Mini 3D for shallow gas reconnaissance

Abstract

Abstract The Mini 3D project was undertaken by TOTAL and ELF with the support of CEPM (Comite d'Etudes Petrolieres et Marines) to define an economical method of obtaining 3D seismic HR data for shallow gas assessment. An experimental 3D survey was carried out with classical site survey techniques in the North Sea. From these data 19 simulations, were produced to compare different acquisition geometries ranging from dual, 600 m long cables to a single receiver. Results show that short offset, low fold and very simple streamer positioning are sufficient to give a reliable 3D image of gas charged bodies, The 3D data allow a much more accurate risk delineation than 2D HR data. Moreover on financial grounds Mini-3D is comparable in cost to a classical HR 2D survey. In view of these results, such HR 3D should now be the standard for shallow gas surveying. Introduction Advantages of the 3D seismic method compared to classical 2D for oil and gas exploration are obvious, particularly in applications requiring high spatial resolution. However, exploration 3D puts heavy constraints on data acquisition and processing. Costs are consequently high. High resolution 3D seismic investigation is not commonly used so for such purposes mainly because cost reasons in spite of the technical advantages. The objective of the Mini 3D project was to investigate economic HR 3D seismic for shallow gas risk assessment. The strategy was to try to define the simplest data acquisition configuration capable of 3D visualization of hazardous shallow gas that is consistent with the equipment currently used in the site survey industry. The work progressed as follows : The first step was to acquire a full, state of the art HR 3D seismic and to process the data with full processing sequence. The subsequent 3D volume was by definition the reference. Further steps were taken to simulate reduced acquisition geometries by means of successive decimations. The results were then compared to the reference volume Data acquisition Site selection To be representative, to possess the following attributes:proven shallow gas occurrencesprevious sumying for shallow gaswell data in the immediate vicinity the experimental site hadlocation in an active oil and gas producing region Such a site was found in the British sector of the North sea, in block 22/7 operated by TOTAL. Acquisition equipment and recording parameters The objective was to acquire a HR 3D volume corresponding to a drilling site making use, to the greatest extent possible, of standard 2D HR equipment including : vessel, seismic source, and streamer. The desired highest HR geometry was a 6,25 × 12.5 m bin size to be acquired along lines 12.5 m apart.