Iterative multiscale gradient computation for heterogeneous subsurface flow.

• Iterative multiscale gradient algorithms provide up to fine-scale accuracy gradients. • Gradient accuracy can be controlled via the i-MSFV tolerance. • Accurate gradients are obtained for challenging, high permeability contrast models. • The accuracy control can be used to speed-up the computations. We introduce a semi-analytical iterative multiscale derivative computation methodology that allows for error control and reduction to any desired accuracy, up to fine-scale precision. The model responses are computed by the multiscale forward simulation of flow in heterogeneous porous media. The derivative computation method is based on the augmentation of the model equation and state vectors with the smoothing stage defined by the iterative multiscale method. In the formulation, we avoid additional complexity involved in computing partial derivatives associated to the smoothing step. We account for it as an approximate derivative computation stage. The numerical experiments illustrate how the newly introduced derivative method computes misfit objective function gradients that converge to fine-scale one as the iterative multiscale residual converges. The robustness of the methodology is investigated for test cases with high contrast permeability fields. The iterative multiscale gradient method casts a promising approach, with minimal accuracy-efficiency tradeoff, for large-scale heterogeneous porous media optimization problems. [ABSTRACT FROM AUTHOR]