Monitoring Areal And Vertical Sweep And Reservoir Pressure In The Ghawar Field Using Multiprobe Wireline Formation Tester
Authors: AL Shahri Ali M. , Al Ubaidan A.A. , Kibsgaard Paal , Kuchuk Fikri 
Published: December 2007

Saudi Aramco's Ghawar Field is a massive carbonate reservoir with sub-zones of varying reservoir quality and has been under flank-water injection. It is a complex reservoir, with thin super permeability layers (10 feet) that are generally stratiform and in some cases fractured, associated with high productivity. Laterally extensive super permeability beds, in good vertical communication with the rest of the oil bearing reservoir, can significantly increase both well productivity and sweep efficiency.

However, isolated super permeability layers can cause early water breakthrough, which adversely affects oil recovery as well as increases the field operational cost. Furthermore, large permeability contrasts can complicate effective drainage of lower porosity zones in the lower part of the reservoir that contains about 35 % of the original oil in place. For this field, pressure and saturation monitoring have been key factors in achieving the overall reservoir management objective of maximizing recovery at the lowest cost.

Saudi Aramco is currently surveying the key new wells drilled behind the flood front using the multi-probe formation tester for obtaining pressure measurements, performing interval tests, and taking fluid samples along the well-bore. The primary objective of the surveys is to establish whether the super permeability beds as well as the lower porosity zones are introducing differential pressure depletion, which will directly impact the field's completion and production strategy. Obtaining fluid samples across the reservoir zones is also a key part of the surveys, to establish water salinity and movable oil fraction in zones with breakthrough where the injection and formation waters are mixed. Determining the fraction of movable oil in the lower porosity zones, where the conventional open-hole log results are uncertain, is very crucial in optimizing recovery. It is also a powerful method to evaluate the sweep in the lower zones matrix where the diffused fractures density is higher and assist the dynamic interaction between the two systems.

The on-going Uhawar Field monitoring has shown that there is good vertical communication in the higher quality zones, at the top of the reservoir and embody the super- permeability thin beds. However, local and reservoir scale barriers as well as differential depletion has been observed towards the base of the reservoir. These barriers have resulted in poor sweep efficiency with zones containing bypassed oil. These zones are now being targeted by dedicated dual vertical-horizontal completions.

In this paper, we show that the pressure and saturation monitoring integrated with other dynamic and geological data contribute immensely to obtain the best completion for optimizing oil production and recovery. Furthermore, we also show that in areas of good vertical communication, super permeability is advantageous to the field development, due to its high productivity and large drainage area exposure.

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