def dryframe_density(rhob, rhofl, phi):
    """Dry frame grain density

    Primarily used for fluid substitution.

    Args:
        rhob (array-like): saturated rock bulk modulus
        rhofl (array-like): fluid density
        phi (array-like): porosity (frac)

    Returns:
        (array-like): dry frame grain density
    """
    return (rhob - rhofl * phi) / (1 - phi)