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Nonwetting phase retention and mobilization in rock fractures

Perchloroethylene (PCE) was injected into fractured limestone samples (approximately 0.3 m × 0.3 m square) under controlled conditions to assess nonwetting phase fracture retention capacity. Testing was performed using two rock samples containing a single fracture in each, one fractured along a bedding plane and the other fractured along a prominent stylolitic joint. Residual PCE saturation decreased from 21% of the fracture volume at horizontal to 3% at completely vertical in the bedding plane fracture and decreased from 27% to 10% of the fracture volume under the same conditions in the stylolitic fracture. A portion of the retained residual was removed by waterflooding the fractures. The PCE retention capacity of both fractures exhibited a strong correlation with combined capillary and Bond number (Nc + krWNB) provided that initial residual saturation was defined in the absence of both buoyant and viscous forces. Residual PCE saturation decreased as the magnitude of NC + krWNB increased; however, even at Nc + krWNB values approaching 10−2, all residual PCE was not completely removed from the fractures.