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Title: Anisotropy, focal mechanisms, and state of stress in an oilfield : passive seismic monitoring in Oman
Author: Al-Anboori, Abdullah Said
ISNI:       0000 0001 3404 8860
Awarding Body: University of Leeds
Current Institution: University of Leeds
Date of Award: 2005
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Knowledge of the spatial characteristics of stress and fractures in reservoirs is important for optimising production and injection processes. Serni-permanent passive microseismic monitoring is being conducted in the Yibal field, Oman, to better understand reservoir geomechanics. The network comprises 12 4C stations in 5 monitoring wells which can be used for focal mechanism and anisotropy studies. In this Study, I analyse 22 days of data, containing over 600 located events. In the first analysis, 43 reliable fault plane solutions (FPSs) are determined using polarities and amplitudes of direct P-, SV- and SH-waves based on a pure double-couple source. The principal stress directions are estimated using the method of Gephart and Forsyth (1984) from FPSs. Stress Magnitudes are then estimated based on a friction model, and stresses are finally modelled based on a passive basin model. In the second analysis, nearly 400 reliable S-wave splitting measurements of time lag and fast shear-wave strike are determined. Shear-wave splitling modelling is used to interprete the results in terms of fracture orientation and fracture density. In the final analysis. 19 examples of frequency-dependent S-wave splitting are determined and the results are interpreted Using the Chapman (2003) theory to estimate the fracture size. I observe a transition in faulting regime from strike-slip(with a thrusting component) in the shale Fiqa cap rock to pure thrusting in the gas-charged Natih A chalk reservoir. Deeper in the held I observe another transition from strike-slip in the Nahr Umr shale cap rock to normal faulting in the oil-bearing Shuaiba chalk reservoir. The transition at each shale/chalk interface may be attributed to variations in the Friction angles: from low in the shales (12' and 18', respectively) to high in the chalks (39'). The Natih A results suggest a positive anomaly in Poisson's ratio (0.37), which is consistent with the ongoing compaction in this unit. The maximum compressive stress direction varies with depth: horizontal E in Fiqa, horizontal NNE in Natih-A, sub-horizontal E' in Nahr Umr, and sub-vertical in Shuaiba. The splitting magnitudes are high (5-10%) in the SE footwall of the large eastern-most graben fault that runs through the field and low (11/c) in the opposite hanging wall. The highest fracturing (517ca verage anisotropy) and largest fracture sizes (2 rn) are predicted in the Natih A reservoir. In contrast, the Fiqa exhibits moderate Fracture density (31Y)(with fine-scale fractures (
Supervisor: Kendall, Mike ; van der Baan, Mirko Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available