Realistic geological characterization is crucial for the development of carbonate reservoirs specially to overcome some challenges related to Early Water Break Through (EWBT) like those observed in the Upper Cretaceous ‘A’ Formation. The complexity of the depositional, diagenetic, and structural history created unpredictable connected pathway with the aquifer. Therefore, having suitable conceptual geological models tied to well data helps to better anticipate their occurrence and extension to plan for suitable well placement, a completion design, and optimize the production. The objective in this case study was to assess the Deep Shear Wave Imaging (DSWI) technology that can bridge the scales between well Borehole Image (BHI) and conventional Seismic to identify sub-Seismic features up to 100ft away from the well bore. A statistical evaluation of the BHI was used as a reference for this DSWI assessment. Then the result was compared with Seismic coherency to assess the value of the combined data. This cross-discipline integration in 3D from different data measurements with a variety of scales by order of magnitude difference between them (Well and Seismic), has allowed to build a conceptual model of the near wellbore region up to 90ft and predict five significant fault zones that are susceptible of connecting with aquifer. Dynamic data will be used in the second phase of the project such as (liquid rate, water breakthrough and tracers) to analyze their contribution to the EWBT and refine/enhance the interpretation. The way forward is to apply a similar workflow to other wells in the field, to gain a solid understanding of the sub-seismic architecture of the subsurface and hence timely optimize the setup of Inflow Control Device (ICD) design used to mitigate EWBT.
Reservoir monitoring is a key initiative for efficient management and development of hydrocarbon reservoirs. Time-lapse feasibility studies are available in literature, however; actual time-lapse seismic surveys are very limited in Abu Dhabi. A time-lapse Walkaway Vertical Seismic Profile (VSP) was acquired as part of the reservoir monitoring plan during a Water Alternation Gas (WAG) Enhanced Oil Recovery (EOR) pilot in an onshore carbonate field in UAE. The purpose of the study is to monitor the injected fluid in the reservoir. A 4D feasibility study predicted saturation changes >50% after 6 years of water and CO2 injection resulting in >10% change in reflection amplitude. Time-lapse Pulsed Neutron logs showed saturation changes of about 25% for both water and CO2 after 3 years of injection. Walkaway VSP survey design was performed but in practice optimum receiver depths could not be achieved due to well tubing resulting in reduced lateral image extent. Three surveys were recently acquired: Baseline survey after 6 months of CO2 injection, Monitor-1 after 6 months of water injection and Monitor-2 after 3 years of WAG injection. The surveys achieved excellent data quality. There were some small inconsistences between all the surveys that were revealed and partially resolved during detailed parallel pre-image re-processing of the three datasets. 4D image attribute analysis recovered a subtle time-lapse response. The time-lapse response was compared to the observed CO2 and water saturation differences and some correlation was observed. Further modeling is required to investigate these small observed differences. Future monitors will take into account lessons from these surveys. It is hoped that this study will provide confidence to operators for deployment of borehole seismic survey technology for future time-lapse monitoring in carbonate reservoirs. Workflows applied in this study will provide a reference point for future geophone or fiber VSP surveys for 4D monitoring. This study also highlights significance of source consistency and quality control that should be performed at wellsite during acquisition.
The purpose this study is to use geophysical technology, analyze the characteristics of typical sedimentary facies in the study area, extract sensitive seismic attributes, compile sedimentary facies distributions, establish a sedimentary model, and improve the understanding of the distribution law of complex carbonate reservoir in oil and gas exploration and development. The Cretaceous Arabian Basin is a typical gentle slope carbonate basin with rich oil and gas resources. The main reservoirs in the study area have complex lithology, including bioclastic grainstone, bioclastic packstone and rudstone. However, the heterogeneity of carbonate rocks is strong and the sedimentary facies change rapidly, which increases the difficulty of understanding the distribution law of reservoirs. Based on core data, well logs, seismic data and well-to-seismic integration, the sedimetary facies study of the Cretaceous Thamama Group has been completed. Typical lithology and logging facies were identified. The logging facies were established, multi-well facies were carried out; and sedimentary characteristics were analyzed. Multi-attributes have been analyzed including amplitude, frequency, continuity and phase attributes to extract sedimentary facies maps by combining lithofacies and logging facies. Based on the above analysis results, a sedimentary model has been summarized. According to this study, the target strata are mainly carbonate deposits of gentle slope with shoal, inner shoal and grain flat facies deposits. Four typical logging facies were identified consisting of low energy shoal, margin shoal, grain shoal and grain flat microfacies. The favorable reservoirs are mainly bioclastic grainstone and packstone formed in a medium-strong hydrodynamic environment. The sedimentary model of the study area is established to analyze the distribution of target reservoirs and guide the analysis of favorable reservoirs in the area. The Thamama Formation has stable thickness in general, the depositional environments changed from subtidal to inner shoal of restricted platform, to grain shoal and grain flat. It is concluded that the Shuaiba Formation and the Kharaib Formation of the Lower Cretaceous Thamama Group in the middle of the study area are the most favorable reservoir development formations, and the central and northern areas are the most favorable oil and gas accumulation areas. Through this study, a more complete and instructive carbonate slope sedimentary model has been established. In addition, through the multi-attribute analysis technology and optimization method, we have completed and deepened the understanding of the distribution of the target layer sedimentary facies, and provided a new geophysical comprehensive research method. However, due to the complexity of carbonate rocks, further research is needed.
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