Full Field Karst Features Characterization Using Reflection Seismic Attributes and Calibration with Diffraction Seismic in a Giant Offshore Carbonate Field, UAE

Abstract

Identifying key geological features (e.g. karsts, faults, fractures) are extremely important for well planning and mitigating drilling hazards. To identify and characterize these features, a high-resolution diffraction study was conducted in a pilot area which successfully detected and mapped subtle geological features. Subsequently, existence of these karst/cave like features in the field also assist in disposing produced water as two produced water disposal wells were successfully drilled. The field is expected to double water production in next 10 years therefore targeting these karst features is expected to improve disposal well performance Application of diffraction imaging in a pilot area distinctively improved spatial resolution in order to enhance the detections of the edges and confirmed high-resolution subsurface imagining compared to conventional reflection seismic. However, extending diffraction study to full field is costly, computationally extensive and require unique skill set. Hence, an attempt was made to exploit conventional seismic reflection data utilizing advanced seismic attributes (e.g. spectral decomposition) to generate high-resolution subsurface image comparable to diffraction images. Appropriate frequencies were carefully chosen and color-blended to produce such high-resolution image that confidently enhance karst collapse features distribution in the full field. Wells drilled within karsts collapse features identified from diffraction data shows two-fold water disposal increment compared to non-karst areas. Dynamic data (e.g. mud losses & PLT) supports result from diffraction seismic in the pilot area. Subsequent full field feasibility study was conducted exploiting available conventional reflection seismic data to produce similar high-resolution subsurface image. With optimum parameters selection in spectral decomposition, subsurface color-blended image produced using reflection seismic shows good correlation with diffraction seismic results within pilot area and improves confidence in imaging outside the pilot area. Results are successfully calibrated with diffraction seismic and other available dynamic data. Distribution of full field Karst collapse features are outlined confidently, which are helpful to improve future well planning and mitigating drilling hazards. One of the main challenges in the carbonate environment is to map karsts collapse and other subtle features due to their complex shapes and lack of resolution in post stack seismic. No doubt cost inhibited diffraction seismic study produced high-resolution subsurface image however, it was just an application in a small pilot area within a giant field. Integrated workflow presented in this paper save not only outsourcing cost but also produce similar results by utilizing available conventional reflection seismic data.