Patterns of Variation of Permeability Anisotropy in a Carbonate Reservoir

Most classical reservoir engineering concepts are based on homogeneous reservoirs despite the fact that homogeneous reservoirs are the exception rather than the rule. This is especially true for the Middle-eastern carbonate reservoirs, which are known to be highly heterogeneous. The realistic characterization of such types of reservoirs is not an easy task and must include the study of directional variation of permeability. The latter is incorporated in engineering calculations as the square root of the ratio of horizontal to vertical permeability, a parameter known as the anisotropy ratio. Approximate estimates of this ratio are often used in practice despite the fact that it is a crucial parameter in hydrocarbon production. This paper addresses the pattern of variation of the anisotropy ratio in an Upper Jurassic carbonate reservoir located in Eastern Province of Saudi Arabia. Based on whole core data from several vertical wells, statistical distributions of horizontal and vertical permeability measurements as well as the anisotropy ratios were determined. These distributions for each well are characterized by a considerable amount of positive skewness. A comparison of the values of anisotropy ratios along the wells indicates that, except for a few extreme values, a certain amount of uniformity exists among the ratios for each well. However, lateral variations in the anisotropy ratios are frequently observed. This study also indicates that higher fluctuations in the anisotropy ratios can be correlated with low porosity intervals existing within the reservoir. Three statistical techniques, based on the arithmetic, geometric and harmonic means, and commonly used for combining permeability measurements, have been proposed for combining the anisotropy ratios.