Pore Pressure Prediction Using Seismic Inversion and Velocity Analysis

Abstract

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