Wettability alterations due to aging in crude oil; wettability and Cryo-ESEM analyses

Abstract

Several studies of fluid distribution and wettability on the pore scale have been reported, but to our knowledge, none of them have attempted to obtain quantitative measurements. In this paper, quantitative wettability results from Cryo Environmental Scanning Electron Microscopy (Cryo-ESEM) are reported and compared to wettability measurements with traditional methods. The wetting properties of sandstone cores, aged in crude oil, have been investigated using the combined Amott/USBM method. The results show that with increasing aging time, the wetting properties changed from water wet to mixed wetting. Pieces of the same cores have then been analyzed using an Electroscan 2020 ESEM with an attached Cryo unit. Combined with Energy Dispersive X-ray Spectroscopy (EDS), these analyses have given images of the surface, showing the liquid distribution and which phase is contacting the mineral surface. The combination of these methods has proved that using a Cryo-ESEM to investigate wetting properties can give very valuable information although the results are somewhat scattered. Compared to the traditional wetting analyses, this method also gives us an image of the liquid distribution, which is a very valuable input for pore network simulation. It is shown that there is a good agreement between the fraction of the pore walls that are contacted with brine and the Amott/USBM index. A new index based on ESEM data is proposed, with the same range as the Amott–Harvey index. Cryo-ESEM avoids two problems encountered in other SEM methods. First, there is no possibility of a coating layer showing up in the element analyses, since these samples are uncoated. Secondly, the same sample can easily be reexamined by simply refracturing.