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Anisotropy, focal mechanisms, and state of stress in an oilfield : passive seismic monitoring in Oman

By Abdullah Said Al-Anboori


Knowledge of the spatial characteristics of stress and fractures in reservoirs is important for optimising\ud 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\ud the results are interpreted Using the Chapman (2003) theory to estimate the fracture size.\ud \ud 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\ud 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\ud direction varies with depth: horizontal E in Fiqa, horizontal NNE in Natih-A, sub-horizontal E'\ud 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 (<O. I jim in size). Weaker anisotropy is found in the Nalih B-G, which is attributed to moderate fracture density in the Lipper layers and preferred crystal orientation in the lower layers. The splitting orientation results are interpreted in terms of\ud a single set of near -vertical fractures trending: 19'NNE in the Natih A, 90'E in the Fiqa and the lower part of Natih B-G, and 45'NE in the upper part of Natih B-G. The fractures are aligned parallel to the direction of the maximum compressive stress, as determined by the HIS-based stress analysis.\ud \ud Cumulatively, these results show how microseismic data can be used to infer the faulting and stress regime, and the size, density and orientation of fractures in individual formations, with a high level of resolution. Such information is invaluable for field development strategies

Publisher: School of Earth and Environment (Leeds)
Year: 2005
OAI identifier:

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