RIASSUNTO
ABSTRACT
The fate of drill cuttings from offshore wells, and their effect on the marine environment, have been studied by means of sidescan 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.
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.