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Collaborating Authors
EPSLOG SA.
Abstract 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.
- Asia > Middle East > UAE > Rub' al Khali Basin (0.99)
- Asia > Middle East > UAE > Abu Dhabi > Arabian Gulf > Rub' al Khali Basin > Abu Dhabi Field (0.98)
Enhanced Core Analysis Workflow for the Geomechanical Characterization of Reservoirs in a Giant Offshore Field, Abu dhabi
Noufal, Abdelwahab (Abu Dhabi company for Onshore Petroleum Operations Ltd (ADCO)) | Germay, Christophe (EPSLOG SA.) | Lhomme, Tanguy (EPSLOG SA.) | Hegazy, Gehad (Abu Dhabi company for Onshore Petroleum Operations Ltd (ADCO)) | Richard, Thomas (EPSLOG SA.)
Abstract 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, and How 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.
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Mudrock > Shale (0.55)
- Geology > Structural Geology > Tectonics > Compressional Tectonics > Fold and Thrust Belt (0.46)
- North America > United States > Texas > Fort Worth Basin > Barnett Shale Formation (0.99)
- Asia > Middle East > UAE > Rub' al Khali Basin (0.99)
- Asia > Middle East > UAE > Abu Dhabi > Arabian Gulf > Rub' al Khali Basin > Abu Dhabi Field (0.97)
Abstract The papers presents the scratch test, a novel technique born in the late 90's at the University of Minnesota that provides a direct measure of the rock strength (uniaxial compressive strength). It is a simple, non-destructive, fast and robust test that bears the unique advantage of producing a log of strength with a high spatial resolution (centimetric) along the core sample. The high resolution nature of the test provides a unique insight on the scales and distribution of heterogeneities along the core samples, thereby offering new perspectives for routine and special core analysis. In the paper we first show (i) how the mismatch between the resolution scale of plug and well log measurements added to the discrete nature of plugging can lead to large errors in the establishment of correlation between petrophysical properties, and (ii) how early knowledge of small heterogeneity length scales can help avoid these errors and thereby reduce the uncertainty in the integration of core and wireline data for the characterization of reservoir rock units. We then illustrate with some examples how rock strength profiles averaged to the relevant length scale can be correlated with other petro-physical properties measured both on core plugs or inferred from well logs. Finally, we discuss how standard and special core analysis could benefit most from all the features of the scratch test when introduced at a very early stage of the workflows.
- Asia (0.46)
- Europe (0.46)
- North America > United States > Minnesota (0.25)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock (0.31)
- Reservoir Description and Dynamics > Reservoir Characterization > Reservoir geomechanics (1.00)
- Reservoir Description and Dynamics > Formation Evaluation & Management > Open hole/cased hole log analysis (1.00)
- Reservoir Description and Dynamics > Formation Evaluation & Management > Core analysis (1.00)