Rock Types Modelling Impact on Field Development and Volumes; A Case Study from Lower Shuaiba Carbonate Reservoir in Field WU
Authors: Mahrouqi Bashara , AL Salehi Ahmed , Aadi AL Khansa , Olivier Pierre , Qasmi Liali , Suwannathatsa Sakharin , Mahrazy Khalfan 
Published: October 2022

Abstract

Objective/Scope
Facies distribution in the static model of carbonate reservoirs provides a framework for the distribution the other petrophysical properties. Lower Shuaiba (LSH) reservoir model in Field WU is aiming to provide improved facies distribution to improve STOIIP calculation and well placement/count.
Lower Shuaiba reservoir is characterised by 5 lithostratigraphic cycles with different facies and rock types. These facies are consistent in the lower zones, however they are more variable in the top layers 1A (in the oil window). This layer is dominated by skeletal packstones, skeletal wackestones and thin beds of Rudist floatstones/rudstones as characterised from core data. These facies are comparable in their log responses and therefore cannot be differentiated with logs alone. However, they have different properties (Por, Perm) which have a great impact on STOIIP and well placement. Moreover, the Rudist-rich rock type is interpreted as an imbibed layer which adds more complexity to the reservoir model.

Methods, Procedures, Process
Two model scenarios were built based on the different facies distribution methods: (1) core-based method, which is limited for few wells and assuming lateral continuity of rock types between the wells; (2) capillary-based method, which uses the Saturation height function to predict the facies. This method uses the idea that facies differ in their capillary entry-height and thus will have different saturation responses. Whenever possible, these were then compared with core data.

Results, Observations, Conclusions
This new capillary-based model improved the volume estimation compared with the layer-cake model because of the improved mapping of the imbibed Rudist facie. The model allows ranking the proposed wells based on their facies distribution to maximise well production and reduce early water breakthrough. It also explains the anomalies in saturation/production (caused by facies distribution) in some of the existing wells.
The study found that the anomalous high peak resistivity is associated with the imbibed Rudist layer (confirmed by log and production test) and should be avoided while drilling the horizontal section. It also highlighted the important of continuous appraisal in the area to add control points for the facies model.

Novel or Additive Information
The paleo Free Water Level (FWL) for Lower Shuaiba formation was determined based on the best fit between a core-based (drainage) saturation-height models and Archie log saturation. The FWL could not be identified on pressure plot due to the large scatter of pressure points which is typical for this low permeability /low mobility reservoir. Other factors such as wettability heterogeneities, fluid fill history and hysteresis makes it difficult to define the oil gradient and hence FWL. This FWL and saturation height function (SHF) were then used to predict the rock types, based on best match iterations between saturation log and saturation calculated by SHF.