An innovative method to qualitatively measure the wettability of sandstone reservoirs based on the sequential extraction and fluorescence analysis

Wettability is a key factor controlling the flow and distribution of multiphase fluids in reservoirs. The study of reservoir wettability is of great significance to understand the mechanism of oil and gas migration and improving oil and gas recovery. The oil reservoirs have various wetting characteristics, including water wet, neutrally wet and oil wet states. The conventional methods for reservoir wettability analysis, including the contact angle, Amott index and the USBM index, have been currently established in the petroleum industry. The contact angle method is relatively simple, but it is not suitable for the reservoir samples with complex mineral composition and strong heterogeneity. Amott and USBM method based on oil-water displacement are encountering problems such as long test time and large errors, when applied to low-permeability and tight reservoirs. This study takes the low-permeability and tight sandstone reservoirs in the Ordos Basin as an example. Based on the measurement of oil and water content in the rock using the nuclear magnetic resonance, the Amott-Harvey index was tested. Then the twin powder sample was sequentially extracted for 72 hours with the fluorescence spectrum of the extracted organic matter was tested every 12 hours. The fluorescence spectrum of the crude oil in the same production layer was also tested, and then the relationship between the fluorescence characteristics of the extracted organic matter and the Amott-Harvey index was analyzed. Finally, a new method for rapid and qualitative evaluation of reservoir wettability was established. The conclusions were as follows: (1) Crude oil represents the characteristics of free hydrocarbons in the reservoir, and its fluorescence spectrum has a fluorescence intensity peak around 375 nm. (2) The firstly extracted organic matter contains both free hydrocarbons and adsorbed hydrocarbons, and its fluorescence spectrum has a double peak with a wavelength of 375nm as the main peak and 460nm as a secondary peak. The last extracted organic matter has a lower proportion of free hydrocarbons and a higher proportion of adsorbed hydrocarbons compared with firstly extracted one. The fluorescence spectrum mostly shows double peaks with the same fluorescence intensities at the wavelength of 375nm and 460nm. (3) The ratio of fluorescence intensity of the extracted organic matter at a wavelength of 460nm to 375nm (I_460nm/I_375nm) may reflect the coverage proportion of the rock surface by adsorbed hydrocarbons, and thus can be used as a parameter indicating the wettability of the rock. The I_460nm/I_375nm value of the last extracted organic matter has a strong correlation with the Amott-Harvey index, therefore it can be used to qualitatively evaluate the reservoir wettability.