There are three contributing factors of toxicity in drilling waste: the chemistry of the mud formulation, inefficient separation of toxic and non-toxic components and the drilled rock. Typically, the first mechanism is known best because it includes products deliberately added to the system to build and maintain the rheology and stability of drilling fluids. The technology of mud mixing and treatment is recognized as a source of pollutants such as barium (from barite), mercury and cadmium (from barite impurities), lead (from pipe dope), chromium (from viscosity reducers and corrosion inhibitors), diesel [from lubricants, spotting fluids, and oil-based mud (OBM) cuttings] and arsenic and formaldehyde (from biocides).
Inefficient separation of toxic components from the drilling waste discharge stream becomes another source of toxicity through retention of the liquid phase on OBM cuttings, use of spotting pills or indiscriminate practices of on-site storage. Removal of the liquid phase from cuttings separated by the solids-control equipment becomes particularly important while using diesel-based drilling fluids (DOBM). Field data show that the total oil-based mud discharge rate jointly for the mud cleaner and centrifuge is 10 bbl/h. Also, the OBM removal performance is different for various separators as shown in Table 1 (the highest for mud cleaners, and lowest for centrifuges).
|Table 1 Liquid discharge and oil retention on cuttings from oil-based muds (OBM) for various separators|
|Oil content (% w/w)/OBM discharge rate (gal/min)|
|Reported data||Shale shaker||Mud cleaner||decanting centrifuge|
Research revealed that the OBM retention on cuttings is smaller for the mineral oil-based than for diesel-based OBMs, as evidenced by field data in Table 2. The hypothetical mechanisms of oil retention on solids have been attributed to adhesive forces, capillary forces and oil adsorption and were identified as the amount of oil removed from OBM cuttings using centrifugal filtration, npentane extraction and thermal vaporization, respectively. The conclusion has been forwarded that 50 % of the oil–solids bond could be attributed to adhesive/capillary forces, 29 % to weak adsorption and 20 % to strong adsorption, i.e. 20 % of oil on cuttings could not have been removed with n-pentane extraction. The adhesive mechanism was also explained using the wettability preference of drilled rock. The preference was evaluated by measuring the adhesion tension of thin-cut plates of quartz and shales immersed in OBM. The results showed that the rocks、 immersed in diesel OBM became strongly oil-wet, whereas for the mineral OBM, the initially oil-wet surfaces tended to reverse their wettability and became waterwet.
|Table 2 Oil retention on OBM cuttings vs type of oil|
|Diesel OBM||20.0||13 – 16||9.8||10.8|
Indiscriminate storage/disposal practices using drilling mud reserve pits can contribute toxicity to the spent drilling fluid, as shown in Table 3. The data in Table 3 are from the U.S. EPA survey of the most important toxicants in spent drilling fluids. In the survey, sample taken from active drilling mud in the circulating system were compared with samples of spent drilling mud in the reserve pit. The data show that the storage/disposal practices were a source of the benzene, lead, arsenic and fluoride toxicities in the reserve pits because these components had not been detected in the active mud systems.
|Table 3 Toxicity difference between active and waste drilling fluids|
|Toxicant||Active mud||Detection rate (%)||Reserve pit||Detection rate (%)|
The third source of toxicity in the drilling process discharges is the type of
drilled rocks. A recent study of 36 core samples collected from three areas (Gulf of Mexico, California and Oklahoma) at drilling depths ranging from 3000 to 18,000 ft revealed that the total concentration of cadmium in drilled rocks was more than five times greater than the cadmium concentration in commercial barites . With a theoretical well discharge volume in a 10,000 ft well model, 74.9 % of all cadmium in drilling waste was estimated to be contributed by cuttings, whereas only 25.1 % originate from the barite and the pipe dope.
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3.Hou J, Luo Z (1986) The effect of rock cutting structure on rock breaking efficiency. SPE 14868, SPE 1986 international meeting on petroleum engineering