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 Auca field is located in the northern Oriente basin (Ecuador) with hydrocarbon production coming from Cretaceous fluvio-estuarine and shallow marine sandstones. The field has produced more than 547 million barrels of oil since 1972 and by the end of 2015 the field recovery factor was approximately 14%. In December 2015, the reservoir management and the field re-development activities for the Auca field were awarded to Schlumberger Production Management (SPM) under the name of Shaya project. Since then, to sustain the field re-development activities, an integrated reservoir characterization process has been implemented. In this depositional environment reservoir evaluation can be very challenging, especially when using only conventional well logs. It is proposed in this paper that the acquisition of texture dependent measurements is the solution to improve the understanding of the reservoir rocks in highly heterogeneous environments. Based on our experience in Ecuador, incorporating nuclear magnetic resonance (NMR) in the petrophysical model appears to be the best way to collect the needed texture dependent data. The Rock type characterization in the field was based on mercury injection capillary pressure data. This method enables the determination of pore throat profiles for each rock type and the dominant interconnected pore system, which corresponds to a mercury saturation of 35% in a capillary pressure curve. An empirical relationship was used to relate conventional porosity and permeability to pore throat profiles, and this was used to classify rock types. With the purpose of validating reserves and optimizing the field development plan, a model based on rock type characterization was developed using existing core, log and production data. Additionally, this model was calibrated using data from multiple fields in the basin. The propagation of the model from core to logs was accomplished through a relationship between gamma ray, density, neutron and NMR logs with core porosity and permeability in key wells. These relationships are dependent on rock type, and they were used to extrapolate core characterization to those wells without cores. Maps of rock type distribution were used to classify areas according to their petrophysical properties. These maps were also used to delineate the reservoir limits, helping to validate and identify prospective areas for future drilling and workovers. This paper presents the characterization of the reservoir into rock types by integrating geological, petrophysical and production data through Neural Network Analysis, establishing a fundamental input into and support for the development of the exploitation plan.
Excellent results from a reentry campaign developed in 2014 in Ecuador have proven the benefits of implementing new drilling technologies for reentry drilling. In this campaign, rotary steerable systems (RSS) were drivers for directional control, and logging-while-drilling tools acquired critical information while drilling. Together these tools open up possibilities for giving new life to aging oil fields and to produce from unexploited drainage zones by enabling reentry drilling. Drilling the reentry wells in Ecuador involve using the 9.625-in. casing to open an 8.5-in. window and drilling to the geological objectives in just one hole section. For this purpose, the combination of a point-the-bit RSS system, a multi depth laterolog resistivity tool with high-resolution images, and a sourceless tool that delivers density, porosity, spectroscopy, and sigma provided 100% of directional control while acquiring comprehensive formation evaluation information in real time. Because of the casing configuration in most of the wells in Ecuador, the most commonly used reentry option is to make the window in the 9.625-in. casing and drill the 8.5-in. hole section from the Tena formation (claystone) and Napo formation (several intercalations of claystone, limestone, unstable shales, and pay zones of sandstone). This particular configuration of formation layers is a challenge for directional control and for running wireline logs. The use of the bottomhole assembly (BHA) with the RSS and tools described above enables successfully drilling the reentries in just one 8.5-in. run, reaching the geological targets with 100% directional control, in spite of the complexity of the well trajectory, and avoiding risks such as packoff, geometrical sticking, and differential sticking. The smoother well profile obtained by use of the point-the-bit RSS and verified through the LWD measurements guaranteed the successful run of the 7-in. liner in all of the reentries drilled in the campaign. Reentry wells provide operators with the opportunity to have new production at less than half the cost of a completely new well and in less than half of the total execution time. Building on this reentry campaign, continued reentry drilling will be improved with implementation of the new generation of hybrid RSS (push - and point-the-bit RSS), which will enable drilling engineers to plan trajectories to deliver greater dogleg severities (DLS). This will, in turn, enable developing deeper reentries, further reducing cost and execution time.
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