Effect of Lamination-Induced Anisotropy on Fluid Saturation and Petrophysical Properties

Abstract Understanding the rock anisotropy induced by laminations is crucial and may have significant implications for reservoir characterization, especially for fluid saturation estimations in horizontal wells. The matrix of a rock can be anisotropic because of layering during deposition of sedimentary rocks. This study assesses the anisotropy effect of lamination in rocks on petrophysical measurements and fluid saturations for better reservoir characterization. The effect of rock anisotropy on petrophysical properties and water saturation were investigated using nuclear magnetic resonance (NMR), thin section images, low-frequency electrical resistivity, and dielectric permittivity. Three laminated kaolinitic sandstone core samples were cut horizontally, vertically, and at 45°, respectively, based on their relative laminae inclination with respect to the measurement direction. The rock samples were centrifuged at gradually elevated capillary pressure steps using air and water. Water saturation, NMR T2, dielectric permittivity, and electrical resistivity were then measured at each capillary pressure step after attaining production stability. The desaturation process during centrifugation was monitored by dividing the NMR T2 distribution curves into three regions representing three different pore sizes in the rocks. The results of this study show clear variations in the measured petrophysical properties and water saturation of the rock samples depending on the lamination direction. The sample with horizontal bedding laminae exhibited the highest gas permeability, highest dielectric permittivity, and lowest electrical resistivity, whereas the sample with vertical laminae showed the lowest gas permeability, highest irreducible water saturation (Swirr), and highest electrical resistivity. The sample with inclined laminae showed the lowest permittivity and similar cementation exponent to the horizontally bedded sample. Its gas permeability fell between the permeabilities of the horizontally and vertically bedded samples. The higher electrical resistivity reflects more tortuous path in the vertically laminated sample. For the desaturation state, the NMR T2 distribution curves revealed different desaturation mechanisms in the samples where water saturation in the horizontally bedded sample appeared to be controlled by large and medium pores, while water saturation in the vertically bedded sample was controlled by large pores. The water saturation in the laminated samples was mainly controlled by the small pores. Additionally, the resistivity index of the samples was mainly controlled by small and medium pore regions. The horizontally bedded sample demonstrated the highest resistivity index with higher Swirr than that of the inclined bedding sample. In contrast, the vertically bedded sample showed the lowest resistivity index and the highest Swirr. These differences in the Swirr due to lamination direction, must be accounted and corrected for accurate estimation of hydrocarbon reserves. The study illuminates the effect of lamination-induced anisotropy on petrophysical interpretations. Such results can be useful for better characterization of shaly sand reservoirs and accurate determination of their fluid saturations in the field. Overlooking or underestimating the anisotropy effect, specifically in laminated clastic reservoirs, may lead to inaccurate reservoir characterization and subsequent erroneous estimations of hydrocarbon saturation in horizontal and highly deviated wells.