1. A computer implemented method of forming a model of fluid flow rates of a multilateral well in which fluid from a subsurface reservoir enters perforations in a plurality of pipes off a main bore of the well, the model being based on data measurements regarding the fluid, well and reservoir, and comprising the computer implemented steps of: determining an initial measure of transmissibility of the pipes based on the data measurements and on the pipes having fluid flow characteristics of a porous media; forming a linear model of postulated well potentials at perforations in the pipes based on the initial measure of transmissibility; determining a measure of well potential at the perforations in the pipes by computer processing to solve the linear model; testing the determined measures of well potential at the perforations in the pipes for satisfactory convergence within a specified limit of accuracy; and if convergence is not achieved, adjusting the postulated well potentials of the linear model, and repeating the steps of determining and testing measures of well potential; or, if convergence is achieved, determining flow rates for the pipes of the multilateral well based on the determined measures of well potential at the perforations in the pipes.