Comparison of patterns of permeability anisotropy distributions in Jurassic and Cretaceous carbonate reservoirs. 9th Middle East Geosciences Conference, GEO 2010.

Permeability measurements in most reservoirs display strong dependency on the direction. Therefore, it is essential to determine permeability variations in different directions within the reservoirs. Such variation is generally incorporated into engineering applications as the square root of the ratio of the horizontal to vertical permeability, a parameter known as the anisotropy ratio. This ratio may vary from one zone to another and even from one layer to another in the reservoir sequence. The pattern of variation of this ratio provides valuable information about flow behavior within the reservoir. Based on the whole core data from several vertical wells, permeability anisotropy distributions in three carbonate reservoirs, including an Upper Jurassic, and two Cretaceous (lower and middle) reservoirs from the Arabian Gulf region, were determined. The open-hole log data and the whole core permeability measurements were plotted together with the calculated anisotropy ratio values to aid interpretation. Such plots were generated for each well from each reservoir providing basis for the comparison of anisotropy ratios with the corresponding porosity and permeability values. The results revealed that the anisotropy ratio distributions closely follow the corresponding distributions of permeability. The values of anisotropy ratio vary considerably from reservoir to reservoir. Upper Jurassic reservoir revealed relatively higher values of anisotropy ratios as compared with Cretaceous reservoirs. Considerable variations have also been observed within each reservoir. In Upper Jurassic reservoir, some correlation has been observed between anisotropy ratios and porosity values, indicating close relationship between anisotropy ratios and lithology. In Cretaceous reservoirs, on the other hand, no obvious relationship between anisotropy ratios and lithology has been depicted. In all cases, it has also been observed that very high values of the anisotropy ratios are generally due to unusually low vertical permeability measurements recorded in compact and undisturbed muddy intervals acting as the barriers to the vertical flow.