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Quantifying and classifying pore systems in carbonates is notoriously challenging, particularly in rocks associated with complex diagenetic histories. Here we report novel computed tomography (CT) core and thin section image analyses through key reservoir intervals in the Buah Formation and Khufai Formation, both part of the Precambrian (Ediacaran) Nafun Group, from two wells located onshore Oman. Our primary objective is to constrain the volume, shape, connectivity, and distribution of vugs down-core in two and three dimensions – this is a key control on reservoir quality. We combine classic sedimentological core descriptions with image analysis on a range of data types. In this paper we focus on the analysis of core CT scan data, but we also introduce a high-level analysis of thin section images from discrete samples from the same boreholes, where available (sidewall core plugs, ditch cuttings and conventional cores). Finally, we integrated the results to (1) provide a holistic understanding of pore systems in the Buah and Khufai Formations; (2) identify the key uncertainties and weaknesses in our approach, and (3) plan for further reservoir assessment.

The hunt for less obvious deeper targets below the Base Cretaceous Unconformity (BCU) within the Norwegian North Sea greatly relies on the accuracy of the velocity model as it impacts the definition of the structural traps. The presence of the limestone/carbonate sequence with high velocities overlying the targeted lower velocity mudstone units represents the main challenge in term of velocity model building and is usually out of the reach of diving waves for the full waveform inversion (FWI) application when using streamer-based data with offset limited to 8km. The recent shift of acquisition industry toward hybrid acquisition combining streamer and sparse node opens the road for deeper application of FWI and even more. In this paper, we show how this new hybrid acquisition design can help to build a reliable high-resolution velocity model down to the Brent level. Moreover, we also present how elastic FWI enables us to better explain the elastic effects induced by the large impedance contrast at BCU level.

Time-lapse (4D) surveys have traditionally been reliant on baseline (base) surveys being well repeated by the monitor to decrease 4D noise. In this case study the monitor was acquired independently from the 3D narrow-azimuth, towed-streamer, hydrophone-only base, and “mixes” two different types of acquisition: a multi-sensor, towed-streamer acquisition for prime coverage and a multi-sensor towed-streamer infill. We describe technologies used to overcome the limitations in 4D repeatability and intra-monitor consistency. 3D Ghost Wavefield Elimination and a novel blind signature inversion method were crucial to create a seamless monitor across the target and to reconcile the signature and ghost differences between the base and monitor. Inconsistent azimuth content between base and monitors introduced complexities for the demultiple process; nevertheless, multiples were successfully attenuated using wave equation deconvolution with joint base and monitor reflectivity imaging. Residual non-repeated 4D noise was attenuated using a curvelet domain 4D co-operative denoise workflow.

High-resolution (HR) site survey acquisitions are traditionally utilized for shallow geohazard investigations and infrastructure planning. However, due to cost reasons, these are often acquired in a sparse 2D manner, and supplemented with conventional multi-streamer 3D seismic imaging aimed at deeper targets. We show how products derived from 100 Hz dual-azimuth (DAZ) full-waveform inversion (FWI) using multi- streamer 3D data, including FWI Imaging, provide a superior uplift in spatial resolution and illumination compared to a combination of both conventional 3D imaging using the same data, and 2DHR site survey data. This is shown to be the case for data and attributes compared against those from a site survey report at various near surface intervals where known geohazards occur. The improved spatial resolution with 100 Hz DAZ FWI can reduce uncertainties in predicting hazards and act as rapidly available supplementary information to a sparser 2DHR survey with less need to acquire a denser 3DHR survey.

In this paper, we present an integrated study that combines geophysical and geological approaches to perform porosity estimation and populate the reservoir geological model with the estimated property. For pre-salt oilfields, due to the complex porosity distribution in carbonate reservoirs, predicting a reliable porosity is a fundamental step for reservoir modeling.

The Culzean field, in the North Sea, has been producing since 2019 gas condensate from fluvial sandstones located within dipping rotated fault blocks at approximately 4km of depth. Two surveys have been acquired with ocean bottom sensors to image and then monitor the evolution of the reservoir during production. In addition to classical time-lapse seismic processing, a time-lapse FWI has been performed to estimate the velocity variation over the production time. Due to the thick chalk layer located just above the target structure and the dipping nature of the reservoir, 4D FWI is the ideal tool compared to more conventional 1D approach based on time-shift estimations. This fast velocity layer represents a challenge for velocity model building and processing in general as it prevents the penetration of diving waves even with 7km of offset and also generates strong multiple curtains covering the reservoir interval. Despite the shallow water environment and complex geology, the 4D FWI implemented in this project was able to recover velocity variations as weak as 1% after only 3 years of production, providing crucial information that can help reservoir evolution assessment.

We present a 4D pre-salt reservoir monitoring study from two wide-azimuth towed-streamer (WATS) surveys. Advanced flows were implemented to mitigate complex salt-related challenges and WATS repeatability issues in the image domain. Three models of converted waves interfering at reservoir level were generated via dual salt-flood Kirchhoff demigrations, and then subtracted at the post-migration stage. Repeatability issues were also addressed in the image domain via a novel 4D Least-Squares Wave-Equation Kirchhoff flow. The results reveal an unexpected level of 4D signal in such a complex geological setting.

Deriving an accurate high-resolution seismic quality factor (Q) model is necessary to both compensate for the phase dispersion and amplitude attenuation effects of the Earth’s anelasticity during imaging, and to reduce parameter cross-talk during velocity model building (VMB). Various methods exist for deriving Q, each with potential inherent limitations that can restrict their suitability for scalable applications, such as over the ?14,000 km2 area presented here in the Norwegian North Sea. We describe a multi-process Q derivation workflow, including dual-azimuth Q full-waveform inversion (Q-FWI), time-lag FWI guided ray-based Q tomography, and well-synthetic validations using Q sensitive metrics to derive a regional high-resolution Q model. When incorporated into VMB and subsequent Q migrations, the regional Q model is shown to both locally and regionally compensate for amplitude attenuation and phase dispersion, and providing an associated improvement in imaging.

The merits of Total and Effective porosity approaches have always been a source of discussion within the Petrophysical community globally. In general, an operating company adopts a single approach (Total or Effective) in their modelling workflows and ignores the alternative method. This is normally to have a consistent approach across the company so that the end users know what they are receiving into their subsequent workflows. In the proposed method, both Total and Effective Porosity Methods have been applied. The differences are then used to minimise and improve the resulting porosity and saturation calculations such that the results are mutually comparable. Total Porosity based water saturation equations are dependent on the ‘shale/clay’ volume and porosity to compensate for the ‘shale/clay’ bound water resistivity. Effective Porosity is based on water saturation equations on the shale volume and resistivity. The difference is that the Effective Porosity Water Saturation approach is not directly dependent on the ‘shale/clay’ porosity and can be used as a fitting parameter via the dry clay density (that doesn’t exist in-situ) in addition to compensating for the invaded fluid volume. Examples will be presented in a wider range of geological environments will be discussed.