Characterization of Karst Development using an Integrated Workflow in an Upper Cretaceous Carbonate Reservoir from Onshore Field, United Arab Emirates
Authors: Farooq Umer , Meyer Arnaud , Al Obeidli Aisha Khalil , Ben Maroof Muneera , Baloch Shahid Ali , Kumar Dey Swapan , Almarzooqi Maitha Jawhar 
Published: November 2020

A brown field producing from an Upper Cretaceous carbonate reservoir has 35 years of production history with challenges of water management and sweep efficiency affecting production plateau duration and ultimate recovery. To mitigate the challenges the reservoir characterization must be updated in order to optimize the field development plan. The reservoir is very heterogeneous with evidence of dual medium behavior which is attributed mainly to fractures. In addition to fractures, the presence of karst features in the reservoir were identified in several cored wells as part of the major field review. As a result, a comprehensive karst characterization study was conducted on the three reservoir units with the objective of describing and understanding the karst development across the field. The ultimate goal was to enhance reservoir understanding and update the conceptual reservoir model.

The study was conducted in three phases. The first phase of the study focused on identifying karst features on cores from 41 wells (~10,000ft of core) which are well distributed across the field. All the intervals with different types of karst features in each well were identified and marked. The second phase was the integration of static and dynamic data. The third phase of the study updated the conceptual reservoir model integrating all the findings. The Karst development in the reservoir corresponded to both early and late diagenetic processes and occurred mainly in reservoir Unit-1 and Unit-3. The early diagenetic process attributed to the formation of epikarst (paleosoil, dolocrete, and hardground) mainly in Unit-1. Epigenetic karst resulted in the development of solution collapse breccias occurring extensively in crestal field position below the top of Unit-3. Late diagenetic process attributed to hydrothermal karst formation (dissolution vuggs and presence of saddle dolomite) mainly identified in Unit-3 and correspond to hydrothermal fluid circulation via deep-rooted faults. Integration of karst features from cores, image logs and seismic attributes with dynamic data depicted clearly the dynamic influence of the karst development in the reservoir (i.e. high Kh values from well tests, production from collapse brecciated intervals-PLT, and early water breakthrough). These effects are more profound in the southern crest of the field. Finally, a conceptual reservoir model was developed and was crucial in integrating both static and dynamic data. This study was unique as karst influence on reservoirs are not common and not very well understood in the region. The integrated workflow to characterize karst development using both static and dynamic data have been of profound importance in terms of building a good understanding of the reservoir for future well placement and completion. The integrated approach used in generating the karst conceptual model, adequately will guide the update of the reservoir model.