Comparison of purge and trap GC/MS and spectrophotometry for monitoring petroleum hydrocarbon degradation in oilfield produced waters

Abstract

Results of using a field spectrophotometer and its appropriate protocols as a surrogate method for an oilfield produced water treatment process is presented. Methylene chloride extractions of the produced water before and after treatment maintained a yellow color pigment that was directly proportional to the hydrocarbon concentration. From this, an absorption spectrum and standard curve were developed. A resultant linear plot of the standard curve indicated that there is an excellent correlation (r²=0.9847) between the varying concentrations and the associated absorbance values at a wavelength of 400 nm. Total n-alkane concentration comparisons between the laboratory GC/MS analysis and the spectrophotometry analysis generated data of similar accuracy and precision at concentrations ranging from 1 to 137 mg/l (alpha=0.05). Linear comparisons between GC/MS and spectrophotometric coefficients were near unity, with the constant being near zero, with a correlation coefficient (r²) of 0.99. Based on this study, spectrophotometry is a complimentary method to GC/MS for determining total n-alkane concentrations in oilfield produced water samples.

Introduction

Oilfield produced water is the largest waste stream generated by the oil and gas industry. In many instances, this waste stream is seven to eight times greater by volume than oil being produced at any given oilfield. In the past, treatment of this waste has been very difficult where available technologies (physical/chemical processes) have fallen short in meeting surface water discharge standards due to elevated petroleum hydrocarbon concentrations. However, one promising process is biological treatment. The biological treatment process can be adaptive to a full range of produced water hydrocarbons and total dissolved solid concentrations, with petroleum hydrocarbon removal efficiencies being maintained at 90% to 95% [1]. Water quality monitoring and analysis of such processes must be made in a reasonable time frame in order to make critical infield treatment process changes.

Produced water contains both soluble and insoluble (oil droplets not removed prior to physical separation) petroleum fractions, and are found at variable concentrations. This petroleum fraction consists of a complex mixture of organic compounds similar to those found in crude oils and natural gases. The individual constituents cover a broad range of boiling points, carbon numbers, chemical families, and structural isomers [2]. The major hydrocarbon groups present in produced water include: alkanes, alkenes, alkynes, aromatics, polynuclear aromatics, and complex hydrocarbon compounds containing oxygen, nitrogen, and sulfur. The most prevalent (approximately 90%) groups detected within produced waters are C₁₀ to C₃₀ straight-chain alkanes [3]. The total n-alkanes (TNA) (C_nH_2n + 2), which are present at the highest concentration are C₁₄ through C₁₈ alkanes, with concentrations gradually decreasing with increasing chain length to C₃₄ [4]. Only 25% of the n-alkanes found in produced water are of the higher molecular weight C₂₁ through C₃₄ n-alkanes. This produced water characteristic is illustrated in Fig. 1.

Analysis of crude oils in produced water has shown that the relatively water soluble light aromatics of BTEX comprise only 2% or 3% of most crude oils as a whole [3]. Produced waters from gas production operations generally contain higher levels of BTEX than waters from oil production [5]. Stephenson [6] obtained a mean benzene concentration range of 5.8 to 12.2 mg/l for gas operations, and 1.3 to 8.7 mg/l for oil productions. Because aromatic hydrocarbons are contained within produced waters, above ground aromatic concentrations are dependent on the degree of physical transformation from the water phase to the gas phase. Therefore, any mechanical and/or natural aeration when brought to the surface may alter their concentration.

The preferred method for analysis in petroleum hydrocarbon biological degradation and/or weathering of crude oils is gas chromatography/mass spectroscopy (GC/MS) [7], [8], [9]. GC/MS is used to fingerprint and characterize oil samples giving detailed compositional information unsurpassed by any other methods. One advantage of using the selective ion monitoring mode is that it can give the maximum chemical information on both saturated and aromatic hydrocarbons, with minimal requirement for sample cleanup [10]. It results in a high degree of chemical and spectral resolution from a single analysis that allows quantification of saturated hydrocarbons, polynuclear aromatic hydrocarbons, sulfur heterocycles, and any selected biomarkers [9]. GC/MS has been determined to be a viable and efficient method of analyzing the extent and progress of oil bioremediation and that it is used as a standard in monitoring the biodegradation of oil. However, with all the advantages that GC/MS analysis provides to a biological degradation monitoring project, there are several very important drawbacks (i.e., analytical time delays, analytical costs, sample storage, off-site shipping, etc.).

The use of visual light spectrophotometry allows for simple cost-effective on-site analysis of contamination by petroleum products in ground waters and soils. The cost of a GC/MS instrument is about 10 to 20 times greater than a visual light spectrophotometer, and one must include the cost of accessories such as capillary column and a purge and trap [11]. Many substances important in biology are either pigments that absorb light or colorless substances that can be chemically converted to light absorbing pigments. This property of the absorbance of particular wavelengths of light is the basis for both quantitative and qualitative analysis during biological treatment of produced water. In order for a colorimetric method to be quantitative, it must form a compound with definite color characteristics and in amounts directly proportional to the concentration of the substance being measured. By scanning the electromagnetic spectrum, it is possible to get an absorbance spectrum of the isolated pigment that is characteristic for that pigment in a given solvent. It is also possible to determine the concentration of a substance that has its maximum absorbance at a particular wavelength by measuring the amount of absorbance at that wavelength.

Solutions of the colored compound or complex must have properties that conform the Beer's law and to Lambert's law [12]. The absorbance is related to the ratio of the intensity of the light striking the sample tube (I_o) to the intensity of the light as it leaves the sample (I):

$$A=\log I/I_{\text{o}}\text{ where }A=\text{ absorbance }\text{.}$$

The greater the concentration of the sample the greater the absorbance. The other factor that has a large influence on the result is the distance that the light travels through the sample. The greater the distance, the more likely light will strike a molecule and be absorbed. The distance is equal to the internal diameter of the sample tube and must sometimes be measured. Specific substances have, at a particular wavelength, a constant absorbance, the absorptivity, also referred to as the extinction coefficient. This constant is dependent upon the molecular configuration of the substance, the solvent in which it is dissolved and, to some extent, on the temperature.

In this study, it was hypothesized that spectrophotometry can produce results comparable to GC/MS analysis for total n-alkanes. This paper presents a comparison of results of GC/MS analysis versus spectrophotometry for monitoring petroleum hydrocarbon degradation in a field scale oilfield produced water treatment system.