On the short-term and long-term impact of drilling activities in the Dutch sector of the North Sea

Transcription

1 ICES Journal of Marine Science, 53: On the short-term and long-term impact of drilling activities in the Dutch sector of the North Sea Rogier Daan and Maarten Mulder Daan, R., and Mulder, M On the short-term and long-term impact of drilling activities in the Dutch sector of the North Sea. ICES Journal of Marine Science, 53: A research programme on the effects of drill-cutting discharges on the benthic system around platforms on the Dutch continental shelf has been running since 1985, with emphasis on locations where oil-based drilling muds (OBM) have been used and discharged. A few locations where only water-based muds (WBM) have been used were also investigated. At OBM locations, elevated total hydrocarbon concentrations in the sediment occurred up to m from well sites during the first year after discharges had stopped. Concentrations tended to decrease to natural background levels during the following years at distances 5 m from the platform, but remained far higher than background levels even after 8 yr within a few hundred metres. One year after drilling, biological effects of OBM discharges were detectable up to m by reduced abundances of a few very sensitive species (particularly Echinocardium cordatum and Montacuta ferruginosa). Closer to well sites, increasing numbers of species experienced adverse effects. In the longer term, the macrofauna seemed to recover at distances 5 m, but within that range the macrofauna was still affected after 8 yr. Possible effects of WBM discharges were investigated at a few single-well sites during surveys carried out within 2 mo to 1 yr after discharges were terminated. Adverse effects on the benthic community were not observed, even within m of a discharge site International Council for the Exploration of the Sea Key words: benthos, drill cuttings, drilling fluids, environmental impact, North Sea. R. Daan and M. Mulder: Netherlands Institute for Sea Research (NIOZ), P.O. Box 59, 179 AB Den Burg, Texel, The Netherlands. Introduction During the past 3 years, there has been a proliferation of oil and gas activities in the North Sea. In the Dutch sector alone, over wells have been drilled at about 5 different locations. Activities in this area culminated in the 198s when 4 to 8 wells were drilled per year (Fig. 1). During these years, the use of oil-based drilling fluids (OBM) has been increasing, which substantially improved drilling efficiency in the deeper well transects compared with conventional water-based muds (WBM) and enabled deviated drilling. Because the extensive discharges of cuttings are always contaminated with residuals of drilling fluids, there has been growing concern about the associated environmental impact. Research programmes have therefore been initiated to assess the effects of drill-cutting discharges (e.g. Hartley and Ferbrache, 1983; Davies et al., 1984; Gray et al., 199; Kingston, 1992; Daan et al., 1992, 1994). Field studies have been carried out in the Dutch sector since 1985, particularly in relation to OBM. At most of the about 12 locations where OBM has been used, the No. of drillings Figure 1. Annual numbers of oil and gas wells drilled on the Dutch continental shelf (data from Anon., 1995) (Production, Appraisal, Exploration ). amounts of base oil that reached the seabed were in the order of tens of tonnes, but in some cases several Downloaded from by guest on 3 October /96/636+9 $18./ 1996 International Council for the Exploration of the Sea

2 Drilling in the Dutch sector of the North Sea 37 6 Frequency Oil: tonnes discharged Figure 2. Frequency distribution of OBM discharge locations in the Dutch sector in relation to the amount of base oil discharged (data from Asjes et al., 1995). hundreds of tonnes have been discharged (Fig. 2). Seventeen surveys have been carried out around five drilling locations, most of which were characterized by a relatively large discharge. In this article, an overview of the results of these studies is presented with special reference to the intensively studied location K12a, a production platform located 8 km north-west of the Dutch coast (53 28 N, 3 47 E). In 1983 and 1984, five wells were drilled at this location with low-tox and diesel-based OBM. Drill cuttings, including 38 tonnes mg oil per kg dry sediment 1 Max. background level mg Ba per kg dry sediment 1 Distance to platform (m) Figure 3. Mean elevations of Ba concentrations relative to natural background levels in the sediment at increasing distances from platforms along the residual current transect. Bars represent ranges of values observed (data from van het Groenewoud et al., 1988; van het Groenewoud, 1991; van het Groenewoud and Scholten, 1992; Daan and Mulder, 1993a, b). of base oil, were discharged on the seabed. A first survey was carried out 1 year after the discharge in 1985 to assess the extent of sediment pollution around the platform and associated biological effects. Subsequently, another five surveys were carried out in 1986, 1987, 1988, 199 and 1992 to follow the recovery of the sediment conditions and of the macrobenthic community. The Downloaded from by guest on 3 October Figure 4. Oil concentrations in relation to distance from K12a ( 1985, 1986, 1987, 1988, 199, 1992) (data from Kuiper and van het Groenewoud, 1986; van het Groenewoud et al., 1988; van het Groenewoud, 1991; van het Groenewoud and Scholten, 1992; Daan and Mulder, 1993a).

3 38 R. Daan and M. Mulder 2 years 3 years 4 years 8 years Oil/Ba ratio major results of this monitoring exercise (reported in detail by Kuiper and van het Groenewoud, 1986; van het Groenewoud et al., 1988; van het Groenewoud, 1991; van het Groenewoud and Scholten, 1992; Mulder et al., 1987, 1988; Daan et al., 199; Daan and Mulder, 1993a) are presented here and compared with those obtained around locations where only WBM had been used. Fieldwork approach The seabed was sampled during field surveys with a.2 m 2 van Veen grab at increasing distances from well sites, most of the stations being situated along a transect in the direction of the local residual current. As a rule, replicate grab samples were taken per station, but within m from a platform this number was sometimes reduced for safety reasons. From each grab sample, two small cores (28 mm, depth cm) were taken for chemical analysis. These subsamples ( 2) were pooled, so that one sediment sample per station was obtained. The grab samples were each washed over a 1-mm mesh sieve and the residual macrofauna was preserved in a 6% formaldehyde solution. Some of these samples (usually five to eight replicates) were analysed individually for quantitative species composition in the laboratory. From 1987 onwards, additional Reineck boxcore samples were usually collected at one station to assess vertical profiles of contaminants in the sediment. For more detailed information on collecting and processing of samples we refer to van het Groenewoud and Scholten (1992) for chemical analyses and Mulder et al. (1988) for fauna analyses. Distribution of discharged material 1992 Figure 5. Observed Oil/Ba ratios by year at K12a (from van het Groenewoud et al., 1988; van het Groenewoud, 1991; van het Groenewoud and Scholten, 1992; Daan and Mulder, 1993a). Barite (Ba) represents a substantial constituent of most drilling muds and is considered a useful tracer for the mg oil/kg dry weight cm 2 cm 3 cm Depth interval Figure 6. Vertical profiles of oil concentrations in the sediment at m from K12a (residual current transect) (data from van het Groenewoud, 1991; Daan and Mulder, 1993a). spatial dispersal of discharged material because of its inertia (e.g. Neff, 1987). Figure 3 illustrates that Ba concentrations were always substantially beyond background levels up to 3 m from well sites. Concentrations between 5 m and m were usually still significantly elevated and traces of discharged material have been detected up to 2 m from platforms. Oil concentrations in the sediment at location K12a were substantially elevated up to m from the platform during all surveys (Fig. 4). There is no indication of a substantial reduction within this area. In 1992 (8 yr after the discharges), the concentrations at m from the platform were within the range observed in previous years. Quite remarkably, the survey of 199, after 6 yr, revealed the highest concentrations and substantially elevated concentrations were even found at 5 m. The wide range of concentrations observed, particularly at m and m from the platform, indicate that the spatial distribution of oil is extremely patchy. Moreover, it is not possible to make a reliable estimate of the degradation rate of the oil on the basis of these observations because of apparent redistribution of discharged material between successive surveys. Van het Groenewoud and Scholten (1992) have suggested that changes in the oil/ba ratios might give an indication of degradation rates. Therefore, Figure 5 shows these ratios for years when concentrations of both oil and Ba were determined. The data appear to be quite variable, but ratios in 1992 were not below the 1986 values, suggesting that degradation rate is very low. Figure 6 shows the oil concentrations at three depth intervals at a distance of m from the platform, suggesting that oil has gradually accumulated deeper in the sediment. Downloaded from by guest on 3 October 218

4 Drilling in the Dutch sector of the North Sea , 1 year after discharge , 2 years after discharge No. of species per sample No. of species per sample , 3 years after discharge No. of species per sample , 4 years after discharge Figure 7. Species richness (numbers of species per sample) along the residual current transect at K12a (data from Mulder et al., 1987, 1988; Daan et al., 199; Daan and Mulder, 1993a). N.B.: Samples collected at m in 1987 were taken from the platform itself , 8 years after discharge Downloaded from by guest on 3 October 218 Biological effects The impact of sediment contamination by oil containing drill cuttings may be roughly described as an overall impoverishment of the benthic community. An increase in species richness with distance from the well site was observed at all locations where OBM cuttings had been discharged. This is exemplified by Figure 7 showing that

5 4 R. Daan and M. Mulder Rank Rank years after discharge 5 Distance from platform (m) 8 years after discharge 5 Distance from platform (m) 5 Figure 8. Relative macrofauna abundance along the residual current transect at K12a in 1986 (top) and 1992 (bottom). (Data from Mulder et al., 1988; Daan and Mulder, 1993a.) a reduction in the numbers of species per sample has been observed in most years when approaching the K12a platform. Analysis of variance confirmed that, except for 1988, there were significant differences (p<.5) in species richness between stations. In addition, a Tukey multiple comparisons test revealed no significant differences between individual stations in 1985 and 1988, but significantly (p<.5) lower numbers of species at m compared with the station farthest away from the platform in the other years. Significantly lower numbers were also found at m in 1986 and at m (samples taken from the platform itself) and 5 m in Figure 7 suggests that the slope of the gradient in species richness tends to become less steep in the longer term. Impoverishment of the macrofauna at stations in the vicinity of the platform is also indicated by the relative fauna abundance (Daan et al., 199, 1992), which is low when compared with more remote stations (Fig. 8). On average, most species appeared to occur in significantly (p<.5) lower abundances within m than at stations at 5 m in In 1992, relative abundance was still lowest at the -m station but no longer significantly different from the others, indicating that both the intensity and the spatial extent of effects at the community level had decreased. Table 1. Results of logit regression for presence absence data of individual species in the samples collected at K12a. Positive gradients indicate that the species occur more frequently away from the platform, negative gradients indicate that the species occur more frequently in the vicinity of the platform (tests at 5% significance level) Time after discharge No. of species tested Positive gradients Negative gradients yr yr yr yr yr Possible gradients in the distribution patterns of individual species were tested by logit regression. The procedure is based on presence absence data of species in individual samples and included a test of the model M that the probability of finding a species in a sample is the same at all stations against the alternative model M 1 that this probability follows a sigmoid curve along the sampled transect (see Jongman et al., 1987). Although for some species other response models (e.g. a unimodal curve) might give a better fit than the sigmoid curve, the latter was considered an adequate response model for most benthic species. For each survey, the regression was applied to those species of which at least 2 specimens were found. Table 1 shows that on average model M 1 gave a significantly (p<.5) better fit than M in slightly less than 5% of the species tested. The most common response was an increase in the frequency of occurrence when moving away from the platform (63 positive slopes), whereas relatively few species occurred more frequently in the vicinity of the platform (11 negative slopes). Although there is no evidence that the number of species showing a significant gradient decreased in the longer term, the distance over which their abundance was affected appears to have decreased substantially. This feature is, for instance, clearly illustrated by the distribution pattern of the bivalve Montacuta ferruginosa in successive years (Fig. 9), a species that has been identified as being very sensitive to OBM contaminated sediment in its response (Daan et al., 1994). In the first 2 years after drilling the species was found almost exclusively at the station 5 m from the platform. During the following years M. ferruginosa gradually colonized the area within m and 8 yr after drilling it was absent only at m distances. Distribution patterns of some other sensitive species (in particular the sea urchin (Echinocardium cordatum)) indicated that effects of former discharges on the composition of the benthic community did not extend beyond a distance of a few hundred metres from the Downloaded from by guest on 3 October 218

6 Drilling in the Dutch sector of the North Sea 41 1 year after discharge 2 years after discharge years after discharge platform after 8 yr. Also the absence of the opportunistic polychaete Capitella capitata in 1992, a species frequently found in the vicinity of OBM platforms during the first years after drilling, suggests that the biological impact has decreased. Effects at other OBM locations investigated were qualitatively very similar. At one location, where 44 tonnes of low-tox base oil had been discharged, the effects in the direct vicinity of the platform were less severe than at K12a, where the amount of oil discharged years after discharge 5 (not sampled) 8 years after discharge Figure 9. Abundance patterns of Montacuta ferruginosa along the residual current transect at K12a ( % occurrence in samples, abundance (n.m. 2 ) (data from Mulder et al., 1987, 1988; Daan et al., 199; Daan and Mulder, 1993a). (not sampled) 5 was nine times as high. However, the initial extent of the affected zone was estimated to stretch beyond m from the platform (Daan et al., 1991). The results of surveys carried out near locations where only WBM had been used were completely different and in fact no effect could be detected at any of the locations investigated. At location L3-3 (53 51 N, 4 53 E), which was visited 2 mo and 1 yr after drilling (Daan and Mulder, 1993b, c), there was no spatial trend in the number of species per sample along the transect sampled Downloaded from by guest on 3 October 218

7 42 R. Daan and M. Mulder 1991, 2 months after discharge , 1 year after discharge 5 No. of species per sample (Fig. ). Also, ANOVA did not reveal significant differences in the number of species per sample between stations, which fluctuated between 33 and 37 during the first survey and between 28 and 31 during the second. Even at a station as close as m from the discharge site no reduction in species richness was observed. The difference in species richness between the two surveys may be explained by relatively large seasonal and year-to-year fluctuations that may occur naturally (Rachor and Gerlach, 1978; Ziegelmeier, 1978). Relative macrofauna abundance in the vicinity of the platform appears not to have been affected by WBM (Fig. 11) and ANOVA confirmed that differences between stations were not significant. Logit regression showed that model M 1 gave a significantly (p<.5) better fit than M in five species during the first survey (Table 2), of which four were found more frequently in the vicinity of the location. However, it is argued that, in view of the large number of tests, the chance of finding only five out of 41 cases where M (i.e. no gradient) is rejected must be taken into account (see, e.g. Rice, 1989). In other words, the odds are not unacceptable that five statistical Type-I errors have occurred. It is concluded therefore that logit regression did not reveal significant gradients among individual species. The same applies even more strongly to the results from the second survey. Discussion Figure. Species richness (numbers of species per sample) along the residual current transect at WBM location L3-3 during surveys carried out 2 mo and 1 yr after drilling (data from Daan and Mulder, 1993b, 1993c). Rank months after discharge from platform 5 Figure 11. Relative macrofauna abundance along the residual current at L3-3, 2 mo after drilling (data from Daan and Mulder, 1993b). Table 2. Results of logit regression for presence absence data of individual species in the samples collected at L3-3 (see also Table 1) Time after discharge No. of species tested Positive gradients Negative gradients mo yr 35 1 The spatial extent of initial effects of OBM cutting discharges as estimated around drilling locations in the Dutch sector is in good agreement with results from the Norwegian sector, where the area affected ranged from 3 to 5 km from well sites (Reiersen et al., 1989; Gray et al., 199). Kingston (1992) also reported effects on sensitive benthic species between 15 and 3 m from discharge sites in the English sector. In fact, the radius of the affected zone as estimated here seems large in view of the fact that the locations investigated were either single well sites or production platforms with relatively few wells ( 5 per location) and not multi-well sites such as Downloaded from by guest on 3 October 218

8 Drilling in the Dutch sector of the North Sea 43 occur in the northern North Sea ( to 3 wells per location). The absence of biological effects at WBM locations may seem surprising, because several studies have revealed at least more or less localized effects (e.g. Gillmor et al., 1985; Neff et al., 1989). In the North Sea, Hartley and Ferbrache (1983) and Addy et al. (1984) have reported such localized effects around multiwell sites. The difference may be explained by the greater extent of the discharges at these locations and associated sediment disturbance, resulting in burying and suffocation of the natural infauna and giving advantage to opportunistic species (see, e.g. Gray, 1982). Moreover, elevated hydrocarbon concentrations were determined in both cases, so the observed effects should not necessarily be attributed to the discharge of WBM. Since 1993, discharges of OBM cuttings have been banned in the Norwegian, German, Danish, and Dutch sectors (Anon., 1994). The use of OBM is still allowed but contaminated cuttings have to be brought ashore for further treatment. Although the short-term effects of OBM cutting discharges are no longer of primary interest, the former discharges appear to have a long-lasting impact. At K12a, persistent effects were determined after 8 yr, albeit in a limited area extending not more than a few hundred metres from the location. There is no evidence yet that the total oil concentration in the sediment has substantially decreased in the vicinity of K12a. However, it is conceivable that the toxicity of the oil may have decreased due to weathering, which usually results in a loss of the lighter aromatic fractions and in an increase in relative importance of the unresolved complex mixture fraction (Boehm, 1987). This might explain why the effects were less severe than during the first years after the discharges. The affected zone at locations where less than one hundred tonnes of oil have been discharged will probably be smaller than at K12a, but the fact that relatively high concentrations were found in the deeper anaerobic sediment layers, where bacterial degradation rates are probably extremely low, may indicate that former OBM discharges will form local but long-lasting sources of potential stress, particularly for deep-digging benthic species, and of disturbance of the benthic fauna composition. Acknowledgements The field studies were partly commissioned by the North Sea Directorate of the Dutch Ministry of Transport and Public Works and partly by the oil companies united in The Netherlands Oil and Gas Exploration and Production Association (NOGEPA). Financial support was also provided by the Dutch national programme Policy Linked Ecological Research North Sea and Wadden Sea (BEON). References Addy, J. M., Hartley, J. P., and Tibbetts, P. J. 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