Insights into the deoiling efficiency of oil-based cuttings by surfactant-free microemulsions

Oil-based cuttings (OBCs) produced during shale gas exploration are classified as hazardous waste, and are difficult and expensive to dispose of. Appropriate disposal of such OBCs has become a key issue requiring urgent attention. Here, we have applied surfactant-free microemulsions (SFMEs) to deoil OBCs. The deoiled OBCs, the recovered SFMEs, and the base oil have been characterized. Studies have shown that gentle stirring and mixing of n-hexane/tert-butanol/water (30:65:5 wt%) can form a cleaning agent that serves as an SFME. The oil content of the OBCs after deoiling by SFMEs was less than 1%, and the remaining solid phase showed no hazardous characteristics. The optimum conditions for cleaning OBCs with the SFME have been identified as 30–35 °C, a duration of 20 min, a solid–liquid mass/volume ratio of 1:4, and a stirring speed of 200 rpm. Thereafter, samples are optimally dried at 100 °C for 50 min. The recovered cleaning agent can be reused efficiently. The recovered base oil was essentially unchanged, except for some contamination with organic additives from the oil-based mud. The deoiling mechanism of OBCs with an SFME mainly depends on the strong solubilization ability of the SFME.