automotive industry Consumer goods industry Energy Industry 2 Simulation of Hydraulic Fracturing
Hydraulic fracturing To mine unconventional shale gas stimulation of the reservoir rock becomes necessary for a profitable gas production Hydraulic fracturing is used to create a large and complex network of fractures which connects the production wells with the greatest possible volume of reservoir rocks - Horizontal well driven in the reservoir layer. - Water pressure is fracturing (enhancing natural and create new fracture) the jointed rock. - Proppant is added to keep fractures open after fluids have been removed and pressure has been subsided. 3 Simulation of Hydraulic Fracturing
Simulation of hydraulic fracturing of jointed rock Dr.-Ing. Johannes Will, Dynardo GmbH, Weimar, Germany
Challenge of modeling hydraulic fracturing Shale is a jointed rock Because of bedding plane and natural fracture system anisotropic strength behavior dominate fracture growth Fracture network dominates fluid flow Therefore 3D geometric model including strength anisotropies and fracture flow approach is mandatory Isotropic mechanical material models will fail 2D or pseudo 3D modeling will fail Porous flow approach inadequat Rock mechanical challenge or the question: Discrete or smeared modeling of joints Discrete joint modeling in 3D result in computational and parameter overkill Therefore homogenized continuum approach for seepage flow in jointed rock which was established for 3D FEM simulation in jointed rock in dam engineering in 1980 /90 s is the method of choice 5 Simulation of Hydraulic Fracturing
homogenized continuum approach mechanics Homogenized continuum approach does not model joints discrete. Jointed rock will be modelled as volume having intact rock and sets of strength anisotropies (joints). Matrix and joints will be evaluated at every discretization point! Major fault Sets of joints: K1, K2, Sch (picture from Wittke, W.: Rock Mechanics, Theory and Application with Case Histories, ISBN/EAN: 3540527192 But major faults will be modelled discrete with a layer of volume elements, having plane of weakness and matrix material. 6 Simulation of Hydraulic Fracturing
Mechanical analysis Mechanical analysis For nonlinear mechanical analysis material model from multiplas based on anisotropic Mohr-Coulomb or Drucker Prager model are used. Consistent numerical treatment of multisurface plasticity describes failure of intact rock and 3 joints. In multiplas the joint orientation is defined by a plane x -y in local coordinate system, which is related to global (WCS) by two orientation angles alpha and beta. 7 Simulation of Hydraulic Fracturing
Hydraulic analysis Flow is dominated by laminar flow in joints. Homogenized fluid flow approach superpose intact (porous) rock with fluid flow in joints, resulting in anisotropic conductivity matrix K flow equation (mass balance): q R S h t s Darcy s law (momentum balance): q K h transient seepage equation: K x xx h x K y yy h y K z zz h z R S s h t 8
3D-hydraulic fracturing simulator - 3D simulations of hydraulic fracturing based on coupled hydraulic mechanical FEM analysis using ANSYS+multiPlas was setup + 9
Application of Hydraulic Fracturing Analysis Barnett Shale, Texas, US 2008/2009 calculate and calibrate jointed rock volume as well as related gas production
Analysis Plan 1) Setting up a three dimensional layered model which can represent - all important layers including a three dimensional joint systems (bedding plane + two joint sets) - coupled fluid flow mechanical analysis, including propagation of fractures 2) Calibration of important model parameters with measurements (DFIT/ISIP, seismic fracture measurements) 3) Using the calibrated model for sensitivity analysis and optimization of operational conditions [Will J.: Optimizing of hydraulic fracturing procedure using numerical simulation; Proceedings Weimarer Optimierung- und Stochastiktage 7.0, 2010, Weimar, Germany, www.dynardo.de 11 Simulation of Hydraulic Fracturing
Collecting Jointed Rock Properties Rock properties are extracted from core and log data as well as they are assumed from experience and literature. More than 200 parameters: Geometry, layering Elastic properties of rock and joints Strength properties of rock and joints Permeability In-situ stress and pore pressure Joint system orientation Marble Falls Shale Marble Falls Limestone Barnett Shale A Barnett Shale B Barnett Shale C 3 rd joint 170/80 joint for all rock units Barnett Shale D Ellenburger 12 Simulation of Hydraulic Fracturing
Simulation of Hydraulic Fracturing Parametric model setup and calibration
Parametric one well model and mesh Parametric model geometry and mesh generation N reference points [XX3,YY3] [XX5,YY5] ST4 ST3 Well position ST5 Stage 3 with 4 perforations [XX6,YY6] ST6 14 14
in situ stress and pore pressure state effective vertical stress - SZ hydraulic height Calculation of initial situ stress state: The initialization of the in-situ stress condition is very important The different layers have different stiffness and in-situ stress states Non-linear mechanical analysis for every layer necessary Calculation of initial pore pressure: All nodes get hydraulic height as boundary condition: H = z g pp / g w H hydraulic height z node z coordinate g pp pore pressure gradient (over pressured) g w pore pressure gradient (water) 15 ECF 18 Simulation of Hydraulic Fracturing
Coupled mechanical and fluid flow analysis Mechanical Analysis of Fracture Growth Mechanical analysis: coupling non linear time history analysis Starting from in-situ stress state, forces from the updated pore pressure field are effecting the stress field if effective stresses violate strength criteria plastic strain (fracture growth) occurs Fluid Flow Analysis to update Pore Pressure Frontier Transient hydraulic analysis: Nodes at perforations get hydraulic height according to the bottom hole pressure of the hydraulic fracturing regime Coupling of hydraulic and mechanic analysis via updated anisotropic permeability matrix 16 ECF 18 Simulation of Hydraulic Fracturing
3D-hydraulic fracturing simulator Input parameters Schematics of 3D coupled hydraulic-mechanical simulation FE-model Initial pore pressure Initial effective stresses Conductivity update Main loop Transient hydraulic analysis Mechanical analysis Flow force update Outputs/results 17 Simulation of Hydraulic Fracturing
Hydraulic Fracturing needs calibration Because of the large amount of uncertain jointed rock and reservoir parameter the reservoir model needs advanced calibration procedure. Sensitivity, Calibration & Optimization Calibrator Optimizer 3D-hydraulic fracturing simulator FEA Solver 18 Simulation of Hydraulic Fracturing
Model Calibration Calibration At first, numerical key parameters such as the maximum permeability of open joints or energy dissipation at pore pressure frontier are calibrated. Then with the help of optislang, a sensitivity study of 200 physical parameters is performed to identify the most important parameters. The mechanisms of important parameters are validated and the model is calibrated to the measurements. The calibrated model is later used to optimize the stimulated volume and to predict the gas production rate of the wells. 19 Simulation of Hydraulic Fracturing Blue:Stimulated rock volume Red: seismic frac measurement
Modeling of Hydraulic Fracturing Using the Calibrated Model for Prediction and Optimization
Calculation of Gas Production From the calibrated model of one stage we derive a shape factor between measurement data (maximum width*length*height) and 3D volume from our simulator. Can we use the shape factor for all stages? 21 Simulation of Hydraulic Fracturing
Calculation of Gas Production 25 117 Using the shape factor of stage 1, we estimated the total volume of all 5 stages. Using field correlation data between stimulated volume and gas production, we estimated the gas production. real production FM_estimated dynardo Calibration well 24.48 MMscf 40 25 That was a positive result that the estimation of total gas production is very good using the shape factor! But can the shape factor be used to forecast the neighboring well? 22 Simulation of Hydraulic Fracturing
Forecast of Gas Production Forecast well is located 0,5 mile south of calibration well Forecast well used 6 active stages Stimulated volume of the two wells cross Calibration well Forecast well 23 Simulation of Hydraulic Fracturing
Forecast of Gas Production Seismic fracturing measurement estimated the total stimulated Barnett Shale volume to 266 e6 ft 3 and the 6 month cumulative gas production is estimated to 71.5 MMscf. Using fracture mapping results of maximum width, length and height as well our body shape factor of 4.0 we estimate the stimulated volume to 103 e6 ft 3 and 6 month cumulative Gas production of 27.5 MMscf. 27.5 103 real production FM_estimated dynardo Calibration well 24.48 MMscf 40 25 Forecast well 25.95 MMscf 71.5 27.5 The estimation of stimulated Barnett Rock volume with the shape factor shows again a very nice agreement with the real 6 month gas production. 24 Simulation of Hydraulic Fracturing
Optimization of Gas Production By improving just one fracture design parameter, the stimulated volume (and the gas production) could improve by 25%. initial design stage1 Barnett Volume=24.2 e6 Improved frac design Barnett Volume=30.2 e6 Pressured volume at 193 min (end of pressuring) 25 Simulation of Hydraulic Fracturing
Application of Hydraulic Fracturing Analysis other Reservoirs, US 2010/2011 calculate and calibrate joint network creation including stage and well interaction
Simulator improvements 2010 calculate and calibrate jointed set opening, investigate stage interaction and sensitivities of reservoir and hydraulic fracturing design parameter During 2010 following improvements are implemented: - Parametric model of multiple stages - Improvement of hydro mechanical coupling, calculation of joint set openings and related anisotropic conductivity updates - Introduction of influence of Joint Roughness Coefficient (JRC) and ratio of geometric and effective hydraulic opening to fluid flow in fractures - Introduction of parametric perforation efficiency - the overlapping of stimulated rock volumes is investigated and overlapping factors are derived 27 Simulation of Hydraulic Fracturing
Simulator improvements 2010 Visualization of geometric joint set openings normal to joint plane 1 st joint bedding plane 2 nd joint set 3 rd joint set joint set openings [in] 28 Simulation of Hydraulic Fracturing
Simulator improvements 2011 Industrial projects (short term) - Improvement of parametric to model and calculate multiple stages at multiple well to investigate well interaction and re-stimulation - Introduction of dilatancy functions and non local material models to improve accuracy of permeability update Funded projects (mid term) - Speed up simulation process and minimize memory requirements - Extraction of most probable network of joints, export network to reservoir simulators and CFD codes - Implementation of stress dependent conductivity decline to run flow back and production 29 Simulation of Hydraulic Fracturing
Model and Mesh FE-model Mesh for hydraulic analysis 1 to 8 approach 8x finer mesh of mechanical model mesh of hydraulic model Hydraulic mesh needs to be finer to cover fracture discretization, one element in mechanical mesh covers 8 elements in the hydraulic model, but every fluid element needs 5 elements to introduce anisotropic conductivity matrix 30 Simulation of Hydraulic Fracturing
User defined anisotropic hydraulic element Transient hydraulic analysis Motivation / current state rock with four joints is represented by five SOLID70 elements, with identical nodes one element for intact rock defined in global coordinate system each joint is represented by one element defined in joint coordinate system anisotropy is obtained by rotation of the element contributions in the global assembling procedure total hydraulic conductivity k k k xx xy xz k k k xy yy yz k k k xz yz zz hydr. conductivity intact rock k 0 0 R 0 k = + + + R 0 0 0 kr hydr. conductivity joint 1 kj 0 0 1 k 0 J1 0 0 0 0 hydr. conductivity joint 2 kj 0 0 2 k 0 J 2 0 0 0 0 y y 1, 1 2, 2 x x Problems high numerical effort and high memory demand (up to five super-imposed elements are used) during simulation special mesh generation procedures required for the generation of the super-imposed elements and handling of the element properties (e.g. local element coordinate systems) post-processing is difficult due to super-imposed SOLID70-elements (special post-processing procedures required time-consuming) Solution New hydraulic element with general anisotropic conductivity 31 Simulation of Hydraulic Fracturing
User defined anisotropic hydraulic element Transient hydraulic analysis Element definition implementation of a new element using ANSYS user-defined element API (USER300) 8 node, isoparametric brick element one degree of freedom per node: hydraulic height fully integrated (2x2x2 Gauss quadrature) ansiotropic hydraulic conductivity matrix support for lumped storativity matrix element body load: internal flow generation rate Performance one stage structured mesh, PC 2 proc 100.000 mechanical elements 800.000 fluid elements new old reduction simulation + post processing 41 hours 53,5 hours 24% memory requirements 9 GB 19 GB 47% Get ready for HPC environment running multiple stages at multiple wells 32 Simulation of Hydraulic Fracturing
New hydraulic element testing and performance new old Pore pressure Joint opening 33 Simulation of Hydraulic Fracturing
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