Recently higher gas prices and improved drilling technology have spurred shale gas drilling across the US. Fig. 1 shows the shale plays currently being explored. Some of the more popular areas are the Barnett, Haynes- ville, and Fayetteville shales in the South and the Marcellus, New Albany, and Antrim shales in the East and Midwest. These plays represent a large portion of current and future gas production.

But all shale gas is not the same, and gas processing requirements for shale gas can vary from area to area. As a result, shale gas processors must be concerned about elevated ethane and nitrogen levels across a field. Other concerns are the increased requirements of urban gas processing. In addition, the rapid production growth in emerging shale areas can be difficult to handle.

This article reviews which gas processing technologies are appropriate for the variety of US shale gas qualities being produced and planned to be produced and reviews regional gas processing capacities to handle current and future production of shale gas.

Gas processing

Gas processing removes one or more components from produced gas to prepare it for use. Common components removed to meet pipeline, safety, environmental, and quality specifications include H2S, CO2, N2, heavy hydrocarbons, and water. The technique employed to process the gas varies with the components to be removed as well as with the properties of the gas stream (e.g., temperature, pressure, composition, flow rate) .

Acid-gas removal is commonly by absorption of the HS and CO into aqueous amine solutions. This technique works well for high- pressure gas streams and those with moderate to high concentrations of the acidgas component.

Fig. 1 US SHAll GAS REGIONS

Physical solvents such as methanol or the polymer DEGP, or Selexol may also be used in some cases. And, if the CO2 level is very high, such as in gas from CO2- flooded reser- voirs, membrane technology affords bulk CO2 removal in advance of pro- cessing with an- other method. For minimal amounts of HS in a gas st re a m , sc aveng ers can be a costeffective approach to HS removal.

Natural gas that becomes saturated with water in the reservoir requires dehydration to increase the heating value of the gas and to prevent pipeline corrosion and formation of solid hydrates.

Ln most cases, dehydration with a glycol is employed. The waterrich glycol can be regenerated by reducing pressure and applying heat. Another possible dehydration method is use of molecular sieves that contact the gas with a solid adsorbent to remove the water. Molecular sieves can remove the water down to the extremely low levels required for cryogenic separation processes.

TEXAS BARNETI SHALE

Distillation uses the different boil- ing points of heavier hydrocarbons and nitrogen for separation. Cryogenic temperatures, required for separation of nitrogen and methane, are achieved by refrigeration and expansion of the gas through an expander. Removal of the heavy hydrocarbons is dictated by pipeline quality requirements, while deep removal is based on the economics of NGL production.

Table 1 BARNEFT SHALE GAS COMPOSITION*

Table 2 MARCELLUS SHALE GAS COMPOSITION

Processing requirements

The following reviews six shale gas plays, their compositions, and processing needs: Barnett. Marcellus. Fayetteville, New Albany, Antrim, and Haynesville.

Barnett

The Bamett shale formation is the grandfather of shale gas plays. Much of the technology used in drilling and production of shale gas has been developed on this play. The Bamett shale formation lies around the DallasFort Worth area of Texas (Fig. 2) and produces at depths of 6,500-9,500 ft. The average production rate varies throughout the basin from 0.5 MMscfd to 4 MMscfd with estimates of 300-350 std. cu ft/ton of shale.1 The most active operators in the region are Chesapeake Energy, Devon, EOG Resources, and XTQ

The initial discovery region was in a core area on the eastern side of the play. As drilling has moved westward, the form of the hydrocarbons in the Barnett shale has varied from dry gas prone in the east to oil prone in the west.

Table 1 shows the composition of four wells in the Barnett. These wells appear from east to west with the eastern most well on the top (Well No. 1). As the table suggests, there is a large increase in the amo mit of ethane and propane as the wells move west. One well sample on the western edge of the play (Well No. 4) shows a high level - 7% of nitrogen.