Wettability Of Porous Media From Environmental Scanning Electron Microscopy: From Model To Reservoir Rocks

Abstract

Abstract The study of wettability and fluid distribution in porous media has been studied by the means of ESEM (Environmental Scanning Electron Microscopy). Two types of samples were used : a model porous medium consisting of sintered glass either water-wet or oil-wet, and mixed-wettability reservoir rock samples : a sandstone from the North Sea and a carbonate from the Middle East (preserved state). It was not very easy to observe spontaneous imbibition by water in the samples saturated by oil; but oil imbibition has been followed in real time in the samples saturated by brine. For sintered glass, it was clearly possible to follow its appearance at the upper face of the core sample, along the pore walls. For the sandstone samples, it was possible to observe the rise of oil through the water phase at the junction between quartz and kaolinite. For carbonate samples, oil capillary rise to occurred through the largest pores. Introduction Study of wettability and distribution of fluids in porous media have been approached by means of ESEM (Environmental Scanning Electron Microscope). This work follows a previous study of fluid distribution in reservoir rocks under residual oil saturation by Cryo-scanning Electron Microscopy. This new method allows visualization of rock samples in the presence of fluids (both oil and brine) without freezing consequently, it also allows the observation of dynamic experiments, for instance a wetting fluid imbibition in presence of a non-wetting fluid. The equipment allows a working pressure from about 1 to 50 torr. So it is possible to directly introduce fluids into the observation chamber. Both model and real porous media were used : sintered glass (homogeneous wettability) and two types of mixed wettability reservoir rock samples (imbibing both oil and brine), a sandstone from the North Sea and a carbonate from the Middle East (preserved state). Technique The main advantage of the technique is to work under reduced pressure (few torr) and without high vacuum as in a standard electron microscope. This allows the observation of liquid-containing samples without any special preparation or metal coating. It also allows the observation of samples under the influence of controlled external parameters such as surrounding atmosphere (air, vapor, nitrogen, carbon dioxide, etc.), temperature (from -20 C to 1000 C), hydration rate, reagent injection, etc. The apparatus consists in a "reactor" on which is adapted the column of an scanning electron microscope. The equipment used is an ELECTROSCAN 3-E (Fig. 1). Vacuum gradient. The column is equipped with a multi stage differential pressure pumping unit. Three pumping levels allow a 10-7 torr in the electron beam source and a pressure of about 1 to 50 torr in the observation chamber. Three apertures disposed along the column allow to maintain the pressure constant in the different stages whatever the value of the pressure and the composition of the atmosphere. Secondary electron detection. Danilatos has developed a new electron detector operating under pressures ten thousand times higher than in a classical Everhart-Thomley detector. It involves the molecules of the environment of the chamber to amplify the signal. Ionization of the environmental gas is induced by collisions between the secondary electrons emitted by the surface of the sample. P. 251^