Innovative Technique of Reservoir Modeling in a Heterogeneous Carbonate Reservoir: Mauddud Formation in Raudhatain Field, North Kuwait

Al-Ajmi, Hussain Zayed, Saikh Abdul Azim, Shehab A.M. Abdullah, Abdul Aziz Al-Duwaihi, Kuwait Oil Company, Ahmadi, Kuwait

The Mauddud Formation is a giant hydrocarbon reservoir in North Kuwait over two domal faulted anticlines. Reservoir description and distribution of rock properties are chal­lenging due to inherent reservoir heterogeneity resulting from diagenetic imprints on origi­nal depositional fabric. A robust depositional model driven by sequence stratigraphy, petro­physics tuned to dynamic data and innovative static modeling techniques were used to char­acterize this complex reservoir. In an overall low angle ramp setting, the carbonates were deposited in a transgressive systems tract followed by a highstand systems tract. Primary rock fabric controls the best flow characteristics in high-energy inner ramp grainstone. Reservoir quality deteriorates in mid ramp to inner ramp wackestones and mudstones due to diagenetic carbonate concretions. Main thief zones in the reservoir are in floatstones, radially fractured concretions and small-scale fractures in low-porosity brittle rocks. A deter­ministic permeability model was developed integrating core data with openhole logs, pro­duction logs, and pressure transient analysis. Logs have been reprocessed to identify zones of secondary porosity and fracture–prone zones. Separate Porosity-Permeability transforms for matrix properties and fracture-prone intervals results in log-derived permeability profiles that match production log profiles and well test Kh estimates. A finescale Geological model using major flooding surfaces captures the primary depositional units. The lithofacies asso­ciations have been modeled as composite objects restricted to facies belts. As porosity was observed to be decreasing towards the flank, trend modeling was used to model the effec­tive porosity. The small-scale heterogeneity caused by vuggy zones and fractures were cap­tured in thin layers. Vugs and fractures have been modeled as objects restricted within an area demarcated by poorer seismic coherence. The Matrix permeability was enhanced by vuggy permeability and fracture permeability. The paper describes the challenges in reser­voir description and static modeling of this complex reservoir in detail.