Is a starting pressure gradient necessary for flow in porous media

A starting pressure gradient of flow in porous media is commonly reported in some laboratories. Due to its importance to oilfield development,whether a starting pressure gradient really exists requires further researches. Since a single-phase fluid is usually used in measuring a starting pressure gradient and there exists no capillary pressure for single-phase fluids,the capillary pressure can not be the cause of a starting pressure gradient. Fluids in boundary layers flow in retard rather than keep still,so a boundary layer effect can not be the cause of a starting pressure gradient either. The present paper showed that a starting pressure gradient measured in labs can be probably ascribed to the difficulty in measuring flow rate and to the insufficient time of measurements. The skin effect may strengthen the false phenomenon of the existence of a starting pressure gradient. The starting pressure for water injection is due to the formation damage and the wettability hysteresis at well bottom. Flows in porous media do not need a starting pressure gradient at all.