The article considers the main types of vertical seismic profiling (VSP). Vertical profiling, the location of the source located directly at the wellhead location and Offset profiling – the source point is located some distance from the borehole. Various schemes for working out offset vertical seismic profiling (OVSP) are described, which depend on the geological tasks: the source points (SP) located beyond the fault boundaries for their mapping; offset SPs are located in the given direction (1-2 way) to clarify the geological structure; offset SPs are located at the ends of mutually perpendicular ways to study the geological structure for cluster drilling; offset SP with a variable step intervals are located along the horizontal projection of the deviated well; profiling in the azimuth of the deviated well with a constant SPs step for studying the geological structure at large distances; 4D VSP for monitoring the exploitable deposits. The analysis of advantages and disadvantages of the considered techniques for each case is given. The geological and technical prerequisites for using the OVSP in exploratory drilling are listed. Geological preconditions are the small size of structures, the dependence on the structure type (fault-closure or limited by faults), and the «fault shadow» presence, which in turn gives uncertainties in the construction of structures geological model. Technical preconditions are directional drilling, used for exploration fault-closure structures; the need to reduce the matching log data and seismic data errors – as an example, the result of using a standard VSP in a deviated well is presented. To solve the problem of qualitative matching seismic and log data in deviated wells, a special case of OVSP is vertical incidence vertical seismic profiling (VIVSP) is considered. This method is effective for deviated wells with angles > 15-20 degree and bottom hole location offset > 500 m. It is a combination of standard VSP (zero offset) and OVSP. The technique, results, advantages and disadvantages of this method are described in the article. The results of practical application of the VIVSP technique in the deviated well are shown. One of the examples of using the VSP survey modeling is also given, which results in the selection of the optimal method for solving the geological and technical tasks. Recommendations are given on the use of modeling for qualitative work planning and the use of OVSP in exploratory drilling.
References
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