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Avaliação dos reservatórios do Campo de Ubarana-RN, com baixas resistividades
(Universidade Federal do Pará, 1990-10-12) RAMOS, Eduardo Moreira; EVANS, Hilton Bernard
A new model for quantitative evaluation of shaly sand is proposed based on bibliography and clay mineral distribution in the pores of reservoir rocks. This model takes in account for two extra conductivity types besides the normal electrolytic conductivity one present in porous rocks. One conductivity type is due to surface conductance of clay minerals, which increases or decreases as result of their capability of cationic exchange. The second conductivity type behaves independently as a parallel system due to the microporous network formed by the clay minerals lining or bridging the grains of a shaly rock. The new equation was tested against several other equations and showed the best correlation with the Waxman & Smits equation. Both equations, the proposed one and the Waxman & Smits, have a similar approach to the clay conduction system. The new model was used in three different zones of the Ubarana oilfield (Potiguar basin, Brazil) It was verified that: (1) in two low resistivity zones (there was the possibility of) oil production was possible because the resistivities were supposedly affected by highly conductive minerals; and (2) one effective oil tested and producing tone of highly resistivity. The water saturations calculated with the new equation for the two low resistivity zones were above 50%, suggesting subsaturation and a continous resistivity phase through the salt water (about 85,000 ppm of NaCl) present in the pores. The practical results shown herein suggest that the proposed equation is theoretically consistent and must be exhaustively used in future evaluation projects of the Ubarana and similar oilfields in Brazilian sedimentary basins.