Developing a Fractured Reservoir Concept in an Unfractured Carbonate Reservoir, Offshore U.A.E
Authors: Sellar Christopher , Mohamed Hassan Mostafa , Contreras Perez David Rafael 
Published: October 2022

Reservoir 2 in Oilfield A shows strong evidence of variable chemical compaction. The south of Reservoir 2 is up to 20% thinner than the north with 50% lower average porosity. Stylolites are more abundant in the south than the north. Fractures are observed in multiple data types associated with stylolites. A stratigraphically-constrained, fractured reservoir concept is essential to understand the higher-than-predicted water cut of production wells on the southern flank of the structure.

High quality core is routinely taken in appraisal wells in Oilfield A. A detailed core description was undertaken including recording the precise depth and amplitude of chemical compaction features including stylolites, their associated fractures and their diagenetic cement fill. Core based observations were calibrated to wireline wellbore images (WBI) and from there to logging while drilling (LWD) WBI in horizontal development wells. These data were integrated with information from production logging tool (PLT) runs. As a result it was possible to build a fractured reservoir concept, vertically and laterally constrained by static data and conditioned by dynamic data.

In the south of Reservoir 2, Oilfield A, open or partially open Mode 1 fractures are often observed from core observation propagating 5-15cm above and below abundant stylolites. The more compacted, thinner reservoir in the south is also more cemented, more brittle and therefore more susceptible to fracturing than the north.

As such, core provides a 1D view of the reservoir. The key uncertainty in developing the fracture concept, is to understand the lateral extent and connectivity of such features. WBI interpretation of stylolite-related fracturing was essential to understand their abundance and orientation in 3D. The connectivity of these features is inferred when combined with PLT and well production data.

Core-scale observation, combined with the WBI fracture dataset, was upscaled to the 3D seismic dataset. Acoustic impedance from 3D seismic shows a strong negative correlation with reservoir thickness and porosity. Since stylolite-related fractures are most abundant in the thinnest, lowest porosity part of the reservoir, fractures could be vertically distributed within the reservoir by WBI and laterally distributed by seismic (acoustic impedance) response. Integration of this concept in the dynamic model resulted in a better history match of water cut behaviour in production wells on the southern flank of the structure.

Traditionally the role of stylolites in oil reservoirs has focused on their impact reducing permeability and baffling transmissibility, not on increasing them. All oil reservoirs are fractured to a greater or lesser extent and traditionally more focus has been placed on tectonic fractures. Highlighting the role that short, bed bound, stylolite-related fractures play in enhancing permeability is essential in understanding their impact on fluid movement within carbonate reservoirs.