Heterogeneity of pore throat structures and petrophysical properties of tight sandstone and its influence on seepage: A case study of sandstone in the Yanchang Formation, Ordos Basin, China

Tight sandstone reservoirs are characterized by poor petrophysical properties, strong pore throat structures and heterogeneous permeability, which prevent oilfield water flooding. We take the tight sandstone reservoir of the Yanchang Formation in Ordos Basin as an example, select samples from the Chang 6 and Chang 8 members in the HX block for constant-rate mercury injection and single-core and combined-core model water flooding tests, and discuss the effects of heterogeneous pore throat structures and petrophysical properties on the seepage of the tight sandstone. Constant-rate mercury injection tests showed that the permeability of the tight sandstone was controlled by the throats. Compared with the pore structures, the throat structures were more heterogeneous, and the samples with more heterogeneous throats were more permeable. Therefore, in addition to the morphology of the pore throat structures, the heterogeneity of the throats was also used to evaluate the seepage. When applying the water flooding tests with the combined-core model, the water absorption capacity of the high-permeability core with a high permeability contrast (7.5) was greater than that of the high-permeability core with a low permeability contrast (3.6). Moreover, by comparing the results from the water flooding experiment for the same core between a single core and the combined-core model, the loss of the oil displacement efficiency (70.7%) of the low-permeability core in the combined-core model with a high permeability contrast (7.5) was much higher than that (29.8%) of the low-permeability core in the combined-core model, which had a low permeability contrast (3.6). The water absorption percentage and the recovery of the low-permeability core in the combined-core model with a high permeability contrast were lower, indicating that after water flooding, the remaining oil in the low-permeability reservoir in the area with a strong permeability heterogeneity was enriched. Such enrichment may increase the exploitation potential of tight reservoirs.