How is natural gas from coal formed?

Like conventional natural gas, NGC is the result of heat and pressure acting on organic matter. In the case of NGC, however, natural gas generation is part of the coalification process and the natural gas thus created remains stored within the coal, instead of migrating to other reservoir quality rocks such as sandstone.

Coal originates from the transformation of terrestrial plants into carbon through burial. As trees and other plants growing in ancient forests and swamps died and decayed, their remains formed peat bogs. Over time, the bogs were buried by sediments, and as the depth of burial increased, the peat was subjected to increasing temperature and pressure. This resulted in a decrease in moisture content and an increase in the relative proportion of carbon. By-products of this process include water, carbon dioxide, nitrogen and methane. The organic matter was transformed sequentially through the different stages or ranks of coal - lignite, sub-bituminous, bituminous and anthracite. The higher the rank, the lower the water content and higher the proportion of pure carbon. The rank of coal is therefore a function of time, temperature and pressure. Deeper coals will generally contain higher amounts of methane.

NGC is adsorbed on the internal surfaces and also stored in pores and fissures in coal under pressure from overlying sediments and fluids. Because of the complex nature of the internal surface structure of coal, coal beds can store up to seven times the amount of natural gas that can be stored in a similar volume of conventional reservoir rock. In the case of wet coals, the gas may be held in place by the pressure of the water in the coal. In this event, it is necessary to dewater the coal to release the pressure so that the gas can flow to the surface.

NGC play types in Canada

NGC play types in Canada fall into three general categories.

• "Dry" coal with insignificant produced water
  In the case of dry coals, the geological history has created an "under-pressured" NGC reservoir that has pressures lower than expected for sediments at the target depth in a "normal" hydrostatic setting and with little or no mobile water. The coal seams themselves do not contain significant water volumes, either saline or fresh, in most areas where they are sufficiently deep to contain commercial quantities of gas. Consequently, any small quantities of water produced with the NGC are likely to have condensed from the gas. Dry coal reservoirs do not require dewatering to reduce the reservoir pressure and release the gas and allow it to flow. These "naturally" dewatered coals contain free gas upon discovery, although at a lower pressure than normal. Because producing pressures are typically very low in NGC reservoirs, compression is often required from the outset to optimize production.
  
  
• Coals that produce unusable brine
  In the case of coals that produce unusable brine, this saline water within the coal seams must be produced to dewater the coals, reducing reservoir pressure to initiate NGC desorption and production. Often, wells drilled into such reservoirs produce mostly water at the onset of production, with NGC production building over time and water production decreasing. The produced saline water is reinjected into deeper zones not connected with shallower fresh water aquifers, in accordance with regulations already in place that govern conventional gas production. Coals that produce unusable brines have been found in moderately deep coals in central Alberta.
  
  
• Coals that produce usable water
  Coals that produce usable water are similar to those producing brine in that the water must be produced in order for the NGC to flow. The main difference is that the produced water is sufficiently fresh to be potentially used for some practical applications as potable water or returned to the aquifer. To date, this type of NGC play is rare in Canada; however, it is common in the United States. In Wyoming's Powder River Basin it has generated substantial attention due to the vast NGC resource and large numbers of wells drilled to recover the NGC and accompanying produced water volumes. Current Alberta regulations regarding the production and disposal of usable water associated with coal seams are designed to protect the usable water rather than to recover the NGC. Co-production of usable water and NGC on a commercial scale in Alberta will require regulation changes to optimize management of both these valuable resources. British Columbia has established guidelines that allow for such co-production of usable water and NGC within the capacity of local watersheds. The co-production of NGC and usable water holds the potential to recover two valuable resources. Since the coals are found at depths greater then most water wells, the water in these coals would not likely be a source of water without NGC drilling activity. These zones are generally deep enough to be isolated from the shallower aquifers landowners usually use.