Chimney seal old flues?

[SDJ]

Hello

I am renovating the ground floor and will be installing a fireplace against an existing chimney. There used to be an oil burner installed on the opposite side of the chimney that is now sealed, and I have opened it on the side where I will install the fireplace. I assumed that I would need to do some masonry work for it to be secure, but I’ve run into some issues.

• There is a flue inside and one on the side where I will install the fireplace
• The one on the right has no outlet upwards but is more like a cavity
• The rear cavity also doesn’t have a flue; it ran up into the full brick flue closest to the hole I just made

I want it to be as stable as possible since there is 10 meters of chimney resting on this bottom part. The reason I have opened it so much is that at the very top of my opening, there was an old opening where someone drilled out on the other side for a new burner in the 70s; they likely sealed it a bit haphazardly for these cavities I have now exposed.

What is best to fill these cavities with sand, concrete them up, or just ignore them and build up the walls so there’s a full brick channel where I want it?