Carbonates

Carbonates precipitate in shallow, warm oceans to form sediments of variable sizes, textures and chemical compositions giving it variable pore sizes. On further burial of these sediments with time, compaction and cementation takes place which decreases the inherent porosity and permeability of the rock. Also post depositional processes like diagenesis gives rise to a secondary porosity system which results in fractals of heterogeneity in the reservoir. It becomes very difficult to determine the effective porosity and permeability of the carbonate reservoir in one region and extend it to different regions of the same reservoir. In most sandstones, the porosity and permeability follow a linear relationship, but in carbonates this relation varies from region to region in the same reservoir. A good understanding of the geological processes which take place in carbonates can be very useful in determining the porosity and permeability of the system, taking into consideration the relevant secondary porosity which plays a vital role in determining the flow characteristics of the reservoir.

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Porosity and permeability can also be determined using the log and the core data (a typical carbonate core sample is shown above). Routine core analysis as wells as special core analysis (SCAL) makes use of several experiments carried out on cores to determine those same properties but on a smaller scale. In a reservoir, what we get are the fractals of certain characteristics, i.e., the properties that we observe on a large scale (well logs) are also present in the small scale observations (core data). Integration of the log and core data makes our observations more reliable at different scales of observation. Since the carbonate reservoir has a large scale heterogeneity, a thorough understanding of the log and the core studies, individually as well as together, makes it a bit easier to work out the properties of the reservoir. Experiments are the essence of any research work. When the same experiments are carried out on different core samples numerous times, what we get is a trend of the behavior of the sample owing to that particular property that we are trying to evaluate for the reservoir and hence we get an empirical relation. These empirical relations obtained can then be applied to different reservoirs to see if they hold for them or not. These relations, if present, are of vital importance in determining the reservoir properties.

These empirical relations can then be used to determine various petrophysical and rock physical properties like the Archie’s coefficients, porosity, permeability, P- and S-wave velocities etc. which are then used to determine the flow characteristics. These relations could also relate the petrophysical to the rock physical properties and vice versa, which could help to simplify the evaluation of the reservoir characteristics especially for such a complex system as the carbonates.

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