THE SPECIALIZED USE OF THE VSP TO DEFINE FRACTURE ORIENTATION AND TO HELP IN A MULTICOMPONENT SURVEY DESIGN

摘要

This project describes the implementation of the firstever Vertical Seismic Profile (VSP) in Mexico using compressional (P) and shear (S) wave sources to determine fracture orientation, which is directly related to the reservoir characterization. The relationship between fracture trend and the offset VSP (OVSP) will aid the design of a future multicomponent surface seismic survey (three-component/three dimensional [3C-3D]) and its calibration (PS, PP, and SS images). A dual-source technique allowed analyzing shear waves in an anisotropic medium and identifying the fast shear direction related to the maximum stress orientation in a vertical fracture system. Between the top and bottom VSP levels, a change in the stress orientation was observed, from N48°E to N54°E. These values suggest a new fracture trend direction in this area of the Cuitlahuac field. Before this job was made, the fracture trend was considered parallel to the regional orientation. With the shear zero-offset VSP (S-ZVSP) and all the services that were run, it was found a different orientation. This information will help to use the technique for well designing and reservoir characterization. The VSPs were acquired in Cuitlahuac-832 well belonging to Burgos basin, with objectives in the tertiary sandstone of the Vicksburg formation. Pre-job planning consisted of velocity model building and ray tracing of different source positions to determine the optimal acquisition parameters. From this analysis, an acquisition was planned that better covered the objectives of this work. Finally the acquisition consisted of: 1 ZVSP, 1 OVSP (Offset VSP) with P-wave source, and 2 ZVSP (Zero Offset VSP) using shear wave source (in-line and crossline mode). A very good correlation between P and Ps waves was obtained in the well location. Also, both images showed higher resolution than the surface seismic data. The acquisition of these VSPs with different geometry and different source types (P and S waves) allows optimizing the design of parameters for future acquisitions in surface seismic, helping to reduce costs for the seismic crew and better quality of collected data. Based on the VSP results, the operators of this field are considering the feasibility of a 3D-3C seismic survey proposed to map lateral variations in the seismic anisotropy to gain better insight into the influence of fracturing of production at the Cuitlahuac field. The S-ZVSP results were integrated with borehole images and sonic tools to get a better reservoir characterization, because the VSP yielded more information away from the borehole than the other tools.