When oil-based mud is used, the drilled formation solids (cuttings) are regarded as controlled or hazardous waste. As such, they can be disposed of in three ways: decontamination treatment followed by discharge into the sea, injection of the cuttings into the well, or transfer to a controlled hazardous-waste landfill. The lowest environmental effect for solids disposal, especially for offshore operation, is decontamination treatment followed by discharge. However, conventional decontamination technology exhibits limited efficiency in extracting oil from the drill solids.
Introduction
One function of drilling mud is a washing action to remove cuttings from the wellbore. The mud returns to the surface with entrained cuttings and, typically, flows through shale shakers, desanders, desilters, hydrocyclones, centrifuges, or other devices to separate the cuttings from the mud. Conventional decontamination technology yields an oil content in the treated cuttings of >1% in the dried solids, which does not meet strict environmental regulations in many highly ecologically sensitive countries (e.g., the UK and North Sea countries). Apart from cuttings reinjection, all other treatment methods require land-based waste treatment.
Nanoemulsion technology can improve oil-removal efficiency by providing ultralow oil/water interfacial tension (IFT). Heavy- and light-oil extraction from the cuttings surface is enhanced. The cuttings-treatment process proposed in this paper can achieve oil content of <1%.
Nanoemulsion
Nanoemulsions are clear and thermodynamically stable. In typical nanoemulsions, the droplet size ranges from 10 to 100 nm, which is much smaller than the wavelength of visible light. Hence, nanoemulsions generally are weak scatterers of light, making them transparent. Nanoemulsions can be formulated with a single phase or multiple phases. The IFT between the aqueous and hydrocarbon phases in nanoemulsion systems can be as low as 0.0001 mN/m, compared with an ordinary emulsion or macroemulsion (approximately 0.1 to 30 mN/m). A nanoemulsion might be a dispersion of water in oil or oil in water (in which the second solution is the dispersion medium or solvent). There may also be a bicontinuous structure in which both water and oil are continuous.
Methods
Fig. 1 shows the treatment’s five major processes.
- Process I: Dispersion and oil removal—breaking down the oily solid cluster, simultaneously dispersing the solid and removing oil (degreasing). A nanoemulsion cleaner or an alkaline solution is added to this process to treat the oil-contaminated cuttings.
- Process II: Centrifuging (equivalent to industrial-scale decanting)—separating liquid (water with emulsified/removed oil) from the solids.
- Process III: Washing with water or a mix of cosolvent and water—removing the remaining adsorbed surfactants.
- Process IV: Centrifuging—separating liquid (water with emulsified/removed oil) from clean solid.
- Process V: Disposal treatment
- Process Va: Heat treatment at approximately 80–100°C.
- Process Vb (optional): Deoiling water—water treatment with deoiler (50 ppm).
The operating temperature for chemical treatment is 25°C. The oil-content analysis of cuttings after the treatment process was determined by retort analysis or by Soxhlet solvent extraction (if the oil content was expected to be <1%).
Results
For conventional surfactant cleaning by use of a surfactant aqueous solution to wash the oily component from the cuttings samples, a nonionic surfactant was used. Use of anionic surfactants was avoided because surfactant precipitation occurred in the presence of hard ions, such as Ca2+, in cuttings. The oil content was reduced from 13 to 10%, even with the use of a very high concentration of surfactant (20%). No significant improvement was observed in the cleaning efficiency of the surfactant solution to extract the oil from the cuttings when the surfactant concentration was increased from 5 to 20%. This result implied that the surfactant’s cleaning power is limited, possibly because the surface tension (which refers directly to wettability and emulsification efficiency) of the surfactant solution becomes constant after reaching its critical micelle concentration.
Nanoemulsion Cleaning Process. Tests were run on three cuttings samples—A and B (solids) and C (paste). Nanoemulsion cleaner was more effective than surfactant alone when treating Cuttings A. The nanoemulsion cleaner reduced the IFT between the cuttings and liquid, emulsified the adsorbed oil, and provided hydrophilic surfaces on cuttings. The nanoemulsion cleaner used here was a water-in-oil nanoemulsion with oil/water IFT <0.01 mN/m (measured at 25°C with a Kruss spinning-drop tensiometer). The mean particle size of the nanoemulsion was <50 nm. The surfactant composition in the nano-emulsion cleaner was considered to be the optimum hydrophilic/lipophilic balance (HLB).
When a 10% concentration of nano-emulsion was applied, the oil content in the treated cuttings was reduced to <1%, whereas surfactant alone was not able to achieve this result. However, a high dosage of nanoemulsion may not be economical. Further studies were designed to reduce the dosage of the nanoemulsion cleaner in the treatment.
Nanoemulsion With Alkaline. Alkaline solution was found to improve the degreasing efficiency of the nanoemulsion cleaner. Use of nanoemulsion cleaner in the presence of high pH achieved an oil content of <1%, which was never observed without the help of alkaline solution. High pH induces a negative charge on the cuttings, thus promoting the electrostatic repulsive force between the cuttings’ surface and the oil. The cuttings are dispersed easily at high pH, which enables the surfactant in the nanoemulsion cleaner to access the cuttings and emulsify and carry oil from the cuttings surface to the bulk solution.
When Cuttings A were treated with cleaning solution consisting of 1% nano-emulsion cleaner and 5% alkaline solution and then washed with water without the heat-treatment process in the last process step, the oil content in the cuttings after treatment was approximately 3.6%. Therefore, increasing the pH by adding alkaline solution improved the oil extraction by nanoemulsion cleaner. The oil-removal efficiency of this process was slightly better when the concentration of the nanoemulsion cleaner was increased from 1 to 5%. However, when the heat-treatment process was included as the final cleaning-process step, the oil content was reduced to 0.6 wt% (i.e., <1%). This efficiency is comparable with that of the treatment that used 15% nanoemulsion cleaner without alkaline solution.
Nanoemulsion With Cosolvent. When surfactant adsorbs onto the cuttings at a concentration approaching its critical micelle concentration, it will self-assemble, or form admicelles, in a manner similar to that of the micelle in the bulk solution. These admicelles will have different conformation and physical properties when hydrocarbon solute is partitioned within them.
A 10 vol% glycol was used as co-solvent. The use of cosolvent consistently reduced the oil content in the cuttings to <0.5% after the heat-treatment process in the final step. Theoretically, the cosolvent will help destabilize the admicelle from the cuttings’ surface and remove oil from the cuttings’ surface by destabilizing the surfactant/admicelle structure that is holding oil. However, the use of cosolvent without heat treatment in the final process step did not improve the cleaning efficiency of the nanoemulsion cleaning method, and the observed oil content in the cuttings after final treatment was >2%.
Nanoemulsion Cleaning With Different Compositions of Cuttings. Three kinds of cuttings samples were evaluated. The cleaning solution in the first process step contained 1% nanoemulsion cleaner and 5% alkaline solution. Water was used as the washing agent in the washing step. All experiments used heat treatment. The oil could be removed effectively from Cuttings A and B, reducing the oil content in the cuttings samples from 12–13% to <1%.
However, the same cleaning-solution composition was not effective for Cuttings C (paste) because the oil content may have been too high. The nano-emulsion cleaner might not be sufficient to emulsify, disperse, and replace the oil on the cuttings’ surface. When this issue occurs, the alkaline solution cannot change the surface charge of the cuttings’ surface easily because the thick adsorbed-oil layer on the cuttings’ surface prevented direct contact with the alkaline solution.
Another experiment that used a higher dosage of the nanoemulsion cleaner and alkaline solution was conducted on Cuttings C. However, the oil content in the cuttings after the final process step still was >1%. Improvement was exhibited in a third experiment on Cuttings C by use of a water/cosolvent (glycol) mixture for the washing step. The oil content in the cuttings after final treatment could be reduced to <1%. Therefore, if the wax or oil content is high, achieving oil content of <1% in the cuttings would require the use of cosolvent.
Conclusions
Nanoemulsion cleaning technology can remove oil (especially heavy oil) from cuttings to <1 wt% by use of an optimum HLB surfactant composition, which yields an ultralow IFT to emulsify the trapped and adsorbed oil and wet the cuttings’ surface. Alkaline solution and cosolvent improved the oil-removal efficiency of nanoemulsion cleaning. This nanoemulsion technology consistently reduced the oil content in the cuttings to <1%.
This article, written by Dennis Denney, contains highlights of paper SPE 162401, “Nanoemulsion-Enhanced Treatment of Oil-Contaminated Oil-Based Drill Solids,” by Wasan Saphanuchart, SPE, Yoong Shang Loke, SPE, and Chun Hwa See, SPE, BCI Chemical Corporation, prepared for the 2012 Abu Dhabi International Petroleum Exhibition & Conference, Abu Dhabi, 11–14 November. The paper has not been peer reviewed.