Characteristics and mechanism of imbibition oil recovery in the ultra-low-permeability volcanic oil reservoir in the Santanghu Basin

Abstract

Imbibition oil recovery (IOR) is an important method to improve the recovery factor (RF) of low-permeability reservoirs. To explore the characteristics and mechanism of IOR in ultra-low-permeability volcanic oil reservoirs (ULPVORs) in the Santanghu Basin, this study conducted various imbibition experiments under NMR monitoring using actual cores from the reservoir, including spontaneous imbibition (SI) and forced imbibition (FI) experiments involving different rock sample sizes. Further, we obtained the IOR characteristics of ultra-low-permeability volcanic rocks (ULPVRs). Meanwhile, based on the experimental test results of the pore structure and casting thin section, combined with imbibition theory, the mechanism of the IOR in ULPVRs was obtained. The results demonstrated that the IOR characteristics of the ULPVRs are significantly different from those of conventional tight sandstone and shale. Conventional tight sandstone and shale evenly produce oil on the rock surface during IOR, whereas ULPVRs exhibits extremely few and scattered oil production points on the rock surface during IOR. Unlike conventional tight sandstone and shale plunger cores where the RF of SI is generally approximately 30%, the IOR of ULPVRs is particularly poor, with RFs of SI being 4.59% and 2.70% for plunger and full-diameter cores, respectively, and the RF of FI is only 10.19% for the plunger core. In addition, ULPVRs are extremely heterogeneous, and the pore structure and production characteristics of IOR are extremely complicated. The oil produced by SI in the plunger and full-diameter cores comes from middle pores (0.01 µm<r < 0.1 µm) and large pores(r > 0.1 µm), respectively, while the oil produced by FI in the plunger core comes from large pores. Conventional tight sandstone and shale are not conducive to seepage owing to their small pore sizes, however, their compatibility between the pore and throat sizes makes them favorable for IOR. The extremely poor matching between the pore and throat sizes of the ULPVRs is unfavorable for seepage, and is primarily responsible for its poor IOR effect. For the ULPVORs in the Santanghu Basin, the key to EOR is to improve the seepage capacity of the reservoir throat, such as further large-scale volume fracturing, rather than implementing too many measures for IOR. The results of this study can provide a reference and basis for the efficient development of ULPVORs.