Drilling operators are facing increased challenges in dealing with the drill cuttings produced due to higher demands from regulatory agencies. Government entities such as the US Environmental Protection Agency (EPA) and Norway's State Pollution Control Authority (SFT) set strict limits regarding the retention of oil on cuttings (ROC) for offshore drilling. This paper describes the role played by the hydrophilic-lipophilic balance (HLB) of a surfactant in designing efficient washing solutions (WS) that utilize both non-ionic and anionic surfactants to mitigate the ROC.
A range of anionic and non-ionic surfactant blends was screened for their cleaning ability on field cuttings. The main parameters considered during the surfactant selection were: chain length, branching/linearity, substitution pattern and polarity. For non-ionic surfactants the emphasis was placed on ethoxylated vs propoxylated molecules. Following treatment with WS the ROC was determined by using the retort distillation method (RD) as described in API RP 13B-2. The numbers obtained were graphed against HLB values for the surfactants in order to determine the best correlation between structure and cleaning ability for that particular type of cuttings.
The results show the achievement of a maximum cleaning ability of the WS for a given chain length of the surfactants used. The linearity of the molecules, although initially thought to play a significant role, proved to have a limited influence. In fact, blends containing roughly equal amounts of linear vs branched surfactants displayed the highest cleaning ability. The overall polarity of the mixture seemed to be important too, since blends containing non-ionic surfactants had almost no cleaning capacity. Of major importance in this particular study was the substitution pattern on the non-ionic surfactants. An optimum combination of propoxylated vs ethoxylated molecules was critical for the design of the most successful WS. Our results indicate that ethoxylation on the anionic component of the mixture and propoxylation on the non-ionic portion was critical since tests performed with the opposite combination showed almost no cleaning capacity. Increasing the PO content on the non-ionic molecule has a detrimental effect which is more pronounced than a similar increase in EO content on the ionic molecule.
The present study aims to help elucidate the roles played by the different surfactant molecules in designing efficient washing solutions for cleaning drill cuttings. This will assist the oilfield service companies in addressing the continuously increasing environmental regulations for disposing of drill cuttings while reducing the present costs and carbon footprint on the environment incurred by the current methods.