Oil charging pore throat threshold and accumulation effectiveness of tight sandstone reservoir using the physical simulation experiments combined with NMR

Abstract Tight oil as a significant part of unconventional resources is now considered as one of most important alternative resources. Oil accumulation mechanism is still a research focus for the tight oil. Therefore, it is important to study the effectiveness of micro-nano pore system to oil charging and accumulation. Taking the Chang 7 tight sandstone reservoir of the Ordos Basin as an example, the physical simulation experiment of oil charging is applied to study the pore throat threshold of tight sandstone reservoir under the different charging force using the nuclear magnetic resonance (NMR) combined with displacement techniques. The relationship between charging force and pore throat radius threshold (Rth) can be established when combined with the pore throat distribution derived from MICP. In addition, the effectiveness of oil accumulation for the different type tight sandstone reservoir is also discussed. The result shows that the Chang 7 tight sandstone reservoir is divided into four groups based on pore structure and petrophysical property (porosity and permeability). In the physical simulation experiment, the oil saturation growth trend can be roughly divided into three groups: uniform growth model, slow growth-stable model, fast-slow growth model. This result indirectly reflects the heterogeneity of pore structure. There is a power function relationship between the charging force and Rth of oil charging, which can be used to calculate the effective accumulation space and further determine the accumulation effectiveness. The effectiveness of oil accumulation changes with the increase of the charging force: the effective accumulation space increases with the charging force, which causes that the reservoir (usually not including type III) gradually transits from ineffective accumulation to effective accumulation. That is, the charging force coupled with pore throat radius controls oil charging and accumulation. For the Chang 7 tight oil accumulation, the effective pore throat radius threshold of oil charging is about 0.1 μm and thus only the type I and II1 reservoirs can become the effective tight oil reservoirs, which is the main target of tight oil exploration. This result not only puts insight into the tight oil accumulation mechanism but also plays an important role in the tight oil exploration of Yanchang Formation in the Ordos Basin.