Abstract
Some authors believe that a minimum pressure gradient (called threshold pressure gradient (TPG)) is required before a liquid starts to flow in a porous medium. In a tight or shale oil formation, this TPG phenomenon becomes more important, as it is more difficult for a fluid to flow. In this paper, experimental data on TPG published in the literature are carefully reviewed. What we found is that a very low flow velocity corresponding to a very low pressure gradient cannot be measured in the experiments. Experiments can only be done above some measurable flow velocities. If these flow velocities and their corresponding pressure gradients are plotted in an XY plot and extrapolated to zero velocity, a non-zero pressure gradient corresponds to this zero velocity. This non-zero pressure gradient is called threshold pressure gradient in the literature. However, in the regime of very low velocity and very low pressure gradient, the data gradually approach to the origin of the plot, demonstrating a non-linear relationship between the pressure gradient and the velocity. But the data do not approach to a point of zero velocity and a threshold pressure gradient. Therefore, the concept of threshold pressure gradient is a result of data misinterpretation of available experimental data.
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