Oil charging model and controlling factors revealed by an online nuclear magnetic resonance (NMR) system

Abstract

It is very difficult to simulate the charging process of crude oil under experimental conditions due to the low porosity and permeability of tight sandstone. However, simulations are important for revealing the mechanism of formation of tight oil and the oil-water distribution characteristics, as well as for enhancing the oil recovery efficiency. In this study, we established a new method, called nuclear magnetic resonance coupling displacement (NMR-DP), to simulate the tight oil charging process online. The NMR-DP method has the advantages of high precision, velocity, recyclability, and the ability to reflect the distribution of fluids in the micropores of a tight rock.We conducted a simulation experiment, involving the charging of crude oil into eight sandstone samples from Gaotaizi (in the Qijia-Longhupao area of the Songliao Basin). The results of this experiment were combined with those of microscopic measurements conducted on the pore structure (i.e., nuclear magnetic resonance, nitrogen adsorption, high-pressure and rate-controlled mercury injections, and micron-computed tomography). In this way, we clarified the charging process and its controlling factors (e.g., physical properties and wettability) and established a tight oil charging model. The results indicate that the oil charging saturation was controlled by several factors, including the charging pressure, physical properties, pore throat structure, and mineral composition of the rock. A smaller radius of the pore throat corresponded to a lower oil charging saturation and higher starting pressure gradient. The lower the content of clay and carbonate minerals, and the higher the content of feldspar mineral, the higher the oil saturation. Oil mainly occurred in the larger pores, while residual water mainly occurred in the smaller pores. The charging process was divided into four stages: an exponential rapid growth stage, a logarithmic slow-growth stage, a linear slow-growth stage, and a quasi-stationary stage. The method established in this paper is expected to be helpful for analyzing the distribution law of oil and water in tight sandstone areas, thereby aiding in the prediction of sweet spots and development of tight oil.