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Carbonate reservoirs are heterogeneous systems at multiple length-scales, a characteristic that influences strongly their petrophysical properties and oil recovery procedures [1]. Wettability for example, capillary pressure as a function of water saturation, and partial permeability, are related with the pore structure (pore size and shape) and the rock mineralogy [2, 3]. Acid stimulation treatments are available, which can improve productivity [4, 5, 6]. It is expected that acid stimulation treatment, which opens rock porous channels known as wormholes, will influence strongly the pore structure and therefore petrophysical properties around the wormholes, decreasing wettability and thus increasing oil recovery. In this work we study the role of the pore structure on the water and oil absorption in a number of acidized and non-acidized carbonate cores by employing 1H NMR.

Environment in which carbonate reservoir rocks are deposited was studied by visiting and collecting rock samples from a carbonate reservoir analog located at Jabal Fuwairit in the Northeastern coast of Qatar. The primary objective of this study is to develop methods to characterize carbonate reservoirs. The experimental procedure included collecting samples based on geology, mapping the location and altitudes of the samples using a high-resolution Global Positioning System (GPS) with an altimeter, permeability measurements at selected locations, conducting petrophysical measurements on the samples, Conventional and Micro-X-ray Computed Tomography Scanning (CT Scanning), Scanning Electron Microscopy (SEM), X-ray Energy Dispersive Spectroscopy (EDS) and Atomic Force Microscopy (AFM) imaging. It is observed that the permeability measurements show an alternating pattern as a function of height above sea level. The cyclicity is probably representative of repeating sequences of sea level changes over geological time. CT Scanner and Micro-CT Scanner measurements were employed to obtain vital information about the flow pathways in the rock, thus assisting in calculating the porosity to compare with values measured in the lab. SEM results gave direct visualization of the pore network and information about grain size (500 microns) and intergranular porosity, mineralogy, compared with the EDS results, and lithotype. EDS results showed that reservoir rocks directly exposed to environment had significant alteration due to weathering. However, it is encouraging to learn that samples even few inches away from the surface had minimal environmental influence.