A computer implemented method of reducing displacing fluid breakthrough in a flooding operation in a weak reservoir, said method comprising: a) providing a coupled geomechanics and multiphase flow model ("coupled model") for said weak reservoir, said coupled model including injection well(s) and production well(s) and being capable of simulating a flooding operation using a displacing fluid and rock deformation and rock failure in said weak reservoir; b) said coupled model comprising: i) a reservoir simulator for quantifying displacing fluid flow and oil flow in a flooding operation; ii) a geomechanics simulator for quantifying rock deformation and rock failure induced by said flooding operation; iii) a constitutive model for simulating the rock failure mechanisms including a dilation mechanism, a tensile failure mechanism, and a shear failure mechanism; and iv) a permeability model that can quantify a permeability change induced by rock failure; c) inputting key physical parameters of said weak reservoir into said coupled model; d) simulating flooding operations using the coupled model over a range of injection and production pressures; e) calculating for each simulation in step d) a distribution of stress, strain, volumetric plastic strain, porosity, pressure, and displacing fluid saturation over time and calculating water breakthrough time and cumulative oil production; f) selecting an optimal injection and production pressure based on the results calculated in step e); and g) employing said selected optimal injection and production pressures in a flooding operation for producing oil from said weak reservoir. a) providing a coupled geomechanics and multiphase flow model ("coupled model") for said weak reservoir, said coupled model including injection well(s) and production well(s) and being capable of simulating a flooding operation using a displacing fluid and rock deformation and rock failure in said weak reservoir; b) said coupled model comprising: i) a reservoir simulator for quantifying displacing fluid flow and oil flow in a flooding operation; ii) a geomechanics simulator for quantifying rock deformation and rock failure induced by said flooding operation; iii) a constitutive model for simulating the rock failure mechanisms including a dilation mechanism, a tensile failure mechanism, and a shear failure mechanism; and iv) a permeability model that can quantify a permeability change induced by rock failure; i) a reservoir simulator for quantifying displacing fluid flow and oil flow in a flooding operation; ii) a geomechanics simulator for quantifying rock deformation and rock failure induced by said flooding operation; iii) a constitutive model for simulating the rock failure mechanisms including a dilation mechanism, a tensile failure mechanism, and a shear failure mechanism; and iv) a permeability model that can quantify a permeability change induced by rock failure; c) inputting key physical parameters of said weak reservoir into said coupled model; d) simulating flooding operations using the coupled model over a range of injection and production pressures; e) calculating for each simulation in step d) a distribution of stress, strain, volumetric plastic strain, porosity, pressure, and displacing fluid saturation over time and calculating water breakthrough time and cumulative oil production; f) selecting an optimal injection and production pressure based on the results calculated in step e); and g) employing said selected optimal injection and production pressures in a flooding operation for producing oil from said weak reservoir. <EOS>