First step is to disable packagist properly like this:
"repositories": [
{ "packagist": false },
{
"type": "composer",
"url": "http://local/packages.json"
}
]
Then I guess you may get an error if the packages you need don't exist in your local repo.
That said, if it didn't pick it up, it probably means your satis repo isn't containing all the packages you need, because if they are there they should take priority over the packagist ones. If you only have tags though it might pick packagist ones since they contain dev-master and such higher prio versions.