FATE OF DRILL CUTTINGS IN THE MARINE ENVIRONMENT

The fates of drill cuttings from offshore wells, and their effect on the marine environment, have been studied by means of side-scan sonar, scuba and underwater photography. Adverse effects are minimal and short-lived, and in some instances the accumulation of cuttings may be beneficial. Modification of normal mud and cuttings discharge procedures would be needed only in very unusual circumstances.

DRILLING CUTTING INTRODUCTION

When wells are drilled for oil and gas, the rock penetrated is broken up and is brought out of the well in small chips or ‘cuttings’. These pieces will normally vary in size from fine sand up to pieces one inch long and inch wide, with the majority approximately 1/3 by 1/6 inch.
Cuttings from wells drilled offshore are routinely discharged into the water, and some concern has been expressed by various environmental groups that this might be detrimental to the marine environment. This paper presents the findings of a study conducted to determine just what effect the cuttings have on marine life, and what happens to those chips after they hit the water. To date we have had an opportunity to make 31 dives during the course of this study to observe cuttings under 3 different drilling wells and 11 different platforms in the Gulf of Mexico and 2 platforms off Santa Barbara, California.

CUTTINGS ACCUMULATION

Cuttings come up out of the well bore suspended in a large volume of drilling mud. When this mud reaches the surface it flows over a sloping, vibrating screen, the shale shaker. The mud flows through the screen and is pumped back down the well again. The cuttings, which are too large to go through the fine screen, roll down the screen into a trough, and from there down a pipe to the water. As soon as they touch the water the thin film of mud coating each chip is washed off and the clean cuttings drop to the sea bottom. Figure 1 is a photograph taken under a drilling well at a time when a particularly large pile of cuttings had dropped off the shale shaker so that there would be sufficient mud and chips in the water to make a good picture. The downpipe is poorly shown in the upper center of the photograph, but the clean chips and the mud which has been washed off are clearly visible.
In the course of our studies offshore we have followed cuttings from the downpipe to the sea bottom, and have examined and photographed both new and old accumulations on the sea bottom to determine how various organisms react to these small chips. Cuttings falling through the water column are either ignored by fish or, on some occasions, examined or even tasted to determine whether they might be edible. Since the chips are very small, they do not adversely affect the fish.

Studies of sea floor conditions while drilling is going on show that although the sessile benthic organisms in the area of accumulation may be buried, there is little or no affect on the plankton or mobile benthic organisms.

The amount of biologic damage is dependent on the areal extent and thickness of accumulation of the cuttings, and on the number and type of organisms living there.

The size and shape of the pile of cuttings which may accumulate under a rig is dependent on diameter and length of the hole drilled, type of sediment penetrated, depth of water, and strength, duration and direction of currents. There is no definite rule as to what the shape thickness or extent of accumulation will be since there are so many variables. Side-scan sonar records and diver surveys show that accumulations in the Gulf of Mexico are typically approximately 150 feet in diameter, with the outline being circular, elongate or star burst. Figure 2 is a side-scan sonar shadow-gram of the cuttings accumulation on the sea bottom of a typical Gulf of Mexico well, made approximately 2 months after the well finished drilling. The dark starburst pattern is the areal extent of the cuttings which are denser than the surrounding sediments. Lack of strong shadows from the cuttings indicates that there is no high relief on the accumulation. The light area in the center of the starburst is the shadow caused by the depression left when the casing was cut off below the mud-line and pulled out.

Maximum elevation of a pile immediately after drilling a well appears to be less than 3 feet, thinning rapidly toward the edges. In areas with stronger currents the accumulation is thinner and more spread out. Where currents are very strong, there will be no noticeable buildup of cuttings, they will simply be swept away and be completely scattered as individual chips or sand grains.

Even while wells are drilling and cuttings coming down through the water, mobile organisms move around on top of the fresh cuttings pile. Various types of fish swim around, rest on the cuttings or even nose into holes dug in the piles by the divers. Figure 3 is a photograph taken in 85 feet of water while drilling was going on in South Timbalier block 111, 55 miles southwest of Grand Isle, Louisiana. It shows the fresh cuttings accumulation on the sea bottom, with one small fish resting directly on the cuttings and others swimming near the bottom. A 15 inch close-up lenses was used, and the fish in the center is approximately 2-1/2 inches long. Several types of crabs were noted there, as were snails on rare occasions. More observations would be expected to increase the number of types of animals which move around on top of the accumulating cuttings with no apparent adverse effects.

This new sea bottom, the top of the cuttings, as shown in the photograph in figure 3 contrasts strongly with old sea bottom as shown in figure 4, which was taken approximately 70 feet away on the same day. The area shown in figure 4 where there are no cuttings is characterized by a large number of burrows in the mud and sand bottom, and by scattered small white mollusk shells or fragments of shells. This contrasts sharply with the top of the fresh cuttings shown in figure 3.

However, as soon as drilling is completed both the organisms and the sea itself begin to rework the cuttings. In most areas, fine mud carried by bottom currents will immediately start to cover the area, and within a few months it may be difficult to find cuttings without digging into the bottom. In areas of stronger currents the chips will be covered with sand, or will themselves be swept away. Shale chips, upon contact with the sea water, begin to disaggregate in a slow but continuing manner, eventually to become unconsolidated mud, the same as that discharged by rivers.

Along with the physical changes noted above, mobile benthic organisms from the surrounding sea floor move up onto the cuttings and build homes as they would anywhere else, as do the larvae of sessile organisms. Within a few months there is usually a diverse and flourishing fauna living both on top of and in the cuttings pile. The photograph in figure 5 was taken in 110 feet of water 65 miles southwest of Grand Isle, Louisiana under a platform in South Timbalier block 172. Drilling from that platform had been completed 8-1/2 months prior to our diving there. The water at the time of the dives was quite murky, and photographs had to be taken with a 4-3/4 inch close-up lenses. Apparent slope of the sea bottom is due to tilt of the camera. A number of photographs were taken which, together with visual observation, showed that although some cuttings were still visible, they were being covered with normal marine sediments. As can be seen in the picture, a few chips are still visible, but the sea bottom there is now mainly mud, silt and fine sand which has nearly covered the cuttings. The conical object in the upper right is a live snail grazing on the bottom. Numerous other animals were observed in the immediate area.

A sample of the top two inches of sediment and cuttings was collected from the same spot where figure 5 was taken. The cuttings have been somewhat rounded by partial disaggregation of the clays due to sea water adsorption, and possibly from abrasive current action. They also show a brownish oxidation on the exterior, further evidence that the chips are undergoing weathering and will not remain as entities. We have since that time taken additional samples under other platforms, and found this to be happening at all localities.

A comparison of the included Recent fauna in that sample with that of a sample taken from the old “normal” sea bottom on the other side of the platform shows that both contain essentially the same fauna, and in essentially the same abundance. Present in both were nearly 30 species of foraminifers, more than 15 species of mollusks, several species of bryozoans, echinoid spines, ophiuroid ossicles, crab fragments, various types of fish remains, etc. We have generally found this to be true in other areas as well. There are often minor differences due to differences in hardness of the substrate, or due to the fact that the top of the cuttings is slightly above the surrounding sea bottom. However, the major difference is that samples under or very near the platform will have more barnacles, fish remains, and certain other animal remains, reflecting the increased productivity on the artificial reef formed by the platform.

As time goes on current action, sedimentation and faunal and floral activity bring the area where cuttings originally accumulated more and more into line with the surrounding sea bottom. Figure 6 is one of a number of photographs taken on top of what is left of the cuttings accumulation under a platform in Brazos block A-I, 45 miles south of Freeport, Texas, in 120 feet of water. The last well had been drilled from that platform approximately two years before the pictures were taken. During that time, sedimentation and the activities of various organisms had been such as to hide the presence of cuttings from all but detailed observation. Where once the area had been a pile of fresh cuttings, it now is a highly burrowed sea bottom, with the surface bearing the normal complement of mollusk shells and fragments, worms, algae, bryozoa, small Oculina (coral) colonies, etc. Also, there are the usual types of fish feeding on or near the bottom.

We have observed this same situation time and again under various platforms, and there is little question that this is the standard progression. The major exceptions are those areas where strong currents have swept all the cuttings away and there is only shifting sand around the platform, or where barnacle (Gulf of Mexico) or mussel (off Santa Barbara) shells have dropped off the platform in such numbers as to form a “reef” which has covered the cuttings.

The senior author has had an opportunity to dive under several platforms in the Main Pass Area east of the Mississippi delta, in a region of occasional strong bottom currents. A thorough search of the bottom under several of those platforms has failed to turn up even one cutting chip, and it is quite evident that they have been completely scattered by the currents. The same situation is to be expected in any area where currents are strong enough to form good sized sand ripples, or to sweep rock outcrops clean of sediment.

Cuttings are buried, and incorporated into “reefs” under such platforms as Hilda and Hazel off Santa Barbara, California. There the cuttings from numerous wells are buried in 20 feet high mounds which include and are covered by thousands of large horse mussels (Mytilus edulis) and other organisms which have fallen off or been scraped off the platform legs. Such buildups now have a much richer and more diversified fauna than the surrounding muddy bottom, and undoubtedly increase the biomass of the area.

CONCLUSIONS

The accumulation of cuttings under an offshore well will cover only a small area of the sea bottom, and damage to the marine life will be minimal and temporary. Bottom currents tend to either cover the cuttings with mud, or to wash them away. Benthic organisms rapidly move onto and in the accumulation of cuttings and use the area as they would any sea bottom of the same consistency.