One of the key issues in creating a good reservoir model in carbonate reservoirs is identifying the horizontal permeability conduits--"thief zones"--if there are any. In the Sabriyah field in Kuwait, dynamic measurements showed evidence of thief zones in the Lower Cretaceous (Albian) Mauddud formation. Early water breakthrough has occurred in some wells. Previous studies indicated that it was very challenging to detect the thinly layered thief zones using conventional openhole logs. This paper describes a method of recognizing the different types of thief zones in the Mauddud carbonate reservoirs using high-resolution image logs with calibration from core and dynamic measurements and by integrating image logs with nuclear magnetic resonance (NMR) and conventional openhole logs.
The Mauddud carbonates are Early Albian in age and consist of grainstones, wackstones, and mudstones deposited in a ramp setting. Observations from production logging tools (PLTs) and production data indicated that there are a few thief zones in different levels within the vertical Mauddud sequence. A previous core study shows that the fractures in the Mauddud formation are short (<10 cm) and concentrated in diagenetically cemented layers. The fractured thin layers are believed to be the principal type of thief zone. Another type of thief zone is associated with better-developed vuggy porosity. This study shows that both fractured and vuggy porosity-related types of thief zones can potentially be detected through integration of high-resolution image logs with PLT, NMR, and conventional logs. In addition, methods of estimating fracture permeability and porosity-related permeability based on logs are also proposed. The log-estimated permeability determined using this approach fits better with the production profile and can then be used to evaluate the thief zones in a more quantitative manner.