A pore-scale investigation of heavy crude oil trapping and removal during surfactant-enhanced remediation

The presence of nonaqueous phase liquid (NAPL) in the subsurface presents significant challenges for soil and groundwater remediation. In particular, heavy crude oil, coal tar and/or bitumen present unique difficulties for removal and cleanup due to associated high viscosities, low aqueous solubilities, and limited mobility extraction potential. Although surfactant-enhanced aquifer remediation (SEAR) techniques have shown some promise for source removal, overall remediation (mobilization) performance will depend significantly on interfacial effects between the fluid and solid phases. A pore-scale study, implementing synchrotron X-ray microtomography (SXM), was conducted to understand and quantify the trapping and mobilization mechanisms and in-situ emulsification processes of heavy crude oil distributed within increasing complexity (i.e. physical heterogeneity) unconsolidated sands during surfactant flushing events. Pore-scale imaging analyses were conducted to quantify the changes in oil blob morphology before and after surfactant flushing events to assess the primary factors controlling the recovery. Results showed relatively low (10%) net recovery from the homogeneous sand after 5 pore volumes (PVs) of surfactant flushing and may be, in part, due to the more connected ganglia (i.e. single continuous) oil-phase. Such a condition may have limited the surfactant/oil contact resulting in relatively low interfacial activity and correspondingly inefficient oil mobilization and recovery. Negligible net oil recovery was achieved from the mildly-heterogeneous-sand and is likely due to the lower associated permeability of this particular porous medium. Furthermore, the oil-phase distribution within this medium primarily consisted of small disconnected blobs more readily exposed (in contact with) the surfactant solution. For the highly-heterogeneous-sand experiments, an average of 20% heavy-oil recovery resulted after each flushing event (total of similar to 37% after 5 PVs) and was attributed to more efficient reduction of interfacial tension associated with the increased surfactant-oil contact. The associated higher pH sand/fine-carbonate system may have aided in maintaining a water-wet porous medium, a condition more conducive to higher oil recovery and displacement efficiency.