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Advanced formation evaluation and seismic imaging of the Raudhatain field, Kuwait
Cheng, Liwei
Cheng, Liwei
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2022
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2023-10-22
Abstract
The Raudhatain field in North Kuwait was ranked 18th among the largest known oil fields. Currently, the development focuses are the Jurassic reservoirs. Like other oil fields in the Middle East, the Raudhatain field exhibits a challenging geological setting, such as highly-reflective interfaces, low-relief structures, thick evaporite formations, and high-velocity overburdens, which poses difficulties for petrophysical and geophysical reservoir characterization. In addition, the Jurassic reservoirs in the Raudhatain field demonstrate a complicated flow zone architecture and require quantitative formation evaluation and seismic data analysis to delineate different production units. This thesis focuses on advancing the formation evaluation and seismic imaging technologies, leading to better reservoir development for the Raudhatain field and similar reservoirs in the Middle East.
Correctly estimating the volumes of iterative anhydrite for reservoir development is critical because the anhydrite volume determines the vertical connectivity of flow zones. Due to a high condition number, high uncertainties exist between anhydrite and carbonate volume fractions in multimineral analysis. However, anhydrite and carbonate are distinct in elastic properties. I develop and incorporate rock-physics effective medium models to assist the multimineral inversion of the Jurassic reservoirs. The constrained multimineral result suggests an alternative solution with a reduction of anhydrite by 10% – 30% of volume fractions and an increase of calcite or dolomite by the same amount. This observation may impact the current understanding of the depositional environment and the calculation of hydrocarbon volumes.
In addition, the implementation of Markov chain Monte Carlo (MCMC) with ensemble samplers addresses the uncertainties in multimineral analysis. The resulting posterior probability density functions quantify the uncertainties of volume fractions and endpoints. The method developed here has fewer tuning parameters and is more efficient in convergence than the conventional random walk MCMC methods in high-dimensional problems. Knowing the uncertainties in rock composition assists to make risk-assessed decisions on reservoir development.
For surface seismic data imaging, due to the strongly reflective and low-relief overburden, attenuating internal multiples has been a perennial challenge for seismic imaging in the Middle East basins. The seismic image of the Jurassic formations is especially poor at the center of the Raudhatain structure due to internal-multiple interferences. Because of the similarities between primaries and multiples, applying adaptive subtraction poses a high risk of primary-amplitude damage, preventing quantitative seismic data interpretation. I use the Marchenko method, which retrieves Green’s functions from surface seismic data, for target-oriented imaging without adaptive subtraction. Marchenko imaging has been successfully applied to several offshore seismic data sets, but the onshore application is lacking. To better understand the effects of internal multiples and implement Marchenko imaging, I perform integrated analysis through well log, vertical seismic profiling (VSP), and seismic data. The resulting Marchenko images improve the seismic images by showing recovered amplitudes and more continuities of the Jurassic formations at the center of the Raudhatain structure. Also, I show that the retrieved Green’s functions are consistent with the VSP data.
VSP data has a greater chance to mitigate many near-surface and overburden challenges associated with surface-seismic data imaging because VSP data are acquired by the receivers deployed in the subsurface closer to the imaging targets. I compare different imaging strategies for VSP data using synthetic and field data. A comparison of images suggests that when the velocity model is exact, mirror RTM with up- and down-going wavefields gives the best image, benefitting from utilizing both primaries and multiples with fewer migration artifacts. However, when the accuracy of a velocity model is uncertain, interferometric imaging may provide a better option for extending the illumination range. VSP images with extended illumination is found to better delineate the Jurassic reservoirs around the wellbore.
In this thesis, I show that advanced formation evaluation and seismic imaging provide valuable insights into evaluating and imaging the Jurassic reservoirs in the Raudhatain field. These advances also are of substantial value for the Middle East hydrocarbon fields that faces similar challenges.
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