how did they seal the deepwater horizon well

They sealed the Deepwater Horizon well by capping it, diverting the flow, and then permanently sealing the damaged bore with cement. The final kill came after months of failed attempts, when responders used a relief well to inject cement deep underground and stop the oil flow.

What happened

The well was leaking from about 5,000 feet below the sea surface, which made the work slow and dangerous. Early efforts focused on controlling the flow with containment devices, but those were temporary fixes rather than the permanent solution.

Final sealing method

The lasting fix was a relief well : crews drilled a second well to intersect the damaged one, then pumped heavy drilling mud and cement into the leaking wellbore. That cement plug cut off the pressure and sealed the well permanently.

Why it took so long

The accident was deep underwater, under extreme pressure, so engineers could not simply close it like a surface pipeline. They had to work through multiple failed containment and capping attempts before the relief-well cementing finally stopped the spill after nearly three months.

In one line

They didn’t just “plug” it from above; they ultimately killed the well from below with a relief well and cement.

TL;DR: The Deepwater Horizon well was permanently sealed by drilling a relief well and pumping in cement to stop the flow.