There are times that gas deposits are not an ideal target. Multicomponent seismic is helping to locate these drilling hazards.



The term "what the market will bear" ideally should not apply to research and development. Unfortunately, it often does. Thus it is that the Exploration Geophysics Lab (EGL) at the Bureau of Economic Geology at the University of Texas is studying gas hydrates.

This may seem an unlikely topic for a lab dedicated to multicomponent research. But as Senior Research Geoscientist Bob Hardage explained, what seem like important puzzles to solve and what investors want to study are sometimes different things.

"At the onset we were a little too narrow in our focus," Hardage said. "We were trying to stay only with projects that involved nine-component seismic data, which provides every possible elastic wave mode that can be used to illuminate geology. But we found out that was too restrictive."

Hence the focus on gas hydrates, a huge source of interest for DOE researchers. Gas hydrates are found around the world and are ice-like crystalline solids formed from a mixture of water and natural gas, usually methane. Usually located near the surface, gas hydrates are considered drilling hazards because of the possibility of melting them and releasing the methane, which could lead to wellbore stability problems or even platform failures. Does it make sense to use the latest seismic technology to study near-surface drilling hazards?

Surprisingly, yes. Hardage explained that using shear waves to image the near surface results in vastly improved resolution. Simply put, the shorter the wavelength, the better the image. "There's a little simple equation that defines wavelengths," he explained. "The wavelength equals the velocity divided by the frequency. In deepwater multicomponent, the illuminating wavefield is a P (compressional) wavefield that's created by an airgun towed at the sea surface. Those airgun data have a frequency spectrum that's maybe 10 Hz to 100 Hz.

"In the very near-seafloor sediments, at least in the Gulf of Mexico, it's very high-porosity and very unconsolidated. The result is that shear wave velocity in that muck is very low."

So low, in fact, that it can be upwards of 15 times slower than the slowest P waves, resulting in wavelengths that are 15 times shorter.

The gas hydrate research is studying these odd subsurface occurrences for a variety of reasons. There is hope, of course, of eventually relying on them as a source of natural gas, although those extraction methods are still in the research stages. Gas hydrate accumulations also create headaches for operators drilling deepwater wells, so it's helpful to know their location prior to spudding a well. Hardage added that there can be adverse impacts if the gas is inadvertently released into the environment.

Shallowwater flow

Perhaps the area that's catching the most attention, though, is research that suggests a connection between gas hydrates and shallowwater flow (SWF). SWF is one of the primary drilling hazards in deep water and can cause wells and sometimes entire fields to be abandoned due to lost circulation problems.

Conventional wisdom attributes SWF to overpressured sands that are penetrated while drilling. The fact that the pore pressure of the sands is close to the fracture pressure of the drilling mud makes well control difficult.

Hardage, however, postulates that other factors may be afoot. "One of the oddities with SWF is that when the problem occurs, it almost always happens after the drill bit has gone far beyond the zone where the SWF occurs," he said. "Sometimes it doesn't even occur in a production scenario until wells five and six have been drilled through the same interval. So there's this long time delay between when the bit first penetrates the zones that deliver the water and when the water flow starts."

Secondly, SWF is not an issue in shallower water depths. (The word "shallow" in SWF refers to subsurface depth.) It only seems to occur in water depths that support hydrate stability. The third point, Hardage said, is "the kicker. When I ask people the salinity of the produced water, I get two answers - either they don't know, or they say it's fresh. How do you get fresh water right below the surface out in the Gulf of Mexico?"

His answer is that the presence of drilling activities causes gas hydrates to dissociate, or melt. "The gas is bound inside a water cage of ice," he said. "It explains the time delay - it takes a period of time to change the thermal properties of the interval for dissociation to start kicking in big time."

Two things can cause this dissociation - an increase in temperature and a drop in pressure. Hardage suspects it's more of the former - the circulation of warm drilling muds, the production of heated hydrocarbons - causing the heat to rise within the formation. It only takes a small temperature increase to cause the dissociation, and the resulting SWF can deep-six an entire deepwater project.

Hardage finds it difficult to believe that the overpressured sands theory holds so much sway over the industry that most operators don't even think to test the salinity of the water. "If it's fresh water, you have to scratch your head and ask how it got there," he said. "It just takes one experience with SWF and tens of millions of dollars lost, and it's usually a vice president or head of engineering who makes it a point of your attention."

One interesting anecdote implies a strong connection between SWF and hydrates. The US Minerals Management Service (MMS) has funded a study with Dr. Harry Roberts of Louisiana State University (LSU) to find a correlation between bright seafloor reflections on P-wave data and visible outcrops on the seafloor, which are studied through dives. Hardage said his LSU and MMS colleagues have established an almost 100% correlation between the two. This has caused the MMS to require a study of seafloor reflectivity prior to drilling.

One operator did just that, found no evidence of a hydrate accumulation and started drilling a well. "Once the drill bit got to about 12,000 ft (3.6 m), boom, they got the SWF back up in an interval they'd drilled past a month or two previously. They fought the problem, spent several million dollars and finally gave up.

"A few years later the MMS did a post-mortem to find out what happened. They asked the operator to collect information about seafloor conditions at the drill site. When that was done, lo and behold, hydrates actually were outcropping on the seafloor around the old well bore. They weren't there before. So the presence of the well and the SWF caused them to form."

He finds this story intriguing. "I don't know who the operator was nor the location of the deepwater well, but when I heard this story from some colleagues who were involved in the post-mortem, I thought the chain of events was a great story that established a genetic link between gas hydrates and some, but probably not all, SWF phenomena," he said. "I hope the 'before' and 'after' data across this SWF occurrence can be released in the future for all to see."

Hardage added that the EGL does other research besides gas hydrates, but so far he's not had much success in getting the majors' attention. "Some of them appear not to be that interested in multicomponent seismic technology at this point, and those who are seem to prefer to try to do it internally," he said. In contrast, small independents are eager to support the research.

If the connection between hydrates and SWF is real, it might be time for deepwater operators to get out their checkbooks.