COUPLED HYDRO-MECHANICAL SIMULATIONS Dr Paul Duplancic pduplancic@beck.engineering

Background Simulation Aided Engineering

Why? Hydro-Mechanical Coupling Because the effect of fluid pressures acting on the solid skeleton of a porous medium plays an important role for many geotechnical applications (groundwater, reservoir examples) Effective stress concept presence of pwp influences the constitutive response Because of Similitude / History Matching comparing like-with-like Mechanical Displacements (subsidence) Stresses Plastic strains (damage) RER (seismicity) Hydrological Pore Pressures Flux rates Similitude = good for ‘calibration process‘ Because of the importance of groundwater for geo-technical applications Traditionally the solution of the groundwater models was separate from mechanical models (due to restrictions in computational capacity) However, the mechanical problem and the hydrological problem are coupled-problems in reality (mainly to name the change of conductivity with increase of damage and flow in fractured rock along gaps or in narrowed fault zones) Mine-scale models require minimum-standards of complexity to achieve similitude, hence full coupled solution approaches are desirable but difficult to implement. Very often information for the hydrological aspects of the problem are limited and have higher uncertainty, hence staggered coupling schemes can be used. Due to the complexity required for large-scale models to achieve similitude, both mechanical and hydrological problems have often been analysed separately, de-coupled, in the past. But in reality it is a coupled problem

How? Hydro-Mechanical Coupling For 3D, large-scale (mine- or reservoir-scale) models including high complexity in geometry and sequence Separate solvers for both the mechanical and hydrological analysis Both solvers communicate with each other, exchanging data at runtime 𝝈 eff = 𝝈 tot + 𝑏 𝛼 𝑝 𝑤 𝟏 𝑘 𝑤 = 𝑘 𝑤0 ⋅𝑓 𝑃𝑆𝑇 pw: wetting phase pressure PST: plastic strain (rock mass damage quantity) Mahyari-Selvadurai (1998), Shirazi-Selvadurai (2005) [exponential law, based on equiv. deviatoric (plastic) strain] technically conceptually

How? Hydro-Mechanical Coupling rock mass damage distr. Rock Mass Damage Hydraulic conductivity Minor Sign. Moderate None Very Sign. 𝝈 eff = 𝝈 tot + 𝑏 𝛼 𝑝 𝑤 𝟏 𝑘 𝑤 = 𝑘 𝑤0 ⋅𝑓 𝑃𝑆𝑇 Mahyari-Selvadurai (1998), Shirazi-Selvadurai (2005) [exponential law, based on equiv. deviatoric (plastic) strain] Chin et al. (2000) [power law, based on volumetric strain via porosity] Hsiung et al. (2005) [power law, based on stress and plastic failure strains as well as fracture measures] Mahyari-Selvadurai (1998), Shirazi-Selvadurai (2005)

EXAMPLE: model features Large extent, relatively close to topology surface Sedimentary domains Detailed sequence 𝑘 𝑤,Aquifer ≫ 𝑘 𝑤,Aquitard orthotropic properties

SUBSIDENCE MATCH Subsidence

Rock MASS DAMAGE COMPARISON

PORE WATER COMPARISON Match to piezometric measurements of pore water pressures Capture effect of perched water table above goaf

RESULTS: PWP AND DAMAGE damage and PWP eveolution with sequence  assessment of hydraulic interconnectivity

RESULTS: AQUIFER INTERCONNECTION POTENTIAL heterogenious (here: orthotropic) permeabilities independent damage-dependency

CONCLUSIONS almost any fluid-flow simulator could be used (establishing coupling via datafile-i/o at runtime) fluid-flow simulator reports wetting-phase pressures to geomechanical rockmechanics-simulator rock mass damage is reported back to fluid-flow simulator, which can use updated hydraulic properties much more realistic behaviour of fluid mechanics and geomechanics forecasting additional benefits: obtaining subsidence-plots establishing damage-induced ‘new‘ flow-channels capture fault flow and transient flow robust coupling framework each simulator can run on machines located in different parts of the world (good experience Australia <-> Europe)