Referencial Sistêmico "In Natura" no Arenito Sergi, no Semi-Árido da Bahia, para Estudo Geofísico da Hidrodinâmica da Injeção de Água

Abstract

A detailed study using geophysical methods sensitive to water saturation changes is being conducted in connection with an artificial water storage experiment in sandstones of the Sergi Formation in the pilot area of Cocorobó, Bahia. The aims of this study are to map the propagation of the injection front, as a function of time, and to evaluate the reservoir petrophysical parameters measured at three different times: before injection starts, under partial saturation and after the complete filling of the reservoir. The results of the pre-injection studies including 28 vertical electrical soundings and 7 seismic refraction profiles showed that the Sergi Formation, locally. is as a two-layer system: the upper layer, transitional to the Itaparica Shales. has a resistivity of about 25 Ωm and a seismic velocity (Vp) of 1,100- 2,000 m/s. In the main sandstone body rock resistivity varies from 1,200 to 1,600 Ωm and Vp changes from 2,000 – 2,250 m/1; these variations being related to clay content and porosity of the rock. The area is covered by dry sands with thickness of 6 to 8 m. Shales of the Aliança Formation are the reservoir substratum and exhibit lower resistivity (50 Ωm) and larger Vp (2,950 - 3,000 m/s) values. Self potential maps reflect the geological features of the area and differentiate the Sergi and Itaparica Formations. Isolated small size anomalies are due to the iron well casing and/or to local water infiltration from the surface. These results represent a general reference system for future investigations.