A method of improving hydrocarbon recovery by modelling fluid flow in a geological volume comprising a geological fault, the method comprising: generating a three dimensional grid of cells representative of the structure of the geological volume; classifying a plurality of cells as fault cells corresponding to the fault in the geological volume; classifying one or more sets of fault cells as relay cell sets corresponding to assumed locations of relays within the fault in the geological volume; generating a three dimensional relay grid of relay sub-cells in the place of each relay cell set, the relay sub-cells of each relay grid being representative of a relay structure within the geological fault and having a finer grid-cell dimension than that of the cells of the relay cell set; determining transmissibilities between the relay sub-cells; and determining transmissibilities between the cells based on the determined transmissibilities between the relay sub-cells; and using the modelled fluid flow to recover hydrocarbons from the geological volume; characterised in that: the vertical faces of the fault cells are offset from the fault; and the fault cells comprise cutted fault cells and non-cutted fault cells, the cutted fault cells having a reduced height as compared to the height of the non-cutted fault cells; the method further comprising: detecting hybrid-interface fault cells interfacing with both the fault and the cutted cells; generating pseudo fault cells representing fault cells in which the hybrid-interface fault cells are represented by pseudo fault cells having a height limited to a height of a portion of the hybrid-interface fault cells interfacing with the fault to compensate for the portion of the hybrid-interface fault cells not interfacing with the fault; wherein the one or more sets of cells classified as relay cell sets are pseudo fault cells. generating a three dimensional grid of cells representative of the structure of the geological volume; classifying a plurality of cells as fault cells corresponding to the fault in the geological volume; classifying one or more sets of fault cells as relay cell sets corresponding to assumed locations of relays within the fault in the geological volume; generating a three dimensional relay grid of relay sub-cells in the place of each relay cell set, the relay sub-cells of each relay grid being representative of a relay structure within the geological fault and having a finer grid-cell dimension than that of the cells of the relay cell set; determining transmissibilities between the relay sub-cells; and determining transmissibilities between the cells based on the determined transmissibilities between the relay sub-cells; and the vertical faces of the fault cells are offset from the fault; and the fault cells comprise cutted fault cells and non-cutted fault cells, the cutted fault cells having a reduced height as compared to the height of the non-cutted fault cells; the method further comprising: detecting hybrid-interface fault cells interfacing with both the fault and the cutted cells; generating pseudo fault cells representing fault cells in which the hybrid-interface fault cells are represented by pseudo fault cells having a height limited to a height of a portion of the hybrid-interface fault cells interfacing with the fault to compensate for the portion of the hybrid-interface fault cells not interfacing with the fault; wherein the one or more sets of cells classified as relay cell sets are pseudo fault cells. detecting hybrid-interface fault cells interfacing with both the fault and the cutted cells; generating pseudo fault cells representing fault cells in which the hybrid-interface fault cells are represented by pseudo fault cells having a height limited to a height of a portion of the hybrid-interface fault cells interfacing with the fault to compensate for the portion of the hybrid-interface fault cells not interfacing with the fault; wherein the one or more sets of cells classified as relay cell sets are pseudo fault cells.