The results of several investigations showed that rock properties under in-situ stress conditions can be significantly different from those measured at normal laboratory conditions. The objectives of this study are to (I) develop a Reservoir Quality Index (ROI) using in-situ measurements and correlate to the conventional RQI, (2) study the effect of change in effective stress on RQI and on the description of clean heterogeneous reservoirs, (3) develop new models for better reservoir characterization and flow unit identification using in-situ measurements, and (4) develop a generalized flow unit model for reservoir characterization of clean heterogeneous formations under in-situ conditions.
Results showed that an increase in change in effective stress leads to a severe reduction in RQI. This reduction shifts the position of the flow units that constituted the clean reservoirs. The flow unit models were found to have a common feature in that each flow unit can be represented by a straight line with a unique slope and intercept. The methodology used in developing the models is presented. Also, a step-by-step example calculations is provided to show how the developed models can be used to identify flow units and study the effect of stress on flow units.
When the reservoir pressure declines due to oil production, the reservoir rock is compacted because the load on the formation grains increases causing changes in the characteristics of flow units. When the reservoir pore volume decreases, the overburden shifts as well, causing variation in fluid flow paths through the porous rock. The importance of the effect of sub-surface stress conditions on reservoir rock to the reservoir engineer is based mainly on the following reasons:
evaluating the reservoir pore volume from porosity data obtained under laboratory atmospheric conditions,
evaluating the variation of the reservoir pore volume with the decline of reservoir fluid pressure resulting from reservoir depletion and
using core data such as porosity and permeability for reservoir characterization and flow unit identification over the extended life of the reservoir.
Dobrynin et al1 studied the effect of overburden pressure on several physical properties of sandstone and other geologic formations. The rock properties include: pore compressibility, porosity, formation resistivity factor, and permeability. The results showed the importance of changes in overburden pressure on the petrophysical rock properties. compaction. This study showed the occurrence of change in rock properties such as deformation, absolute permeability, electrical resistivity, pore volume, and seismic wave velocity due to compaction. The study also introduced some semi-analytical equations to simulate these rock properties under various loading conditions up to rock failure. Jones3 introduced several empirical equations describing change in permeability, pore volume and porosity with net confining stress for different rock types.
McKee et al4 laid the foundation for the theoretical relationship between stress-dependent permeability and porosity for coals and other geologic formations. These relationships eliminate the need for variable compressibility in the range of interest, and therefore, are simpler to use and are not limited by maximum stress. The equations are given as follows:
