In tight Cretaceous carbonates with complex reservoir and fluid typing, Nuclear Magnetic Resonance (NMR) petrophysical solutions has become a one of the integral logging technology to provide reliable and robust solution through systematic acquisition and processing methodology. This paper covers a case study from two carbonate fields of UAE applying innovative NMR log analysis techniques to understand varying complex reservoir porosities, permeabilities, identifying irreducible water, movable hydrocarbon and water in complex tectonic-cum-geological area, and identifying difficult gas-condensate fluid regime, validated by fluid sampling and testing to support optimizing well placement, and bringing confidence for fast track appraisal-cum-development program. The Nuclear Magnetic Resonance (NMR) Logs and its derivatives bring lots of value addition to overcome challenges in reservoir and fluid characterization for tight, complex and heterogeneous carbonate reservoirs. In the early life of exploration and fast appraisal stage of any carbonate field, measuring NMR log data for porosity typing (clay, capillary & free fluid), permeability variations, irreducible water saturation, identifying presence of movable water in unknown fluid contacts and resolving hydrocarbon typing through continuous T1T2 (simultaneous longitudinal relaxation-T1 & transverse relaxation-T2) with two dimensional (2D) fluid characterization to assist in differentiating condensate, gas, and water in reservoir static condition are such useful technical information coming out-off single logging tool to properly access reservoir and field potential. NMR data like porosity, rock matrix density and irreducible water saturation continuous profiling is already filling the gap for routine core analysis and capillary pressure data. These outputs from NMR can be easily integrated with other logs and geoscience data to plan perforation and testing in such challenging reservoirs. Despite NMR having shallower depth of investigation, with proper log acquisition plan, a good NMR data can be acquired and interpreted to get reliable interpretations for rock and fluid characterization, overcoming borehole and its fluid influences. Further all these NMR valuable data can be used for the application of reservoir rock typing in such complex carbonate reservoirs, if required. In today’s lower oil price and cost optimization phase, NMR logging is becoming popular acquisition tool providing most of time efficient petrophysical answers to reservoir and fluid characterization. The NMR log analysis products namely porosities (clay, capillary & free fluid), permeability, irreducible water, and fluid typing in difficult gas-condensate fluid regime validated by fluid sampling and testing has helped in trusting this systematic approach for fast track appraisal-cum-development program in these exploration-cum-fast appraisal carbonate fields. The NMR based log evaluation has been integrated with other logs and geoscience data to bring clarity on reservoir characterization, fluid typing and fluid contact to update the static model accordingly.
The studied Fields cover about 1900 sq km covering fourteen wells in Abu Dhabi, UAE. It was discovered in 1962 and developed by Abu Dhabi Petroleum Company. The objective of the current study is to illustrate the comparison between prestack stochastic & prestack deterministic seismic inversion results to better understand vertical and lateral heterogeneities of upper Jurassic reservoirs and their impact on property distribution during static modelling and further into dynamic model construction.
Throughout the UAE and the wider region, several broadly E-W orientated structural lineaments are observed on seismic within the Cretaceous successions and are described as strike-slip faults. However, in the studied field, these features have not been readily observed in well data. Instead, networks of fractures and deformation features are present in core and borehole images. A study was carried out in an attempt to calibrate well and seismic data and to understand the relationship between the seismically-resolved faults and the fractures observed on core. This study focuses on a dataset from the north-east part of the field, which includes BHI images, cores, full 3D CT scans and conventional logs in four penetrations, three of which are horizontal, drilled through the faults; as well as 3D seismic data and relevant derived horizons and fault polygon interpretations. The available data have been investigated in detail, with all structural features in core, circumferential CT scans and BHI images systematically classified using simple and reproducible descriptive schemes. All the structural features have been orientated using directional data from BHI. The understanding of the character and fill of the fractures observed in core has also been incorporated. A further calibration with seismic and integration of results with information from previous studies allowed a full description of the fracture networks, of their densities within and outside the potential fault corridors of the studied field, as well as an assessment of their potential for reactivation and their possible impact on localised formation compaction. On the BHI images, several sub-vertical fractures have been identified, consisting mainly of mixed resistivity and resistive fractures, striking dominantly WNW-ESE. Particular zones along the wells have noticeably higher fracture densities, where features are organised in clusters; they are intercalated with zones where fractures are rarer. The clustering of fractures within fracture corridors are believed to be fault-related, subvertical and tabular fracture clusters that traverse an entire reservoir unit vertically and extend for several hundreds to thousands of feet laterally. These zones are believed to represent fracture corridors, which correlate with the structural lineaments observed on seismic. The fracture corridor network in the study area shows a variable deformation signature at the different scales of observations, but consists mainly of sub-vertical (dominantly >60°) deformation bands (c.50% of the features identified) and partially-cemented fractures (c.25-40%). Some of these features show a small displacement and it is believed this scaled variation in deformation within the corridors accounts for the overall larger, but relatively minor displacement observed on seismic (c.10-40ft vertical throw and possibly up to c.500m cumulative strike-slip observed in seismic).
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