1. A method for reservoir simulation, comprising: applying, using a processor, traction boundary conditions at a surface of a grid representing a subterranean reservoir; and finding, using the processor, a rock state, representing initial rock displacements and porosity in the grid, that satisfies a first equation of V e NTGφ e =V 0 NTGφ 0 =PV 0 where V e is a bulk volume after application of the traction boundary conditions, NTG is a user-specified net-to-gross ratio, φ e is porosity after application of the traction boundary conditions, V 0 is an initial bulk volume which is directly calculated from Cartesian or corner point geometry, φ 0 is an initial porosity, and P is pore pressure; initializing, using the processor, the grid based on the rock state to generate an initialized grid; determining, using the processor, a set of rock displacements based on the initialized grid and mechanical properties measured by a sensor disposed about the subterranean reservoir; and generating, using the processor, a simulation result based on the rock displacements, wherein an operation in the subterranean reservoir is performed based on the simulation result.