Experimental Study of Imbibition Oil Recovery from Unconventional Rocks with Dual-porosity and Dual-wettability Pore Network

We conducted a series of laboratory experiments to analyze wettability and other petrophysical properties of twelve core plugs, drilled and preserved from two wells completed in the Montney tight-oil Formation. Interpretation of the measured results led to insightful correlations among imbibition oil recovery, wettability, and the shape of MICP pore-throat size distribution profiles. These correlations are particularly important for identifying sweet spots and for designing EOR fluids for developing tight oil formations similar to the Montney.

We used reservoir oil and brine to conduct contact angle and spontaneous imbibition experiments on the core plugs. We examined the effects of initial oil/water saturation on the imbibition behavior, and investigated the possible correlations between oil wettability index and petrophysical properties including pore-throat size distribution (from MICP data) and tight rock analysis data. Finally, we conducted soaking experiments on oil-saturated core plugs using brine and fresh water as fracturing fluids. We investigated the relationship of oil recovery to pore structure and wettability, and compared oil production when using different soaking fluids.

We observe higher and faster oil imbibition compared with brine imbibition into the core plugs, suggesting the strong affinity of the samples to oil. Comparative analysis of the imbibition and MICP profiles show that the normalized imbibed volume of oil (Vo) is positively correlated to the initial oil saturation, and the volume fraction of small pores, represented by the tail part of MICP profiles. The results of soaking tests show that imbibition oil recovery is positively correlated with the average porosity and permeability. This can be explained by the trapping of oil in small hydrophobic pores. Also, the oil recovery results show that fresh water expels more oil out compared with brine due to the effect of osmotic potential.

The results of this study lead to an improved understanding of wettability of tight rocks and the mechanisms controlling oil recovery. The discovered correlations can be applied for quick evaluation of wettability, sweet spots with high initial oil saturation, and imbibition oil recovery. The results of soaking tests can be used for designing optimum fracturing fluids for enhanced oil recovery.