Utilize Joint Seismic VVAz-AVAz Inversion to Better Understand, Delineate and Develop Carbonate Reservoirs with Fractured Area in Middle East, Offshore Abu Dhabi
Authors: Alderei M. H. , KHEIDRI L. H. , Alkaabi A. S. , Kirstetter O. H. , Michou L. , AlJaberi F. A. , Agrawal P. R. , Goyal P. A. , Alhashmi A. H. 
Published: November 2024

Lower Cretaceous reservoirs are one of the main reservoirs in the UAE petroleum system. They are very well developed Onshore/Offshore where it is producing in different fields. In our Offshore field, the Lower Cretaceous reservoirs have always been considered as marginal reservoirs because of their low permeability. Hence historically, they have only been developed in the crestal part of the field where they are known to be fractured.

In accordance with the ADNOC objective to reach oil production target by 2027. A specific development plan was set for this low permeability reservoir in our field. This later consists in the development of the flank areas targeting the non-fractured area with a line drive, avoiding the fractured crestal part of the field. In a line drive pattern, an injection wells are in a straight line parallel to the production wells. Consequently, the mapping of the fractures area is becoming fundamental for the realization of this development plan.

In the past, several attempts to map the fractured area were made by different companies with small success as only the limit of the fractured area was defined with high uncertainty. Currently, the main technologies used in the industry for azimuthal anisotropy characterization are Velocity Variation with Azimuth (VVAz) and Amplitude Variation with Azimuth (AVAz). These two technologies have their respective strengths and limitations and are often used separately to predict reservoir fracture networks. In our field, a joint azimuthal velocity and amplitude inversion workflow is used on a high density WAZ & Broadband OBC 3D seismic survey. This workflow has enabled us to generate attribute maps linked to the fracture orientation and their intensity, hence it helped in the mapping of the fractured area of the Lower Cretaceous reservoirs.

The use of these attribute maps has helped in the interpretation and integration of the dynamic data, mainly the well test results and their spatial distribution along with the injected gas movement. From PTA analysis, the result shows an enhancement in the permeabilities that are within the AVAz/VVAz polygon. Several RMS maps were extracted from AVAz joint inversion DT on R1, R2, and R3. The result shows good response with the wells that have P32 conductive logs. Lastly, the drilling losses map of R2 was generated and was overlayed with the RMS map extracted from AVAz joint inversion DT on R2. These maps show high losses in the areas that have high DT values.

The use of these maps has also allowed us to identify more heterogeneities in the crestal area of the field, mainly NE-SW fractured corridors not seen on the conventional seismic.

Accordingly, these maps were integrated in the building of the dynamic model of Lower Cretaceous reservoirs and were used during the history match process. This model was the main tool for the definition of the new development plan of these reservoirs. Finally, these maps were used in the well planning in terms of well position and well length.

In the future, these maps will be optimized using the results of the newly drilled wells and then will be re-incorporated in the update of the dynamic model and thus the optimization of the development plan. As this methodology is showing very good results in Lower Cretaceous reservoirs it might be applied to other reservoirs of our field.