The Role of Sand /Shale Interfaces in Saturated Zone NAPL Trapping

Abstract

Abstract Spilled nonaqueous phase liquids (NAPLs) can be trapped by relatively small-scale structures at sand/shale interfaces just as hydrocarbons are naturally trapped in larger scale structures in oil and gas fields. We measured microscale closure at the upper sand/shale interface for 62 locations at three sites for fluvial/alluvial and tidal flat depositional environments and found that closure volume ranged from 0.56 to 2.9 L/m2 and averaged 1.2 L/m2. The area of each measurement ranged between 1024 and 2090 cm2. We could not measure closure height for individual features, so we measured the topographic variation at each of the 62 locations. Topographic variation ranged up to a maximum of 3.3 cm, and site means ranged between 1.3 and 2.4 cm. We also conducted sand box experiments to see if microscale features have NAPL trapping potential. Our results suggest that for NAPL to be trapped at greater than residual saturation, the trap closure height must be greater than the capillary rise of water into the NAPL. A closure height of 4.0 cm effectively trapped Marvel Mystery Oil when the average grain size was 1.5 mm but failed to do so at smaller grain sizes. Our experimental results are reproduced adequately by Hobson’s Formula which states that the height of capillary rise is inversely proportional to grain size and is also a function of the physical properties of the NAPL. Further calculations using Hobson’s Formula suggest that traps containing coarse-grained material are effective for some NAPLs at closure heights approaching 1 cm, whereas fine-grained sands require minimum closure heights of slightly <1 to 5 m or more to trap NAPLs.