Reservoir rock heterogeneity is an ever-present occurence in most reservoirs. One among reservoir properties that are the most affected by its presence is permeability. In most cases, reservoir rock heterogeneity causes permeability anisotropy or variations in permeability with directions. As most theoretical bases used in many aspects in petroleum engineering (e.g. waterflooding, well completion, well productivity, well stimulation, and reservoir simulation) are developed under an assumption of homogeneous reservoir rocks, this permeability anisotropy certainly imposes a special problem that needs to be dealt with.

Indonesia's geological setting is characterized by its high level of complexity. Permeability anisotropy in Indonesian reservoirs varies very widely, and careless approach in picking values representing the level of heterogeneity for individual reservoir may lead to various problems. This paper presents results of a study on permeability anisotropy - in this case vertical-to-horizontal anisotropy - in Indonesian reservoirs through the use of core analysis results. The study involves 14,634 core samples, of which 6,689 are pairs of vertical and horizontal plug samples and 1,256 are full-diameter core samples. They are taken from 259 wells representing 157 fields in all 15 Indonesia's productive sedimentary basins to date. The core samples cover a wide lithology spectrum from sandstones to various limestones.

The study that is carried out through a series of classifications and comparisons has indeed shown the complexity as expected, even though differences can still be distinguished among groups of classification. Evidence has also been observed that results for sandstones and limestone/carbonates core samples as two lithology groups are significantly different even though there are also differences among either group. One very important lesson that can also be drawn from this study is a reconfirmation upon the common opinion suggesting the requirement of a careful study for individual reservoir before adopting KV/KH values for the reservoir of interest.

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