The goal of this DOE, Aera Energy LLC, and Kennedy/Jenks Consultants funded project was to evaluate the potential for treatment and beneficial reuse of produced water from an oilfield in San Ardo, California. A 10 - 30 gpm reverse osmosis (RO) pilot plant was constructed and operated for nine months. The key constituents of concern include total dissolved solids (7000 mg/l TDS), temperature (190 º F), boron (25 mg/l), ammonia (20 mg/l ammonia as N), and organics (75 mg/l TOC). Although water quality requirements vary with the type of end use, treated water quality goals for this pilot were set at 400 mg/l TDS, 1 mg/l boron, 5 mg/l ammonia as N, and 1 mg/l organics. Highlights of the project include an evaluation of a recently developed boron rejection membrane for boron removal at a lower pH than for a conventional brackish water membrane. The lower pH (9.5 versus >10.5) could result in significant savings on the caustic requirement.
All of the water quality goals, except for ammonia, were achieved during the pilot study using both the conventional and the new boron rejection membrane systems. Because ammonia and boron have conflicting pH requirements for removal by RO, a separate ammonia removal step was necessary to meet the ammonia treatment goal. Analysis of
the low and high pH clean-in-place (CIP) solutions used following membrane cleanings, as well as evaluation of transmembrane pressure drop after cleaning, showed that most of the pressure drop was caused by inorganic scaling from magnesium and silica, rather than organic fouling. Although the new boron rejection membrane successfully removed more boron at a lower pH than the conventional brackish water membrane, this membrane was more prone to scaling.
Oil production generates a large amount of by-product water, commonly known as 'produced water.' As oil is produced from an oil field, the amount of produced water can account for over 90 percent of the fluids pumped from a well. The most prevalent method of using or disposing of oil field produced water is to inject it underground. A significant portion (50 to 65 percent) of produced water from onshore sources is currently reinjected into oil producing zones where it enhances oil recovery (using water flooding and steam flooding) or for subsidence control. The other 35 to 50 percent is disposed of via deep well injection or other methods. Unfortunately, deep well injection disposal may increase reservoir pressure and, in steam floods, lead to lower oil recovery and increased production costs. In such cases, eliminating deep well injection by finding a beneficial use for the treated water may increase oil production, increase recoverable oil reserves, and reduce production costs. Furthermore, treatment of oilfield produced water
may provide a new reclaimed water supply for specific uses in water-short areas such as California.
The feasibility of produced water reclamation depends on a number of factors. For example, the chemical composition of the produced water, which is typically very saline, can significantly impact the treatability of these waters. The total dissolved solids (TDS) of produced waters in the United States can range from about 3,000 to more than 350,000 mg/l, with sodium and chloride generally comprising 70 - 90 percent of the TDS (U.S. Geological Survey, 2002). The produced water may also contain high concentrations of calcium, iron, manganese, ammonia, boron, and dissolved organics. Often, the complexities involved in treating these waters (e.g. TDS > 10,000 mg/l) may render produced water reclamation cost-prohibitive. In addition, regulations adopted pursuant to the Clean Water Act prohibit the discharge of treated produced water from onshore oil and gas wells into surface waters, except in areas west of the 98th Meridian (a north-south line approximately running from just west of Minnesota down through Dallas, Texas). Discharge of treated produced water directly into surface waters west of the 98th Meridian is allowed only if the treated water is of acceptable quality for agricultural use or wildlife propagation. If the treated water is intended for any other beneficial use, it must be delivered through direct piping or alternate means, which may not be cost effective. Finally, large storage facilities (hundreds of millions gallon capacity) would be needed if there is a large seasonal variation in water demand for the identified end use.
This project, jointly funded by DOE, Aera Energy LLC, and Kennedy/Jenks consultants, evaluated reclamation of produced water from an oilfield at San Ardo, California. The major tasks included evaluation of treated water end use options, regulatory requirements, and a pilot study to evaluate the technical and economic feasibility of treating this produced water. This paper presents data from the pilot study performed to treat the San Ardo produced water for potential off-site use.