Application of the Vertical Seismic Profile to the Piper Field

Abstract

Summary The technique of vertical seismic profiles (VSP) now is being used as part of a successful approach to optimal development of a major North Sea oil field. The information obtained is applied both in the deconvolution of surface seismic data and in the direct delineation of subsurface structure. Introduction This paper discusses the cost-effective acquisition, processing, and application of VSP data in attempts to processing, and application of VSP data in attempts to improve the siting of wells in the highly permeable, faulted, and layered Piper reservoir. In particular, the technique was used in Well P35 to define the location of a fault not determined precisely by other means. The well was redrilled successfully and subsequently produced about 6.5 × 10 3 STB/D [1 × 10(3) stock tank m'/d] produced about 6.5 × 10 3 STB/D [1 × 10(3) stock tank m'/d] dry oil from a selective interval. In Well P38 the VSP was used to deconvolve surface seismic data and demonstrated the possible existence of an appreciable extension of the reservoir to the northeast of Block Ill toward the main Piper field. Fig. I shows the location of Piper wells in which VSP's have been run with respect to the major fault trends. Theory A VSP is the record obtained from a conventional surface geophone with an energy source on the surface that is lowered into a well. A standard velocity or "checkshot" survey is extended by decreasing the distance between successive downhole geophone positions and recording for several seconds the seismic data that occur after the arrival of the direct wave. The principles of the VSP and the separation of down and upward waves were first covered by Gal'perin et al. from the USSR. For the purpose of seismic deconvolution work a brief summary of the theory of the VSP is given, but Refs. 2 and 3 give more detailed accounts. It can be seen on the VSP section shown in Fig. 2a that the first arrivals define the time-depth curve as in a classical velocity log, but the continuity is improved greatly because of the small interval between traces. After the first arrivals, two distinct types of reflections can be noticed (Fig. 2b):arrivals parallel to the first breaks, A, which correspond to waves that reach the geophone from above-i.e., downward energy, andarrivals with a gradient, B, which have approximately horizontal alignment and correspond to waves that reach the geophone from below-i.e., upward energy. The downward energy consists of direct first arrivals and waves reflected an even number of times by horizons above the geophone (Fig. 3a). Then by subtracting the time corresponding to the first break from each of the traces the first arrivals and multiples appear coherently (see Fig. 3b). However, since the reflection coefficient of the seabed is usually appreciably higher than those of subsequent horizons, the strongest multiple pattern normally can be identified as resulting from this interface. Similarly, the upward energy arrivals can be defined as corresponding to reflections from horizons that lie below the geophone position, as shown in Fig. 4a. By suitable processing and addition of the time corresponding to the first break to the reflection travel time, the equivalent of the two-way time of the conventional reflection seismic record is obtained (see Fig. 4b). This demonstrates the power of the VSP to separate the up- and downward wave fields. The latter can bused to design precise deconvolution operators for use on the VSP and surface seismic data to separate out the true events from the reverberant system. A typical VSP processing flowchart is shown in Fig. 5. processing flowchart is shown in Fig. 5. Deconvolution of Surface Seismic Data By Using the VSP The autoconvolution method of VSP deconvolution of surface seismic data was used in this study. This procedure involved convolving the surface seismic traces procedure involved convolving the surface seismic traces with the inverse of the autoconvolution of the VSP downwave. The latter is extracted from the normal processing route (in Fig. 5). processing route (in Fig. 5). This step replaces the normal deconvolution before stacking. Apart from this one substitution, the processing route for marine seismic data is unchanged (Fig. 6). processing route for marine seismic data is unchanged (Fig. 6). Since the VSP wave field is derived from a data source within the earth, a more accurate estimate of the multiple wave train is obtained compared with the conventional approach. Hence it is generally possible to examine a broader spectrum of frequencies within the processed sections because of better attenuation of multiples and decreased processing noise. This leads to greater fidelity in the final seismic sections and, consequently, greater confidence in their interpretation. JPT P. 1517