Sam Fletcher
OGJ Online
HOUSTON, Feb. 21�Investigators are using a combination of water-well rigs and coiled-tubing units to find and vent shallow migrating pockets of natural gas that recently sparked explosions killing two people in Hutchinson, Kan.
The exact source of the gas has not been determined. But the �working assumption� is that the mishap is connected to the loss of an estimated 73 MMcf of gas from one of six salt caverns at the Yaggy storage facility, located 7 miles down-dip from the central Kansas community of 40,000 people.
Officials said a �fingerprint� of the escaping gas, taken by the Kansas Department of Health and Environment (KDHE), apparently matched the gas stored at Yaggy. That facility is operated by Kansas Gas Service Co., a division of Oneok Inc., Tulsa. Kansas Gas Service provides gas to 660,000 customers in Kansas and northeast Oklahoma.
Investigators found what some described as �a fist-size hole� in the casing of an injection well through which the gas in the S-1 cavern may have escaped. That hole is at a depth of 600 ft and in an area where, according to regulatory records of the KDHE, a drilling crew encountered problems in 1992 while drilling through the plugged well to convert the former propane storage facility into a gas storage unit.
According to those records, the crew wore out several bits trying to drill through a steel coupler that had dropped into the well bore during the earlier cementing operation. That has caused some speculation that the casing may have been damaged at that time.
That hole has since been plugged and the S-1 cavern was shut down.
However, there apparently was no indication of a problem until geysers of gas and brine erupted in Hutchinson Jan. 17.
The first eruption triggered an explosion that destroyed two businesses and damaged 26 others in downtown Hutchinson. About 6 hr later, another geyser shot up on the east edge of town. On Jan. 18, escaping gas exploded in a trailer park, injuring three people, two of whom later died.
Those eruptions may be related to salt mining operations around Hutchinson that date back to the late 1800s, said Rex Buchanan, associate director of the Kansas Geological Survey, a research division of the University of Kansas that is working with the gas utility and state officials to trace the gas migration path.
No one knows how many abandoned unplugged and uncased saline wells may exist in Hutchinson as potential vents for migrating gas, he told OGJ Online in an interview Wednesday.
The Kansas Geological Survey ran two small seismic surveys early this month to try to find a subsurface route by which gas could be migrating. One survey was on a 4-mile north-south line between Hutchinson and the gas storage facility.
The other was a 500-yard line next to a gas vent on the west side of town. Two wells drilled on the basis of that seismic survey encountered gas pockets, Buchanan said.
He said the seismic data suggested �two working hypotheses� about the type of conduits through which escaping gas may have traveled the 7 miles between the storage facility and Hutchinson.
�The first is a channel sand of some sort�that sort of jumps out at you,� said Buchanan. �It seems to fit the pattern. Pockets of gas have been found at a fairly constant depth of 285-400 ft.�
That theory of essentially a buried river bed through which gas could easily flow is so popular, he said, that �one geologist accused us of having �sand-tunnel� vision.� That critic favored the second hypothesis of gas flowing through fractures in the subsurface rock.
�And he could be right,� Buchanan said, �although a fracture system so connected that it can do this is hard to buy into.�
So far government and industry officials have concentrated on finding and venting pockets of gas to prevent any additional deaths or destruction. But that means workers have taken few cores that would tell more about the subsurface conditions in the rock and perhaps provide clues about gas migration, said Buchanan.
Officials have employed water well drilling contractors to drill through the bedrock at 70 ft and then used coiled tubing units to drill for gas pockets below that, he said. Those smaller units were necessary because of the physical restrictions in some of the downtown areas that were drilled.
Moreover, in the rush to find and vent gas pockets, the most accessible city-owned areas were drilled first. The result was a random shotgun approach. But with more information, Buchanan said, a pattern seems to be emerging.
When pockets of gas are found, workers set a surface casing and attach a flare to burn off the escaping gas, said Conrad Koehler, spokesman for Kansas Gas. Those flares burn brightly at first, but fade rapidly as the gas is vented, he said. Of the 10 flares installed so far, he said, four have burned out after depleting the gas pockets.
Other sources put the number of flares at 12, apparently including the fires resulting from the two explosions.
The vents are low-pressure at 60-120 psi, Koehler said.