Zero Offset VSP in a Deviated Well and Q Factor Estimation

Abstract

Vertical Seismic Profiling (VSP) is a technique in geophysical exploration, which involves recording downgoing and upgoing wavefields propagating through the stratigraphic section. In a Zero Offset VSP the receivers are located in the wellbore, allowing the wavefront produced by the source to have a shorter path to reach them. As a primary consequence of that, there is less attenuation of of high frequencies, which means better resolution. A Time – Depth relationship and a corridor stack are generated in to correlate with surface seismic at 1D response (i.e. well seismic tie). However, a rig-source VSP survey acquired taken on deviated wells can also provide a 2D image below the wellbore trajectory. A rig-source VSP was conducted on a deviated well in a field located in the Gulf of Paria, Trinidad and Tobago. Three (3) component data was recorded and 3-C processing was used to obtain a corridor stack as well as a 2D image. This information is to assist in the reservoir delineation and characterization, as well as in the development of a velocity model for ongoing 3D seismic processing in this field. This 2D image has a frequency content of approximately about 90Hz, which showed higher frequencies in comparison to surface seismic acquired in 2005. The correlation of the image with the seismic section was excellent, illustrating deep structural features with better resolution than seen on surface seismic. In addition, Q absorption values using spectral and multi-spectral methods were calculated, resulting in values of 63.00 and 62.85 respectively. This was interpreted as medium values of absorption for these formations. The Q values are presently being used to balance amplitudes in an ongoing 3D seismic processing project. This example confirmed the importance of the VSP as an excellent tool for calibration, interpretation and processing of seismic data that will ensure more accurate planning and placement of new wells.