Thinly bedded reservoirs are common throughout the Columbus Basin, but have traditionally been overlooked as productive targets. The presence of interbedded shales suppresses resistivity values within the thin sand beds (<2 feet thick), resulting in low-resistivity log signatures that do not meet conventional pay cut-offs. Core data and advanced high-resolution log analysis are required to accurately evaluate reservoir quality. In the absence of such data, thin bedded pay can still be identified on conventional logs as low resistivity intervals and a modified petrophysics workflow can be used to estimate gas-in-place. An example is taken from a recently drilled well where 180 feet of conventional core was acquired together with an advanced logging suite over a thinly bedded interval. The resulting petrophysical analysis indicates lower water saturation than predicted by conventional logs, which is independently supported by wellsite plug measurement. Core data also demonstrates that the porosity and permeability distributions in the cleaner, thin sand beds show similar ranges to conventional "thickly bedded" sands, irrespective of bed thickness. Porosity and permeability decrease in the more heterolithic zones (with increasing silt content) as would be expected. These results endorse the concept that reservoir quality of thin sands is comparable to that of thicker sands, although this may not be evident from conventional log analysis. As a result, a well-by-well screening was carried out for a number of fields in the Columbus Basin to identify and classify potential thin bedded intervals based on typical log signatures. The reservoir properties of these intervals can then be evaluated using a modified petrophysics approach based on calibration to the core and advanced logging technology. This workflow has proved to be an important tool for the identification of thin bedded intervals, highlighting opportunities for targeted surveillance and providing an inventory of potential targets for future access.
A petrophysical refresh of the bpTT fields in the Columbus Basin in Trinidad has been carried out. The objectives of the refresh were to provide continuity and consistency in petrophysical interpretations in this mature basin where over the years multiple vendors and differing interpretational approaches have been employed. In an effort to create a more robust core data set, data gaps were identified in the existing core analyses and supplemental analysis performed. The core data set was expanded to include Co/Cw measurements on plugs from 2 wells to augment legacy data to investigate log-based water saturation methods. New models were developed for permeability and water saturation and each of these models were calibrated against the core dataset. Permeability was re-evaluated with the new model being based on core-derived measurements and tuned to dynamic well test data to incorporate upscaling heterogeneities. Both log-based and core-based water saturation models were explored. The new core conductivity measurements provided support for the log-based method selected. Air-brine capillary pressure data have provided a key input to the development of a new saturation height function. The match between the new saturation height function water saturation and that derived from resistivity-based saturation is good, reinforcing its validity.
The Columbus basin offshore Trinidad is a mature hydrocarbon province. It contains multiple, stacked, discrete reservoirs which are supported and driven by complex displacement mechanisms. The reservoir surveillance challenges in the basin are compounded by the interaction of low salinity formation water, multiple fluid phases, thin beds, and completions that present difficult conditions for cased hole reservoir monitoring instruments. Challenging current monitoring practices resulted in the implementation of new strategic measurements in the surveillance plan that delivered valuable insights and clarity to complex reservoir management problems. The results obtained using existing procedures and technologies highlighted their shortcomings and uncertainties. To address these issues emerging technologies were evaluated under these challenging conditions. The results obtained clearly prove that tangible benefits could be realized through the use of the new surveillance techniques. The benefits of applying new technology as part of an integrated surveillance strategy will be described in this paper. This new approach has helped reduce the uncertainty in both fluid contact movement and remaining hydrocarbon saturations. This has had a direct impact on reservoir simulation and the definition of future reservoir targets.
In the past, leaks were found by pressuring the process systems first with water and then with hydrocarbons. This low cost but hazardous method of leak detection used gas detectors and soapy water to detect gas leaks. The disadvantage of this method is that large leaks contaminate the environment, risk a possible explosion and fire, and can result in a considerable loss of product.In the last few years, NOWSCO Well Service Ltd has developed a leak test process utilizing nitrogen mixed with helium as a tracer gas. With this innovative technology the industry is now able to eliminate the use of hydrocarbons during leak testing, and detect leakage rates as small as 0.1 cubic foot per year. This nitrogen/helium leak testing method was used on North Rankin 'A' platform.Some 559 leaks were found and rectified, of which 178 occurred in systems that had been previously leak tested with water. This highlights the inadequacy of water leak testing for gas handling systems.In May of 1984 an essentially leak-free start-up of the process equipment was accomplished on North Rankin 'A' Platform, which is attributed to the helium leak detection method used in the commissioning programme.
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