Influence of stress on the petrophysical properties of flow units in clean and shaly heterogeneous formations.

Abstract

The results show that increasing effective stress reduces the value of reservoir quality index under stress (RQIs) for clean formations. In addition increasing effective stress significantly reduces the value of reservoir quality index for shaly formations under stress (SRQIs). Also increasing the change in effective stress leads to change in the fluid flow path patterns and the position of flow units. Therefore, identification of flow units should be updated. The results also show that increasing stress effect leads to a shift of the J-function versus water saturation (S$\sb{\rm w})$ curve in both clean and shaly reservoirs. Therefore, there is a need to correct laboratory acquired J-function values for stress effect.

This study is devoted mainly to the reservoir characterization of clean and shaly heterogeneous reservoir rocks under stress effect. This study is divided into five different categories: (1) characterization of clean heterogeneous formations (without stress effect). (2) characterization of shaly heterogeneous formation (without stress effect), (3) characterization of clean heterogeneous formations under stress effect using well-logging derived data and investigation of effect the of stress on petrophysical properties of flow unit in this type of formation, (4) characterization of shaly heterogeneous formations under stress effect using well-logging derived data and investigation of the effect of stress on petrophysical properties of flow units in shaly heterogeneous reservoirs, and (5) investigation of the effect of stress on the J-function and development of new J-function models in clean and shaly reservoirs using well-logging data.

With respect to the first and the second categories, several practical and theoretical flow unit models were developed for identification and characterization of hydraulic (flow) units in clean and shaly heterogeneous reservoirs. Characterization of clean and shaly heterogeneous formations under stress effect was achieved by developing two new Reservoir Quality Index of formation under stress including Reservoir Quality Index of clean formation under stress (RQIs) and Shaly Reservoir Quality Index under stress (SRQIs). These newly-developed RQIs and SRQIs were used to study the effect of stress on petrophysical properties of flow units and to derive several new models for characterization and identification of flow units constituting clean and shaly formations under stress effect. Finally, this study has developed new J-function that considers stress effect and presented a new approach by which the utility of well-logging data can be used to estimate the J-function in clean and shaly heterogeneous reservoirs. Validation of these newly-developed J-function models has been proven using actual field data.

Reservoir characterization has been considered as a critical component of reservoir development because its main goal is to provide better description and distinguish the essential features of geological/petrophysical parameters affecting fluid flow in the producing formations.