Location of Repository

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:
oai:etheses.whiterose.ac.uk:208

Provided by:
White Rose E-theses Online

Downloaded from
http://etheses.whiterose.ac.uk/208/1/uk_bl_ethos_423619.pdf

- (2003). 2003,3D model for rock strength & in-situ stresses in the Khuff formation of Ghawar field, methodologies & applications,
- (1984). A geology for engineers,
- (2003). A geornechanical modeling approach in deep horizontal well development designs,
- (2005). A one-way wave equation for modelling seismic waveform variations due to elastic heterogeneity,
- (2005). A proposed data acquisition program for successful geornechanics projects,
- (1981). A simple method to calculate Green's functions for elastic layered media,
- (1984). An improved method for determining the regional stress tensor using earthquake focal mechanism data: application to the San Fernando earthquake sequence,
- (1994). Anisotropic effective-medium modeling of the elastic properties of shales,
- (2004). Attenuation anisotropy and the relative frequency content of split shear-waves,
- (2003). Comment on: 'The 3D shear experiment over the Natih field in Oman: the effects of fracture-filling fluids on shear propagation' by C. A Van
- (2000). Constraining the interpretation of AVOA for fracture characterisation,
- (2004). Coupled stress and fluid flow using a finite element method in a commercial reservoir simulator,
- (1997). Details of stress directions in the Alaska subduction zone from fault plane solutions,
- (1965). Determination and some uses of isotropic elastic constants of polycrystalline aggregates using single-crystal data, in
- (1976). Dynamics of an expanding circular fault,
- (1984). Effective anisotropic elastic-constants for wave propagation through cracked solids,
- (1977). Elastic moduli of a cracked solid,
- (1996). Estimates of stress directions by inversion of earthquake fault-plane solutions in Sicily,
- (2003). Estimating geornechanical properties using an integrated flow model,
- (2005). Estimating signal polarisations in seismic array data: theory and applications,
- (2004). Fault mechanisms of induced seismicity at the superdeep German Continental Deep Drilling Program (KTB) borehole and their relation Z: 1 to fault structure and stress field,
- (1974). Formation velocity and density - The diagnostic basics for stratigraphic traps,
- (2003). Fracture-induced anisotropy study using shear-wave splitting in the Yibal oil field,
- (2001). Frequency band dependence of S-wave splitting in China mainland and its implications,
- (2003). Frequency-dependent anisotropy due to meso-scale fractures in the presence of equant porosity,
- (2003). Frequency-dependent anisotropy: effects of multi-fracture sets on shear-wave polarizations,
- (2003). Frequency-dependent seismic anisotropy and its implication for estimating fracture size in low porosity reservoirs, The Leading Edge,
- (2004). Frequency-dependent seismic anisotropy of porous rocks with penny-shaped cracks, Exploration Geophysics
- (1978). Friction of rocks,
- (2005). Full moment tensor retrieval for two earthquake swarms at the alps-dinarides junction,
- (1989). Fundainentals of rock- inechanics, Chapman and Hall, London, 3rd edition. 182
- GEOScience, Karstic and tectonic fracture study, Natih-A reservoir, Ylbal Gas Field, Northern Oman, Unpublished report, prepared by Baker Atlas GEOScience,
- (1998). in press, Kinematic interpretation and structural evolution of Northern Oman,
- (1999). In-situ stress analysis, Unpublished report, prepared by Baker Atlas GEOScience,
- (1993). Instantaneous spectral bandwidth and dominant frequency with applications to real seismic reflection data,
- (1989). Introduction to rock mechanics,
- (1998). Maslov shear-waveforms in highly anisotropic shales and implications for the shear-wave splitting analyses,
- (1996). Mechanical properties of materials with interconnected cracks and pores,
- (1996). Microearthquakes induced by a hydraulic injection in sedimentary rock,
- (2005). Microseismic monitoring and spatial variations in anisotropy, an example from
- (1988). Modeling of marine seismic profiles in the t-x and tau-p domains,
- (1993). Modelling seismic waveforms in anisotropic inhomogeneous media using ray and maslov asymptotic theory: applications to exploration seismology,
- (2001). Modelling the wide-band laboratory response of rock samples to fluid pressure changes,
- (2005). Numerical simulation of fully coupled fluid-flow / geornechanical defori-nation in hydraulically fractured reservoirs,
- (2003). Observation and analysis of frequency-dependent anisotropy from a multicomponent VSP at Bluebell-Altamont field,
- (1980). Overall properties of a cracked solid,
- (2004). Petrofabric analysis of seismic anisotropy in siliciclastic sedimentary rocks,
- (2001). Petroleum reservoir simulation coupling fluid flow and geomechanis,
- (2005). Quantitative seismic interpretation : applying rock physics tools to reduce interpretation risk,
- (1980). Quantitative seismology: theory and methods,
- (1978). Seismic wave propagation through a cracked solid: polarization as a possible dilatancy diagnostic,
- (1979). Seismic waves in a stratified half-space,
- (1986). Shuaiba reservoir geological study, Yibal field,
- (2004). Some elastic properties of shales,
- (2003). Stress field in the Northern Rhine area, Central Europe, from earthquake fault plane solutions,
- (2004). Stress field in the Ryukyu-Kyushu Wadati-Benioff zone by inversion of earthquake focal mechanisms,
- (2001). Stress orientations obtained from earthquake focal mechanisms: What are appropriate uncertainty estimates?,
- (2001). The dynamic fluid substitution problem, 71st SEG Meet.,
- (2003). The effect of geological and geornechanical parameters on reservoir stress path and its importance in studying permeability anisotropy,
- (1952). The elastic behaviour of a crystalline aggregate,
- (1977). The energy release in great earthquakes,
- (2000). The role of microearthquake monitorinĂ½,,, in hydrocarbon reservoir management,
- (1997). The stress-depletion response of reservoirs,
- (1997). The use of borehole imaging logs to optimize horizontal well completions in fractured water-flooded carbonate reservoirs,
- (1996). Type of faulting and orientation of stress and strain as a function of space and time in Kilauea's south flank,
- (2000). Use of passive seismic monitoring in well and casing design in the compacting and subsiding Valhall field, North Sea,
- (1981). Wave speeds and attenuation of elastic waves in material containing cracks, Geophys. I Roy. Astr Soc.,

To submit an update or takedown request for this paper, please submit an Update/Correction/Removal Request.