It is obvious, from the preceding discussion, that drilling waste contains a large amount of base fluid, whether that fluid is diesel oil, mineral oil, oolefin, ester, or water. A more detailed discussion about the nature or characteristics of the waste should consider the place of disposal. In a broad sense, this can be accomplished by considering that all waste must be disposed in the water, on land, or in the air. For example, the characteristics of drilling waste when discharged offshore (disposal in
water) will be viewed from the potential effects between the waste and water. These are effects to the seabed, to the water column itself, and to the air/water interface at the surface. In this scenario, diesel oil is an obvious contaminant. Diesel oil creates a sheen on the water surface, disperses in the water column, and creates a toxic effect in cuttings piles on the seabed. For this reason, diesel oil-based drilling fluids and the cuttings generated while using them are not discharged into the sea.
While it is beyond the scope of this text to fully discuss the nature of drilled cuttings, it is important to at least identify some of the common characteristics. Water-based fluids are generally considered relatively benign. The main concern is with the smothering effect of potential cuttings piles, although the creation of piles can be somewhat moderated by the manner of discharge, water depth, and strength of prevailing currents. There is also a concern for entrained oil, either from the formation or from surface additions. With modern emulsifiers, it is possible to entrain fairly large amounts of oil (say, 34%) without detection by standard rig site testing. There is also a concern for toxicity, as defined by the standard toxicity test run in the Gulf of Mexico. This is not truly a test of toxicity, but simply an indicator with a discharge/no discharge implication. Modern drilling fluids formulated for high inhibition can run close to the boundary of this test. Another concern is with heavy metals. With the use of barium sulfate (barite) to increase the drilling-fluid density, there is little direct concern with barium solubility or the biological availability of barium. However, there is concern for trace heavy metals within barite, such as mercury and cadmium.
All of the water-based considerations are also considerations with NAFs. In addition, there are specific concerns with the NAF itself. Generalized concerns associated with offshore discharges and NAFs include:
- benthic smothering
- toxicity (aquatic or in sediments)
- sheen or entrained oil
- biodegradability (aerobic and anaerobic)
- taint (alteration of flavor or smell of fish)
- heavy metals.
Most of these concerns are addressed by some sort of stock (base fluid) limitation and by limiting the amount of fluid to be discharged. Some areas restrict the type of base fluid that can be discharged based on biodegradation rate. There may also be limits on the amount of fluid retained on the cuttings when discharged. In this manner, any fluid on cuttings discharged (whole fluid is not discharged) will biodegrade rapidly and any effects will be short term.
The preceding discussion applies to discharges at sea when no special environmental condition exists. Special environmental conditions might be reefs, oyster beds, kelp beds, subsistence fishing grounds, or sites near shore. In freshwater environments such as lakes and rivers (or enclosed brackish waters), discharges may also pose a hazard due to simple sedimentation.
When considering land disposal options, the concerns are of a different nature. The concern with oil is still present, but to a much less extent. The type of oil is also important. Oil can be incorporated into dirt or soil and will biodegrade. The major concerns are about the concentration of oil remaining after biodegradation and potential plant toxicity of some portions of diesel oil. Some types of NAF will biodegrade to very low concentrations and do not exhibit toxicity to plants. Salts are a major concern. Salt is toxic to plants even at fairly low concentrations. Associated with the salt is the concern over sodium from sodium chloride. Sodium replaces calcium and magnesium in clays, causing a condition known as sodicity. Sodic soils collapse, causing a low permeability to water and a hard surface. Since water cannot infiltrate the soil matrix, there is no water available to support plant life. Further,
salt inhibits the transport of water via osmosis to the plant.
Heavy metal content is the third major concern with drilled cuttings disposed onshore. While barium from barite has low solubility and bioavailability, there is still a concern with the concentration of barium in dirt or soil. Other heavy metals of potential concern that are found in drilled cuttings are lead and zinc, although these are found to be a problem far less often.