Low-power capacitive deionization method shows promise for treating coalbed methane produced water

A test unit for Redwine Resources demonstrated the effectiveness of the small-footprint technology in the Atlantic Rim play. 

Robert Atlas, Aqua EWP, LLC; and John H. Wendell, Redwine Resources

A new, low-power technology using capacitance-based deionization is being used to treat the highly saline waters produced from coalbed methane wells. The mechanism for ion removal during purification and wastewater regeneration is a hybrid of Capacitive DeIonization (CDI) and Electro-DeIonization (EDI).

The technology developed by Aqua EWP, called the EWP X3, uses hybrid electrodes comprised of activated carbon, nano-materials and a semipermeable coating. The hybrid electrodes are electrically charged to opposing polarities using a DC power supply, and ionically charged contaminants in the water are attracted to the electrodes of opposite charge, thus facilitating their removal from the water.

In early 2008, Aqua EWP and Redwine Resources, a producer in the southern Atlantic Rim coalbed methane trend, field-tested a 250-bwpd processing unit on a CBM well cluster in Carbon County, Wyoming, as a first step to a full-scale development including a larger, full-sized treatment unit. Initial results demonstrate the unit’s utility as an effective, small-footprint produced water treatment technology with low operating and maintenance costs.

INTRODUCTION

The Atlantic Rim has become the most significant new producing area in the progressing development of coalbed methane from the Mesaverde coals residing in the Upper Cretaceous formations. This play is a further expansion of the coalbed plays that stretch from the Powder River development, to the north, through Wyoming and now continuing into Colorado and Utah. One of the most critical aspects of this play is the disposition of the formation water that is produced from these wells, its cost and environmental impact.

Prospective acreage for Atlantic Rim CBM exists in a gentle north/south arc adjacent to the west flank of the Continental Divide. Currently identified prospective acreage extends in this arc from the Colorado border near Baggs, Wyoming, about 60 mi north with an identified fairway that varies from 6 to 10 mi in width. Spacing of the initial wells varies, but current rules allocate 80-acre drilling locations, which indicates potential for 3,000 wells within the currently envisioned development area.

Major producers in the southern Atlantic Rim CBM trend are Anadarko Petroleum Corporation, Double Eagle and Redwine Resources, Inc. Anadarko has drilled about 135 wells in seven clusters, Double Eagle has drilled 14 wells, and Redwine has drilled 11 wells. Initial water production rates have averaged 1,000-1,250 bwpd per well for the down-dip wells. Salinities of the produced water have ranged from 2,500 down to 1,200 TDS, which is above the surface discharge limits for drainage into the Colorado River.

In addition to having high salinity, CBM produced water tends to be high sodic, which means that it has a highly level of sodium relative to calcium and magnesium levels. Sodicity is expressed as Sodium Absorption Ratio (SAR). Highly sodic irrigation water can damage soil’s ability to absorb water.

Disposal wells in the southern Atlantic Rim trend are generally completed in various sandstone members of the Haystack Mountain Formation, encased in the massive Steele Shale. The lensatic depositional nature of the disposal zones provides varying injection rates and capacities depending on areal extent. The Lower Haystack has salinities above 10,000 TDS, which allows disposal wells there to be permitted as Class II water disposal wells, into which higher-salinity effluent may be discharged.

Injection pressures into the disposal wells are limited by a regulatory requirement that they not exceed the fracture gradient of the receiving formation.

Permeabilities of the disposal zones nominally allow injection rates of about 6-8 bbl of water per minute for each 50 ft of disposal zone engaged. Based on a water production rate of 1,250 bwpd per well, with an average disposal zone, a disposal well can accept produced water at a rate of about 10,000 bwpd. This would require one disposal well for each group of eight producing CBM wells. This disposal capacity will be degraded over time as the bounds of the disposal reservoir are reached and progressively higher pressures are required to inject into the formation. When the permitted allowable pressure is reached, the well must either be shut in or injected at a lower rate to keep the pressures low enough to avoid fracturing the disposal zone and allowing invasion of adjacent formations. Generally, these permitted pressures will not exceed 900-1,000 psig at the wellhead. The profusion of disposal wells necessary to service the potential total of 2,500 CBM wells is a major concern of environmental agencies, regulatory bodies and operating companies. A gross estimation of one disposal well for each eight producing CBM wells would require more than 300 Water Disposal Wells (WDWs).

Experience indicates that a fully equipped WDW will cost upwards of $1 million. Over $300 million of developmental capital would be required to provide enough WDWs to service the potential total of CBM producers. Furthermore, these disposal wells are not unlimited in their capacity and do not inherently have unlimited lives. And there is no guarantee that suitable disposal zones will be present within the convenient vicinity of the producing wells.

Several issues provide cause for justifiable concern on environmental grounds. First is the fact that a large number of WDWs would present considerable risk of surface water contamination through mechanical integrity and migration problems. Second is the ethical dilemma presented by taking moderately saline waters and then sequestering them into more saline zones, rendering them potentially untreatable for future use. If there is an economically feasible alternative, is this course of action justifiable?

A number of technologies have been maturing, and some are currently being fielded, to try to mitigate these concerns. Outgrowths of methods proposed for wastewater remediation, desalination and other water treatments have spurred a host of potential solutions. Flash distillation, reverse osmosis, ion exchange and capacitance deionization are but a few of the methods available.

These methods are all judged by their suitability to the application intended. Usual engineering and economic factors are in play as well as the ethical desire to cause as little harm as possible, or even to enhance the environment through beneficial uses of the processed water.

TREATMENT SELECTION

Over the last several years, literature searches and engineering investigations have narrowed the choices down to one process that fits well with the operational and ethical requirements of CBM production. Not the least of these requirements is low initial and operational cost. Ability to accept some coal fines in the produced water without disruption or destruction of the remediation equipment, the cost of required power and simplicity of system operation are also driving factors. Another important component from both a personnel-safety and an environmental perspective is the preference to limit the use and handling of caustic chemicals such as strong acids and bases.

Flexibility and portability of the system and the size of its footprint are environmental, operational and marketing design goals. Low-profile observables and the ability to mitigate the adverse impact of the facilities in visually sensitive areas are also important environmental ends. Small size eases the measures necessary for concealment. The ability to meet surface discharge criteria and the ability to obtain permits for injection into reservoirs, impoundments and streams, as well as for other municipal, agricultural and operational beneficial uses, are especially important. Finally, overall cost of processing per barrel of fluid defines the utility of the method.

Capacitance DeIonization (CDI) appears to best meet all these criteria. Redwine Resources, Inc. of Dallas has entered into a joint venture with Aqua EWP to develop and field CDI produced water remediation facilities for both its own use and for marketing to industry.

Tests have shown that CDI is capable of reducing the TDS of up to 95% of CBM produced water to surface discharge standards. In other words, only 5% or so of the produced water need be disposed. The rest may be used beneficially or discharged at surface. This ability effectively extends the service life or capacity of a WDW by a factor of up to 20. Instead of one WDW for each eight CBM wells, as few as one WDW for 160 CBM wells may be required. Different configurations with lower processing costs may yield a higher percentage of effluent requiring disposal-up to 20%- and the processing cost vs. disposal cost will drive the decision of which configuration to use. Even in this case, reducing the number of WDWs to one per 40 CBM wells from one per eight CBM wells yields major disposal cost savings. This treatment option would potentially reduce the number of WDWs required for the entire Atlantic Rim CBM play to 15-60 from the 300 initially estimated.

Flexibility and simplicity of the CDI process allows the units to be packaged in cargo carriers. This is ideal for varying capacity requirements where either an increase or decrease of processing capacity may be needed. Present design shows that 12,000 bwpd of capacity may be housed in a medium-size cargo container. The ability to easily truck these modular units into and out of a facility and set them up with low vertical projection and a small footprint eases concealment and preservation of the surface disruption area, if required. Power requirements are minimal, as are personnel support and facilities required. Combine this with an indicated per-barrel cost reduction of 50% or more for processing and lower capital expenditure than other available technologies, and it is easy to see why Redwine has selected CDI for prototyping and development.

In early 2008, Redwine and Aqua EWP ran a field test at Morgan Run, in southwest Wyoming. Full-scale development and deployment of a larger, full-sized unit are planned to follow shortly.

TECHNOLOGY OVERVIEW

For years, membranes and ion exchange have been used to lower TDS from water and wastewater. These methods are not economical, practical or efficient as TDS levels increase beyond 10,000 ppm. Power sufficient to purify the primary pollutants and low power consumption have not been available in a single technology platform. The produced water treatment field is characterized by mature technology, so the product cycles are measured in decades by the consumer.

The Electronic Water Purifier (EWP), however, is a new technology developed in the last 10 years that has low operating costs, low rejection wastewater volume, low capital expenditure, no chemical requirements, a small footprint and is now available in sizes ranging from under-the-sink water purifiers to large commercial units. These advantages put EWP in position to challenge traditional technologies.

The basic concept for separating compounds that are dissolved in water using electrical means is quite old-dating to 1950. The technology began to be refined in a 25-year period starting in 1980 by about 12 inventors. The system being used by Redwine is one of the most widely installed CDI systems, with more than 1,000 units installed in consumer, commercial and industrial applications for water purification. The Redwine installation is the first commercial application of this system for treating produced water associated with hydrocarbon production.

Various dissolved salts and silica in water are the major components of Total Dissolved Solids (TDS). These dissolved salts need to be removed in many applications or they will form deposits. Ultimately this will affect equipment performance, safety and taste of the water.

CDI removes these dissolved salts using electrodes that are charged with opposing polarities. The ionically charged components of the dissolved salts are attracted to the electrodes of opposite polarity and migrate to them, thus removing them from the water.

The new class of purifier produced by San Antonio, Texas-based Aqua EWP, LLC-the EWP X3-uses a CDI process to remove dissolved ions from water, but also emulates Electro-DeIonization (EDI) by using a semipermeable membrane that coats the electrodes. The device consists of multiple layers including chargeable coated electrodes or layers that work in response to an applied DC potential above 1 VDC nominally.

Each electrode on the device contains a conductive surface sandwiched between layers of activated carbon. A non-conductive spacer material separates the plates from one another. These electrodes are alternately connected to the two sides of a DC power supply via appropriate connecting leads.

The device works on the principle of capacitive deionization to purify water, with the application of a low-voltage DC potential to attract and discharge ions on the electrode surface. The high-surface-area carbon electrode layers attract and hold ions on their surface as the ions move out of the produced water stream flowing through the device. The positive ions are attracted to the negatively charged plate (anode), and the negative ions are attracted to the positively charged plate (cathode).

Eventually, all the charged sites are filled, and the device must then be regenerated by discharging the ions from the electrode surfaces. This is accomplished by shorting the electrodes and reversing the polarity of the applied DC potential. Once a substantial number of the newly displaced ions are flushed into the waste stream, after a length of time, the unit begins to charge again by attracting ions from the feed solution under the influence of the reverse potential. This action then begins a reversible service cycle, Fig. 1.

Fig. 1. The produced water treatment system removes dissolved salts using electrodes charged with opposing polarities. The ionically charged components of the dissolved salts are attracted to the electrodes of opposite polarity and migrate to them. The contaminants are then adsorbed through the coating and onto the activated carbon electrode surface by a process called “electrochemical diffusion,” thus removing them from the water. When sufficient contaminants are deposited on the electrodes, the electrodes are regenerated and the contaminants fall off the electrode into the wastewater stream, which is discharged through a valve.

What makes this device different from any CDI or EDI system on the market today is the use of three different flow mechanisms. The electrode and the semipermeable coating are not in contact with each other, so ions can 1) flow along the spacer, 2) diffuse through the semipermeable coating onto the electrode, and 3) flow between the semipermeable coating and the electrode. The system doesn’t use chemicals to regenerate the electrodes as in EDI. The three flow mechanisms result in shorter regeneration times, faster flush with a greater concentration of contaminants in the wastewater, and 25% less power consumption, compared with previous EWP systems.

TEST UNIT RESULTS

A test unit was field tested on a CBM well cluster at Morgan Run, in southwestern Wyoming, to treat produced water associated with coalbed methane production from the Mesaverde coals residing in the Upper Cretaceous formations. Each well produced about 800-1,000 bwpd.

The test unit consisted of two smaller modules run in parallel for single-stage treatment. The feed flow was 410 bwpd. Sediment was filtered using a 30-micron filter upstream of the unit. The system was designed to be flexible, and was housed in a container that could be easily moved around to various wells, Fig. 2. The test unit could also be set up so as to operate the two modules in series, to treat water of higher salinities but at a reduced flow of 205 bpd.

Fig. 2. The test unit consisted of two smaller modules run in parallel for single-stage treatment. The unit was designed to be flexible, and was housed in a container that could be easily moved around to various wells.

The field test demonstrated that the CDI/EDI hybrid technology could purify CBM produced water to acceptable discharge standards, Table 1. By measuring the conductivity of the produced water before and after treatment, it was determined that a TDS reduction of 80% was achieved from feed water with an initial conductivity of 1,700 microsiemens per centimeter (µmho/cm), with 83% water recovery. It was also determined that the Sodium Absorption Ratio (SAR) could be brought to an acceptable discharge level of 3.0 by blending 50 mg/L of calcium into the effluent. Looking at the likely economics over a five-year span, it was determined that using the CDI/EDI hybrid technology, including the total capital and operational expenditures, would be significantly cheaper than using the currently employed remediation and discharge options.

TABLE 1. Summary of results from test unit

The test unit is flexible enough to allow flow that is 15% higher than under normal operation and still achieve a 60% reduction of TDS. To meet the maximum allowable SAR of 3.0, 150 mg/L of calcium would have to be blended instead of the 50 mg/L used under normal operation. The economics of amortizing the capital cost over 15% more flow might be favorable when compared with the incremental cost of blending more calcium and using more pumping power to overcome the greater pressure drop. This technology platform can vary the flow inverse to the desired level of purification to “dial in discharge limits.”

CONCLUSIONS

The CDI/EDI hybrid water purifier has already shown its utility in more than 1,000 applications for domestic, commercial and industrial use. With the success of Redwine Resources’ field test of the technology at the company’s well cluster at Morgan Run, Wyoming, the system has shown its usefulness for treating coalbed methane produced water as well. Redwine plans to drill 12 more wells in the area in 2008, bringing the total water processing capacity to 10,000 bpd by the end of the year. A larger, full-sized version of the treatment unit is planned to accommodate the flow from the full-scale CBM development.