Print Email Facebook Twitter 4D seismic reservoir characterization, integrated with geo-mechanical modelling Title 4D seismic reservoir characterization, integrated with geo-mechanical modelling Author Angelov, P.V. Contributor Arts, R.J. (promotor) Wapenaar, C.P.A. (promotor) Faculty Civil Engineering and Geosciences Department Geotechnology Date 2009-12-10 Abstract Hydrocarbon production induces time-lapse changes in the seismic attributes (travel time and amplitude) both at the level of the producing reservoir and in the surrounding rock. The detected time-lapse changes in the seismic are induced from the changes in the petrophysical properties of the rock, i.e. (visco-)elastic constants as a consequence of saturation, porosity, and stress-strain changes, and by direct changes in the layer-thickness due to compaction or elongation. Usually the production effects in the surrounding rock and the effect of changes in the layer thickness are neglected, which can lead to misinterpretation of the recorded time-lapse information. In this study we have first investigated how large each of these effects can be and what factors have the main influence. To this end we have developed different synthetic models, for which the parameters have been based on a real field cases. In more detail we adopted the following steps to investigate the time-lapse changes in the stress field both in the reservoir and in the surrounding rock for different scenarios: - We developed a petrophysical model of hydrocarbon-saturated sandstone reservoir, based on the Hertz-Mindlin contact theory, to investigate the time-lapse changes in the seismic parameters (velocities and density) following from 4D changes in the rock parameters. The influence of the different rock properties and environmental conditions (pore pressure, water saturation and porosity) on the seismic parameters inside the reservoir has been investigated. As expected it was demonstrated that the hydrocarbon substitution by water causes an increase in the aforementioned seismic parameters, whereas an increase in the porosity and pore pressure will decrease the values of these parameters. - We developed three different geo-mechanical models based on the North Sea reservoirs and ran several scenarios with each of the models to understand the development of the stress and strain fields as result of production. -- The first model consisted of a 2D layer-cake model and has been used to investigate the stress distribution and vertical strain in the reservoir and in the surrounding rocks. We observed that the distribution of the stress changes in the surrounding rock depends on the elastic properties of the reservoir and surrounding media and is linked to the lateral boundaries between the reservoir and the surrounding rock. -- The second model was also a layer cake model, but now with parameters and layering based on the gas field Shearwater. This model has also been used to investigate the effect of offset on the development of the time-shifts. -- The third and final model considered of a 2D model now also with the geometry of the gas field Shearwater. In that model we ran several scenarios changing the shape and the depleting segments of the reservoir in order to investigate their influence on the stress distribution and vertical strain. Overall we concluded that the main factors influencing the changes in the stress distribution and vertical displacement in a depleting gas reservoir are: 1) the distribution and the magnitude of the pressure drop in the reservoir, 2) the geometrical shape of the reservoir and the overlaying rocks, 3) the presence of faults, and 4) the elastic properties of the layers. We investigated the time-shift variation as a function of offset using the second geo-mechanical model. We created synthetic time-lapse seismic data for a simple 2D model. This was done by combining the results of the geomechanical modelling with the time-shifts representative for the Shearwater data, given in the literature. We measured the time-shifts from the synthetic 4D data for different stacks in order to find the optimal value of signal-to-noise ratio without violating the requirement for vertical travel paths. From the case study we concluded that the near offset partial stack (500-1300 m) can be used in the real data example to give reliable results of the measured time-shifts. As a second step in this study we introduced a new workflow in order to separate the effect of the changes in petrophysical properties and environmental conditions in the surrounding and reservoir rock and the physical displacement of the layers. The proposed workflow has been applied to the Shearwater 4D seismic data. The following steps have been taken: - We measured the time-shifts using 4D data from the Shearwater field. The differences in the two-way travel time at the main reflectors were estimated and stabilized using vertical stacks of estimated time-shifts around the interfaces in order to reduce the effect of multiple reflections. Further the time-shifts are smoothed using a lateral median filter to remove the effect of the outliers, and time-shifts horizontal maps are produced for each of the main reflectors. - We calculated the differential time-shifts in each of the geological layers using the relative ratio between the measured time-shifts at the top and at the bottom of the layers. We used the results from the 2D geo-mechanical modelling (vertical strain) to remove the time-shifts caused by changes in the displacement of the layers. We observed that the calculated differential time-shifts follow accurately the modelled changes in the stress field for the seismic 2D line which corresponds to the geo-mechanical model. We concluded that the results of geo-mechanical modelling can be effectively used to eliminate the effect of displacement from the measured timeshifts. The resulting time-shifts are thus induced by changes in the seismic velocity. Furthermore, the changes in the seismic velocity are consequence of changes in the petrophysical properties and environmental conditions. This allows us to use the calculated differential time-shifts to map directly the changes in the stress field. In the Shearwater field the effect of physical displacement appeared to be negligibly small for the selected 2D line, where the geo-mechanical modelling has been applied. Furthermore, we demonstrated that the calculated changes in the seismic velocity follow accurately the modelled stress field variations. We used the inverse relation ship between the modelled stress changes and the calculated time-shifts (induced by velocity changes) to define the changes in the vertical stress over the entire field. We conclude that the stress changes in and around a depleting hydrocarbon reservoir will always induced changes in the seismic velocity. As demonstrated in this example the correlation between the two allows an estimation of changes in the petrophysical properties and in the environmental conditions from observed velocity changes. Subject 4Dreservoirseismicgeo-mechanics To reference this document use: http://resolver.tudelft.nl/uuid:18feed38-e97d-4b55-a226-6cffba45a567 Publisher Angelov ISBN 9789090249858 Part of collection Institutional Repository Document type doctoral thesis Rights (c) 2009 Angelov, P.V. Files PDF phd_thesis_Petar_Angelov.pdf 22.68 MB Close viewer /islandora/object/uuid:18feed38-e97d-4b55-a226-6cffba45a567/datastream/OBJ/view