Sammendrag
For heterogeneous reservoirs, fast, stable and accurate methods are hard to obtain. Large changes in the velocity field leads to severe time step restrictions in explicit schemes or expensive time steps in implicit schemes. In the absence of gravity, the exact velocity field will be loop free in the sense that there are no closed integral curves. For streamline methods, the absence of closed integral curves ensures that all streamlines have endpoints in wells. Likewise, this property implies that the Jacobian matrix of an implicit scheme with an upwind flux approximation can be reduced to a triangular matrix by a permutation, or reordering, of the unknowns. For the above methods the effect of gravity is usually handled by operator splitting in the transport equation. Gravity will introduce rotation in the total velocity, which may yield closed integral curves. Even when the effect of gravity is small, it can limit the efficiency and the robustness of streamline and reorder methods. To overcome this problem, we split the total velocity field in a loop-free part without the effect of gravity, and a part driven by gravity only. This is achieved by solving the pressure equation two times with different righthand sides. ...