Particulate Metal Tracers of Petroleum Drilling Mud Dispersion in the Marine Environment Robert P. Troclne and John H. Trefry" Department of Oceanography and Ocean Engineering, Florida Institute of Technology, Melbourne, Florida 3290 1
rn Drilling mud discharges to the marine environment have received considerable attention as petroleum exploration expands along the continental shelves. In this study, an offshore discharge was monitored by using suspended matter Ba, Cr, and Fe concentrations as tracers. The techniques developed permit (1)observation of differential transport, separation, and resuspension of drilling mud components, (2) distinction between drilling- and nondrilling-related source materials in particle-rich layers, and (3) identification of residual drilling mud in a water column with uniform and/or background suspended matter concentrations. Highlights from the data set include observed restriction of vertical settling by a pycnocline, the resistance by barite to resuspension, fractionation of drilling mud components with time and distance, and the presence of a barium "haze" in the water column a t the rig site.
Introduction A primary environmental concern during offshore drilling for oil and gas is the discharge of spent drilling fluids (or "muds") to the marine environment. This concern focuses on two categories of possible adverse effects: (1)a massive influx of particulates that may attenuate light penetration and thereby lower surface water productivity, cause excess siltation on sensitive benthic communities, and/or introduce a very different substratum; (2) immediate and long-term biological uptake of potentially toxic components from the drilling fluid. Assessment of these possibilities requires accurate and complete drilling fluid composition and dispersion data. To enhance the available dispersion data base and provide a framework for future studies, we traced a 27 000-L, 3-h surface discharge from a production platform in the northwest Gulf of Mexico. This exercise made use of water column hydrography, total suspended matter (TSM) concentrations, and chemical analysis of suspended particulates and bottom sediments to follow the dispersion process. Previous drilling mud dispersion studies (2-3) have shown a return to background suspended solids concentrations following a discharge a t distances of 200-1000 m from the platform as a function of release rate, volume discharged, and current velocity. Similar distances for a return to background have also been estimated from particulate Ba and Cr data (1-3). Of the focal elements in our study, particulate Ba (as BaSO,) is used to control the density of the mud and may account for as much as 90% of its solid mass. Barium serves as an ideal drilling particle tracer with concentrations on the order of 200 000 hg of Ba/g of solids in some 0013-936X/83/09 17-0507$01.50/0
drilling muds relative to the 200-600 hg/g common in most nearshore sediments. Chromium, as ferrochrome or chromium lignosulfonate, is used to keep formation cuttings and drilling mud solids in suspension and is also a useful tracer with concentrations as high as 6000 pg/g in some drilling mud solids (4). Iron is a useful indicator of bentonite clays in the upper water column near the platform. These clays, the second most abundant solid phase in most drilling muds, are added for their gelling and suspending properties and function in fluid loss control. The utility of Ba, Cr, and Fe as tracers can be greatly extended by considering concentrations in both nanograms of particulate metal per liter of seawater and micrograms of metal per gram of solid and by using metal ratios. With this geochemical perspective, drilling mud dispersion can be traced over longer distances/time spans, and differentiation of drilling mud components from normal suspended matter may be facilitated.
Methods Study Site. The study site is situated on the outer continental shelf (OCS) of the northwest Gulf of Mexico a t a water depth of -90 m (Figure 1). The 27000-L discharge was in the form of a fine spray released 10 m above the sea surface over a 3-h period beginning a t 2 p.m. local time, May 31,1980. Samples were taken once during the discharge and seven times in the following 17 h (Figure 1, inset A). Initially, the surface manifestation of the drilling mud plume was oriented toward the northwest with a current velocity of -20 m/min and a wind from the southwest a t 300-500 m/min, as determined by the ship's set/drift and anemometer. During the next 2 h, the surface plume shifted almost due south with surface water movement a t 15 m/min. The release of the drilling mud as a spray and active water movement led to rapid dispersion of the visible plume. During the discharge period, numerous conductivity/ temperature/depth profiles (Neil Brown CTD) showed the water column to be highly stratified with a sharp density gradient (pycnocline) at a depth of -15 m (Figure 1,inset B). Sampling. Sample collection was carried out from the NOAA vessel R/V Researcher. To achieve the optimal conditions required for ultratrace metal work, the ship was equipped with a portable, fiberglass-shellclean laboratory (Grasis Corp., Kansas City, MO) with interior design and construction by Environmental Air Control, Inc. (Hagerstown, MD). The double-chambered unit consisted of an inner Class 1000 clean room with a Class 100 vertical
0 1983 American Chemical Society
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Figure 1. Map of Gulf of Mexico showing locations of study sites. Inset A shows expanded view of discharge site with rig locations (W) and sediment Ba concentrations for the top 1 cm. Inset B shows profiles of temperature, salinity and density [ut = (water density in g ~ m - -1.00 ~ g~m-~)10 during ~ ] the study.
laminar-flow clean bench and an outer room for garment changing, sampler preparation, and sample transfer. General Oceanics 10-L, Teflon-lined GO-FLO bottles were used to collect the water samples. These samplers were prepared in the outer chamber of the clean lab and remained closed until 10 m beneath the sea surface. Prior to use, the sample bottles were carefully washed with 1N HC1, rinsed with 18-megohm Milli-Q water and flushed with mid Gulf of Mexico seawater. The GO-FLO bottles were mounted on a metal-free Kevlar hydrowire connected to a sealed bottom weight and triggered by Teflon messengers. Water sample depths were selected from CTD profie data and distance from the platform. Confirmation of depth placement was by way of the ship's acoustic tracking system. Once closed a t the desired depth, the samplers were returned to the outer chamber of the clean lab and attached to a pressure filtration system using filtered N2 a t 85-105 kPa. In-line, 47-mm diameter, nylon Millipore filter holders containing acid-washed, preweighed, 0.4-pm pore size Nuclepore polycarbonate membranes were connected to the lower ports of the bottles for a 1-2-h fitration period. The filter holders were then transfered to the clean bench in the inner chamber of the clean lab to be rinsed with two 30-mL aliquots of 18-megohm water prior to storage in acid-washed plastic petri dishes. Nontalced, polyvinyl chloride gloves were used in all manipulations. Sediment samples were obtained by using a 30 X 30 X 50 cm stainless steel box corer. Duplicate subcores were taken with 7.6-cm diameter plastic core liner and sealed with plastic end caps. Drilling mud was collected during the discharge directly from the drilling platform in acid-washed, 500-mL conventional polyethylene bottles. The drilling mud being released was part of the final flushing of an 1800 m deep bore hole prior to the addition of a packing fluid. Total Suspended Matter Quantification. In preparation for the cruise, the Nuclepore filters were washed in warm 5 N HN03 (double distilled from Vycor) for 24 h to ensure low trace metal blanks, especially for Cr. Three rinses with 18-megohmwater were carried out during the next 12 h. The filters were then placed in acid-washed plastic petri plates and allowed to dry in our clean laboratory. All land-based filter handling, weighing, and particulate digestion were carried out in this clean environment. After drying, the filters were weighed three times to the nearest microgram by using a Mettler M5 S/A balance 508
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with a zlOPoa! source in place to control static. Filters were randomly separated into experimental and control groups. The control filters were later used as filter blanks for chemical analysis. Following suspended matter collection, the filters were dried and reweighed. Less than 0.1% variation was found on reweighing the control filters. Particulate Trace Metal Analysis. Particulate matter digestion followed the procedure of Eggiman and Betzer (5) using 6 N HC1, concentrated HN03 (both double distilled from Vycor), and concentrated HF (Suprapur, MCB). Stoppered Teflon test tubes were substituted for the Teflon bombs specified. Analysis of Fe and Cr was by flameless atomic absorption spectrometry (AAS) using a Perkin-Elmer 460 instrument equipped with an HGA-400 graphite atomizer, an AS-40 autosampler, and deuterium background correction. Combined acid/filter blanks averaged 1.0 ng of Cr/mL and 4.1 ng of Fe/mL relative to mean sample concentrations of 8.0 ng of Cr/mL and 2200 ng of Fe/mL. Barium was determined by neutron activation analysis (NAA) based on the measurement of 139Baactivity following a postirradiation separation of Ba as a BaSO, precipitate (6). Acid/filter blanks for Ba averaged 26 ng/mL; the mean sample concentration was 600 ng/mL. Sediment Trace Metal Analysis. Sediment cores were split and sectioned into 1-cm samples over the top 10 cm of the core, 2-cm sections from 10-20 cm, and 5-cm subsamples for the rest of the core. These samples were then freeze-dried and gently powdered with a Teflon rod. Digestion was carried out in 50-mL Teflon beakers with Teflon watch covers using 0.5 g of sediment and concentrated "OB, HF, and HClO,. Analysis for Cr and Fe was by flame AAS. Barium was measured by NAA of the pulverized sediment using a lBIBa method (6). Acid blanks were below detection limits for the three elements. Multiple analyses of USGS standard rock G-2 (-2 mg for particulate digestion and 0.5 g for sediment analysis) averaged 7.9 f 0.8 pg of Cr/g, 1.82 f 0.05% Fe, and 1800 f 80 pg/g of Ba compared to the standardized averages of 7.0 pg of Cr/g, 1.82% Fe, and 1870 pg/g of Ba reported by Flanagan (7). Drilling Mud Analysis. The drilling mud collected on the platform was analyzed for particulate trace metals and grain size distribution. For trace metal analysis, whole drilling mud was freeze-dried and homogenized. Digestion followed the procedure outlined for the sediments. Grain size distribution of untreated whole mud was carried out by the sieve and pipet analyses described by Folk (8). Selected drilling fluid components (provided by IMCO, Inc.) were also analyzed for several trace metals. These included "blended" and "ferrochrome" lignosulfonate (IMCO RD-111 and VC-10) and barite (IMCOBAR). The grain size distribution of IMCOBAR was also determined by pipet analysis using a density correction.
Results and Discussion Conditions at the Discharge Site. To provide a reference for postdischarge conditions, background data from the rig site were selected from control stations and recent literature values (Table I). TSM concentrations in the area generally range from 50 to 125 pg/L, are comparable with data reported by Manheim et al. (13) and require strict attention to detail to establish. This range of suspended matter concentrations is below the detection limits of most acoustic and transmissometry systems (-200-400 pg/L); therefore, TSM could be enhanced by drilling mud remnants yet not be detected by these methods. Background particulate Cr, Ba, and Fe concentrations in the vicinity of the platform were also quite
Table I. Observed Background Particulate Metal Values and Metal Concentrations in Representative Suspended Matter background concn metal Cr Ba Fe concn (ref), metal pg/g, dry wt
cr
Ba Fe
2.4 6 (9) 4 (10) < 1 (11) 80 (12) 30 (11) 50 (11) 740 210 200 (11) 210 (11) 46 000 (12)
ng/L 4 30" 200
8 200a 260
selected source materials zooplankton zooplankton zooplankton phytoplanktonC Mississippi River susp matter zooplankton phytoplanktonC Mississippi River susp matterb zooplanktonb zooplankton phytoplanktonC Mississippi River susp matter
a Value for the Texas Flower Gardens area (Figure 1); values at the rig site, 80 ng of Ba/L and 400 p g of Ba/g. This study. Group I phytoplankton as described by Martin and Knauer (11 ).
low (Table I) and more closely resemble those of plankton than of aluminosilicate detritud. A distinct near-bottom nepheloid layer (high particle concentrations) of resuspended sediment was observed a t several of the sampling sites. Drilling Mud Characterization. Effective identification of drilling mud components in the marine environment requires an accurate initial characterization. Selected information on the drilling mud solids discharged during this study is summarized in Table 11. These data agree with values calculated from the drilling mud log and suggest that the samples collected accurately represent the bulk of this discharge. Relative to background TSM levels, suspended solids concentrations in the drilling mud (450 X lo6 pg/L) are 4 X lo6times greater. At such high levels, suspended solids dispersion (i.e., degree of dilution) can be quantitatively followed by using a suspended solids dispersion ratio (SSDR) based on eq 1. From initial SSDR = (pg of suspended solids/L of drilling fluid)/[(pg of suspended solids/L of sample) (background value in pg/L)] (1) drilling fluid characterization (Table 11), a theoretical maximum detectable SSDR can be calculated, in this case 3.6 X lo6. Increased sensitivity in tracing drilling fluid dispersion may be obtained by calculating the particulate Ba dispersion ratio (PBDR): PBDR = (ng of particulate Ba/L of drilling fluid)/[(ng of particulate Ba/L of sample) (background value in ng/L)] (2) The detectable PBDR maximum calculated for this discharge study is 5 X W. Similar calculations for particulate Cr yield an observable PCDR maximum of - 5 X 10'. Thus, for each discharge situation, the degree of dispersion can be calculated on a per sample basis and the minimum traceable levels identified. At times, even though the observable dispersion ratio maximum has been exceeded, it may be possible to establish the percent drilling mud character (% DMC) of a particulate sample by using metal concentrations as micrograms of metal/g of solid. For example, an upper
Table 11. Drilling Mud Data Drilling Mud Solids 15% barite CaCO, 6% (3content in 3.4-3.9% lignosulfonate 40 x l o 6 pg/L of fluid particulate Ba (90 000 pg/g of solids) 0.22 x l o 6 pg/L of fluid particulate Cr (500 pg/g of solids) i2 x l o 6 pg/L of fluid particulate Fe (27000 pg/g of solids) Whole Drilling Mud total solids 450 X lo6 pg/L (360mg/,g) density 1.26 g cm11 PH barite 8% lignosulfonate 0.5% water 64% Grain Size Distribution drilling mud solids >62 pm, 2% 62-2 pm, 40% < 2 um. 58%
barite >62 pm, 1% 62-4 pm, 81%
