Characterization of Soluble Organics in Produced Water

Soluble organics in produced water and refinery effluents represent treatment problems for the petroleum industry. Neither the chemistry involved in the production of soluble organics nor the impact of these chemicals on total effluent toxicity is well understood. The U.S. Department of Energy provides funding for Oak Ridge National Laboratory (ORNL) to support a collaborative project with Shell, Chevron, Phillips, and Statoil entitled ''Petroleum and Environmental Research Forum project (PERF 9844: Manage Water-Soluble Organics in Produced Water''). The goal of this project, which involves characterization and evaluation of these water-soluble compounds, is aimed at reducing the future production of such contaminants. To determine the effect that various drilling conditions might have on water-soluble organics (WSO) content in produced water, a simulated brine water containing the principal inorganic components normally found in Gulf of Mexico (GOM) brine sources was prepared. The GOM simulant was then contacted with as-received crude oil from a deep well site to study the effects of water cut, produced-water pH, salinity, pressure, temperature, and crude oil sources on the type and content of the WSO in produced water. The identities of individual semivolatile organic compounds (SVOCs) were determined in all as-received crude and actual produced water samples using standard USEPA Method (8270C) protocol. These analyses were supplemented with the more general measurements of total petroleum hydrocarbon (TPH) content in the gas (C{sub 6}-C{sub 10}), diesel (C{sub 10}-C{sub 20}), and oil (C{sub 20}-C{sub 28}) carbon ranges as determined by both gas chromatographic (GC) and infrared (IR) analyses. An open liquid chromatographic procedure was also used to differentiate the saturated hydrocarbon, aromatic hydrocarbon, and polar components within the extractable TPH. Inorganic constituents in the produced water were analyzed by ion-selective electrodes and inductively coupled plasma (ICP)-atomic emission spectrometry (AES). The WSO found in produced water samples was primarily polar in nature and distributed between the low and midrange carbon ranges. Typical levels of total extractable material (TEM) was about 20 mg/L; that associated with the aromatic fraction was present at 0.2 mg/L and that in the saturated hydrocarbon fraction was present at less than 0.02 mg/L. Formic, acetic, and propionic acids were also found in the produced water, occurring at a total concentration of 30 mg/L. It was estimated that the presence of 30 mg/L organic acids would artificially overstate TEM content by 2 mg/L. Of the five tested parameters, the factor that most controlled the total WSO in produced water was that of aqueous phase pH. Beyond a value of pH7 significant quantities of C{sub 10}-C{sub 20} range material become markedly soluble as they deprotonate in a basic aqueous phase. Both the absolute and relative volumes of GOM brine and crude additionally affected total WSO. Produced water appeared to reach a saturation level of WSO at a.50% water/oil ratio. Pressure slightly enhanced WSO by increasing the relative quantity of C{sub 6}-C{sub 10} range material. Temperature primarily altered the relative ratio of carbon ranges within the WSO without significantly elevating the total WSO in the GOM brine. Salinity had the least affect on the chemical character or the carbon size of WSO in produced water.