Quantification of Oil Stain and Residual Oil Saturations (ROS) Within Cored Injector and Breakthrough Wells in a Giant Offshore Field, Abu Dhabi
Authors: Edwards Ewart , Al Ali Abdulla , Lyons Ronald , Dodge Scott , Wischer Stephanie 
Published: October 2011

Investigation of sweep efficiency and water movement within both relatively homogeneous and heterogeneous bedded carbonate reservoirs of Early Cretaceous age in a large field offshore Abu Dhabi, can be directly quantified by means of residual oil analytical methods in wells both cored after significant volumes of water have been injected and after water breakthrough has occurred. These results can be cross-checked against petrophysical logs flagged to show either water encroachment over time or specific water movement layers. To supplement these results observations of oil distribution in core from other wells provides a qualitative data set that can be used comparatively, utilizing a population of already cored wells from similar specific settings.

By developing an innovative means for logging oil stain in cored wells, the scheme presented robustly captures details of the intensity of staining (percentage), the general type of hydrocarbons present (e.g. light oil, bitumens, etc.), paleo-fluid flow behaviour (potential fairway paths and small-scale fluid flow heterogeneities during charge) and recent development-effected changes (heterogeneously flushed zones). This paper concentrates on the application of such a scheme to a water-flood setting and how the oil-stain seen in many cored wells can be used to better understand the displacement mechanism and the details of specific high-permeability and low-permeability heterogeneities, including reservoir layers and fractures, under production.

Results of analytical determination of ROS through Dean-Stark analyses display good affinity with the qualitative logged oil stain intensity and type observed from the core and seen on UV light photographs. The results show the following patterns: ‘Piston-like’ displacement of oil occurs at short distances (10's metres) away from injectors, irrespective of the reservoir heterogeneity;With limited injection, or at greater distances (10's-100's metres), the more heterogeneous reservoirs can display differential sweep, manifest as patterns of increased residual oil or flushed rock related to permeability contrast between different layers and small-scale phenomena such as burrows, fractures and cemented nodules;At greater distances from injectors (100's metres/ up to 1.5km), as seen in cored breakthrough wells, the oilstain patterns display more specific layer-based differential movement. Also observed is small-scale fluid flushing of specific high permeability features or retention of oil within other adjacent lower-permeability features or layers.Comparison of water breakthrough flags and computed water saturations in logs from these cored wells displays excellent agreement with qualitative oil stain observations. Specific oilstain patterns display close relationships with layer heterogeneities, core poro-perm data, breakthrough zones, at varying scales from 10cm to 10m.

The value of the approach adopted is that it provides a means of deriving observations from a suite of wells for which there is no analytical data, thus increasing the relevant data-base to be used on subsurface models on which layers and features facilitate sweep and water movement in the reservoirs.