The goal of this project is to determine the optimal blending ratio of salinity and sodicity required for irrigating crops with produced water treated with various pretreatment methods, followed by reverse osmosis (RO), ion exchange, or electro-dialysis reversal (EDR). The optimal blending ratio is necessary to maintain the long term physical integrity of representative soils from the Powder River Basin (PRB) and to achieve normal crop production.
Western Research Institute (WRI)
Given the current price and strong demand of natural gas, the continued development of the coalbed methane (CBM) industry in the Powder River Basin (PRB) is a certainty. Coalbed methane reserves in the PRB represent a major recent play for the industry and are of great importance to the region and the nation. CBM reserves are estimated at over 25 trillion cubic feet in the PRB of Wyoming and Montana alone. This is equivalent to the gas reserves of the Gulf Coast. According to the Final Environmental Impact Statement (January, 2003), an anticipated 51,000 wells are expected to be in service within ten years.
There are a number of issues that impact the CBM industry. The primary issue is the vast water quantities needed for CBM production. The draw down of water in the coal aquifers is causing concern from landowners and others who are worried about the availability of water for future generations. Another concern is the large quantity of CBM produced water being discharged in the process of releasing gas from coal seams and how this water might be used in worthwhile, environmentally sound applications. Significant quantities of produced water discharge are anticipated, with estimates of over three million acre feet being produced over the next ten years and an estimated 4-8 trillion gallons over the potential 30-35 years of the PRB CBM play (FEIS, 2003). The number of producing wells and the volume of produced water in the PRB has increased in recent years while total gas production has remained relatively constant (Figure 1.)
The practice of discharging large volumes of water into drainage channels or using it to irrigate rangeland areas has the potential to cause problems with regard to salinity and sodicity of soils. The primary problem associated with salinity is the ability of plants to take up water to facilitate the biochemical processes of photosynthesis and plant growth. As the solution electrolyte concentrations of the soil increase, plants become less able to absorb sufficient volumes of water. As a result, the plants will not function at high levels and will grow at slow rates or die. The major impact of sodicity on soils is associated with soil structure. Soil structure is important to maintain the flow of gases (oxygen) and solution (water plus nutrients) to the plant roots. Poor soil structure can cause severe erosion of once-productive soils. High levels of sodium can cause the structure of a soil to completely disperse.
An important aspect of the sodicity and salinity chemistry of soils is that the two are closely related. For example, a highly sodic (high sodium adsorption ratio (SAR)) soil can maintain its soil structure if the salinity level (electrical conductivity (EC)) of the soil is high. However, if the salinity level is low, a highly sodic soil will slake and disperse and the structure will be lost. If a soil is characterized by a low SAR, the application of clean water or water characterized by low EC can cause degradation of soil structure. If the electrolyte concentration of the water applied to the soil is high, the soils will maintain their structure. Therefore, it is important to develop produced water management practices that assure that water applied to soils meets the favorable combination of salinity and sodicity that will allow plants to grow at good production levels and maintain the structure of soils. Each soil will react differently to the chemistry of the water applied and the method of application. Therefore, research is necessary to understand these interactions in order to develop improved irrigation practices, and to assess the long-term consequences of irrigation on salt movement, loading, and plant productivity. This will be necessary in order to assess CBM producer liabilities.
Some waters currently being disposed of in the PRB are of a high enough quality that land application should not cause any significant problems. However, there is evidence that some of the CBM-produced waters will cause problems with salinity and sodicity in soils. The geographical distribution of SAR values for the CBM waters within the PRB are shown in Figure 2. The SAR values increase to the north and west. In fact, the Big George coal seam, the next big play, is expected to produce higher water discharge/quantity of gas and very poor quality (very high SAR) water.
Demonstrating effective treatment technologies and beneficial uses for oil and gas produced water is essential for producers who must meet environmental standards and incur the high costs associated with produced water management. Proven, effective, produced water treatment technologies coupled with comprehensive data on blending ratios for productive long-term irrigation will improve the state-of-knowledge surrounding produced water management in the West and other regions. This knowledge will help managers find beneficial uses for the produced water that will provide an economic return in the form of crop production that utilizes a windfall water source that would otherwise be managed as a contaminant stream at an additional cost to the producer. Irrigating with treated produced water will also ensure that the cropland remains viable and productive once extraction projects have ended and irrigation resumes using traditional irrigation sources or a dry-land ecosystem relying on natural precipitation. Irrigating with produced water will also relieve some of the water-volume strains on natural streams and rivers in areas like the PRB where low water quantity sometimes causes more problems than poor water quality. Furthermore, effective produced water management scenarios, such as cost-effective treatment and irrigation, will discourage discharge practices that result in legal battles between stakeholder entities. Meeting these challenges will help to maintain safe and efficient oil and gas production in the U.S.
During the first year of this project researchers studied currently available technologies for treating produced water and the effects of irrigating with highly saline and/or sodic water sources. Soil and water quality data from areas that have been, or are currently being, irrigated with treated produced water were analyzed and relevant information was used to evaluate blend ratios and application rates for use during planned experiments. Reverse osmosis and ion exchange will be used to treat CBM water in this project. Reverse osmosis and electro-dialysis reversal will be used to treat conventional produced water.
Three soil types were collected from areas near Riverton, WY, Sheridan, WY, and a short distance north of Sheridan, WY(X). Preliminary analyses completed on the soils indicates that the Riverton and X soils are a clay-loam and the Sheridan soil is a sandy-loam.
This soil was used to reconstruct an A to Bw profile in PVC columns and potted plants. Plants were started from seeds in each planted treatment. Homogenized soil pots and cores composed of three representative soil types from the PRB and two representative plant species grown in the PRB were irrigated with a range of treated produced water and a range of treated blends, with or without mineral amendments including calcium. The produced water was treated using RO, ion exchange, or EDR along with dissolved air floatation (DAF) and/or granulated activated carbon (GAC) pretreatments as necessary.
Petri-dish seed germination experiments were completed as part of the preliminary testing currently underway in support of the greenhouse experiments. Results indicate that significantly fewer alfalfa seeds germinated when watered with untreated conventional oil and gas produced water (12% germination) than with any of the other waters (68-92% germination). There were no significant differences in the germination of alfalfa seeds when watered with other treated or untreated water chemistries.
In the greenhouse study, plants demonstrated differences in alfalfa growth in some soils irrigated with oil and gas produced water treated with EDR. Alfalfa plants responded well to the EDR treatment of oil and gas produced water. As the amount of treated water increases, plant production appears to increase. Plants treated with at least 50% oil and gas EDR appear comparable to the highly treated produced water and may exceed growth of the control. Preliminary chemistry data of the extracts are demonstrating the expected interactions between SAR and EC resulting from comparisons between treatments.
The greenhouse irrigation activities were completed. Biomass has been measured for alfalfa and wheatgrass irrigated with various blends of treated CBM and oil and gas produced water. Preliminary results show that wheatgrass biomass did not increase significantly with the treated versus untreated produced water. Alfalfa biomass did significantly increase with treated produced water versus untreated water. Biomass results are undergoing more rigorous statistical modeling. Coal-bed natural gas (CBNG) produced water was associated with higher biomass production compared to produced water generated during oil and gas production. Plant materials have been dried and prepared for chemical analysis. The soil column experiment results show a general trend of higher hydraulic conductivity measurements through soils irrigated with more highly treated waters versus the untreated water. Treatments associated with oil and gas produced water and the clay loam soil appear to cause more notable changes between levels of treatment.
This project ended September 30, 2010. The final report is available below under "Additional Information".