Coupled Thermo-Hydro-Mechanics in Fractured Rocks: Modelling Status and Data Needs Chin-Fu Tsang Lawrence Berkeley National Laboratory USA Imperial College London UK Uppsala University Sweden

Talk Outline Introduction to coupled thermo-Hydro-mechanical (THM) processes in geological systems Two examples of major field studies Study of FEBEX Experiment Study of YM Heater Test More recent Studies Coupled THM processes in CO2 injection storage Coupled THM processes in methane production from gas hydrates General lessons learned, challenges and data needs

Introduction A number of geoscience problems require the understanding and modeling of the effects of coupled processes involving Temperature gradient (T) Hydrologic flow (H) Mechanical deformation (M) Chemical reactions (C) in fractured rock and bentonite (and argillaceous clay) systems One major example is the concern over their role in the performance of a radioactive waste geologic repository However, the problem is of wider interest, such as — geothermal energy extraction, — earthquakes induced by fluid injection, — fluid injection into deep petroleum reservoirs, — disposal of solid waste — sequestration of CO2 and — gas production from methane hydrates

Scientific Challenges The processes, T, H, M and C have widely different characteristic time and spatial scales Thermal effects (T) in rock material has relatively large time and spatial scales Mechanical effects (M) have a short time scale, responses can propagate with the speed of sound (Deformability is controlled mainly by the presence of large discontinuities, such as faults and shear zones) Groundwater flow and transport (H, C) are sensitive to small-scale heterogeneities and characterized by long time periods for flow and solute transport

Numerical Modeling Challenges Numerically, T, H, M and C processes are usually modeled by different techniques, such as finite-element methods (FEM) discrete-element methods (DEM) finite-difference methods (FDM) discrete fracture network (DFN) methods In addition, many of the coupled processes are nonlinear and anisotropic Constitutive equations typically contain different parameter sets, with uncertain parameter values To combine all these processes into a coupled numerical model is a major challenge

Recent International Projects for studying THM(C) Processes DECOVALEX Project (1992-2007) Gone through 4 phases About 10 countries Many publications D-2011 Project (2008-2011) THERESA Project TIMODAZ Project Others

Talk Outline Introduction to coupled thermo-Hydro-mechanical (THM) processes in geological systems Two examples of major field studies Study of FEBEX Experiment Study of YM Heater Test More recent Studies Coupled THM processes in CO2 injection storage Coupled THM processes in methane production from gas hydrates General lessons learned: challenges and limitations

Two Examples of THM Field Studies Study of FEBEX THM(C) experiment by ENRESA at Grimsel 5-year heating with temperature to 100°C Bentonite clay with fractured crystalline rock Unsaturated bentonite and fractured rock unsaturated near tunnel wall (due to ventilation) Study of coupled THMC (C=chemistry) test at Yucca Mountain, USA 8-year experiment with 4-year heating and 4-year cooling Temperature up to 200°C Unsaturated fractured porous medium (tuff)

FEBEX (FULL-SCALE ENGINEERING BARRIERS EXPERIMENT) AT GRIMSEL TEST SITE IN SWITZERLAND (1997-2002 with followup tests) Multi-national project coordinated by ENRESA (Spain) Max heater temperature = 100 °C

THM PROCESSES OF FEBEX (after Gens, 2003)

Model of FEBEX In Situ Test ROCMAS: A 3D finite element program for analysis of coupled THM processes in unsaturated/saturated geological media (Rutqvist et al, International Journal of Rock Mechanics, 2001) Processes modeled: Finite element grid: Fully coupled THM Liquid water and vapor flow with evaporation-condensation Heat transfer with conduction and convection Thermal expansion Mechanical elastoplastic deformation Moisture swelling and shrinkage Porosity and permeability changes caused by deformation 56 m 66 m

SEQUENCE FOR THM MODELING OF FEBEX 1) Calibration of rock mass properties against measurements during and after excavation of the drift 2) Back-analyses of bentonite properties from laboratory tests 3) Prediction of THM behavior during the heater test (1000 days)

Laboratory Experiments for Back-analyses of Coupled THM Properties of Bentonite Process Experiment T Thermal Conduction Tests Suction Tests (water retention) Isothermal Infiltration Tests (relative permeability) Thermal Vapor Diffusion Tests (diffusion constants) Thermal Expansion Tests Permeability vs Porosity for Gas Flow and Liquid Flow Oedometric Tests with Controlled Suction (swelling behavior) H TH TM HM

TH Properties of Bentonite Buffer Material Water retention: Van Genuchten’s P0 = 35 MPa ( = 2.910-8 1/Pa), m = 0.45 Intrinsic permeability: k = 2.0×10-21 m2 Relative permeability: kr = S3 Tortuosity factor:  = 0.8 Thermal conductivity:  = 0.6 to 1.3 W/(m C) Specific heat (solid part) Cs = 138T + 732.5 J/(kg C) Thermal diffusion factor ftv = 2 (ftv enhances vapor diffusion under thermal gradient ,Philip and De Vries, 1957)

Back-analysis of state surface parameters from laboratory tests Bentonite Properties Back-analysis of state surface parameters from laboratory tests Oedometric test with controlled suction (s) State surface: Measured: s z Relation between state surface and measured quantities: dz M  z  Numerical modeling of lab tests to back-calculate a, b, c, d

Back-analysis of state surface parameters from laboratory tests Bentonite Properties Back-analysis of state surface parameters from laboratory tests Swelling under constant external load 1) Apply external load on unsaturad sample 2) Saturate sample under constant external load and measure strain Moisture z z dz Saturation S Si  50%

Back-analysis of state surface parameters from laboratory tests Bentonite Properties Back-analysis of state surface parameters from laboratory tests Swelling under constant external load  100 % saturation S dz z = 0.5 MPa  98 % saturation  50 % saturation Experiment Modeling

Back-analysis of state surface parameters from laboratory tests Bentonite Properties Back-analysis of state surface parameters from laboratory tests Swelling under constant external load z z = 0.5 MPa dz z = 1 MPa S z = 5 MPa Experiment z = 10 MPa Collapsing behavior Modeling

Prediction of THM behavior at FEBEX heater test Calculation sequence: 1 Pre-heating) Starts 8 months before the heat is turned-on to take into account the wetting of the benonite during experimental setup. 2 Heating) The heater power is increased step-wise during the first 53 days of heating. Then the power of each heater is individually controlled by a constant heater temperature of 100 C Heating Pre-heating

COMPARISON OF MODEL RESULTS WITH FIELD MEASUREMENTS The figure shows examples of comparison of temperature in the rock water saturation and stress in the buffer Good agreement between simulated and measured results, especially for temperature and water content. Measured delay in swelling stress during the first several months was probably caused by the existence of gaps between the bentonite blocks.

Yucca Mountain DST Thermal Test Heat released from radioactive decay of the waste gives rise to T H C M processes within the unsaturated rock mass, which would impact the transport of radionuclides Test objective is to acquire a more in-depth understanding of the coupled processes Drift Scale Test, 60 m scale, ~190 kW for 4 years (max temperature 200oC on drift wall) Close integration of detailed numerical modeling along with testing program

TH Coupled Processes Heat Transfer Conduction, convection Counterflow of liquid and vapor - heat pipe Moisture Redistribution Fast transport of vapor in fractures Condensation Imbibition into matrix Gravity drainage in fractures Drying front moves out from heat source with time As heat output declines, dryout zone contracts

THM Coupled Processes Processes Thermal expansion and thermally induced stresses Open or close fractures Change porosity Change permeability Impact Altering flow paths and effect on water seepage into drifts Far field flow and transport

THC Coupled Processes Chemical evolution of waters, gases and minerals coupled to TH Drying concentrates aqueous species in remaining liquid phase Pure water in condensation zones promotes dissolution of minerals Reaction rates Generally increase with elevated temperatures pH affected by CO2 degassing and transport Mineral dissolution and precipitation Changes porosity and permeability Alters chemistry of water that could contact waste package if seepage into drift occurs

Drift Scale Test (DST) Heat Turned on: Dec 3, 1997 Heat turned off: Jan 14, 2002 Cooling phase monitoring and testing until April 2006

} Probing THMC Processes-Drift Scale Test Periodic Active Testing during Heating and Cooling Pre-Test Characterization Laboratory T-Properties H-Properties M-Properties MIN/PET Pore water Field Rock Classification Fracture Mapping Borehole Videos Air Permeability } GPR Neutron log ERT Matrix Liquid Saturation Air Permeability Gas Sampling Water sampling Passive Monitoring during Heating and Cooling Temperature Displacement Strain Humidity Pressure Acoustic Emission (microfracturing)

Comparing Model Results with Measurements Close integration of sophisticated and detailed numerical modeling with measurements Pre-heat characterization to measure site-specific properties important for coupled processes Model predictions prior to commencement of test Use of early test results to discriminate alternative conceptual models applied in pre-test simulations Numerical Tools TH: TOUGH2 THC: TOUGHREACT THM:TOUGHFLAC Model predictions compared to data: selected examples THC: CO2 evolution, dissolution and precipitation TH, THM: Fracture air permeability evolution from moisture redistribution and stress-induced fracture aperture changes

DETAILED ANAYSIS OF GROUPS OF AIR-PERMEABILITY DATA TO DISTINGUISH TM FROM FULL THM EFFECTS TM only Away from Drift Mid TM only Near Drift

Measured and Modeled CO2 over time Gas Sampling Degassing of CO2 from pore water with elevated temperature: thus CO2 concentration in gas samples initially increases with temperature As temperature exceeds boiling, rock mass dries out and CO2 concentration drops Model Predictions Successfully captured trend of CO2 field data without calibration Changes in CO2 partial pressure play important role in pH and water chemistry and subsequent mineral alteration

Talk Outline Introduction to coupled thermo-Hydro-mechanical (THM) processes in geological systems Two examples of major field studies Study of FEBEX Experiment Study of YM Heater Test More recent Studies Coupled THM processes in CO2 injection storage Coupled THM processes in methane production from gas hydrates General lessons learned: challenges and limitations

CO2 Injection Into a Brine Aquifer Effects of stress change on the performance of an injection site Hydraulic properties is a function of effective stress:  = f(’) k = f(’) Pc = f(k, ) Hydraulic fracture when P > h Expansion of aquifer rock:  = f(’,E) Fault slip when  > (n - P)tan

Model Simulation of Injection Operation CO2 Injection Pressure (constant rate injection)

Geomechanical Effect of CO2 Injection - 2D Multi-Layer System at Depth with Faulted Caprock Failure Criteria for Fault Slip and Hydraulic Fracturing Stress-Induced Hydrological Property Changes

Hydromechanical Changes After 30 Years Reduction in Vertical Compressive Stress CO2 Saturation Faults allow for significant leakage from storage aquifer Upflow is partially diverted sideways into middle and upper aquifer Significant pressure and stress changes in upper aquifers, causing hydromechanical changes

Potential for Fault Slip Fault Stress Regime (h= 1.5v) Fault Stress Regime (h= 0.7v) Potential for injection-induced slip reactivation may be largest in upper layers, depending on initial stress field For fault stress regime (h= 1.5v), reactivation occurs on subhorizontal faults. Faulting limited to the region of strongly increased fluid pressure. For fault stress regime (h= 0.7v), reactivation occurs on subvertical faults. Faulting predominantly in the upper zones of multi-aquifer system.

Observations on CO2 Geomechanical Effects A general reduction in the effective mean stress (due to pressure increase and local stresses) induces strongly coupled hydromechanical changes in the lower part of the caprock. Shear reactivation in the lower part of the caprock could take place at an injection pressure below the lithostatic pressure. For multi-layer systems, the potential for injection-induced slip reactivation may be largest in upper layers, depending on initial stress field The type of stress regime (e.g., compressive or extensional types) is a key parameter that determines whether fracturing and shear slip are likely to take place along subhorizontal or subvertical fractures. Once the CO2 fluid leaks to the upper part of the caprock (for example through a permeable fault) the upward CO2 migration is accelerated because of the combined effects of relative permeability and viscosity changes, as well as changes in intrinsic permeability caused by pressure-induced hydromechanical effects.

Methane Hydrate A solid form of water that contains a large amount of methane within its crystal structure (a clathrate hydrate) One liter of methane clathrate solid contains about 168 liters of methane gas (at STP) Methane hydrates occur naturally in hydrate-bearing sediments (HBS) offshore in shallow depths below the ocean floor and onshore beneath the permafrost If economically producible, it could contribute significantly to future energy supplies Several production methods, including depressurization, thermal methods, and inhibitor injection, are being considered for extraction of gas from HBS "Burning ice". Methane, released by heating, burns; water drips. Inset: clathrate structure (University of Göttingen, GZG. Abt. Kristallographie). Source: USGS 37

Geomechanical Performance During Gas Production Deposits that are suitable targets for production often involves unconsolidated sediments that are usually characterized by limited shear strength The geomechanical response of hydrate-bearing sediments and potential wellbore instability and casing deformation are serious concerns that need to be addressed and understood before industrial gas production from hydrate deposits can be developed During production, the dissociation of the solid hydrates (a strong cementing agent) can significantly degrade the structural stability of hydrate-bearing sediments the evolution of pressure, temperature, saturation distribution, and salt concentration in hydrate-bearing systems undergoing hydrate dissociation or formation A coupled geomechanical numerical simulator based on the hydrate simulator TOUGH-HYDRATE and the geomechanical simulator FLAC3D 38 38

Geomechanical Properties ? Data on geomechanical properties of hydrate-bearing sediments are very limited; laboratory methods challenging Currently, geomechanical properties are taken from laboratory experiments on hydrate-bearing Toyoura Sand (Masui et al., 2005, 2008) Assumed elasto-plastic, Mohr-Coulomb model and modified elastic and strength properties for pore-filling hydrate (and ice) (Masui et al. OTC2008) 39 39

Numerical Test of HBS Mechanical Behavior During Methane Production Pressure, temperature, and stress conditions correspondent to an oceanic HBS Simulate constant rate production for 15 days 40

Numerical Test of HBS Mechanical Behavior During Methane Production: At 15 days Hydrate Saturation Ice Saturation 41

Numerical Test of HBS Mechanical Behavior During Methane Production: At 15 days Bulk Modulus Cohesion 42

Numerical Test of HBS Mechanical Behavior During Methane Production: At 15 days Volumetric Strain Vertical Displacement 43

General Lessons Learned, Challenges and Needa (1/3) Significant advances has been made in thermo-hydro-mechanics of geological systems, both In understanding and insight in modeling techniques A number of challenges has been met through studies under a number of international projects Research cooperation at a deep and detailed level Use of alternative approaches, multiple conceptual models, and different simplification methods advanced insight and understanding Advanced models and test against field data 44

General Lessons Learned, Challenges and Needs (2/3) A number of challenges remain ahead as we get into new problems such as those related to CO2 injection storage and methane production from gas hydrates. Examples are Constitutive relationships, probably mainly through comprehensive laboratory investigations on core samples --- a major investment is needed Efficiency in handling highly non-linear problems Efficiency in handling complex geologic geometry Large scale realistic field studies with extensive monitoring and measurements are needed 45

General Lessons Learned, Challenges and Needs (3/3) Limitations to the current approaches include How to improve laboratory measurements How to handle heterogeneities How to upscale THM from small scale measurements to larger scales How to predict far into the future How to calculate prediction uncertainty ranges Need new ideas and approaches convert the limitations to challenges and then work on challenges to obtain solutions!!! 46

Work continues: To see order out of disorder from multiple THM-C couplings!

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