Tertiary to Cretaceous age karsts and collapse disturbances were observed from recent 3D seismic interpretation of an offshore Abu Dhabi oil-field. The seismic evidence of karst features was investigated using full-stack, spectral whitened, and discontinuity volumes. In addition, circular features were detected at specific sequence boundaries after examining curvature maps, disturbed amplitudes, velocity effects, and attenuation attributes. Several karsts and collapse disturbances tend to be associated with anhydrite beds overlaying thick carbonate intervals and seem to be limited to the Tertiary stratigraphic column; other karsts were observed to be limited to Cretaceous dolomite and limestone reservoir intervals. The Tertiary age karsts were observed to cause seismic image and amplitude disturbance at various depths, whereas the Cretaceous age karsts tend to be limited in radius and depth and have more limited effect on seismic response.In the attempt to better understand the limit of some karsts and collapse disturbances, well data (wireline log, conventional core, and thin-section) were investigated within the karst areas and integrated with 3D seismic data. 3D seismic based geometries and attributes were analyzed to evaluate the possibility of detecting the damage-zone limit versus velocity effect with depth due to fill material. Another effect investigated is that of fracture-fault zone distribution relative to karst localization.Analogue karst features of a similar age-range (Tertiary and younger) to the seismic examples occur within Jebel Hafit in the onshore UAE, where solution effects, debris fill, mineralization, and collapse effects can be observed and compared with the offshore examples. Here, structural discontinuities also enhance the karstic features.

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.

The ability to identify subtle structural and paleo geomorphological features within carbonates may significantly reduce wide range of drilling challenges including stuck pipe, breakouts, casing failure, lost circulation, hole collapse, washouts, etc. Current study emphasis on integrated approach to discuss drilling challenges, their possible mitigations and subsurface risk assessment. This study is a multi-disciplinary application of integrated borehole and advanced seismic attributes to identify hazards, mainly linked with drilling problems. In this regards, paleo structure mapping is initially utilized to broadly outline areas in the field that may be associated with potential karst collapse features development. These karst collapse features are mainly responsible for dynamic mud losses and may also cause other problems (e.g., casing failure, etc.). One of the main challenges observed to confidently characterize karst collapse and other subtle features is that seismic data contains a lot of overburden noise hence an improved approach is proposed to eliminate seismic noise and enhance S/N ratio. Advanced seismic attribute volumes are then produced to capture full field lateral as well as vertical distribution of subtle features. Integrated results are calibrated with borehole image, core, logs, PLT, mud losses, Hall plot and CUM injection in vicinity. A novel approach is introduced to calculate unmonitored injection area of influence and other associated subsurface risks assessment. An improved Integrated approach used in this paper shows promising results. Full field distribution of karst collapse features is outlined confidently and results are validated with dynamic data and diffraction seismic imaging. Vertical extent of karstified zone is clearly captured in cross sectional view, which ties with petrophysical zonation, PLT, core and dynamic mud losses. Furthermore, the novel approach of Injection area of influence calculation improves subsurface understanding and highlight other challenges e.g. pressure anomalies or future disposal wells planning, etc. Two examples are presented in this paper for reference. An approach defined in this paper improves confidence in subsurface hazards identification, well plan optimization and minimizing unexpected costs associated with mud losses, sidetracks, etc.