Most CBM operators are desperate to get rid of their water. We are more than happy to take it and put it to good use, not only improving environmental quality but also making a profit at the same time.

—Ted Ladd

The extraction of the methane (natural gas) contained in coal seams (Coal Bed Methane) has boomed recently, and now accounts for approximately 7.5% of the total natural gas production in the US.

According to the United States Geological Survey (USGS), the Rocky Mountain Region has extensive coal deposits bearing an estimated 30-58 trillion cubic feet (TCF) of recoverable CBM. The Wyoming Oil and Gas Conservation Commission (2002) estimates 31.8 TCF of recoverable CBM in the Powder River Basin of Wyoming alone.

In the late 1980s there were fewer than 20 CBM wells in Wyoming; today there are more than 13,600 producing and shut in CBM wells across the state, according to the Wyoming State Geological Survey. The most active development is currently in the Powder River Coal Field.

Schematic diagram of a CBM well. Click to enlarge. Source: Wyoming State Engineers Office.

To extract CBM, operators drill wells into the seam and pump out water to reduce the water pressure holding gas in the seam. The CBM readily separates as pressure decreases, allowing it to be piped out of the well separately from the water. Water moving from the coal seam to the well bore encourages gas migration toward the well.

The amount of water produced is high, although it varies from deposit to deposit. The Powder River wells have one of the higher water-gas ratios, according to the USGS, at an average 2.75 barrels per thousand cubic feet (MCF) of gas. On average, a Powder River CBM well produces about 400 barrels (16,800 gallons) of water per day per well.

Over time, the water ratio decreases and gas production increases as the beds are dewatered.

The product water must be disposed of; it is not re-injected into the field as in some oil production. The application of the water is determined in part by its composition—the type and amount of total dissolved solids (TDS). But CBM product water generally has a moderately high salinity hazard and often a very high sodium hazard based on standards used for irrigation suitability.

There are currently four primary approaches to dealing with CBM product water, according to Montana State University:

• Discharge into a stream channel. Direct stream discharge is no longer permitted on new wells, but existing operations were grandfathered and are still discharging directly into streams.

• Impounded in holding ponds. Most impoundments are not lined and do discharge to the subsurface. Some percentage of seepage flow from impoundments is likely to reach stream channels via subsurface flow.

• Irrigation.

• Other uses, such as dust control.

Ladd’s concept would use this product water, alleviating some of the disposal problem. He speculates that the salts in the water might be beneficial to the electrolytic process.

Resources:

• USGS: Water Produced with Coal Bed Methane