Thickness sealing effect of relative caprock and its application

The concept of the thickness sealing effect of relative caprock is put forward,and the thickness sealing effect of relative caprock is proven according to theory,experiments and practical application.Theoretically,it is known from the seeping formula of non-Darcy flow that petroleum can leak through caprock only when the driving pressure gradient is greater than the starting pressure gradient of the caprock;the starting pressure gradient is determined when the properties of stratum and the fluid are given,so with the increase of the caprock thickness,the caprock can afford the greater driving pressure of petroleum and the sealed petroleum column is higher.According to experiments,the breakthrough pressure of the connected caprock core increases with the increase of the length of the connected core and it is direct proportion to the sum of the breakthrough pressures of the connected cores.Practically,it is well interpreted using the thickness sealing effect of relative caprock why commercial biogenic gas pool can be formed in the Quaternary of Qaidam Basin under the caprock condition of high-porosity,high-permeability and low breakthrough pressure.