Abstract: A method of determining capillary pressures in a fluid reservoir comprising three phases g, o and w, the reservoir being considered as comprising a plurality of gridblocks, and the method comprising, for each gridblock of the reservoir: determining six representative two-phase capillary pressures, the representative capillary pressures being weighted based on saturation, and using the weighted representative capillary pressures to determine at least two of the three capillary pressures g-o, o-w and g-w. The three phases g, o and w may be gas, oil and water respectively. The method may comprise using the weighted representative capillary pressures to determine the gas-oil and oil-water capillary pressures. The method may comprise using the weighted representative capillary pressures to determine two of the capillary pressures, and implying the other capillary pressure from the two capillary pressures determined using the weighted representative capillary pressures. Normalized gridblock saturations may be used.