1. A multi-scale computer-implemented method for use in modeling fluid flow in a subsurface reservoir, the method comprising: creating a fine grid defining a plurality of fine cells associated with a geological formation of the subsurface reservoir, a coarse grid defining a plurality of coarse cells having interfaces between the coarse cells, the coarse cells being aggregates of the fine cells, and a dual coarse grid defining a plurality of dual coarse control volumes, the dual coarse control volumes being aggregates of the fine cells and having boundaries bounding the dual coarse control volumes; calculating basis functions on the dual coarse control volumes by solving local elliptic problems; integrating a source term of an elliptic pressure equation over each coarse cell; and calculating a pressure using an iterative method comprising, for each iteration: (i) applying a smoothing scheme to a solution for the pressure over the fine grid from a previous iteration to provide a smoothed fine-grid pressure; (ii) calculating correction functions using the smoothed fine-grid pressure from step (i); (iii) applying a restriction operation comprising the correction functions from step (ii) to solve for the pressure over the coarse grid; and (iv) applying a prolongation operation to the pressure solved over the coarse grid from step (iii) to reconstruct an updated solution for the pressure over the fine-grid; wherein the pressure calculated using the iterative method is used to model fluid flow in the subsurface reservoir.