Texaco UK North Sea is operator and 100% license holder of the Captain Field which contains a greater than 1.5 billion barrel, viscous oil accumulation. Oil production from the Captain reservoir will be dominated by the water / oil mobility ratio of 30, resulting in severe water coning from an aquifer underlying a large section of the field. For this reason the field will be developed with pumped horizontal wells with completion intervals of up to 6000 ft. Key elements which will affect reservoir and well performance are horizontal to vertical permeability ratio, critical water saturation, mobile transition zone height, and horizontal well inflow profile. To evaluate these parameters a 30 day vertical water coning test and a separate 90 day extended production test in a prototype horizontal development well, were performed and the measured water cut performance and bottom hole pressures matched with simulation models. Additionally inflow performance in the horizontal well was assessed by production logs which demonstrated that inflow occurred along the majority of the well length. These two tests were conducted in close proximity to each other in a reasonably homogeneous sand, providing the opportunity for simultaneous and consistent interpretation of the observed data. This integration of the results from two tests of different types has resulted in interpretations which have greater levels of confidence attached to them than would have been possible if the two tests had been performed in isolation.