This paper presents a new structural model for the North Kuwait Carbonate fields as well as its implications in term of fracture modelling and field development. It also describes a workflow which can be used as foundation for further fracture modelling study at production and exploration scales alike. This workflow consists of a four step approach: 1) elaboration of a regional structural model, 2) creation of 3D conceptual fracture diagrams, 3) elaboration of constraints capturing the key elements of the conceptual diagrams and 4) creation of fracture model properties for further dynamic simulation. The application of this workflow resulted in the creation of a series of fracture models for the North Kuwait Carbonates fields. During the first step of the study, a new structural model has been elaborated based on key kinematic observations from well and seismic data, as well as experimental and field analogues which have been linked to the known regional phases of deformation. These main phases of deformation are 1) post Triassic rifting, 2) Alpine 1-late Cretaceous transtension and 3) Alpine 2-Mid Tertiary compression related to the Zagros formation, which has the greatest impact on the formation of the pre-Gotnia structures and fracture development. The major difference between the new model and previous structural thinking is that the formation of the compressional folds in the Carbonate fields (an event that shaped the current outline of the fields) has happened during the Tertiary time instead of Jurassic time. The proposed structural evolution has been used to define characteristic structural domains. These structural domains have defined a foundation to elaborate conceptual fracture diagram to support fracture modelling study work. The fracture conceptual models have potential implications on fracture development and preferred direction of horizontal and deviated wells. Greater fracture connectivity is expected in compressional ridges developed in Tertiary time, while in the area between the compressional ridges, less dense fractures and probably more cemented fractures (likely to have developed before hydrocarbon emplacement) are expected. The new view on the timing of the structural development (i.e., late uplift of compressional ridges regionally) also has possible implications on maturation/charge history as well as reservoir properties development. The new proposed model for structural evolution is now being used as a foundation for appraisal and fracture modelling activities of the pre-Gotnia carbonate reservoirs. A fracture characterisation study integrating all available static and dynamic data is ongoing.
The North Kuwait Carbonate (NKJG) reservoirs are currently under development by KOC (Kuwait Oil Company). The appraisal and development of the NKJG offer challenges such as lateral variations in reservoir quality, tight to very tight reservoirs and natural fracturing to a varying degree spatially. The presence of open, connected fractures is one of the key elements to achieve a successful development. Also, the presence of fracture corridors increase the risk associated with drilling. Numerous fracture modelling studies have been supporting both appraisal and development strategies of the fields. This paper illustrates how small scale detailed DFN (Discrete Fracture Network) can support the planning and drilling activities of future appraisal wells. A series of detailed DFN models has been built around existing wells. The DFN models are based on a thorough structural understanding, detailed fracture characterization using bore-hole image (BHI) and core data around the wells of interests. In addition to the fracture characterization work, mechanical stratigraphy has been elaborated using E-facies and geomechanical logs. Fracture connectivity analysis has been carried out to calibrate the DFNs to the static and dynamic well data. Scenarios of DFN models can now be used to communicate with drilling in order to illustrate the potential fracture corridors distribution in the sub-surface.
In this paper we show how to extend seismic-driven earth model building into the domain of geomechanics and drilling. A mechanical earth model (MEM) is a quantitative description of rock mechanical properties and in-situ stresses in the subsurface. Formation strength and in-situ stress are key components that impact well design. Most mechanical earth models, even today, are one-dimensional (1D), based on well and drilling data alone. The concept of using seismically derived horizons and velocities to extend the MEM into 3D space was introduced a few years ago. Very recently, a few authors have demonstrated the power of seismic inversion to improve the resolution and quality of a 3D MEM. We present a case-study from Kuwait (Sabriyah field) where a 3D geomechanical model was built using a combination of wellbore geomechanics, geologic structure, and seismic inversion-derived lithofacies and elastic properties. We show critical challenges facing seismic-based geomechanical model-building, demonstrate current solutions, and discuss future strategies.
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