Hydrocarbon production optimization in Pre-Salt carbonate reservoirs is a main focus for oil and gas research in Brazil. Stimulation treatment design optimization requires good knowledge of the reservoir properties and excellent understanding of the interaction between rock formation and treating fluid. This paper investigates these interactions through laboratory tests determining the compatibility of fluids used in matrix stimulation with different Pre-Salt carbonate rock types. The objective of this work is to relate the geology, petrophysics, and geomechanics of the Pre-Salt reservoirs to their expected stimulation response. Because of the difficulty in obtaining downhole cores and the destructive nature of most tests, the study focused on samples collected from a Pre-Salt carbonate analog: the Coquinas formation (Schafer 1973) from the Sao Miguel quarry, northeast Brazil (Chagas de Azambuja Filho et al. 1998). A thorough geology-based study of the Coquinas formation, including routine core analysis (FZI) microtomography, and thin section study was conducted. Usually these grain-supported carbonates show different amounts and types of primary porosity, closed and reopened by multiple diagenetic phases. Throughout the 25-m thick Coquinas reservoir, five rock types in 13 layers with permeability ranging from microdarcy to almost 1 darcy were identified. All rock types were subjected to routine mineralogy evaluation and various petrophysical, geomechanical, and spectroscopic measurements. Six of the thirteen layers were selected to perform core flow tests with a viscoelastic surfactant technology based diverting acid fluid (Al-Mutawa et al. 2005; Chang et al. 2001; Samuel et al. 1997). This is the first extensive study reporting the efficiency of a viscoelastic diverting acid system in the Pre-Salt analogue Coquinas carbonate formation outcrop cores. Spectroscopic measurement showed wormhole creation and, in some cases, rock texture alteration or fine migration. Through the study we identified the flow units and characterized the rock behavior when chemically stimulated. The conclusions from this study will enable us to tailor and optimize stimulation treatments of Pre-Salt carbonate reservoirs. Introduction The offshore Pre-Salt in Brazil comprises a group of recently discovered fields with promising oil reserves in the Coquinas formation or the above the microbialites section. For example, Lula (ex-Tupi) field, the lead field of the Santos cluster, is believed to hold between 5 to 8 billions barrels of oil equivalent (Beltrao et al. 2009). The Pre-Salt reservoirs are currently the focus of research in Brazil; however, the scarceness of downhole samples collected makes destructive tests very difficult to perform, and so analysis must be performed on analogues. The onshore Coquinas formation from northeast Brazil is taken here as analogue of the Pre-Salt carbonates.

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Carbonate reservoirs often show great heterogeneity in their inner rock structure, and stimulation treatments are often necessary to maintain or establish fluid production. Therefore, core-flow tests are usually conducted to test and model stimulation treatments within a laboratory scale to predict their performance. The visualization of wormholes that were created within core-flow tests requires novel technologies for evaluation and pathway-prediction purposes. Unfortunately, past visualization techniques were always associated with the destruction of the core sample, creating a demand for nondestructive methods.Nuclear-magnetic-resonance imaging (NMRI) is such a method that fulfills the approach of being nondestructive. The technology is widely known by medical applications, and this study developed a procedure on how to use the NMRI technology to visualize wormholes with NMRI in 3D.The study was started by initially choosing and obtaining various core samples that have different contents of calcite and dolomite. These core samples were imaged with the NMRI and microfocus-computed-tomography (mCT) technology in their unchanged state, and basic petrophysical experiments were conducted for initial experiments. The lCT technology was used as a reference visualization technique, because it provides a very high resolution with a corresponding high level of detail. Afterward, core-flow tests were conducted on the core samples with various acid systems and wormholes generated. Finally, the core samples with wormholes were imaged again with the NMRI and lCT technology, whereby the NMRI acquisition technique was improved toward imaging of rock samples, and the results were compared with the lCT results. The NMRI results showed moderate imaging achievements for the unchanged rock samples and high-quality imaging achievements for the extracted wormholes. IntroductionAcidizing applications are one of the most important stimulation strategies for carbonate reservoirs with low porosities and low permeabilities. Predicting the performance of such treatments is very difficult and normally performed with small-scale laboratory experiments in a core-flow tester. These experiments are evaluated by the measured parameters such as the used acid that was needed to create one or more wormholes through the core sample, but the wormhole pathway within the core sample is more difficult to identify. In the past, it was performed by techniques that destroyed the core sample by, for example, taking thin slices. One of the biggest disadvantages of conventional measures is thereby the mechanical destruction of the core samples, and damage to the wormhole structures themselves cannot be completely avoided.The nuclear-magnetic-resonance-imaging (NMRI) technology is already far developed through its medical applications and can be easily adapted from human/animal bodies toward rock

Carbonate reservoirs with their high hydrocarbon potential show often a great heterogeneity in their inner rock structure, wherefore stimulation treatments are often necessary to maintain or establish fluid production. Therefore, core flow tests are usually conducted to test and model stimulation treatments within a laboratory scale in order to predict their performance. The visualization of wormholes that were created within core flow tests requires novel technologies for evaluation and pathway prediction purposes. Unfortunately, past visualization techniques were always associated with the destruction of the core sample and in that way, an increased demand raised in non-destructive methods.Nuclear Magnetic Resonance Imaging (NMRI) is such a method that fulfills the approach of being non-destructive. The technology is widely known by medical applications, but has never been used before in correlation with 3D rock imaging. Hence, this study developed a procedure on how to use the NMRI technology to visualize wormholes with NMRI in 3D.Therefore, a comprehensive study was started by initially choosing and obtaining various core samples that have different contents of calcite and dolomite. These core samples were imaged with the NMRI and µCT technology in their unchanged state as well as basic petrophysical experiments were conducted for initial experiments. The µCT technology was utilized as reference visualization technique, since it provides a very high resolution with a corresponding high level of detail. Afterwards, core flow tests were conducted on the core samples with various acid systems and wormholes generated. Finally, the core samples with wormholes were imaged again with the NMRI and µCT technology, whereby the NMRI acquisition technique was improved towards imaging of rock samples and the results were compared to the µCT results. The NMRI results showed moderate imaging achievements for the unchanged rock samples and high quality imaging achievements for the extracted wormholes.