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A couple of weeks back had a chance to see the Coal Bed Methane (CBM) blocks in Shahdol district of Madhya Pradesh. It was an experience to see the pilot wells drilled by M/s Reliance producing huge quantities (100s of cubic meter) of produced water.

In subsurface CBM formations, naturally occurring rocks are generally permeated water, and gas The less dense hydrocarbons migrated to trap locations, displacing some of the water from the formation in becoming hydrocarbon reservoirs. Thus, reservoir rocks normally contain both petroleum hydrocarbons and water. This water is frequently referred to as “connate water” or “formation water” and becomes produced water when the reservoir is tapped and brought to the surface.

When hydrocarbons are produced, they are brought to the surface as a produced fluid

mixture. The composition of this produced fluid is dependent on gaseous hydrocarbons, produced water, dissolved or suspended solids, produced solids such as sand or silt, and injected fluids and additives that may have been placed in the formation as a result of exploration and production activities.

Production of coal bed methane (CBM) involves removal of formation water so that the

natural gas in the coal seams can migrate to the collection wells. This formation water is also referred to as produced water. It shares some of the same properties as produced water from oil or conventional gas production, but may be quite different in composition.

For the methane to be removed from the crystalline structure of the coal, the hydrostatic head, or reservoir pressure, in the coal seam must be reduced. CBM produced water is generated when the water that permeates the coal beds that contain the methane is removed. The produced water from a CBM well is known to come in large volumes in the early stages of production; as the amount of water in the coal decreases, the amount of methane production increases.

The quality of CBM produced water varies with the original depositional environment,

depth of burial, and coal type and is also known to vary significantly across production areas.

As CBM production increases and more water is produced, concern about the disposition of these waters on the receiving environment is increasing, since uncertainties abound regarding the impact of these waters, as regulators and operators try to ensure protection of the environment.

Although sufficient data on CBM produced water quality data is available in US and UK this is bound to be of different quality as against the one that is produced in Indian context. These Specific Produced Water Constituents  are salinity, and presence of Heavy Metals

Much of the CBM produced water may be put to beneficial use, but for the above

constituents and their concentrations may limit the use of these waters in certain areas.

The final determination of whether a CBM produced water can be used for agricultural

purposes (generally irrigation or stock watering), for example, will depend not only on

the quality of the produced water but also on the conditions of the receiving areas. These conditions include soil mineralogy and texture, amount of water applied, sensitivity of plant species, and the length of time the water has been stored in impoundments prior to use

Because of the lack of data and the site-specific nature of these potential impacts, specific threshold values for protecting riparian plant communities have not been developed. In some cases, CBM may be considered for domestic supplies and drinking water.

In areas where CBM produced waters have dissolved constituents that are greater than

those in the receiving water, stream water quality impacts are possible. Which include the following:

- Surface discharges of CBM produced water can cause the infiltration of produced

water contaminants to drinking water supplies or sub-irrigation supplies.

- Surface waters and riparian zones can be altered as a result of CBM constituents.

- New plant species may take over from native plants as a result of changes in soils

resulting from contact with CBM produced water.

- Salt-tolerant aquatic habitats in ponded waters and surface reservoirs may increase.

- Local environments can be altered as a result of excess soluble salts, which can

cause plants to dehydrate and die.

- Local environments can be altered as a result of excess sodicity. Excess sodicity

can cause clay to deflocculate, thereby lowering the permeability of soil to air and

water, and reducing nutrient availability.

- Oxygen demand in produced water can overwhelm surface waters and reduce the

oxygen level enough to damage aquatic species.

As the amount of produced water increases, the amount of produced water constituents entering the water will increase, even assuming concentration discharge limits are met. Also, because actual impacts of produced water constituents will depend on the produced water as a whole in the context of the environment into which it is released, it will be important to understand effects of site-specific produced waters rather than addressing individual components.

A variety of potential additive, synergistic, and antagonistic effects of multiple constituents can affect actual impacts. Cross-media impacts can occur when technologies designed to address one environmental problem  create other problems (e.g., increased energy use, air emissions, contamination of aquifers from CBM reinjection), which could result in a greater net impact to the environment.

 I shall be grateful if some more information on Produced water quality in Indian context and any observations on the impact of produced water on environment is available