If it's not published anywhere, be it in a peer-reviewed journal, conference proceedings, some book or whitepaper, then don't put it as a reference.
You could write,
According to our work log, we first frobnicated the machine, …
The idea of including references is for interested readers to follow up on them. If it's some internal document nobody will ever see, then it does not qualify as a reference. It also makes it impossible for peer review to happen: you could claim anything and back it up with a reference to your internal document—and nobody could ever (dis)prove it.
The APA blog has an entry on that subject (emphasis mine):
Likewise, if you have other nonretrievable sources (personal communications, like personal e-mail and phone calls), these do not get reference list entries either (although they do receive in-text citations, because they involve other people than just yourself).