Analytical solution of a double moving boundary problem for nonlinear flows in one-dimensional semi-infinite long porous media with low permeability

摘要

Based on Huang’s accurate tri-sectional nonlinear kinematic equation(1997),a dimensionless simplified mathematical model for nonlinear flow in one-dimensional semi-infinite long porous media with low permeability is presented for the case of a constant flow rate on the inner boundary.This model contains double moving boundaries,including an internal moving boundary and an external moving boundary,which are diferent from the classical Stefan problem in heat conduction:The velocity of the external moving boundary is proportional to the second derivative of the unknown pressure function with respect to the distance parameter on this boundary.Through a similarity transformation,the nonlinear partial diferential equation(PDE)system is transformed into a linear PDE system.Then an analytical solution is obtained for the dimensionless simplified mathematical model.This solution can be used for strictly checking the validity of numerical methods in solving such nonlinear mathematical models for flows in low-permeable porous media for petroleum engineering applications.Finally,through plotted comparison curves from the exact analytical solution,the sensitive efects of three characteristic parameters are discussed.It is concluded that with a decrease in the dimensionless critical pressure gradient,the sensitive efects of the dimensionless variable on the dimensionless pressure distribution and dimensionless pressure gradient distribution become more serious;with an increase in the dimensionless pseudo threshold pressure gradient,the sensitive efects of the dimensionless variable become more serious;the dimensionless threshold pressure gradient(TPG)has a great efect on the external moving boundary but has little efect on the internal moving boundary.