JPT | 11 February 2015
Beyond the Headlines: Can Waste Water Be Disposed of Safely?
Editor’s note: Professionals in the oil and gas industry often receive questions about how industry operations affect public health, the environment, and the communities in which they operate. Of particular concern today is the impact of hydraulic fracturing on the environment. In this new column, JPT is inviting energy experts to put those questions and concerns about industry operations into perspective. Additional information about the oil and gas industry, how it affects society, and how to explain industry operations and practices to the general public is available on SPE’s Energy4me website.
Shale oil and gas wells use a lot of water in fracturing operations. Each well may use up to 8 million gallons, and as much as 35% of this can return as flowback water. Safe disposition of this waste water is an industry priority especially because of the widely reported past missteps in Pennsylvania. Safe disposition is completely feasible and, in fact, is being broadly practiced today.
The Nature of Flowback Water
Even if fresh water is used as the base fracturing fluid, what returns to the surface is salty. This is because the water found in association with hydrocarbons has high salinity. Shale oil and gas flowback water salinity typically range from 16,000 parts per million (ppm) to more than 300,000 in some instances. For comparison, sea water runs around 35,000 ppm. The chemicals introduced into the fracturing fluid will also be present in some proportion. These will be low in concentration because even the original fracturing fluid contains only up to about 0.5% chemicals.
Finally, one could also encounter species present in subterranean rock. These could include aromatic compounds (such as benzene) and radioactive species. Some state regulations, such as the ones pending in North Carolina, prohibit the use of aromatics in fracturing fluid so, if present, they could only have come from the subsurface. The same holds for radioactive species. In most instances, subterrestrial bacteria will also be present. All of these render the flowback water unsuited for direct discharge.
Reuse of Flowback Water
Flowback water may be reused to formulate fracturing fluid for the next operation. But because only about one-third of injected water returns, additional water is needed (makeup water). In the early going, reuse was rendered costly because of the need to desalinate down to fresh water. However, more recently, all service companies have announced that they can tolerate salinities in excess of 250,000 ppm.
To accomplish this, they had to invent substitutes for certain chemicals. In particular, these were the cross-linkers (for thickening the gel in the fluid to fracture the rock more effectively) and the breakers (the chemical that breaks down the cross-linked gel to thin it for removal at the end of the operation). This salt tolerance suggests that even the makeup water could be a salty water of convenience rather than fresh. Brackish water is ubiquitous in shale oil and gas operations, and yet few companies use it. Not using fresh water would go a long way toward community acceptance of the operations.
While salinity per se may not be a bar to reuse, some treatment may be required. The operator may choose to remove divalent ions. These tend to form scale and radioactive elements, and tend to concentrate in the scale even though the concentrations in the water may be too low to be a concern for personnel safety. Manual descaling operations could constitute an operational hazard if radioactive elements were present. Most operators would also attempt to remove the bacteria in some way. But these operations are straightforward. Divalent ion removal is known as water softening, found in many homes using well water.
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