The implications of shale geomechanics and pressure diffusion for 4D interpretation
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Shales in the reservoir and the surrounding rocks are often regarded as mechanically active for stress deformation and inactive barriers for fluid flow transportation. In clastic reservoirs experiencing pressure depletion due to production, the sands naturally compact to some degree. Consequently, the much lower permeability intra-reservoir and non-reservoir shales may experience mechanical tension. It is well documented that the dilation in the overburden and underburden shales leads to a detectable time shift in the seismic (Hatchell et al., 2003; Tura et al., 2005; Sayers, 2010). Less well know is that the combined effect of stress deformations and pressure diffusion in the non-reservoir and intra-reservoir shales may alter the predicted effective seismic response of the reservoir interval and the surrounding rocks. In this thesis, the combined effect of geomechanics and pressure diffusion process in the reservoir and non-reservoir shales is examined. The integration of these coupled mechanisms into forward seismic modelling is performed to assess the 4D seismic implications. For this, both; synthetic and field data are used in this thesis. It is commonplace to regard shales as barriers in the simulation of reservoir fluid flow induced by hydrocarbon production. Whilst this appears correct for fluid exchange, this is not the case for the fluid pressure component of this process. Based on this work, I observe that pore pressure reduction due to reservoir depletion can propagate significant distances into the intra-reservoir and non-reservoir shale over the production time scale. This diffusion process opposes the geomechanical effects. Numerical computation for a range of shale permeabilities suggests that intra- reservoir shales of 1m to 10m thickness should be considered as active when quantitatively assessing the 4D seismic signature with frequent acquisitions of 3 to 12 months. The critical set of parameters required to carry out accurate calibration of these predictions is not yet fully available from published literature. Also it is observed that pressure depletion in the reservoir can ‘propagate’ distances of as much as 50m into the shale over/under burden during the production time scale. Consequently this could leads to different polarity of time shift above and below the reservoir. In this thesis, I consider two field case studies. In the first case study I focus on the Schiehallion field. Here, the coupled mechanisms of geomechanics and pressure diffusion is integrated into the forward modelling of time lapse seismic. It is found that the polarity of the P-wave acoustic impedance changes and the corresponding synthetic 4D amplitude changes obtained from the coupled mechanisms are different from those modeled using geomechanics alone. Remarkably the synthetic 4D seismic amplitude generated using the coupled mechanisms is in agreement with the observed 4D amplitude. The second field case study is the HPHT Erskine field from the UK central North Sea. Here, 4D seismic data have been used to investigate the combined effects of geomechanics and pressure diffusion on the intrareservoir and non-reservoir shales. Modelling of synthetic seismic time shifts capturing these effects allow a quantitative evaluation of the observed 4D seismic time shifts. In particular, the comparison between synthetics and observations helps to calibrate the range of permeability for the Heather formation and the Erskine shale units. This calibration gives rise to a model which should more reliably predict the pore pressure and stress tensor changes, allowing more confidence in selecting safe well paths, mud weights, and casing schemes. For this particular case, the results suggest that the Heather formation undergoes pressure diffusion and consequently the trapping mechanism at the Heather shale is highly uncertain. The Erskine shale, however acts as an effective pressure barrier, and hence it is overpressured relative to the surrounding formations and could be a high-risk formation for future drilling programs. The results of this work strongly indicate that the understanding of shale geomechanics and pressure diffusion is essential to adequately understand the elastic wave stress sensitivity of the reservoir and the surrounding rocks. Based on the field case studies, the observed timelapse seismic data are consistent with the combined effects of shale geomechanics and pressure diffusion, and are preferable to taking into account only the geomechanical effects. In addition to the above, the results suggest that the need for shale properties should be recognized and measurement become more common practice in order to calibrate the predictions made in this study.