Dynamics of mass exchange within tight rock matrix/fracture systems induced by natural gas ‘dynamic’ soaking and oil recovery prediction

Abstract

The objective of this work was to elucidate the mass exchange dynamics (MED) between the tight rock matrix and fractures induced by natural gas ‘dynamic’ soaking (NGDS) during Huff-n-Puff. The equilibrium interfacial tension (IFT) between natural gas (NG) and oil decreased rapidly with an increase in the NG C2/C3 fraction and equilibrium pressure (Peq). The IFT curves were separated into two distinct regimes because of the transition of the MED from gas dissolution to oil vaporization. The apparent diffusion coefficient of NG ranged from 4.9 × 10−11 to 3.35 × 10−9 m2/s. The dissolution of NG into the oil resulted in a linear swelling behavior with Peq. Mass exchange between matrix and fracture took place intensively from the onset of NGDS, driven by gas diffusion, oil swelling, and concentration gradient-driven flow, which stimulated the oil recovery up to 59.3% original oil in place. The NGDS oil recovery curves exhibited three distinctive regions because of the MED evolution over time. The average cutoff pore radius for NGDS-movable oil was less dependent on the operational pressure but was affected by the C2/C3 fraction. We modified a dimensionless number to better correlate the NGDS oil recovery and predict field-scale production.