Drilling advances

	Vol. 228 No. 01
LES SKINNER, PE, CONTRIBUTING EDITOR

Wellcontrol. The very phrase brings images of John Wayne and a cast of thousands jumping on a private jet to fly to some desert site where pre-positioned bulldozers, cranes and valves are already on-station along with hundreds of welders, pipe fitters and reporters. The crew spends an hour or two setting up, then they extinguish the massive fire with the explosion of a few hundred pounds of dynamite placed at just the right elevation on the first attempt. After the cheering dies down, a crane lowers a valve assembly onto the wonderfully clean wellhead flange and John and the boys bolt it down in record time while black-colored water runs off their hardhats (oil doesn't flow like that, folks).

This, dear friends, is the stuff of action movies—not real life. Sure, there are real-life firefighters and blowout specialists, and they still use the occasional shot to blow out a flame. Most of them are very good at what they do, and the work is still very risky. When a blowout occurs, meaning that there is a loss of pressure and flow containment from the well with no conventional means of restoring it, the oilfield firefighters are needed to perform highly specialized emergency response measures to get the well back under control. The work they do is reactive in nature—the blowout is already in progress—and they are paid in accordance with the risks they take (which means they get paid a whole lot!).

Well control is much more, however. It is an art that is practiced countless times everyday on rigs all around the world. It requires correct mud density, good hole cleaning, slow trip speeds, copious testing and maintenance of blowout prevention equipment (BOPE), crew training and endless drills. The primary and secondary flow and pressure barriers (i.e., the mud column and the BOPE) must be present at all times to avoid the worst case scenario—a blowout. So, pressure control and flow containment is a much broader, and often much deeper, subject than the emergency response efforts to restore it.

Once again, it seems that the men and women around the world on the rigs are the ones that should be acclaimed as the real heroes and heroines of well control by not letting wells get out of control in the first place. Maintaining well control at all times is now the norm, so the contributions of that army of professionals drilling new hole every day are often taken for granted.

Think of what would happen if they didn�t keep the wells under control. There are simply not enough people with the embroidered flames on their funny-colored coveralls to put out all the fires and cap all the flowing wells that would result. Kuwait, after the invasion in 1990, should have taught us that. So, the lowly drillers continue keeping the wells under control, but the blowout specialist is the one interviewed by the reporters.

Wells do not always behave themselves. What happens when the unexpected event occurs, like drilling into an over-pressured interval or losing circulation, allowing the mud column in the annulus to drop? If one has ever experienced a hard kick, especially a vicious gas kick, one realizes that this issue of maintaining well control is not as easy as it sounds. We go through all the steps taught to us in well control school (everyone has gone to well control school, haven�t they?). Under ideal conditions, the kick is supposed to be circulated out through the choke manifold easily, while maintaining constant BHP. Unfortunately, conditions are never ideal. Just getting the well shut-in is sometimes a battle.

The debate on whether to use the hard or the soft shut-in technique has been going on for as long as I can remember. I�ll admit my memory is getting just a little weaker these days, but when I first took well control training in about 1978, the soft shut-in was the preferred method. The justification for using this technique was that the wellbore underwent less total stress than the hard shut-in. Now, the hard shut-in is almost universally taught to limit the volume of the kick inside the wellbore.

In the hard shut-in, the pipe is raised or lowered to ensure that a tool joint is not across a pipe ram in the BOP stack. Then, a pipe ram is closed around the drill pipe which stops the flow immediately. In the soft shut-in, the HCR valve is opened first (the hydraulic choke is open), then a ram is closed which diverts the flow through the choke manifold. Then, the choke is closed shutting in the well at a fast but measured pace.

Here�s one argument that is rarely considered in the debate. Let�s assume that we have a 15,000-ft well with a 12 1⁄4-in. hole and we�re drilling with 41⁄2-in. drill pipe. The hole is filled with 16.5-ppg mud. A hard kick is taken, and the well is flowing mud out of the annulus at a high rate. Imagine that the annular velocity of the mud column is, say 90 ft/sec (not an unusual velocity in a real kick) and a ram is closed on the drill pipe (i.e., the hard shut-in). In about two seconds, the mud column goes from a high vertical velocity to a dead stop. The inertia of the moving mud column is exerted on the bottom of the ram and BOP stack in what�s called spike loading. The spike in this scenario is about the same as that of a fully-loaded semi-trailer truck hitting a brick wall at 60 miles per hour. I contend that�s why the well control specialists see lots of stretched bolts at the flanged connection between the wellhead and the BOP stack. A soft shut-in would allow the mud column to decelerate before stopping.

Recently, a friend in the well control industry mentioned that as we drill in deeper, hotter reservoirs with increasing concentrations of toxic gasses, the challenge of maintaining well control becomes ever more difficult. The small kick that was a nuisance in shallower wells can no longer be tolerated. A light spot in the mud that might be inconsequential in some wells could result in a critical failure for a deep HPHT well. Better kick detection equipment and procedures will be needed. Now, instead of filling the hole every five stands on a trip out of the hole, we will probably need to pump across the BOP from the trip tank. This will ensure that the hole stays full, and it will enhance our ability to detect a flow very quickly.

Regardless of how it�s done, it will be up to the guys and gals on the rig to keep these monsters under control. Keep your heads up and your eyes moving. You sure don�t want to snooze when it comes to well control. Be safe.