Optimal Zonation of Geological Formations and Identification of Pore Structures for Sandstone Reservoirs, Including Variation in Cementation and Fining Sequences

Abstract

Abstract Hydraulic Flow Zone Unit (HFZU) analysis for characterising cored formations using the Carman-Kozeny (C-K) equation was first proposed in the late 1980s. More recently this formulation was amended as for diverse formations, the C-K equation typically works in less than 50 percent of the cases. The modified formulation includes a cementation factor to handle C-K non-compliant cases. The objective of this paper is to build on these formulations, covering the full range of geological situations. The study presented compares traditional reservoir characterisation approaches with the latest geological zonation techniques, showing how the full range of relationships may be universally accommodated. Such zonation is the best preparation for optimal reservoir simulation. In particular, it is shown how variable grain size and sorting may be visualised. Results are presented in traditional space, logarithmic permeability vs. porosity, and in model space: hydraulic radius, or reservoir quality index vs. porosity group, porosity fraction divided by the solid fraction. It is shown that the methodology presented can deal with any sandstone (clastics) situation: C-Kcompliant and non-compliant, further categorising variable cementation and other diagenetic features, and including interbedded intervals and fining sequences. The zonation of a geological interval using the methods presented is the best basis for choosing vertical gridding for a dynamic reservoir simulation model, circumventing traditional upscaling, often fraught with inconsistencies. It has been found that the Flow Zone Indicator, a measure of grain size (or pore throat size) is the best parameter for zonation, followed by sorting of grains or pore throats. A Global Characteristics Envelope, encompassing 6 dimensions, is used to further validate final results. The paper presents several case histories, covering fluvial and marine environments and a range of geological depositions and facie, including pore-fill (typically kaolinite). Fields are located offshore Australia and overseas.