Seismic velocity is important input data to predict and analyze abnormal pore pressure, particularly in the area of limited well control. In the normal pressure environment, porosity decreases as depth increases. This normal trend can be captured by the increase of seismic interval velocities versus depth. In this case, if pore pressure is increased, then the expected seismic interval velocity trends would be decreased. This paper describes a methodology of three-dimensional (3D) seismic velocity cube analysis for pore pressure prediction consisting of four major components—time-depth conversion, seismic velocity validation, seismic velocity to well calibration, and prediction of pore pressure and fracture gradient. The input data for pore pressure analysis consisted of the wireline data and vertical seismic profile (VSP) checkshot from the existing well along with 3D seismic and seismic velocity, which were derived from velocity analysis during seismic processing.
This case study uses data from the Heidrun field in the Norwegian North Sea. The original discovery well was drilled based on a gas amplitude bright spot at the crest of the structure. A significant overpressure zone of 1.2 g/cc difference existed in the pore pressure gradient encountered in the discovery well. As such, it was necessary to perform detailed pore pressure analysis to optimize the drilling program at the target appraisal well location. A comprehensive review of seismic velocity was completed to validate the data quality and the impact of uncertainty on the results of pore pressure analysis. The validation process, referred to here is an interpretive velocity analysis, requires access to prestack seismic to verify whether seismic velocity functions have an adequate vertical resolution and slower velocity anomalies respond consistently to changes in pore pressure. In addition, more detailed control points can be added to improve the vertical resolution of seismic velocity functions.
This results in a 3D velocity model that utilizes all available velocity data from well and seismic for time-depth conversion. Furthermore, structural controls and well picks ties are also taken into account. Consequently, seismic velocities are calibrated to well VSP checkshots along the structure controls. Finally, a 3D pore pressure prediction workflow is applied to calculate the density, overburden, pore pressure, and fracture gradient cube from the calibrated interval velocity cube. The parameters of 3D workflow were optimized to gain an improved correlation between seismic velocity and the well log data one-dimensional (1D) analysis results.
In summary, it is crucial to validate the integrity of seismic velocity before applying it to pore pressure prediction. The interpretive velocity analysis on prestack seismic data is effective for validating and refining the velocity functions. Consequently, a better calibration between velocity functions and a sonic delta T trend was obtained by adding more controls points to the existing velocity function. As a result, the slower interval velocity was confidently interpreted, representing an abnormal pore pressure for calculating safe drilling window parameters at the target well location.
AAPG Datapages/Search and Discovery Article #90324 © 2018 AAPG Asia Pacific Region GTW, Pore Pressure & Geomechanics: From Exploration to Abandonment, Perth, Australia, June 6-7, 2018