Fate and Effects of Offshore
Hydrocarbon Drilling Waste

T.G. Milligan, D.C. Gordon, D. Belliveau,
Y. Chen, P.J. Cranford, C. Hannah, J. Loder, D.K. Muschenheim

The Canadian East Coast offshore frontier contains extensive reservoirs of oil and gas and current government policy, at both the federal and provincial levels, is to encourage their development. PanCanadian Nova Scotia Ltd. (formerly LASMO) began production at the Cohasset and Panuke (CoPan) fields on Sable Island Bank in 1992 and the first Grand Banks well started production in the fall of 1997 by the Hibernia Management and Development Corporation. Development proposals for the Sable Offshore Energy Project and Terra Nova have recently passed regulatory review and are now being implemented. Further development of hydrocarbon resources on both Sable Island Bank and the Grand Banks is possible, and exploration on Georges Bank is again being considered.

Hydrocarbon exploration, development and production activities produce environmental risks and impacts. Of particular concern in the offshore frontier are the impacts on fisheries and wildlife of not only accidental discharges resulting from blowouts or spills, but also permissible discharges of operational wastes such as drilling mud, cuttings, and produced water. Protection of these renewable resources and their habitats, which will be present long after the drilling rigs and pipelines disappear, is a high priority and requires objective, credible and timely scientific information. It is important to understand exactly what the potential effects are, if and how they can be mitigated and the significance of any effects that can not be mitigated. This information needs to be incorporated into a continually evolving, scientifically based regulatory framework for the offshore hydrocarbon industry that protects valuable renewable living resources. At the same time, it must be efficient and not impose unnecessary constraints on this rapidly developing industry.

In the late 1980´s, the Panel on Energy Research and Development (PERD) began funding a program on the fate and effects of drilling wastes that has been conducted by the Department of Fisheries and Oceans (DFO), primarily at the Bedford Institute of Oceanography. At that time, despite extensive exploratory drilling on the East Coast frontier, there was limited Canadian scientific experience on the fate and effects of drilling wastes. The proposal by Texaco to drill exploratory wells on Georges Bank met with strong opposition, which resulted in the 1988 drilling moratorium that closed the area until 2000. Therefore, the initial focus of the PERD program was on Georges Bank to determine if the concerns raised about potential impacts on fisheries, especially sea scallops, were valid. With time, the geographic coverage of the program expanded to include Sable Island Bank and the Grand Banks in order to conduct collaborative studies with industry at hydrocarbon development and production sites. The scientific collaboration has also expanded in recent years to include scientists from DFO laboratories in St. John´s and Mont Joli as well as Natural Resources Canada scientists from the Geological Survey of Canada (Atlantic) at BIO.

A series of focused and well-integrated research projects was designed that would provide the information necessary to understand the fate and effects of drilling wastes on benthic resources, in particular the sea scallop. Because of the nature of the questions being asked, these projects have covered a wide range of scientific disciplines. The research has included physical oceanographic field programs on Georges Bank, sedimentological field studies on Georges Bank, Sable Island Bank and the Grand Banks, laboratory studies on the flocculation of drilling mud and on the effects of various drilling wastes on sea scallops, the development of new instrumentation for measuring drilling wastes in the offshore environment, and the development of numerical circulation and dispersion models.

Figure 1 Illustration showing the lethal and sublethal effects of drilling wastes and some of their components on the growth of sea scallops (Placopecten magellanicus). Scallops were exposed to various concentrations over periods of up to 70 days.

Physical Oceanography
The first project undertaken in the program was a detailed field study of frontal circulation and mixing over the scallop grounds along the northern edge of Georges Bank (Loder et al. 1992). It included moored current measurements, drifter tracking, turbulence profiles, and detailed hydrographic surveys. The study has revealed intermittent surface convergence at the tidal front and enhanced vertical mixing at the Bank edge due to the breaking of internal waves, which can have a pronounced influence on the trajectories of oil spills and the dispersion of drilling wastes released in this region. The study has also provided a comprehensive observational data set for the forcing and validation of circulation and dispersion models.

In collaboration with US scientists, a three-dimensional finite-element circulation model of the Gulf of Maine region has been developed (Greenberg et al., in press). This modeling approach provides high spatial resolution in key areas, which is important for such a complex system as Georges Bank. The model has provided vertically-varying mean and tidal flow fields on a seasonal basis which are being used as input to the benthic impact models described below.

Figure 2: Disaggregated inorganic grain size distributions for used drill mud (solid), and two of its major components bentonite (dashed) and barite (dotted). The characteristic shape of the bentonite distribution is used to identify drill mud in samples collected offshore

It has also been used successfully in studies of the early life history of cod, haddock and scallops on Georges Bank. Similar high-resolution models for the Scotian Shelf and Grand Banks are being developed.

Figure 3: BOSS being recovered after sampling near the Hibernia Platform on the Grand Banks. Suspended sediment samples are collected from the benthic boundary layer by spring loaded syringes at 7 depths between 5 and 50 cm above the sea floor.

Biological Effects
Initial studies demonstrated that the exposure of sea scallops to mineral oil-based mud cuttings has potentially damaging implications for production, reproductive success and population survival in the vicinity of a drilling platform (Cranford and Gordon, 1991). These were followed by a lengthy series of laboratory experiments under environmentally representative conditions in flume tanks in which adult scallops were exposed for up to 70 days to different kinds and concentrations of drilling fluids and major constituents to determine the effect of chronic exposure on mortality, tissue growth and physiological responses (Cranford and Gordon, 1992). The results indicate that sea scallops are very sensitive to low concentrations (10 mg·l^-1 or less) of drilling waste and associated contaminants in their food supply (Fig. 1).

Threshold concentrations causing effects are now available and can be used to interpret the output of impact assessment models. The relative toxicity of the four wastes tested (in increasing order) is water-based mud, bentonite, barite and oil-based mud.

Instrumentation has been developed whereby sea scallops can be placed just above the sea floor in the field and their growth response to changing environmental conditions accurately measured. This allows the detection of sublethal responses to chronic exposure to drilling wastes contaminating food supplies under actual exposure conditions. These techniques will be tested at Hibernia beginning in 1999 and will hopefully be suitable for use in industry-funded environmental effects monitoring programs.

Figure 4: Graph showing the % of bottom covered by floc containing drill mud along a transect running from the west to the Rowan Gorilla III on Sable Island Bank

Figure 5: The camera tripod Campod being deployed near the Rowan Gorilla III drilling on Sable Island Bank. A high resolution video camera is mounted on the central axis of the tripod looking down (A). Also mounted on the Campod are a 35 mm camera and flashes (B) and special floc sampler called Slurper (C).

Fate of Drilling Wastes:
Under present guidelines, used water-based drill mud and cuttings may be discharged to the ocean without limitation. When drilling with oil-based mud, only cuttings containing <15% oil by dry weight are permitted. A major component of this study has been to determine what happens to these materials after they are discharged. Essential to this study was the ability to differentiate drilling wastes from naturally occurring material.

Particle size analysis of the principal components of drill mud, bentonite clay and barite, and of used water-based drill mud shows that they consist mainly of fine silts and clays. More precise analysis of the disaggregated inorganic grain size of these materials using a Coulter Multisizer shows that bentonite has a very distinct size distribution with a modal diameter <1mm and, when plotted as log concentration vs. log diameter, a steep negative slope between 1 an 10mm (Fig. 2)(Muschenheim et al, 1995, Muschenheim and Milligan, 1996). This characteristic size distribution can be seen in a sample of used drilling mud obtained from the inclined shaker discharge of the Rowan Gorilla III drilling at CoPan (Fig. 2). In contrast, the disaggregated inorganic grain size of naturally occurring suspended particulate material (SPM) collected during studies on Georges Bank, Sable Island Bank and the Grand Banks have very low concentrations of fine sediment the size distribution for which is either positive or flat in the 1-10mm range. Using a specially designed benthic boundary layer sampler called BOSS (Fig. 3), it has also been shown that SPM concentrations are highest in this near bed region and vary with current speed.

In the past it has been assumed that material such as drill mud that consists of very small particles with settling velocities <0.001 mm·s^-1, would readily dissipate to negligible concentrations on the energetic offshore banks of the Canadian East Coast. Laboratory studies conducted as part of this study, however, have shown that drilling wastes readily flocculate in seawater to form fragile aggregates on the order of 0.5-1.5 mm in diameter with settling velocities > 1 mm·s^-1. This means that drilling wastes could sediment rapidly and be concentrated in the benthic boundary layer near a drilling rig. In 1991, a sampling program carried out from an industry support vessel near the RGIII used particle size distribution data to confirm the presence of drilling wastes not only in the surface plume but also in the benthic boundary layer.

The fragility of flocs makes it impossible to sample them using conventional methods. A second study conducted at CoPan in September 1993, after an extended period of drilling, confirmed that drilling mud does flocculate and concentrate in the benthic boundary layer. Images collected using a SONY 3-CCD high resolution colour video camera mounted in a grab, found dense accumulations of dark flocculant material on the seabed with the highest concentrations near the RGIII (Fig. 4). Size analysis of the BOSS samples confirmed that the flocs contained drilling mud and that drilling mud could be detected as far away as 8 km from the platform. Further observations indicated that these flocculated drilling wastes were alternately resuspended and deposited over a tidal cycle.

In the spring of 1994, a third survey, 5 months after drilling operations at CoPan had ceased, was carried out using a newly designed, light weight camera frame called Campod (Fig. 5). The open profile of Campod allows video and still images to be taken with minimum disturbance of the floc layer found at the seabed. Although there were very abundant flocs in the water column and benthic boundary layer, no trace of drill mud was found. The hypothesis that this was the result of resuspension and dispersion through the winter months was later supported by data collected during a 10 day deployment of MIMS (moored impact monitoring system) tripod (Fig. 6). A digital floc camera designed and built at BIO collected silhouette images of suspended sediment during a storm. The new instrument uses a single CCD to allow characterization of sediment properties critical for obtaining settling velocity. Results showed that during the height of the storm, flocs were dispersed and sand was in suspension at 0.5 m above the bottom, the height of the camera windows (Fig 7a.). As the storm abated, flocs reformed and settled back to the bottom (Fig 7b.).

The new sampling methods developed during this project are now being used to study drill waste dispersion at Hibernia. Results from October 1997 suggest that in contrast to CoPan, drill wastes were not reaching the bottom but were trapped in the surface water. This could be the result of the greater water depth and stratification of the water column. In June 1998, however, flocs were observed on the seafloor near the platform. It has not yet been determined if they contain drill mud.

Figure 6: MIMS tripod being deployed near the Rowan Gorilla III drilling on Sable Island Bank. Visible sensors are A) Digital silhouette floc camera, B) S$ Current Meter, C) VDV current meter and D) Tranmissiometer.

Numerical Modeling of Impacts
Existing plume dispersion models have been used to determine the likelihood of drilling wastes reaching the seafloor under different discharge conditions at the CoPan site on Sable Island Bank and on Georges Bank (Andrade and Loder, 1997). Factors that significantly affect the depth of descent include the mud density, depth of release, initial downward flux of the discharge, current strength and water column stratification. At the relatively shallow CoPan site, wastes generally reach the seafloor rapidly under most anticipated conditions. On Georges Bank, where depths are generally greater and tidal currents stronger, there is greater dilution and horizontal displacement during descent, and impacts on the seafloor are quite variable, depending on location.

Figure 8: Example output from the bblt model showing predicted concentrations of drilling waste in the bottom 10cm at 1 and 4 days after bulk discharge of 95 tonnes of drilling mud located at a frontal site overlaying the scapllop grounds on the Northeast Peak of George´s Bank. The simulation assumes a setting velocity of 0.5 cm for the flocculated mud and is forced with moored current meter observations.

A new benthic boundary layer transport model called bblt has been formulated to simulate the dispersion and transport of drilling wastes near the seabed (Hannah et al, 1995). A key feature of the model is the simplified representation of vertical mixing through the random shuffling of packets of material and the use of observed suspended sediment profiles as probability density functions for the overall vertical distribution. Horizontal transport is represented through the advection of the packets in the vertically- and time-varying flow fields taken either from current meter records or the three-dimension finite-element circulation model described above. Model output includes contours of near-bottom concentrations of drilling wastes around the release point (Fig. 8) as well as concentration time series at fixed locations. Model applications to Sable Island and Georges Bank indicate sensitivity to the effective settling velocity of drilling wastes, as well as important spatial variations in the dispersion and drift characteristics of offshore banks such as those illustrated in for Georges Bank (Fig. 9).

Researchers are now in the process of linking the various pieces of the puzzle together to develop an impact assessment model. Results from the biological impact work on scallops are being used to interpret the output of the bblt model in terms of loss of growth resulting from exposure to drilling wastes. Simulations of realistic discharge scenarios on Georges Bank are being linked to both real-time series and to output from the Georges Bank finite element model to predict zones of impact at various locations. Initial results indicate that impact levels would be very dependent on the site of the drilling operation.

Figure 9:Spatical variablity of drift and dispersion on George´s Bank illustrated for the September-October period with a setting velocity of 0.5cm. The mean height

Summary:
One of the most important general results of the overall program is the realization that the fate and effects of drilling wastes are dependent upon a large number of factors including the type of waste, discharge properties, physical oceanographic setting and the time of year. There is no such thing as a typical situation and each drilling proposal must be assessed and evaluated on its own particular conditions. Existing offshore drilling waste treatment guidelines tend to be generic and only consider the properties of the discharge from a platform. More realistic guidelines must take the properties of the receiving environment into consideration. Some discharge conditions may be acceptable at one location but not at another.

As a result of the research conducted in this program, government and industry are much better prepared to manage future offshore hydrocarbon developments in Canada. There is a much better overall scientific understanding of the fate and potential effects of drilling wastes in the offshore environment that can be used to evaluate concerns raised by the public. There are improved numerical models that can be used to predict the environmental impacts of specific development projects. New instrumentation and methodology are available to conduct more efficient and meaningful environmental effects monitoring programs at approved development sites. The results of the effects monitoring programs can provide immediate feedback into the daily operation of the project (e.g. adaptive environmental management) as well as be incorporated over the longer term into improved discharge regulations. They can also be used to improve the numerical predictive models. This iterative process will help to protect valuable renewable resources without placing unnecessary burdens on the offshore hydrocarbon industry.

References:

Andrade, Y. and J.W. Loder. 1997. Convective descent simulations of drilling discharges on Georges and Sable Island Banks. Can. Tech. Rep. Hydrogr. Ocean Sci. 185: vi + 83 pp.

Cranford, P.J. and D.C. Gordon Jr. 1991. Chronic sublethal impact of mineral oil-based drilling mud cuttings on adult sea scallops. Mar. Poll. Bull. 22: 339-344.

Cranford, P.J. and D.C. Gordon, Jr. 1992. The influence of dilute clay suspensions on sea scallop (Placopecten magellanicus) feeding activity and tissue growth. Neth. J. Sea Res. 30, 107-120.

Greenberg, D.A., J.W. Loder, Y. Shen, D.R. Lynch and C.E. Naimie. 1997. Spatial and temporal structure of the barotropic response of the Scotian Shelf and Gulf of Maine to surface wind stress: a model-based study. J. Geophys. Res. (in press).

Hannah, C.G., Y. Shen, J.W. Loder and D.K. Muschenheim. 1995. bblt: Formulation and exploratory applications of a benthic boundary layer transport model. Can. Tech. Rep. Hydrogr. Ocean Sci. 166: vi + 52 pp.

Loder, J.W., R.I. Perry, K.F. Drinkwater, G.C. Harding, W.G. Harrison, E.P.W. Horne, N.S. Oakey, C.T. Taggart, M.J. Tremblay, D. Brickman, J. Grant and M.M. Sinclair. 1992. Physics and biology of the Georges Bank frontal system. DFO Science Review 1990 & 91: 57-61.

Muschenheim, D.K., T.C. Milligan and D.C. Gordon, Jr. 1995. New technology and suggested methodologies for monitoring particulate wastes discharged from offshore oil and gas drilling platforms and their effects on the benthic boundary layer environment. Can. Tech. Rep. Fish. Aquat. Sci. 2049: x + 55 p.

Muschenheim, D.K. and T.G. Milligan. 1996. Flocculation and accumulation of fine drilling waste particulates on the Scotian Shelf (Canada). Mar. Pollut. Bull. 32: 740-745.

Naimie, C.E. 1996. Georges Bank residual circulation during weak and strong stratification periods. J. Geophys. Res. 101: 6469-6486.