No abstract

No abstract

No abstract

In this paper a workflow that integrates the most advanced tools for seismic data processing and imaging to overcome limits imposed by poor seismic data in the definition of reservoir geometries with the details needed to properly locate the appraisal wells is presented. The area of study is the north-western coast of Australia. Here the reservoir, Middle Jurassic sandstones, is affected by a very poor signal to noise ratio due to poor energy penetration and illumination associated with reefs, faults and a high level of multiples content that completely masks the target area. In the following, we will describe our approach to depth imaging with robust velocity analysis, effective attenuation of multiples, choise of most suitable migration algorithm and constant interaction between geological and geophysical expertise. The application of this integrated approach was the added value of the project and the main driver for guiding the interpretation of the reservoir, providing a strong support for well planning.

The accurate prediction of the depth of top carbonate while drilling carbonate reservoirs is important to avoid losses and well control problems. This can have a significant financial impact. This paper demonstrates the use of innovative look-ahead VSP technology in Vietnam to refine depth prediction of the top of carbonate ahead of the bit for accurate geo-stopping. An offshore exploration well was being drilled in an area with drilling challenges. This was the first well in the field and lacked offset well velocity information, so there was a high degree of depth uncertainty to the top of the carbonate target from surface seismic interpretations, making drilling operations risky. An intermediate VSP was requested to look for reflections ahead of the intermediate TD using Schlumberger VSI1 tool. The VSP data was acquired in open and cased hole using an air gun cluster as the seismic source. The drilling program required the casing to be set immediately above the top of the carbonate. By acquiring an intermediate VSP, the depth uncertainty was reduced and the target depth prediction was refined. This was achieved using the look-ahead VSP technique, from which the acoustic impedance and velocity ahead of the bit was estimated. This was combined with time-depth from the VSP data over the logged interval to help refine the predicted depth to the top of the carbonate. This was used to optimize the depth at which the casing was set. The error range for the top carbonate depth from the surface seismic interpretation was +/−50m. The final prediction depth from the VSP interpretation is 6m shallower than the actual carbonate reservoir. Guided by the inverted acoustic impedance log and velocity ahead of the bit from look-ahead VSP, a further 100m was safely drilled before setting the casing. The casing point was set successfully without penetrating the carbonate. The intermediate VSP results helped to decide drilling ahead in a safe manner with confidence, after setting the casing before penetrating the carbonate target.