In a giant offshore UAE carbonate oil field, challenges related to advanced maturity, presence of a huge gas-cap and reservoir heterogeneities have impacted production performance. More than 30% of oil producers are closed due to gas front advance and this percentage is increasing with time. The viability of future developments is highly impacted by lower completion design and ways to limit gas breakthrough. Autonomous inflow-control devices (AICD's) are seen as a viable lower completion method to mitigate gas production while allowing oil production, but their effect on pressure drawdown must be carefully accounted for, in a context of particularly high export pressure. A first AICD completion was tested in 2020, after a careful selection amongst high-GOR wells and a diagnosis of underlying gas production mechanisms. The selected pilot is an open-hole horizontal drain closed due to high GOR. Its production profile was investigated through a baseline production log. Several AICD designs were simulated using a nodal analysis model to account for the export pressure. Reservoir simulation was used to evaluate the long-term performance of short-listed scenarios. The integrated process involved all disciplines, from geology, reservoir engineering, petrophysics, to petroleum and completion engineering. In the finally selected design, only the high-permeability heel part of the horizontal drain was covered by AICDs, whereas the rest was completed with pre-perforated liner intervals, separated with swell packers. It was considered that a balance between gas isolation and pressure draw-down reduction had to be found to ensure production viability for such pilot evaluation. Subsequent to the re-completion, the well could be produced at low GOR, and a second production log confirmed the effectiveness of AICDs in isolating free gas production, while enhancing healthy oil production from the deeper part of the drain. Continuous production monitoring, and other flow profile surveys, will complete the evaluation of AICD effectiveness and its adaptability to evolving pressure and fluid distribution within the reservoir. Several lessons will be learnt from this first AICD pilot, particularly related to the criticality of fully integrated subsurface understanding, evaluation, and completion design studies. The use of AICD technology appears promising for retrofit solutions in high-GOR inactive strings, prolonging well life and increasing reserves. Regarding newly drilled wells, dedicated efforts are underway to associate this technology with enhanced reservoir evaluation methods, allowing to directly design the lower completion based on diagnosed reservoir heterogeneities. Reduced export pressure and artificial lift will feature in future field development phases, and offer the flexibility to extend the use of AICD's. The current technology evaluation phases are however crucial in the definition of such technology deployments and the confirmation of their long-term viability.
The application of segmented, smart completion strings is expanding to enhance the hydrocarbons production along extended horizontal drains that are drilled across heterogeneous carbonate reservoir units, Offshore Abu Dhabi. Conclusive formation evaluation answers are required to support the well completion design. Nuclear Magnetic Resonance well logging while drilling "NMR-WD" supports the operations efficiency, HSE and adds a wealth of input to the completion design in a time-efficient process. A quantitative matrix independent porosity and permeability are required. In this case, we present the value of laboratory NMR measurements to maximize the value of the log interpretation process, adding a conclusive picture of the pore size distribution and reservoir quality as critical inputs. Representative core plug samples of the different major rock types have been selected along the Upper Jurassic Carbonate sequence, Offshore Abu Dhabi. The plugs have undergone Soxhlet cleaning, and routine core analysis measurements before Mercury injection capillary pressure measurements were carried out on plug trims too. Saturation of the plugs was conducted using synthetic formation brine and desaturation has been conducted over porous plate. A complete scanning of the pore size and pore throat distributions was made available and the transverse relaxation time T2 cutoff for the irreducible fluid was identified, then applied to the NMR-WD dataset. The laboratory measurements could be used to reprocess the log data, bridge the gap between the Coates permeability computation and the offset core values based on the measured benchmark T2 cutoff values. The integration between the porous plate, MICP and NMR laboratory measurements is utilized to qualify the NMR-WD as a vital tool for the future similar formation evaluation operations across the same target reservoir. NMR-WD log data interpretation is enforced with a representative pore size distribution. The quantitative matrix properties representation improves the well completion design.
Nuclear Magnetic Resonance logging measurements (NMR) provide detailed information about rock texture and pore distribution. The main objective of this study is to highlight a carbonate reservoir characterization example in a mature field, offshore Abu Dhabi; providing qualitative porosity, permeability and pore type classification in real time (while drilling), to support efficient field development decision making. Different logging while drilling vendors tools (NMR-WD) operate at different concepts; some use the longitudinal relaxation time (T1) measurements, others apply the transverse relaxation time (T2). In this case, a low magnetic field gradient (T2) tool type was deployed in a tight formation horizontal oil producer. The well objective is to expose the maximum reservoir contact (MRC). Primarily, the acquired (NMR) spectrum was used to deliver accurate total porosity, to compute Archie's water saturation. However, delivering a quantitatively reliable permeability become very challenging in the complex carbonate environment subject to study as it was well linked to (NMR) pore size distribution. At first, a standard (T2) cutoff value was applied. The computed (bulk irreducible water – BVI) was too low and hence the permeability was too high, resulting in inaccurate NMR interpretation. Next, a varying T2 cutoff – per zone was applied based on the changing spectrum profile itself. Finally, a Gamma Inversion technique by the service company was introduced to better quantify the different pore types and the corresponding permeability. The (NMR) log analysis was validated with well core data in addition to production logging results. The data was applied to design the well stimulation and completion programs resulting in a healthy oil producer drain added to the asset. Integration of Gamma Ray-resistivity-NMR and borehole image logs helped to consolidate the interpretation findings hence supporting decision making for mature field development.
Full field development of the Upper Jurassic carbonates, offshore Abu Dhabi is exceedingly challenging. The heterogeneous texture, complicated pore systems and intensive lithology changes all mark the regressive cycles of sedimentation. Such complicated characteristics obscure formation evaluation of these formations. Advanced well logging tools and interpretation methodologies are implemented to minimize the petrophysical uncertainties to qualify the products as field development critical elements. This case study highlights a newly applied NMR log interpretation approach. The results help to understand the complex pore system in a tight carbonate layer, along a horizontal drain drilled close to the oil-water contact. NMR log data was acquired in real-time while drilling simultaneously with Gamma Ray, Resistivity and Image Logs. Earlier field studies recommended swapping standard T2 free fluid relaxation cutoff values by actual laboratory NMR measurements for a higher precision suitable for the reservoir texture heterogeneity, the study itself supported the application of higher cutoff values to better discriminate the free fluid in well-connected macro pores from the irreducible which will have a direct impact on the computed permeability. In this case study, a variable free-fluid T2 cutoff was firstly implemented based on arbitrary estimations to match the computed Coates permeability to the offset core values. Free-fluid, irreducible fluids were sequentially computed. A unique NMR-Gamma Inversion (NMR-GI) workflow is further utilized as a mathematically defined approach to process the raw data using probabilistic functions. The result is a more precise pore size distribution, coherent with the geological variations. NMR Capillary pressure was computed. The complex formation texture could be accurately tracked for thousands of feet drilled along the horizontal drain. After validation with offset core, the NMR-GI interpretation was combined with, Archie saturation and Image log analysis for a conclusive assessment. Hydraulic flow units were combined. Successful completion design and production zone selection articulated on the defined open hole log interpretation. NMR while drilling logging and the applied (NMR-GI) methodology prove to be leading tools to assist in resolving carbonate reservoir complexities. Not only that they help to understand the pore system characteristics, but they effectively support well placement, completion and production.
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