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The Importance of Ground Water Aquifer Identification on Well Planning — An Example From the São Sebastião Aquifer, Northeast Brazil

Abstract

São Sebastião aquifer is located in the Recôncavo Basin, northeast Brazil, and it is one of the most important fresh water reservoir in the state of Bahia. Currently, it is forbidden to drill wells using oil base mud in fresh water zones, as had been done before. Thus, the understanding of the ground water aquifer distribution is important to predict the bottom of the fresh water zone prior to drilling. The Recôncavo aquifer system was identified as two different components: (1) The ground water resources, composed by Barreiras and Marizal formations, and the upper layers from São Sebastião formation. This system is an open aquifer that allows the vertical water flow from surface down to the São Sebastião formation. (2) The subsurface water resources are multiple layers in a confined system composed by the São Sebastião formation, which has up to 1600 m thickness. The fresh water in Recôncavo is classified as very good quality. A map of the bottom of fresh water zone was created for the entire Recôncavo basin and the aquifer depths range from 0m to 1600m. Higher thicknesses are found mainly in the Alagoinhas Miranga and Camaçari structural lows. The aquifer host formations were eroded at some of the structural highs of Recôncavo, mainly at the east margin and northernmost compartment of the basin. Thus, there is no subsurface fresh water resource in these areas, but some shallow ground water can be found if there are sandstones from the syn-rift stage formations near the surface. Alvopetro Energy has drilled four wells, one of which was drilled before this study while the other three were drilled after. In the first well, the fresh water bottom was detected 57m deeper than the estimated. In the three other wells drilled after this study, Alvopetro Energy was able to reduce the discrepancy to a maximum of 5m. The identification of fresh water distribution is extremely important for well drilling planning, as it is essential to determine the surface casing depths in a correct location to be successful in isolating all subsurface aquifers before changing the drilling mud from water to oil base. Aquifer mapping is supporting Alvopetro Energys efforts to reduce significantly the error on estimating the bottom of the fresh water zone in drilling operations. Greater precision on the fresh water aquifer determination increase the drilling performance, reduces the operational risks and increase the confidence of the environmental department in the company operations.