CBM Paper for IPEC, November 3-5, 2009

Coalbed Methane Produced Water – An Evolution in Treatment

By James Welch, Siemens Water Technologies

James.P.Welch@siemens.com

Abstract

This presentation will discuss the pretreatment evolution for the reverse osmosis/ recovery

reverse osmosis process (RO/RRO), treating produced water from Powder River Basin

coalbed methane wells. The RO/RRO process is approved by Wyoming Chapter 3 permit.

In 2008, Siemens was awarded a contract for the treatment of Powder River Basin, Coalbed

Methane produced water. As of the date of this writing, the plant, Mitchell Draw, has not

been commissioned. The plant was designed to treat 72,000 bpd (477 m3/h or 2,100 gpm).

Coalbed Methane Production

Coalbed methane recovery techniques are unique when compared with other production

methods, as hydrostatic pressure holds the methane in the coal seam so that gas production

requires removing the formation water, or dewatering. Removing the formation water

depressurizes the formation, releasing the gas. Initial water production is high, but rapidly

decreases to allow the methane to be released. Producers must manage the large volumes

of water generated during the dewatering process. The Powder River Basin CBM produced

water is of high quality, such that much of the water can be disposed of by direct discharge.

However, operators must manage produced water of a lower quality according to

environmental compliance and economic objectives. This includes volume of water

produced, proximity to surface water, rights-of-way, influent chemistry, discharge quality

requirements, land use provisions and recycle objectives.

The reverse osmosis/recovery reverse osmosis design (RO/RRO) used at the Mitchell Draw

plant is a permitted design through the Wyoming Department of Environmental Quality/Water

Quality Division (WDEQ) Chapter 3 process, requiring review and monitoring by the

department’s water and wastewater division engineers.

The Mitchell Draw plant optimizes costs by maximizing system recovery using pretreatment

techniques and an aeration pond for evaporating and concentrating the brine. An example

evaporation pond is shown in Figure 1. The plant design includes a bypass and blend

provisions so that the plant can ratio blend the product water according to discharge

specifications. The plant maximizes membrane performance with filtration and scale control,

and it is the scale control that is the focus in this presentation. Also discussed are lessons

learned relative to alternative scale control strategies used in an earlier plant design, also in

the Powder River Basin.
Figure 1

Influent, Effluent Criteria and System Design

One must clearly understand feedwater characteristics for proper plant design. This includes

seasonal variability that may identify influent extremes or complex chemistries. Waste and

product stream characteristics must also be understood so that service factor, redundancy,

and compliance can be addressed in the plant system design. CBM water is high in sodium

and bicarbonate and low in hardness, and may also include suspended solids, iron, silica,

and barium.

Application engineers use solubility indices to understand the relationship of the dissolved

ions as they move through the treatment process. For instance, one technique for predicting

calcium carbonate solubility considers the bicarbonate and calcium concentrations to access

the potential for hardness scale formation. This is the concept behind the Langelier

Saturation Index (LSI). A positive LSI denotes an increased potential for calcium carbonate

scale formation while a negative LSI denotes that calcium carbonate may dissolve in the

solution.

Another constituent common in CBM water is silica. Because of its unique chemistry, silica

poses special treatment challenges to design engineers. While the silica concentration in

Powder River Basin produced water is moderate, the high recovery rate of the membrane

system creates ideal conditions for silica to scale membrane surfaces. Silica precipitation

control is further complicated because control techniques for other ions conflict with methods

for controlling silica.
Treatment systems are designed with sufficient flexibility to meet changing effluent targets.

The state permitting authority defines effluent standards to protect aquatic life and

downstream uses of the water. Some discharge standards change on a monthly basis

according to state permits indexed on assimilative capacity. Sodium is a closely monitored

aspect of the plant effluent given its affect to downstream agricultural application. Soils with

an excess of sodium ions, as compared to calcium and magnesium ions, affect the way

plants adsorb water. The ratio of the sodium to calcium and magnesium is referred to as the

Sodium Adsorption Ratio (SAR).

Plants operating with intent to discharge require a Wyoming Pollutant Discharge Elimination

System (WYPDES) permit issued by the WDEQ for construction, operation and discharge of

the produced water. The treatment plant subject to this paper is designed to discharge

greater than 95% of the influent water into the Powder River.

Geographical, Environmental Concerns

Given the pristine remoteness of the Powder River Basin, where inclement weather is a

given, inventory controls must incorporate the possibility for restricted site access during

seasonal extremes. For instance, one must carefully design acid feed systems to minimize

risks to personnel and facility. The volume of acid needed to neutralize the alkalinity inherent

in the CBM water is considerable. Plant designs may require tanker truck deliveries of acid

down lease roads and potentially during severe weather. Treatment plants designed in this

fashion should:

 Store the acid outdoors in double-contained tanks

 Provide storage volumes that consider order delays and weather interruptions

 Have double-contained feed lines and valves.

 Locate tanks as close as possible to injection points to minimize feed line length.

The Mitchell Draw Plant

In 2008, Siemens was awarded a contract for treating Powder River Basin CBM water at

Mitchell Draw. To date, the company has not yet commissioned the plant, designed for

treating 72,000 b/d. Siemens’ engineers focused on system recovery and eliminating acid

feed and therefore added ion exchange softening into the process flow. Ion exchange

removes polyvalent cations from the feed water. The process removes constituents such as

calcium, magnesium, barium and soluble iron to very low levels by exchanging them for

sodium on the ion exchange resin.

Ion exchange is routinely used in front of reverse osmosis applications to protect the

downstream membranes as the reduced hardness lowers the LSI of the feed stream.

Given the low hardness of the CBM water, softener regeneration frequency is manageable

and sodium contribution to the feed stream moderate. The softener provides several

advantages. First, it reduces the potential for scale formation by removing dissolved cations

such as calcium, magnesium and barium. This reduces the antiscalant and acid chemical

requirement typically used for controlling solubility when influent concentration or system

design affects the solubility limits. The process typically feeds acid to a neutral or slightly

acidic pH range to control hardness scale. Without acid feed, the bicarbonate alkalinity

concentration increases, resulting in an alkaline feedwater condition and scale formation.

The high pH in the Mitchell Draw water offers preferred operating parameters that increase

the solubility of residual organics, thus reducing the potential for organic fouling on the
membrane surface. The higher pH shifts the boric acid to borate equilibrium so that the

membrane more easily rejects the boron, resulting in lower boron concentrations in the

effluent water. Hardness removal reduces the potential for silicate scale formation enabling

higher system recovery while the higher pH increases silica solubility, thus lowering the

potential for silica fouling.

Figure 2 shows a flow schematic of the Mitchell Draw plant.

Figure 2

Water Analysis

Table 1 illustrates the changes to feed water quality while applying the pH-adjusted scale

control technique versus the softened feed water technique. The feed water data are

approximated from actual Powder River Basin produced water analyses. The analyses are

then modeled using the Dow Chemical Co’s Reverse Osmosis System Analysis (ROSA)

program to illustrate the changing conditions. As noted in the table for softened feed water,

the design reduces the divalent cations calcium, magnesium, strontium and barium to less

than 0.3 ppm in the adjusted feed water. There is a corresponding increase in sodium

content as an equivalent amount of sodium is exchanged into the water.

Unlike the acid feed case technique, the pH and total dissolved solids (TDS) stay about the

same. The LSI scaling calculation shows –1.3 in the adjusted feed water. Furthermore, the

barium sulfate and calcium fluoride scaling calculations also show reduced scaling potential

with the softened feed water as used at Mitchell Draw.

Influent for iron

reduction and

solids settling.

Iron

Removal &

Media

Filtration

Chlorine

Polymer

Dechlor

Primary RO

Recovery RO

Waste Brine

Bypass water

Mitchell Draw

Softener

(NaZ)

Plant

Effluent

Analytical

Testing
Table 1

Feed Water Units Feed Adj. Feed Conc. Feed Adj. Feed Conc.

Potassium mg/L 35.2 35.2 35.2 35.2

Sodium mg/L 880 880 880 941

Magnesium mg/L 14.6 14.6 14.6 0.0

Calcium mg/L 28.0 28.0 28.0 0.1

Strontium mg/L 0.9 0.9 0.9 0.0

Barium mg/L 1.4 1.4 1.4 0.0

Carbonate mg/L 40.7 2.3 40.7 40.7

Bicarbonate mg/L 2,416 2,118 2,416 2,416

Chloride mg/L 28.4 247.0 28.4 28.4

Fluoride mg/L 1.0 1.0 1.0 1.0

Sulfate mg/L 1.0 1.0 1.0 1.0

Silica mg/L 12.0 12.0 12.0 12.0

Boron mg/L 0.2 0.2 0.2 0.2

Carbon dioxide mg/L 18.9 262.5 18.9 18.9

TDS mg/L 3,460 3,342 3,463 3,479

pH 8.2 7.0 8.2 8.2

Scaling Calculations Feed Adj. Feed Conc. Feed Adj. Feed Conc.

pH 8.2 7.0 8.3 8.2 8.2 9.5

1.2 -0.1 3.8 1.2 -1.3 2.6

Calcium Sulfate (% Saturation) 0.0 0.0 0.2 0.0 0.0 0.0

Barium Sulfate (% Saturation) 24.6 24.6 532.0 24.6 0.0 0.0

Strontium Sulfate (% Saturation) 0.0 0.0 0.2 0.0 0.0 0.0

3.7 3.7 30,251 3.7 0.0 108.0

Silica dioxide (% Saturation) 9.2 11.3 173.0 11.5 11.5 83.5

0.0 0.0 0.4 0.0 0.0 0.0

Langelier Saturation Index

Calcium Fluoride (% Saturation)

Magnesium Hydroxide (% Saturation)

Softened Feed

Softened Feed

pH Adjusted Feed

pH Adjusted Feed

Conclusion

After the Mitchell Draw plant is commissioned, Siemens expects the following benefits from

the system design:

 CBM water is characteristically high in sodium and low in calcium and magnesium

hardness. The low hardness concentration makes it an ideal application for sodiumform ion exchange. Removal of the hardness ions will reduce the risk of hardness

scale formation, eliminate the need for acid and increase system recovery.

 CBM water contains relatively high concentrations of bicarbonate alkalinity. Given

the concentration affect across a membrane surface, the pH of the feed water will

increase to increase the solubility of silica and residual organics and improve the

rejection of boron.

 The process eliminates acid for scale control, resulting in improved plant safety and

system reliability. Public safety is also improved with fewer acid deliveries.

 The new design removes acid feed equipment and controls from the system because

pH control is no longer a primary concern to system operation. In addition, it

improves system reliability because inventory control is less vulnerable to delivery

interruptions.