A Sea in Flames - Carl Safina [8]
Added complications. The oil and gas—in the pay zone, or “production zone”—lay between 18,051 and 18,223 feet. The well was drilled to below the zone, to 18,360 feet below the sea surface, which allowed cement to be placed under the oil and gas reservoir as well as around it.
Because this was an exploratory well, the idea was to find the oil, then seal the well shut so a different rig could later tap it for commercial production. Cement is the main barrier for preventing the pressurized oil and gas from entering the well. So it was crucial that the cement job at the bottom of the well absolutely seal off the oil and gas reservoir from the well casing. A bad cement job could let oil and gas into the well.
The environment at that depth means cementing is not a matter of getting a few bags of concrete from the hardware store. Temperatures and pressures at the bottom of a well like this—it’s hotter than boiling, 240° Fahrenheit—make cementing a highly technical endeavor, requiring calculations and tests to select several chemical mixtures, which will be used in layers.
Earlier heavy losses of drilling fluid told technicians that they could be into very loose rock and sand. If you are nervous about a soft zone, you also worry that when you insert cement to seal the well bottom, your cement may ooze into the loose stuff. This complicated the cementing deliberations. John Guide: “The biggest risk associated with this cement job was losing circulation. That was the number one risk.”
If you’re worried that the well walls may be so porous that they’ll suck in cement pumped under pressure, you might add some nitrogen gas to the cement mixture, to get it to form foamy bubbles; this would prevent the cement from leaking into the loose spots.
From the well’s training resources document: “Foamed cement is more expensive than regular cement and it works better than regular cement in some applications. One of the advantages is that the bubbles stiffen the wet cement so that it is less prone to being lost into a zone or being invaded by fluids in a zone. A remote analogy is that when a sink is drained after washing dishes, the water flows out the drain while the soap bubbles remain in the sink.” But, the document notes, while foamed cement is good at sealing off shallow areas, “use of nitrogen foam is less common for deep high-temperature, high-pressure zones.”
Halliburton cement specialist Jesse Gagliano first proposed including nitrified cement. After some back-and-forth, BP agreed. But because nitrified cement is usually used for shallower jobs, the depth created concern on the rig.
Transocean offshore installation manager Jimmy Harrell: “That nitrogen, it could be a bad thing. If it gets in the riser, it will unload the riser on you.… Anything can go wrong.”
There were three parts to the cement and three formulations. “Cap cement” topped the cement in the space between the casing and the oil-bearing rock and sand formation of the well’s sides. Below that, the nitrified “foamed cement” filled the rest of the narrow space outside the casing and along the formation. “Tail cement” filled the “shoe track” at the bottom and was used inside the lower part of the casing itself.
So in various ways this was going to be a difficult cement job. As late as the afternoon of April 14, BP was still reconsidering the chosen long string casing design, with its heavier reliance on the integrity of the cement deep at the well bottom. And the porous surrounding rock was on everyone’s mind. Cement has to be pumped in under some degree of excess pressure in order to fully fill the gap and get a good bond to the rock and sand on one side and to the outside of the production casing on the other. You need enough pressure both to keep the hydrocarbons contained and to