Carbonates are known to differ significantly from conventional sandstone reservoirs and this mainly associated with the rock structure and rock-fluid interactions. The differences also influence on the study of the carbonate cores, especially on core handling and preparation procedures prior to laboratory tests. The main distinguishing features are following: high heterogeneity (anisotropy) of properties; presence of special features (cracks, caverns and the like) of sizes larger than core samples; initial wettability (intermediate or hydrophobic); rock-fluid interactions (ion exchange, chemical absorption, dissolution); relatively low rock strength. These differences impose a whole list of limitations and changes on carbonate core handling and preparation procedures.
A more than 10-year development history of a group of oil fields of the Central Khoreyver Uplift in the Nenets Autonomous District of the Timan-Pechora oil and gas province, has come up with new approaches and a system of internal regulations. The reservoir rocks of the Central Khoreyver Uplift are mainly carbonates (up to 96-98%) with little impurities and rock wettability ranging from totally hydrophobic to intermediate.
Oil oxidation and evaporation from cores generally affect the results of subsequent studies. For better results, the initial wettability of carbonate cores must be maintained in any study. This can be achieved by carrying out mild cleaning procedures during core preparation. Core orientation coupled with stress anisotropy is also shown to greatly improve carbonate data quality and their correlations. Considering the above mentioned factors in core analysis techniques can bring about a change in the general understanding of the in-situ processes occurring in such reservoirs during field development. Studies, carried out to compare the results obtained from the study of preserved and unpreserved carbonate cores, showed the great impact core handling procedures (from the well site to the laboratory) has on subsequent lab results.
References
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