Nonexistence of Riemann solutions for a quadratic model deriving from petroleum engineering

The study presented in this work shows that the viscous profile entropy criterion is too selective in reducing the number of solutions to guarantee existence of stable weak self-similar Riemann solutions to conservation laws. This result is shown on a particular quadratic model derived from the three-phase flow equations used in petroleum engineering. The viscosity matrix considered in this work derives from capillary pressures. The Riemann initial data are hyperbolic and correspond to a Lax 1-shock that does not admit a viscous profile. The nonexistence of a profile in this example is due to the presence of a limit cycle in the vector field associated with the viscous profile entropy criterion. To establish the main result of this work, a complete list of possibilities that could lead to a solution, is examined. This list includes solutions that consist of only classical waves and the solutions that contain at least one nonclassical (shock) wave. The construction of solutions breaks down because either the shock waves do not satisfy the viscous entropy criterion or the speeds of the waves that comprise a solution are decreasing. To the author's knowledge, this is the first result on nonexistence of stable solutions for models that allow nonclassical (transitional) shock waves. The results presented in this paper are a combination of analytical and numerical work. The theoretical ideas and techniques derive from the bifurcation theory of vector fields and the theory of weak solutions of conservation laws. These are combined with numerical results when no theory is available. (C) 2002 Elsevier Science Ltd. All rights reserved.