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Heterogeneity of the Ara Group Intrasalt Reservoirs: Implications for Development by Miscible Gas Injection

Tull, Simon *1; Oraimi, Mahmood 1; Anneke, Kleinpenning 1
(1) PDO, Muscat, Oman.

The Ara Group intrasalt reservoirs of the South Oman Salt Basin are characterized by a high degree of reservoir heterogeneity at the core plug, well and inter-well scales. Plug data exhibit Dykstra-Parsons coefficients in the range 0.85 to 0.97. Vertical inflow profiles constructed from plug and PLT data indicate that in many cases, as little as 10-20 percent of the reservoir thickness actually contributes fluid flow into the well. Large variations in well-test permeability and Productivity Index are observed between adjacent wells. These variations in permeability are the result of the complex interplay of a range of diagenetic and depositional processes. High permeability is particularly associated with sub-unconfomity dolomitised, vuggy microbial limestones. Tight intervals are variously associated with non-microbial depositional facies, calcite cementation, the emplacement of reservoir bitumen and salt/anhydrite plugging. In some cases, reservoir degradation or salt cementation halos around faults have resulted in compartmentalization, adding an additional degree of complexity.

An extensive appraisal drilling and data gathering campaign has contributed greatly to the understanding of these reservoirs. Appraisal drilling and primary depletion has greatly reduced volumetric uncertainty. Pressure build-up, PLT and core data have improved our understanding of permeability distribution.

This paper will demonstrate how the integration of these data has guided the planned development of the Ara reservoirs by miscible gas injection. Although key aspects of the behaviour of individual wells, such as the vertical distribution of permeability, are understood, others - particularly patterns of sweep and gas breakthrough will remain unknown until after miscible gas injection commences. A programme of future data gathering, surveillance and reservoir management activities has been planned accordingly and a strategy for the incorporation of these data is in place.

 

AAPG Search and Discovery Article #90141©2012, GEO-2012, 10th Middle East Geosciences Conference and Exhibition, 4-7 March 2012, Manama, Bahrain