When defining a non-isotropic porous media, e.g. with Pxy = Pyy = 0 for a 2D case, the calculation may diverge and shows oscillatory behaviour in the Darcy pressure. When changing Pyy = 1e-12 (insignificantly small number), convergence is quickly restored. More generally, the permeability property should be represented by a regular tensor. For a 3D case, care should also be taken for the Pzz component of the permeability tensor.