Reservoir Engineering – Permeability
1. To what extent, the concept of ‘scale-dependent permeability’ (as against, single-valued ’average reservoir permeability’) remains critical – while characterizing a petroleum reservoir – given the presence of spatial heterogeneity @ various scales (assuming that nano-scale heterogeneity with few nano-meters remains captured using nano-CT imaging; pore-scale heterogeneity with few micro-meters remains captured using micro-CT imaging; while, core-scale heterogeneity with few centi-meters remains captured using medical-CT imaging)?
2. How exactly the increasing spatial heterogeneity with the increasing ‘reservoir areal-extent’ / ‘reservoir volume’ – remains correlated with the size of the Representative Elementary Volume (REV)?
3. To what extent, ‘REV based on permeability’ is going to be different from that of ‘REV based on porosity’ (or specific surface area)?
4. Feasible to deduce an approximate (non-linear) relationship between ‘reservoir permeability’ and ‘reservoir volume’ - for both sandstone and carbonate reservoirs – based on the degree of reservoir heterogeneity – upon the availability of pore-throat radius distribution?
5. How complex would it remain to characterize the reservoir permeability – if both increasing as well as decreasing trends - remains associated with the scale-dependence of - either sandstone or carbonate reservoir permeability?
6. For a given data set, if a reservoir permeability has a huge variance (or a huge standard deviation) for a given mean value, then, can we still go ahead using multi-variate regression scheme to up-scale and predict reservoir permeability from pore-scale to Darcy-scale – just by deducing an up-scaled porosity-permeability relationship?
7. With increasing reservoir heterogeneity, would it remain feasible to estimate the up-scaled reservoir permeability – directly deciphered from low-resolution CT images (associated with a relatively larger-scale) – by taking into account the smaller-scale heterogeneities?
8. How easy would it remain to integrate – the heterogeneities associated with a smaller-scale – that has been deduced by scanning the (low resolution) CT images captured @ core-scale?
9. Will we be missing the actual complex three-dimensional pore-geometry (or pore-network) while deducing the heterogeneity details from any low-resolution CT image? Or, Is it fine for Reservoir Engineers to have the detailed CT images of the reservoir - as they no more remain interested in capturing the actual flow velocity field @ pore-scale?
10. In the absence of details on flow velocity field @ pore-scale, how do we deduce the relation between ‘absolute permeability’ and ‘relative permeability’ – in the absence of precisely capturing the pore-space geometry?
The Representative Elementary Volume (REV) is a concept used in porous media characterization to describe the size of a representative volume of the material that can be used to obtain meaningful measurements of its properties.
The REV is the minimum volume of the porous material that is large enough to be statistically representative of the whole material, meaning that the measurements taken from it can be extrapolated to the entire material with reasonable accuracy.
The REV is typically defined based on a specific property of the porous medium, such as porosity or permeability. Therefore, the REV based on permeability and that based on porosity (or specific surface area) can be different, as they are related to different properties of the porous medium.
The REV based on permeability is the volume of the porous medium over which the permeability remains constant and can be accurately measured. This means that the REV based on permeability is defined based on the flow of fluids through the medium, and it is related to the size of the pores, their connectivity, and tortuosity.
On the other hand, the REV based on porosity or specific surface area is related to the surface area of the pores and the size distribution of the pores. It is the volume of the porous medium over which the porosity or specific surface area remains constant and can be accurately measured.
Therefore, the REV based on permeability and that based on porosity (or specific surface area) can be different because they are based on different properties of the porous medium.
The choice of which REV to use depends on the specific property of the porous medium that is being investigated and the type of analysis being performed. For example, if the flow of fluids through the porous medium is of interest, the REV based on permeability is more appropriate, whereas if the surface area of the pores is of interest, the REV based on porosity or specific surface area is more appropriate.
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