"could well" is correct and usable in written English. You can use this phrase to indicate that something is likely or probable. For example: "This decision could well have far-reaching consequences for the industry.".
This could well happen.
They could well be.
That could well be.
November could well prove difficult.
They could well be right.
This could well be true.
It could well happen again.
Thanks to Ludwig my first paper got accepted! The editor wrote me that my manuscript was well-written
Listya Utami K.
PhD Student in Biology, Bandung Institute of Technology, Indonesia