Abstract

Temporal and lateral resolution of seismic data is principally determined by the shape and duration of the propagating wavelet. Its accurate estimation and the ability to change its shape are both of paramount importance to achieve the maximum resolution afforded by the available bandwidth.

A proprietary space adaptive wavelet processing (SAWP) system is described herein. SAWP is an integrated multi-line, multi-well wavelet estimation and shaping system. It involves borehole analysis, extraction of wavelets at well locations and performs implicit spectral inversion to condition the seismic pulse to a common, broad band, zero phase wavelet.

SAWP was applied to a dataset consisting of well and 3D surface seismic data from the Heidrun field, located offshore Norway, operated by Conoco Norway Inc. and it consisted of well and 3D seismic data. The objective here was to increase the resolution and fidelity of the seismic data to minimise the risk associated with the interpretation of geologic units, faulting and truncation boundaries. This interpretation was to be used by Conoco to assist the selection of locations for the pre-drilling phase of the Heidrun field.

The field will be developed with a Tension Leg Platform (TLP) which allows the pre-drilling of wells prior to platform installation. The reason behind the selection of the Heidrun pre-drill well locations was to minimise the risk associated with geological and reservoir conditions that could affect the recovery and economic return from the field.

Analysis of the existing 3D seismic data covering the field showed poor correlations between the well synthetics and surface seismic. In addition, the seismic data had a low frequency content with a dominant frequency, over the reservoir interval, of approximately 20 Hz.

Wavelets extracted at the well locations from the seismic data showed that the embedded wavelet within the seismic to be non-zero phase and spatially unstable.

Further analysis of the 3D seismic data after SAWP proved that the bandwidth of the data had been increased from approximately 10-40 Hz to 8-60 Hz at the threshold level of -20 db from the maximum response. The ties between the well synthetics and surface seismic were improved and the extracted wavelets at the well locations were close to zero phase and spatially stable.

Space adaptive wavelet processing was proven to be an effective method of improving the temporal and spatial resolutions of the 3D seismic and thereby minimising the geologic risk associated with the interpretation of the data.

The SAWP 3D seismic was interpreted, converted to grid data and contoured. From these maps, locations for the pre-drill wells were selected.

At the time of writing two wells of the Heidrun pre-drilling phase have been completed. The results from the analysis of borehole data, indicate that the prognosis for both wells is within the expected resolution from the wavelet processed seismic.