This is a repository copy of Basement structure of the United Arab Emirates derived from an analysis of regional gravity and aeromagnetic database.
This paper is focused on the integration of two laboratory centimeter-resolution logs of mechanical properties (strength and compressional elastic-wave velocity Vp) into an enhanced core analysis workflow for the geomechanical characterization of unconventional reservoirs in a giant field in Abu Dhabi, where fracking is the cornerstone for producing the unconventional oil. The design and placement of hydraulic fratures rely strongly on the a-priori knowledge of the stress profile and brittleness index, which were estimated via a mechanical earth model constructed from wireline logs and correlations based on US shales analogues. With most of the stratigraphic column in the Abu Dhabi field composed of carbonates, the calibration of the mechanical earth models was found critical as the US shales based correlations would otherwise not have been suitable to the geomechanical characterization of these tight carbonate reservoirs. With this case study we illustrate:How the combination of the continuous profiles of rock strength UCS (Uniaxial compressive strength) and P-wave velocity measured directly on dry cores with the scratch tests contributes to the identification of different Geomechanical Facies,How the mapping of several Geomechanical Facies enables the building of a simple yet robust relationship between the UCS measured directly on cores and properties such as the total porosity and acoustic velocities of sonic waves, obtained from wireline logs, andHow the centimeter-resolution profiles of strength and elastic wave velocities measured on dry cores enable the proper upscaling of geomechanical properties measured on plug samples to the entire cored section and the computation of a horizontal stress and brittleness profiles derived from unbiased geomechanical properties. From this case study follows a general discussion on the relevance of wireline sonic logs relative to centimetric resolution data (scratch profiles or plug measurement) acquired on dry cores for the geomechanical characterization of reservoirs. We conclude that measurements on dry cores enable the more robust calibration of mechanical earth model and in turn better description of the reservoir mechanical response. The upscaled profiles of horizontal stress and brittleness index derived from dry core measurements would ultimately lead to an alternative strategy for the design and placement of hydraulic fractures along producing wells.
This paper focuses on a tight carbonate reservoir in a giant field in Abu Dhabi by identifying shortcomings in conventional modeling strategies for geomechanics and demonstrating the benefits of continuous core data to build more reliable 1-D Mechanical Earth Models (MEM). A 1-D MEM was built from the sonic wireline log, which shows significant difference with a profile of ultrasonic P-wave velocity (Vp) measured on cores. However, results of rock mechanical tests (RMT) on plug samples (including ultrasonic Vp measurements at different stress conditions, and stress-strain curves from triaxial tests) are consistent with the core-based Vp profile. We investigate the impact of stresses, resolution and fluid saturation on sonic velocities to reveal the possible shortcomings of sonic wireline logs as an input for geomechanical models and the greater relevance of using core based ultrasonic velocities measured on dry cores for the upscaling of static elastic moduli. Finally we propose an empirical relation to correct sonic wireline logs for geomechanical modeling in offset wells. The following conclusions can be drawn from this study: The core based Vp profile, which is highly consistent with the RMT results, ultimately leads to opposed trends in the in-situ horizontal stresses predictions compared to those of a 1-D MEM based on the non-calibrated wireline sonic log.Only unrealistic reservoir stress conditions could reconcile ultrasonic Vp measured on plugs at different stress states with wireline sonic velocities;Using a low resolution Vp profile at reservoir stress conditions (combining Vp from plug samples and core based continuous Vp profile), we show that differences in stress only partially explain the discrepancy between velocities measured on plugs and wireline sonic velocities.Although a conventional Gassman fluid correction could explain the remaining differences between core measurements and the wireline sonic, its practical application would require the detailed knowledge of the rock mineralogy and of the saturation along the well.Conversely, a profile of the bulk modulus of the rock mineral fraction can be derived from the sonic log and the ultrasonic P-wave velocities measured on dry cores corrected for stresses effects.Evidences in the drilling data suggest that the discrepancies between the core based sonic velocities and the wireline sonic could be due to natural fractures in the borehole vicinity.An empirical relationship involving wireline logs only was established to correct the sonic wireline log to enhance the reliability of geomechanical models for offset wells. These findings have important implications for the practical applications of 1-D MEM, such as the design of hydraulic fractures. Quality control of the sonic logs with extensive data acquired on dry cores reduces the uncertainty when upscaling static elastic properties. Continuous velocity profiles acquired on dry cores are therefore highly valuable to calibrate empirical corrections of sonic logs for geomechanical modeling in offset wells.
Kinematically salt-related faults and fault linkage in Abu Dhabi were classified based on the faults geometry, linkage patterns, deformed layers, and associated salt bodies. The outcrops observations and subsurface numerical modelling address fault patterns and styles. The study is based on analysis of cored data, well logs, and seismic and potential field data and linked to the exposures from outcrops. Salt updoming in the strike slip regions can be formed in the transtensional and in the transpressional regions of the fault segments. Strike slip faults prevailing Abu Dhabi triggered salt movements; as the inherited basement faults reactivation triggered salt in the transpressional regions due to compressional forces. In the transtentional regions, the pull-apart basins is favouring zones for salt updoming. The faults showing asymmetric arrays of tectonic and salt related faults. The tectonic inherited faults are the master segments evolving transtensional/transpressional zones and the diapirs crestal faults are radial in nature linked to the tectonic faults. These faults dip primarily basinward and sole into a subhorizontal salt layer; ramp faults, that extend upward from the landward margin of mushroom salt stocks. The fault segments found triggering salt movement, principally those showing zone of overlap in both transpressional and transtensional regions. In addition, the salt piercements are usually evolved in the transpressional zones and the diapirs not reached the surface formed in the transtensional regions. Therefore, the salt domes are only updoming in these locations answering the question, why not distributed all over Abu Dhabi. Furthermore, the salt mainly allocated in the offshore relative to the onshore, because of the presence of the NE transtenciona shear zone relative to the master NW-fault trends, creating zones of pula-apart basins. The areas of extension between the fault segments are not favoring salt piercement movement and hence forming deep-seated diapirs. Potential field data revealed that the salt is deep seated in the onshore of Abu Dhabi and most of the anticlines forming the giant hydrocarbon fields are rooted by salt diapirs. Understanding the origin of salt movements and their relation to the fault systems of Abu Dhabi help in exploration of the new ventures and development of those fields related to salt tectonics.
In recent years, advanced and integrated seismic processing and interpretation workflows have been adopted in ADNOC. The application of new processing routines to legacy seismic data has shown to be valuable in improving the quality of the seismic data. This improvement has had a positive impact on our understanding of complex geological settings, in particular in areas where salt movement has had an influence on the structural framework of the field. In this paper, case studies are presented for salt-related structures in Abu Dhabi. Salt structures have been classified into three classes according to their complexity and each of these classes asks for a different type of seismic data volume and a different interpretation workflow. With the help of better reprocessed seismic data and seismic attribute analysis, complex structural uncertainties are better appraised, allowing less operational problems in wells due to drilling through faults and more optimal field development plans.
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