Abstract

Conventional offshore 3D acquisition is still being performed mainly with narrow azimuth streamer configurations, even in structurally complex areas. Attempt at breaking this paradigm have been recently made by the industry through the successful acquisition of some unconventional "Multi-Azimuth" (MAZ), "Wide-Azimith" (WAZ) and "Rich Azimuth" (RAZ) marine surveys. Eni Indonesia and WestenGeco conducted "the first of its kind" full 3D Circular Shooting survey (Coil) over the Tulip Discovery in Indonesia between August and September 2008. Circular Shooting is a technique developed to acquire a full azimuth dataset with a single vessel towed streamer operation: this is achieved sailing in circles. The circular geometry introduces several differences and a number of new challenges in the whole modeling, acquisition and processing workflow. This paper describes the experience made on the Tulip discovery, focusing primarily on the acquisition planning, deployment efficiency, special QC phases and preliminary processing results. This paper demonstrates also that despite the new technical challenges inherent to circular shooting, it was possible to achieve since the "fast track" volume results, a superior imaging of the Tulip discovery, comparing with the conventional techniques. The improved imaging led interpreters to place with confidence a new appraisal well.

Introduction

Tulip is a complex structural feature located in the Bukat Block offshore Kalimantan (Indonesia). The water depth is varying and ranges from 350 to 1800 meters. Several unfavourable geological conditions cause a very poor seismic response in the whole area. The presence of methane hydrates represents the main problem; this is indicated by the BSR reflection covering the whole structure. The hydrates layer generates in this area a very reflective sea bottom that causes the presence of up to seven multiple bounces. The presence of shallow free gas is suggested by an abrupt frequency-amplitude dimming below the BSR. The well drilled on the structure indicates a Q factor of less than 70. Moreover the rough geomorphology of the water bottom, characterized by canyons and irregularities causes a non homogeneous behaviour of the illumination in the subsurface and a complex 3D raypath for surface and internal multiples. Complex deep geology, a steeply dipping thrust with 2 culminations, represents a third critical factor, posing severe structural illumination doubts as well. Last but not least challenge is the low reflectivity of the target sequence, often falling below the noise level. Due to the mentioned factors, the vintage seismic in the area is of very poor quality (Fig.1). In order to get a better image of the subsurface for the appraisal campaign, it was decided to design a new seismic survey which would adequately address the geophysical problems described above. Conventional towed streamer marine acquisition generates a dataset with a limited azimuth range, of the order of 10o. In areas of complex geology, this limited azimuth collection, can result in very poor illumination of the reservoir by the seismic wave-field, due to bending and scattering of the ray-paths in the over-burden. Over the last five years, efforts to mitigate this poor illumination effect, with marine towed streamer surveys, have focused on acquisition solutions to generate seismic datasets with better azimuth distribution. The efforts included multi-direction and multi-vessels geometries to acquire surveys rich in azimuth. These techniques have proven to provide significant value in complex imaging environments; however they are constrained by the cost and availability of the multi-pass, multi-vessel operations. Circular Shooting [French, 1984; Durrani, 1987] is an acquisition technique where the vessel follows a circular pre-plot line. More circles are repeated in the X and Y directions to build up fold, offset and azimuth distribution. This method allows for full azimuth (FAZ) acquisition using a single vessel, shooting on a continuous turn. Line changes are short, of the order of minutes, resulting in high acquisition uptime and efficiency. The offset and azimuth distribution of Coil Shooting is excellent; as good as and sometimes better than any of the wide, multi azimuths (WAZ/MAZ) techniques.

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