Presented here is a method by which we are using a shared earth model in the planning of development wells. The latest 3D modelling packages give us the ability to design non-conventional well trajectories interactively, inside a 3D geological model. This means that we can easily generate a number of possible well paths designed to penetrate particular features of the reservoir. Final well placement decisions are normally based on performance predictions from a reservoir simulator, but the time required to run the simulator for all possible wells would be prohibitive. There is clearly a need for a rapid screening or ranking technique to identify the well paths most likely to be suitable, and we describe here some recent work to meet this need.