Impact of hydrocarbon charges on the wettability of the low-permeability to tight sandstone reservoirs

The original sedimentary reservoirs in sedimentary basins are usually water-wet. However, when the crude oil charges into the reservoir, the polar components in the crude oil may be adsorbed or precipitated on the mineral surface, causing the mineral surface wettability to change. Wettability is the important factor in controlling fluid occurrence and displacement in reservoirs. Therefore, the alteration of reservoir wettability, especially the wettability of low-permeability to tight reservoirs, will have an important impact on the fluid distribution. The basins rich in petroleum in China have often experienced multiple periods of tectonic movement, and the petroleum-bearing reservoirs are often the result of multiple periods of oil and gas charging. Then, how does the reservoir wettability change under the influence of multiple hydrocarbon charges? This is significant for analyzing fluid occurrence and displacement in reservoirs during geological historical periods. However, previous researchers have not gained a clear understanding on this aspect. In this study, the oil-bearing sandstone samples from the low-permeability to tight reservoirs of the Yanchang Formation in the Ordos Basin were selected. First, a sequential extraction experiment was carried out on the sample. The extraction reagents used were dichloromethane, followed by a mixture of benzene and alcohol. Then, the environmental scanning electron microscopy was used to analyze the pore-scale wettability at different extraction stages. Finally, fluorescence spectrum and GC-MS analyses were carried out to analyze the geochemical characteristics of organic matter related to the rock wettability alteration. The conclusions were as follows: (1) As the sequential extraction proceeds, the maximum fluorescence intensity revealing the content of extracted organic matter gradually decreases. The organic matter extracted in the early stage is mostly free hydrocarbons, and the maximum wavelength of the fluorescence spectrum is about 375nm, which is similar to the crude oil produced from in the reservoir. The organic matter extracted in the later stage is mostly adsorbed hydrocarbons, the maximum wavelength of the fluorescence spectrum is about 460nm, indicating the enrichment in asphaltene. (2) The sandstone samples with the free hydrocarbons removed behave in oil wetting which is the same as the original samples’ states. However, after the adsorbed hydrocarbons is extracted out, the samples become water wet. This difference of wettability indicates that the adsorbed hydrocarbons make the alteration of wettability (3) Biomarker analysis shows that the maturity of free hydrocarbons is lower than the adsorbed hydrocarbons. It is inferred that the adsorbed hydrocarbons are charged in the early stage, whereas the free hydrocarbons are charged in the late stage. Therefore, the wettability of the reservoir sandstones is mostly altered to oil wet states after the early hydrocarbons, however, the impact of late charged hydrocarbon is small.