1 Introduction Definitions and some challenges of reservoir geomechanics. Modeling of coupled phenomena. 2 Constitutive Laws: Behavior of Rocks Fundamentals of Pore-Mechanics. 3 Constitutive Laws: Behavior of Fractures Geomechanics of Fractured Media. 4 Reservoir Geomechanics Elements of a geomechanical model and applications. 5 Unconventional Reservoirs Naturally fractured reservoirs, hydraulic fracture, proppant and fracture closure model, validation (microseismicity). 6 Advanced Topics Injection of reactive fluids and rock integrity. Introduction to Reservoir Geomechanics Earth Sciences and Engineering: Problems Scales micro lab wellbore reservoir basin Earth Reservoir Geomechnics micro lab wellbore reservoir basin Earth Reservoir Geomechnics micro lab wellbore reservoir basin Earth Reservoir Geomechnics Geophysics Structural Geology Reservoir Engineering Geomechanics (mechanics of deformable porous media): integrating data from different scales… Underground stresses Normally, an underground formation has to carry the weight of the overlying formations. The vertical stress at the bottom of a homogeneous column of height z is: σv = ρgz where ρ is the density of the material and g is the acceleration of gravity. If the density varies with depth, the vertical stress at depth D becomes: The average density of sediments in the overburden is between 1.8 and 2.2 g/cm3, so as a rough number, the vertical stress increases downwards with about 20 MPa/km (typically 1 psi/ft). Note: ) ( ) 1( o o w w s S S           Off-shore The sedimentary rocks encountered during oil well drilling and production are porous and hence contain fluids. One refers to the pore pressure at depth D as normal if it is given by the weight of a fluid column above, the normal pore pressure pfn is The pore fluid density in case of brine with sea water salinity is in the range 1.03–1.07 g/cm3, so the pore pressure increase with depth is roughly 10 MPa/km (0.45 psi/ft). The effective vertical stress, σ’v, is then also increasing with approximately 10 MPa/km. In many important cases, however, the pore pressure deviates from the normal value pfn (abnormal pore pressures). Effective stress tensor: Fluid Pressure σ’ = Underground stresses L. Cabral (MSc, 2007) Vertical stress - Integration of density values (obtained from the density profiles); r(z) - Specific weight along the depth. Pore Pressure - Direct measurement ; Bottom well pressure transducers Density Profile Sonic Profile Density Pressure Fluid Pressure Fluid Pressure Fluid Pressure Fluid Pressure Underground stresses Horizontal Stress: K’ (lateral earth stress coefficient) may vary significantly. At shallow depths (0–150 m) it may vary from 1 to 10 or even higher. At larger depths it may vary from 0.2 to 1.5. Chemical compaction increases in importance at depths below 2–3 km (Bjørlykke and Høeg, 1997). It will contribute to horizontal stresses by altering the trend seen from pure mechanical compaction above this level. It has been suggested that, with time, K’ → 1 , so that the stress state becomes hydro- static (Heim’s rule) due to creep (very slow process). Underground stresses Models for estimating the in situ stress states: Formation laterally constrained ( ) rock behaves according to the theory of linear elasticity: There are many reasons to apply the relationship above with great care... (lateral stress coefficient at rest, K’=K0) In a fluid, where νfr = 1/2, K0 = 1. For a rock with νfr = 1/3, K0 = 1/2 Stress history analysis must be performed (Warpinski, 1989), incorporating variations in mechanical properties over time: Consolidation, diagenesis, changes in pore pressure due to gas generation, temperature gradients, and various tectonic and thermal episodes. Viscoelasticity appeared to be more relevant for stresses in shale than in sandstone (Warpinski, 1989). Underground stresses Models for estimating the in situ stress states: Rock at failure: reasonable assumption in areas of active tectonics. Mohr-Coulomb: Assuming coesion=0: If the friction angle is 30°, then K’ = 1/3. A lower friction angle will result in a higher value for K’. Relevant information from the faults: relative magnitude of principal stresses. Faults and the stress state: Brittle behaviour leading to fault formation is characteristic of rocks subjected to low confining pressure, i.e. in some respect close to the surface of the Earth.