oa Pore Pressure Prediction Using Seismic Inversion and Velocity Analysis

The concept of abnormal pressure, especially geopressure, is most important in hydrocarbon<br>exploration and production. Overpressured formations, in which the pore fluid pressure is higher than<br>the corresponding hydrostatic pressure, form an excellent trap for hydrocarbons. However, if the pore<br>fluid pressure exceeds a threshold dictated by the strength of the rock, the seal may have been<br>breached in the geologic past. This will cause the hydrocarbons to migrate away. This process will be<br>further facilitated by the presence of hydrocarbons in dipping formations due to fluid migration and<br>buoyancy effects. Thus, reliable estimates of formation pressure are critical to understanding the<br>hydrocarbon habitat, from regional to prospect scale.<br>In addition, Drilling through geopressured zones is challenging, and requires extra care. Knowledge of<br>the pore pressure in an area is important for several reasons. In overpressured zones, there is often<br>little difference between the fluid pressure and the reservoir fracture pressure. In order to maintain a<br>safe and controlled drilling, the mud weight must lie in this interval (i.e. between fluid pressure and<br>fracture pressure). If a too low mud weight is used (underbalanced drilling) while drilling through high<br>pressure zones, there is danger of well kicks.<br>Generally pore pressure can be estimated from elastic wave velocities using a velocity to pore-pressure<br>transform. velocities obtained from processing seismic reflection data are clearly required, but these<br>velocities often lack the spatial resolution needed for accurate pore-pressure prediction. This low<br>spatial resolution results from assumptions such as layered media and hyperbolic moveout.<br>In this study we obtain velocities at much finer scale at one of the Iranian south east oil fields using<br>either seismic inversion of amplitudes in conjunction with any acceptable low-frequency model, such as<br>SCVA or Dix. This is a new approach for generating velocity model and our aim is to obtaining a high<br>resolution velocity model that is more appropriate for pressure prediction. The next step is calculating<br>effective stress with Bower's equations (1995) that calibrated using available wells within the basin.<br>Also the main factors that causes pore pressure to rise abnormally within this field explained in detail.<br>Finally high resolution pore pressure cube which have enough detail for drilling applications can be<br>obtained with the use of Terzaghi's empirical equation.