Nisar Ahmed has developed a workflow for seismic inversion methods to estimate the rock-properties in a reservoir.
Nisar Ahmed's research project dealt with the problem of seismic reservoir characterisation and improved oil recovery. It focused on the development of an appropriate theoretical background and workflow, for including frequency dependence into the 4D (time-lapse) seismic amplitude variation with offset (AVO) inversion and analysis to estimate viscoelastic properties, pressure and fluid saturation
changes in the reservoir.
PhD thesis: Adjoint-state method for seismic AVO inversion and time-lapse monitoring
Ahmed's dissertation presented seismic inversion methods to estimate water saturation and effective pressure quantitatively in elastic and viscoelastic media. In geophysics, seismic inversion is the process of transforming seismic reflection data into a quantitative rock-property description of a reservoir.
Knowledge of the saturation and pore pressure properties from seismic measurements has been an area of interest for the geophysical community for decades. However, the focus on the existing inversion methodologies and explicit expressions to estimate saturation-pressure variables or changes in these properties due to production or fluid injection has been based on so-called elastic AVO models. These conventional methods do not consider the seismic wave attenuation effects on the reflection amplitudes and therefore can result in biased prediction.
Numerous theoretical rock physics models and laboratory experiments have demonstrated the sensitivity of various petrophysical and seismic properties of partially fluid-filled porous media to seismic attenuation. This makes seismic wave attenuation a valuable time-lapse attribute to reliably measure the saturation and effective pressure properties.
"Therefore, in this work, I have developed two AVO inversion processes; the conventional AVO inversion method for elastic media, and the frequency-dependent amplitude versus offset (FAVO) inversion technique for the viscoelastic media," Ahmed explains.
Nisar Ahmed defended his dissertation for the degree of PhD (philosophiae doctor) at the University of Stavanger Monday 23 October 2023. His work was part of the National IOR Centre of Norway.