1. A computer-implemented method for simulating fluid flow in a fractured subterranean reservoir, the computer-implemented method comprising: (a) providing a reservoir model representative of a fractured subterranean reservoir, the reservoir model including porous matrix control volumes and a plurality of networks of fractures overlying the porous matrix control volumes, wherein each network of fractures comprises fracture control volumes, and wherein each fracture control volume of a particular network of fractures is highly connected to at least one other fracture control volume of the particular network of fractures; (b) for each network of fractures, splitting a pressure vector corresponding to the network of fractures into an average pressure value and remainder pressure values; (c) constructing a system of equations based on scale separation for pressure values in the porous matrix control volumes and the average pressure value of each network of fractures; and (d) for each network of fractures, solving sequentially the system of equations based on scale separation by: (i) approximating a pressure field on a coarse-scale grid responsive to a matrix pressure vector and the average pressure value for the corresponding network of fractures; (ii) obtaining a fine-scale pressure solution responsive to the pressure field on the coarse-scale grid; (iii) computing the remainder pressure values for the corresponding network of fractures; (iv) smoothing the fine-scale pressure solution; and (v) repeating steps (i) through (iv) until the remainder pressure values have converged to a convergence criterion.