Given fracking injections are a lot shallower than 10km, I'm not sure it would have to be injected that deep. But it's a good point, that temperature will be a

Given fracking injections are a lot shallower than 10km, I'm not sure it would have to be injected that deep. But it's a good point, that temperature will be a factor. Getting a high-melting-point fluid down there might be done by dissolving or suspending it in water that is allowed to boil off, or having a heated injection pipeline. Silicone and silicate lubricants are viable at those temperatures. Might not be viscous enough, though. A low melting phosphate glass might work.