Print this pageComplex Reservoir Evaluation in the Pre-Salt Carbonates of Santos Basin — The Wildcat of Tupi — a Case History
Heterogeneities and the variability of the pore space in microbiolitic reservoirs pose an unique evaluation challenge in the log interpretation of these complex carbonates. Conventional well log analysis fails to fully address the petrophysical behavior of the reservoir, potentially increasing the risk of miscalculating the porosity, water saturation and netpay thickness. Additionally, the application of conventional porosity and water saturation cutoffs can also deteriorate the interpretation results, affecting the decision-making process.
The complex reservoir evaluation process starts with the well being drilled with OBM (Oil base mud) in order to minimize the formation damage, increasing the success in obtaining pressure gradients and fluid sampling. The variability of the pore system and matrix composition require the use of the NMR tool for a reliable porosity measurement and reservoir identification. Additionally, NMR porosities can also be processed to obtain an Archie-independent Sw calculation, increasing the confidence in the volumes of hydrocarbon in place. The evaluation workflow ends up with a set of logging crossplots to evaluate the presence of free water, qualitative permeability, matrix composition and cutoffs determination.
Despite the apparently low porosity of the reservoir, Tupi well was tested with a pre-stimulation DST in order to investigate the reservoir performance at original conditions. The test flowed oil at a constant rate, allowing the determination of the basic reservoir parameters such as productivity index, damage ratio, effective permeability and PVT sampling. The pre-stimulation test was a key point in the evaluation workflow of Tupi since any stimulation procedure before knowing the original reservoir condition could seriously endanger the real performance of the reservoir.
AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil