Can't give you much of an answer with that broad of a question. Wastewater and oil industry covers a huge range.

I'd start by identifying the various waste water streams and their compositions. Then identify the water requirements for your process and see if there are any potential matches.

Good data and lab testing on the streams is essential to evaluate the effect of mixing water streams together to avoid precipitation or scaling issues from various contaminents in the stream(s). You'll then need to consider what treatment options may allow you to use other streams as replacement.

You can use waste water again for injection in the wellheads after purification.
Depending on the region of plant, and availability of fresh water it may change.