To coax gas out of the concrete-like Barnett Shale, operators pump large amounts of water down their wells to fracture the rock. One horizontal well uses about 3 million gallons of water. Most of the water for these so-called "frac jobs" comes from groundwater.
According to the Texas Railroad Commission, in 2005, about 2.6 billion gallons (or 8,000 acre-feet) of water were used for frac jobs in the Barnett Shale. That represents 1.6 percent of the water pumped from the Trinity Aquifer for all human uses.
That might not seem like a large percentage. It is an average, though. In some areas, gas drilling might represent 10 or 20 percent of the local usage.
"On average for the aquifer, this is not a big deal," says Jean-Philipe Nicot, a geological engineer at the Bureau of Economic Geology. "But for some heavily drilled areas like Denton County, it may be an issue. If that drilling expands elsewhere in the area it may become significant."
If drillers have one vertical well every 40 acres, and if each corresponds to one water well nearby, water use is well distributed across the landscape. In the Barnett Shale, though, it's typical to use horizontal drilling with multiple wells originating in a much smaller area. Pipelines are also used to deliver water from one spot to many wells.
"If you have one location heavily pumping water to hundreds of wells," says Nicot, "then people in the vicinity would see an impact."
In January 2007, Nicot and Potter released a report for the Texas Water Development Board estimating the water use for frac jobs in the Trinity Aquifer from 2007 through 2025. They noted uncertainties due to potential changes in the price of gas which might dampen or accelerate drilling and uncertainties due to new technologies or recycling techniques that might lower the amount of water used in each individual well. Under a mid-range scenario, the team projected a total groundwater use in that time period of 183,000 acre-feet (or 59.6 billion gallons or 226 million cubic meters).
About 80 new wells are drilled in the area each month. As the rush to capture more gas from the Barnett play intensifies, the amount of water used for frac jobs will likely rise. At some point, it could compete with water for drinking and farming. Neighbors with shallow water wells might see their supplies drop.
To complicate matters, the region has experienced a severe drought since the beginning of 2005. According to John Nielson-Gammon, Texas State Climatologist, this drought is one of the most severe the region has felt in the past century. "There's been a shortage of hay and grass for cattle to graze, crop failures and reduced yields," he says.
"We're expecting a wetter than normal winter because of El Nino," says Nielson Gammon. "But it's going to take a lot more rain to make up for the deficits. I wouldn't be surprised to see droughts continue next summer."
The oil and gas industry might eventually be forced to use less water in artificial fracturing. Researchers at Texas A&M University are studying techniques for recycling used frac water.
Of course, concerns over water use would evaporate if the price of natural gas dropped to a point that made drilling uneconomical.
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