Drilling advances

	Vol. 231 No. 1
JIM REDDEN, CONTRIBUTING EDITOR

Subsalt drillers learn to take it easy

Not so very long ago, telling a driller to keep steady pressure on the brake would likely draw a reaction akin to what you’d get advising a race car driver that a particular track had a speed limit. Slowing things down simply was not in their DNA.
But talk to well control specialists and they’ll tell you that if ever there was a time for that long-held drilling philosophy to make an about-face, it is now, with the technically demanding and risk-prone unconventional plays unfolding around the world. Specifically, they point to the double-whammy presented when drilling subsalt and presalt targets in the deep and ultra-deepwater Gulf of Mexico, Brazil, West Africa and elsewhere.
A double-whammy, because here is an environment where the potential well control issues unique to the deep water are compounded with a myriad uncertainties in the lower salt sections, where the preservation of well control can intimidate even the savviest drilling engineer. Multiple hazards lurk in the low-temperature and high-pressure regime of the deep and ultradeep water, related to equivalent static density, low frac gradients, trip gas and kick circulation, not to mention the formation of gas hydrates that can clog up the BOP and choke or kill lines. Hydrates tend to crop up after a kick has been taken and the mud has been allowed to cool during a period of non-circulation. Add to these issues the unknown pressure regimes and kick tolerance challenges once the sub- and pre-salt sections are encountered, and the risks rise through the stratosphere.
If a driller manages to safely penetrate the whole salt body, there’s still the matter of getting safely through the underlying basal shear zone, known colloquially but accurately as the rubble or thief zone. There, the threats connected to failed rock and the very real risks of severe lost circulation and stuck pipe are magnified by pressure regimes that are all but impossible to detect or clarify with seismic or offset experience, so controlling the rate of penetration becomes a must-do drilling practice.
That’s in stark contrast to the widely held practice of making hole as fast as possible. In today’s economic environment, where the day rates of semisubmersibles and drillships can add up over time to approach the gross national products of any number of small countries, it’s understandably reasonable for boardrooms to want the well down and the rig off location as soon as possible. However, in these drilling environments, nearly everyone agrees that conducting business as usual must go the way of telephone booths and typewriters, especially since coping with a single well control event here can easily cost between $5 million and $20 million.
 As a well control specialist for Boots & Coots told me, “To a lot of people, the philosophy is ‘Turn to the right as quickly as possible.’ That doesn’t apply here. I liken it to driving a car. If you back into a mailbox at 3 mph you’ll do little damage. You back into it at 80 mph, and it’s a different story.”
While advancements in seismic have leveled the playing field to an extent, the structure and stratigraphy of subsalt, the specific boundaries and the pressure regimes remain nearly impossible to detect from snowy seismic images. Other new technologies can help reduce the risk; Wild Well Control, for example, uses proprietary modeling software that simulates kick behavior as part of a real-time modeling package of the full dynamic conduct of the wellbore and its contents.
Although technology is helping give the drillers a subtle edge, the industry says the best defense is simply to train drillers to slow it down. That’s why, over the past two years, a number of companies, the Society of Petroleum Engineers and other industry organizations have sponsored workshops and training sessions dedicated to advising personnel on the hazards. And, as many have pointed out, many of those hazards can be addressed simply by changing the “faster is always better” mindset.
For instance, when approaching what is believed to be the top of the salt, the specialists agree that it is advisable to reduce the ROP and/or weight-on-bit appreciably to allow more time for interpreting and reacting to any risks or lithological changes they may encounter prior to entering the salt. Others say the shoe should be installed as close as possible to the base of the salt, thereby reducing the length of an openhole salt section should trouble arise. “With the uncertainties, you don’t want to come screaming out of the salt and hit a low-pressure rubble zone or a high-pressured sand and have all this salt open,” advised the Boots & Coots specialist.
In addition, owing to the unknown pressures, some operators will go with a higher density of often-premium drilling fluid—sometimes as high as 90% overburden—and risk expensive fluid loss rather than the chance of taking a kick.
While subsalt and deepwater have always been inseparable, it may come as a surprise to some that the perils of lower salt bodies are not limited to offshore. For instance, it has been said that the subsalt zones of the giant and elderly Hassi Messaoud Field in the Algerian desert are responsible for nearly half of the hundreds of wells drilled there sustaining total circulation loss—a precursor to potential well control incidents.
Then there is the so-called wholesale crew change and the influx of inexperienced personnel it has drawn into the industry, which many fear will create serious safety concerns, especially in high-risk environments like the subsalt. While intensive training is paramount, they say, it should in no way be limited to those a bit wet behind the ears. As one operator said, few highly experienced hands routinely kill wells and many may never have to respond to a well control incident throughout their careers.
Once those hurdles are jumped, the next act should focus on also convincing some of the folks controlling the purse strings that slower actually can be better, regardless of the immediate costs.