The "Oriente" basin is located in eastern Ecuador between the Andes Mountains and the Amazon rainforest. In 2012, daily oil production reached 505,000 barrels. The three main oil-bearing Cretaceous formations in the basin are the Hollin, T and U formations. Results from recent extensive coring of the U and Hollin formations showed that the pore size significantly affects oil saturation and production. Therefore, understanding pore size distribution can greatly enhance the success of a well. It is a major challenge to characterize and classify reservoir type and heterogeneity in reservoirs with pore-size variations using only well log data. We used core data from three wells in the U and Hollin formations to validate a new nuclear magnetic resonance (NMR) spectral analysis technique, applied in the echo domain, to estimate the pore-size distribution. In certain carbonate reservoirs in the Middle East, the distribution of pore size classes can be accurately determined by fitting the NMR pulse echoe. The method was blindly tested on three siliciclastic wells from the Oriente basin, and the results were compared with pore-size analysis from mercury-injection and capillary-pressure data. Additionally, a multi-mineral petrophysical model was built for each eall from log measurements, omitting the core data. The porosity derived from the multi-mineral model was used as a porosity input to guide the time-domain inversion of the NMR echo trains. The inversion solves for continuous logs of the porosity, attributed to three pore families, representing the range of pore-body sizes from small to medium to large. After completing the log-based classification into three pore families, the resulting porosity logs were compared to the analysis of core samples for several oilfields. For all formations and in all fields, the core-analysis inversion data was in good agreement with the time-domain NMR inversion results. These results were used to select optimum intervals to be completed and to predict production in the studied fields.
The Eden Yuturi field, first exploited in 2002, is located in the northeast of the Oriente basin, Ecuador. The field was initially operated by Occidental Petroleum and has been operated by Petroamazonas EP (PAM) since 2006. From its peak production of 83.8 million B/D in August 2004, the field declined and now produces less than 30 million B/D, with a water cut of 92%. In 2014, PAM awarded an incremental production based service contract to the Kamana Services consortium, with the objective of reversing the field decline and increasing the recovery factor by optimizing production and applying enhanced oil recovery techniques.The Lower U in the Cretaceous Napo formation is one of the most important reservoirs in this mature field and has a cumulative oil production of 48.1 million barrels (as of July 2015) from 110 wells. The production is supported by a water drive mechanism and a strong aquifer.The major challenge for the consortium is to define an optimal field development plan that maximizes the current recovery factor of 34%. To achieve this goal, it is crucial to enhance the reservoir characterization by integrating all the available information from multiple disciplines and use this knowledge to understand the fluid dynamics in the reservoir.The sedimentary column of the Lower U is interpreted as fluvial at the base follow by tide-dominated estuarine tidal shelf deposits grading into lower shoreface deposits towards the top of the reservoir. This transition subdivides the reservoir column into different hydraulic units. However, the vertical stacking of these hydraulic units is not present across the whole field, and amalgamation has been observed in specific zones. The fluid dynamics changes inside the reservoir have a large impact on the waterdrive mechanism sweep efficiency, as identified in the pressure and production performance analysis of the reservoir across the area, and differences in pressure levels and water cuts trends are evident.Therefore, a proper stratigraphic characterization is the key to optimizing the field development strategy of the Lower U reservoir to improve the ultimate recovery factor.In this paper, we describe how the stratigraphic characterization has been performed and the final framework obtained by the multidisciplinary team, to be used for the optimized development strategy of the Lower U reservoir.
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