A method for modeling fluid depletion in a reservoir is disclosed. A map is divided into cells. For each of the cells a value is stored that is based at least in part on a physical characteristic of the cell. At least one cell that contains a depletion location is identified along with a depletion amount corresponding to that location. An amount of walkers associated with the depletion location is determined. For each walker, a plurality of steps are calculated with each step to an adjacent cell. Each walker starts in the cell containing the depletion location associated with that walker. The visits of all the walkers are recorded by cell. The fluid depletion of each cell is then assessed based at least in part on the number of walker visits for each cell.