CALIFORNIA STUDY COMPARES NATURAL/ DRILLING DISCHARGE CONTAMINANTS OFFSHORE

May 4, 1992
William G. Steinhauer Battelle Ocean Sciences Duxbury, Mass. Eiji Imamura Marine Research Specialists Ventura, Calif . Joan Roberts Barminski Minerals Management Service Camarillo, Calif. Jerry M. Neff Arthur D. Little Co. Cambridge, Mass. An analysis of drilling fluid and cuttings discharges in the southern Santa Maria basin offshore California indicates that the amount of metal and hydrocarbon contaminants from drilling operations is small relative to that from natural sources.
William G. Steinhauer
Battelle Ocean Sciences
Duxbury, Mass.
Eiji Imamura
Marine Research Specialists
Ventura, Calif
.
Joan Roberts
Barminski Minerals Management Service
Camarillo, Calif.
Jerry M. Neff
Arthur D. Little Co.
Cambridge, Mass.

An analysis of drilling fluid and cuttings discharges in the southern Santa Maria basin offshore California indicates that the amount of metal and hydrocarbon contaminants from drilling operations is small relative to that from natural sources.

The metal and hydrocarbon discharges were calculated for only one of the three platforms discharging between 1986 and 1989 in the Point Arguello field. However, assuming concentrations are similar on each platform, the combined input of metals and hydrocarbons over the 3-year period was still low (except for barium and lead) compared to the average annual flux from natural sources.

Located off the coast of southern California, the Santa Maria basin contains 50 of the 102 active lease tracts in the Minerals Management Service (MMS) Pacific Outer Continental Shelf (OCS) region. Once the necessary permits are obtained and the offshore fields in these tracts and associated transportation facilities are fully developed, oil production in the Pacific OCS region is expected to increase from 78,000 to 178,000 b/d by the mid-1990s. 1 Future development of the Point Arguello field in the southern Santa Maria basin and the nearby Santa Ynez Unit in the western Santa Barbara Channel is expected to account for a majority of the increased production. The Point Arguello field is one of the largest fields discovered in OCS waters. The onshore Gaviota processing facility serving the Point Arguello field is currently permitted for 100,000 bo/d. However, a number of unresolved issues concerning the transportation of oil and gas from the field have delayed the full development of the field.

The MMS is monitoring the Santa Maria basin to understand possible long-term environmental effects of oil and gas development (California Monitoring Program, Phase II; and Effects of OCS Production Platforms on Rocky Reef Fishes and Fisheries). A site-specific study area was established to determine effects of drilling-related discharges at Chevron U.S.A. Inc.'s platform Hidalgo in the Point Arguello field (Fig. 1). Part of the study included review of discharge records for platforms Hidalgo, Hermosa (Chevron), and Harvest (Texaco Exploration & Production Inc.).

The discharge records were provided to the Environmental Protection Agency (EPA) by the platform operators in accordance with National Pollution Discharge Elimination System (Npdes) permit conditions. The compositions of the drilling fluids were obtained from the MMS Santa Maria district office. Additionally, drilling fluids and cuttings were analyzed for metal and hydrocarbon content.

This article summarizes all permitted drilling-related discharges between November 1986 and January 1989 from the three platforms in the Point Arguello field and compares contaminants from platform Hidalgo and natural sources.

ENVIRONMENTAL SETTING

The Santa Maria basin extends from Point Conception in the south to Monterey Bay in the north. Fig. 2 shows the Point Arguello field in the southern portion of the basin between Point Arguello (3434'N) and Point Conception (3428'N). The Arguello Canyon and adjacent submarine canyons are prominent features of the basin. The topography generally lacks the complexity of the southern California borderland south of Point Conception.

The shelf extends seaward to a depth of about 110 m (360 ft) and varies in width from about 3.7 km (2.3 miles) in the Point Conception area to about 9.3 km (5.8 miles) between Point Conception and Point Arguello. In the Point Arguello area, the seabed rapidly drops to a depth of about 1,000 m and is cut by the Arguello canyon.

There are four major coastal rivers draining into the southern Santa Maria basin: the Arroyo Grande Creek, the Santa Maria River, the San Antonio Creek, and the Santa Ynez River. Each drainage basin is distinct geologically and contributes sediments with a distinct mineral and trace elemental signature. The sediments in the study area are carried from these rivers, from other smaller streams, and by currents flowing north from areas east of Point Conception.

The California current system is a predominant oceanographic feature, as is seasonal coastal upwelling, which leads to significant increases in primary biological production.

Currents near the Point Arguello field are generally aligned with topography and show frequent flow reversals. There are periods of northwest or southeast flow parallel to the coastline and less frequent periods of south-southwest flows that provide a cross-shelf component.

The local circulation has a variety of transient features including eddies, swirls, filaments, meanders, and narrow jets that are obscured in time-averaged current data. Some of these features may be responsible for significant cross-shelf transport of heat, nutrients, and pollutants. Interannual variations, such as El Nino/Southern Oscillation, can obscure or alter the generalized circulation pattern. Thus, the study area has a complex circulation pattern.

OIL SEEPS

Natural petroleum seepage rates have not been estimated for the waters north of Point Conception, but based on previous studies in the neighboring Santa Barbara Channel, seepage may be a principal natural source of petroleum to the sediments. Macroscopic tar particles have been reported in bottom sediments throughout the study area. The hydrocarbon chromatograms of sediments and sediment trap samples from the study area display a prominent unresolved complex mixture characteristic of petroleum hydrocarbons.

Reports of fouled fishing gear and sightings of oil slicks and tar mounds provide additional evidence of seeps in the study area. Tar from natural seeps frequently washes ashore in the Point Arguello vicinity.

Approximately 20 oil and gas seeps have been recorded between Point Conception and Coal Oil Point south of the study area. 2

Compared to other coastal regions, the OCS north of Point Conception is relatively uncontaminated by human activities.

Along the coast there are only a few cities and only minor industrial development; thus, this region has little domestic and industrial waste discharge. Also, Channel Islands National Park is approximately 33 km south of the Point Arguello field, and its sanctuary extends 5.6 km into the surrounding waters.

The relatively limited public access to the national park and the marine sanctuary status further limits the amount of anthropogenic contaminants reaching the southern Santa Maria basin.

DRILLING HISTORY

Although 76 exploratory wells have been drilled in the Santa Maria basin in the past 25 years, there had been no development or production activities in the basin until 1986 when drilling started at Unocal Corp.'s platform Irene in the Point Pedernales field. Drilling at platform Irene, approximately 12 nautical miles north of the Point Arguello field, began in April 1986 and ended in October 1989. Several extended reach wells have since been drilled. Platform Irene came online in April 1987.

In the Point Arguello field, drilling started at platform Harvest in November 1986 and ended with the last well drilled at platform Hidalgo in January 1989 (Table 1).

Sustained hydrocarbon production began in the Point Arguello field in May 1991. Additional production wells are tentatively scheduled to be drilled in the field in 1992-93.

PLATFORM DISCHARGES

A variety of solid and liquid wastes are generated during drilling and production. The Npdes permits may allow discharge of these wastes to the ocean. Deck washdown and sanitary waste are relatively minor discharges that continue throughout the life of a platform; during drilling operations, however, large amounts of drilling fluids and cuttings are discharged. These discharges are of some environmental concern.

Discharges at platforms Harvest, Hermosa, and Hidalgo are allowed under a general Npdes permit issued by EPA Region IX on Feb. 18, 1982, and reissued on Dec. 8, 1983 (Table 2). The permit covers mobile exploration activities and development and production activities. Produced waters are included in the materials permitted for discharge, but there have been no produced water discharges in the Point Arguello field to date.

Table 3 summarizes the constituents in the drilling fluids discharged in the Point Arguello field, as compiled from data provided by platform operators as a stipulation of their Npdes permits. The major inorganic constituents discharged during drilling at all three platforms were barite (BaSO4) and bentonite clay (sodium montmorillonite). Large amounts of potassium chloride, sodium chloride, and potassium hydroxide were also discharged at platforms Hermosa and Hidalgo, with lesser amounts discharged at Harvest.

It should be emphasized that the relative consumption (discharge) of individual drilling fluid constituents, particularly the amounts of commercial products, is a reflection of the drilling conditions. However, it is also a reflection of the management philosophy of the operator. Many of the major constituents and commercial drilling mud additives perform similar functions and can be used interchangeably. For example, a relatively large amount of a commercial polyanionic cellulose polymer (used to modify fluid viscosity) was discharged at platform Harvest; whereas, a small amount was used at platform Hermosa, and none was used at platform Hidalgo. Chrome lignosulfonate, an additive of environmental concern because of potential chromate toxicity, was not used at any of the platforms.

Data were not available for the amount of water used to formulate the drilling fluids at these platforms. The amount of water may range from 76% for a low-density fluid used at shallow well depths to 30% for a high-density fluid used near well completion.

Generally, the washed cuttings, still carrying some drilling fluid solids, were discharged to the ocean more or less continuously at a low rate.

The summaries of the daily Npdes drilling fluid discharge records show that drilling fluid discharges were intermittent and that volumes varied from 0 to 3,100 bbl in a single day (Fig. 3). Intermittent discharges are common during drilling and reflect the type of activity on a given day. For these platforms, the usual discharge was 310 b/d. However, larger volumes were occasionally discharged when the drilling fluid composition was changed substantially or when the volume of drilling fluid increased to the capacity of the fluid tanks. It is not uncommon for these bulk discharges to occur several times during the drilling of a well.

At platform Harvest, 19 wells were drilled with an average of 220 metric tons of solids discharged per well. Platform Hermosa had 13 wells drilled with an average discharge per well of 280 metric tons, and platform Hidalgo had 7 wells drilled with an average discharge per well of 500 metric tons. The volumes discharged from these wells seem typical of production wells drilled in the OCS (200-2,000 metric tons of solids discharged per well).

DISCHARGE CONTAMINANTS

The chemicals of most concern in discharged fluids and cuttings are metals and petroleum hydrocarbons. In samples of drilling fluid at Hidalgo, zinc and barium were the only metals found in concentrations significantly higher than in surrounding marine sediments (Table 4).

Except for mercury and barium, the metal concentrations in the drill cuttings were higher than in the drilling fluids from the same wells. However, only concentrations of lead, zinc, and barium were significantly greater in the drill cuttings relative to the concentrations in marine sediments.

The high concentrations of lead and zinc in cuttings and of zinc in drilling fluids might come from drill pipe thread dope. These metals are present as fine granules in the pipe dope. Elemental lead and zinc dissolve readily in soft freshwater at low pH but are only slightly soluble in seawater because of the high concentrations of carbonate, hydroxide, and sulfate (at a pH around 8.0). Therefore, metal granules probably are relatively inert in marine sediments, unless ingested by animals with strongly acidic digestive processes (e.g., birds).

Barium is derived from the mineral barite which is used as a weighing agent in the fluid. Barite is highly insoluble in seawater (about 50 mg/l. as barium). Except for barium, the metals showed no concentration differences with depth. The barium concentration in the drilling fluids ranged from 25,000 to 180,000 mg/g, reflecting the increased use of barite in drilling fluid formulations with depth.

HYDROCARBONS

Petroleum hydrocarbons were found in all composite drilling fluid and cuttings samples analyzed (Table 4). These hydrocarbons may come from two sources: refined oil intentionally added to the drilling fluid and crude oil from the formations drilled. Refined petroleum products are occasionally added to lubricate the drillstring, particularly for slant holes, and to help free stuck drill pipe.

The use of oil-based drilling fluids is allowed, but the discharge of oil-based fluids or cuttings is not permitted in any U.S. territorial waters.

If the drillstring becomes stuck in the hole, a pill of oil or an oil-based drilling fluid may be pumped down the drillstring and spotted in the annulus where the pipe is stuck. Because oil-based drilling fluids may not be discharged to the ocean, industry practice is to isolate the oil-based pill from the bulk drilling fluid for subsequent disposal onshore.

Traditionally, diesel fuel (No. 2 fuel oil) has been used for stuck pipe. Because MMS Pacific OCS Region policy discourages use of diesel in drilling fluids, the oil industry has begun using specially formulated mineral oil. Mineral oil contains low concentrations of aromatic hydrocarbons and is much less toxic than diesel fuel.

Although as much as 2-4% mineral oil may be added to the bulk drilling fluid to reduce drillstring torque and drag, drilling fluid inventories showed no such practices occurred at wells in the Point Arguello field. Also, the drilling fluid analyses for petroleum hydrocarbons showed no indication that any significant concentrations of petroleum products were added to the fluids used in the Point Arguello field (Table 4).

In the final stages of drilling, the cuttings brought to surface may contain relatively high concentrations of crude oil.

At Platform Hidalgo, concentrations of total petroleum hydrocarbons in the drilling fluids increased from 160 to 990 mg/g and concentrations of polycyclic aromatic hydrocarbons (PAH) increased from 0.9 to 50 mg/g from surface to total depth. The concentration of PAH in drill cuttings increased from 2.3 to 120 mg/g from surface to total depth, but the concentrations of total petroleum hydrocarbons showed no correlation with well depth (600 to 525 mg/g). Drilling fluid and cuttings containing small amounts of crude or refined petroleum may be discharged to the ocean if they can pass the "bucket-sheen" test (i.e., no sheen of oil can be visible at the surface of a sample of the material collected in a bucket).

NATURAL SOURCES

Table 5 lists the average metal and hydrocarbon input to the marine environment from discharges at Platform Hidalgo and from natural sources. Depth-averaged concentrations are given because the Npdes discharge information does not relate discharges to depth of well penetration. This approach averages the well-to-well concentration differences (seven wells) and within-well depth concentration differences (four depths: surface, two middle depths, and bottom), and provides only a rough estimate of average metal and hydrocarbon concentrations in the actual discharges.

For drilling fluid constituents other than barium and petroleum hydrocarbons, this approach appears reasonable because concentration differences with depth are small. However, concentrations of barium and petroleum hydrocarbons in the drilling fluid increased dramatically with well depth as more barite was used in the fluid formulation and as higher concentrations of hydrocarbons were found in the formation rock. Thus, the use of average concentrations most likely underestimated the total amounts of barium and hydrocarbons discharged in drilling fluids. For example, the barium discharge calculated from the drilling fluid analysis and total discharge data is 370,000 kg. However, the barium discharge calculated from the amount of barite consumed on the platform is considerably more (1,810,000 kg barite consumed at platform Hidalgo = 1,070,000 kg barium discharged, assuming all the drilling fluid consumed was discharged).

Table 5 shows the estimated amounts of metals and hydrocarbons reaching the Santa Maria basin from rivers and coastal petroleum seeps. Although significant amounts of metals and hydrocarbons enter the marine environment as a result of drilling operations, the amounts are relatively small, except for barium, compared to the average annual flux of materials entering the southern Santa Maria basin from natural sources. At platform Hidalgo, barium was released in amounts roughly comparable to the average annual rate of input from coastal riverine sources: 320,000-1,070,000 kg released over 14 months compared to 650,000 kg/year from riverine input.

The input of other metals from drilling operations, present in only trace levels in drilling fluids, is low compared to the average annual input from natural sources. Concentrations of zinc and lead are relatively elevated in cuttings, which may reflect high levels in the pipe dope.

This region of the California coast was in a drought during the drilling period from 1986 to 1989; thus, the input of suspended matter from riverine sources was probably below the average low-flow discharge. However, riverine input during low-flow conditions still represents a bench mark against which to compare input from drilling activity.

Although naturally occurring hydrocarbon input to the southern Santa Maria basin has not been estimated, there are numerous hydrocarbon seeps charted in the nearby Santa Barbara Channel. Allen, et al., estimated the natural input entering the marine environment at Coal Oil Point as 7,600-11,400 l./day. If this is indicative of natural input to the southern Santa Maria basin, then the average annual input of total hydrocarbons and PAH from natural sources far exceeds input from drilling operations in the Point Arguello field.

CONCLUSIONS

The total amounts of metals and petroleum hydrocarbons associated with marine discharges can be estimated from an in-depth analysis of the drilling fluids and cuttings. Comparison of the relative contaminant flux to the southern Santa Maria basin from drilling operations at platform Hidalgo to that from natural sources showed that, except for barium and lead, the relative amount of metals and hydrocarbons discharged at sea from drilling operations was small. However, comparison of time-averaged metal and hydrocarbon flux from drilling operations over wide spatial scales to that from natural sources does not address issues of localized or event-specific impact to the marine environment.

Daily discharges in the Point Arguello field varied dramatically, from 0 to 3,100 b/d. Drilling fluid trajectory modeling has shown that most drilling mud is deposited rapidly at first. Initial deposition may cover a broad area but is concentrated around the platform. This deposition pattern is supported by the results of sediment-trap studies. Although the overall flux of metals and hydrocarbons from drilling operations may be small compared to those from natural sources, there may be localized accumulations of some components of the drilling fluid and drill cuttings solids.

The California Monitoring Program, Phase II, was designed to determine and detect localized environmental changes in hard bottom benthic (reef) communities within the Point Arguello field during drilling. This program was also designed to determine if any changes are caused by drilling-related activities or by natural processes. Results from the program indicate that most species observed did not undergo population changes during the drilling period, but that small changes in species abundance was detected in some hard-bottom epifauna, and that these changes could be related to discharge events in the field.

PHASE III

Phase III of this program will continue to monitor epifauna near the Point Arguello field for the next 3 years, during which several more wells at the existing platforms are expected to be drilled. In addition, the program will devote resources to the following:

  • Platform-specific monitoring to further define the area of impact and to refine trajectory modeling

  • Laboratory and field studies designed to resolve issues of natural population change in hard bottom communities

  • Studies to determine the toxicity of drilling fluids to indigenous species of marine animals.

These studies will provide knowledge on separating natural background variation from potential low-level cumulative environmental impacts caused by drilling-related activities.

ACKNOWLEDGMENT

The authors thank Eugene Bromley, EPA Region IX, for providing Npdes discharge information; Jeffrey Hyland, University of California at Santa Barbara, for providing information about the survey region; and Fred Weiss, Donald Boesch, and Robert Spies for review of the manuscript.

REFERENCES

  1. Personal communication with S. Wolfson, MMS Pacific OCS office, Camarillo, Calif.

  2. Personal communication with Danenberger, MMS Pacific OCS office, Camarillo, Calif.

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