Experimental study of spontaneous imbibition for oil recovery in tight sandstone cores under high pressure high temperature with low field nuclear magnetic resonance

Abstract

Spontaneous imbibition (SI) plays a key role in production mechanism of unconventional reservoirs. Trivially, with the goal of getting higher efficiency there always has been an incessantly research activities to develop the technology progressively. In this regard, thermodynamic variables (i.e. pressure and temperature) can directly influence the process performance of SI. Thus, numerous papers have been published by considering normal conditions (e.g. 20 °C, 0.1 MPa), high pressure (HP) or high temperature (HT). To bridge an existing gap in the literature, an inhouse built imbibition experimental setup was used to carry out the SI under high pressure-high temperature (HPHT) conditions (50 °C, 10 MPa). The core samples used in this study were tight sandstone taken from Yanchang formation, Ordos Basin. Experimental conditions were: 1) simulated formation conditions (50 °C, 10 MPa), 2) normal condition (20 °C, 0.1 MPa), 3) pulse imbibition (20 °C, 8/4 MPa in cyclic alteration) and 4) simulated formation conditions (50 °C, 10 MPa) for fractured cores (1/3 and 2/3 length). Moreover, NMR was employed to characterize the dynamic behavior of imbibition for each sample. It was found, samples tested under HPHT conditions yielded higher imbibition efficiency (average of 63.49%) compared to ambient conditions (27.31%). and imbibition efficiency of HPHTSI (max, 73.47%, min, 55.23%) showed early imbibition with higher rate. Ultimately, micropores had the largest imbibition efficiency and matrix core samples exhibited lower recovery than fractured ones (74.6%). Collectively, this study presents the importance of fracture-matrix interaction in tight sandstones which is a crucial component for higher recovery rates.