Published in Petroleum Transactions, AIME, Volume 216, 1959, pages 78–84. Abstract The existence of fluid migration across fixed boundaries in oil and gas reservoirs has been known for many years. Several techniques have been developed in the past for estimating the rate of migration across fixed boundaries as an aid in planning field development and in the valuation of oilfield properties. The principal deterrent to the use of these techniques lies in the rather extensive reservoir and field data required for calculations of fluid migration. For this reason, a new, simplified procedure has been developed which makes possible the calculation of fluid migration with a minimum of field and reservoir data. This new technique is based on certain solutions of the differential equations describing flow in the reservoir which assume, for the portion of the reservoir of interest, that the formation can be approximated by a homogeneous rock of uniform thickness, that only a single mobile fluid phase exists, and that fluid production at the well is solely a result of expansion of the reservoir fluids. The results of the present work are compiled in a set of curves. These curves can be used to calculate both rate and cumulative fluid migration when the above assumptions are justified. The only data required for such calculations are the production histories of all wells in the field, the permeability and porosity of the reservoir, the compressibility and viscosity of the fluid, and the shape of the reservoir. Introduction The migration of fluids across fixed boundaries in oil and gas reservoirs has long been recognized as an engineering and economic problem, but no methods for estimating the extent of migration during the development stage of a reservoir have been published. The investigation reported here has been directed toward providing a simple technique for estimating the extent of such migration and has therefore been restricted to the simple case of a homogeneous reservoir of uniform thickness containing compressible liquids. It is assumed that only a single mobile fluid phase exists and that fluid production at the wells is solely by expansion of the reservoir fluids.