  In this work we propose a novel block preconditioner, labelled Explicit
Decoupling Factor Approximation (EDFA), to accelerate the convergence of Krylov
subspace solvers used to address the sequence of non-symmetric systems of
linear equations originating from flow simulations in porous media. The flow
model is discretized blending the Mixed Hybrid Finite Element (MHFE) method for
Darcy's equation with the Finite Volume (FV) scheme for the mass conservation.
The EDFA preconditioner is characterized by two features: the exploitation of
the system matrix decoupling factors to recast the Schur complement and their
inexact fully-parallel computation by means of restriction operators. We
introduce two adaptive techniques aimed at building the restriction operators
according to the properties of the system at hand. The proposed block
preconditioner has been tested through an extensive experimentation on both
synthetic and real-case applications, pointing out its robustness and
computational efficiency.
