Energy Norway

Assessments of fault geometries and fault-risk parameters are crucial when evaluating the integrity of a structurally controlled CO 2 storage site. To perform these assessments, seismic data, recorded in time, must be converted to depth. The velocity models used for this time to depth conversion influence the final depth image and, consequently, the geometry of the interpreted faults. Against this background, we created four velocity models for depth conversion, assessed the impact on fault throw, dip and thickness of the primary seal, and, subsequently, a fault-risk assessment of the Vette Fault Zone in the Smeaheia CO 2 storage site. We found that depth conversion had a larger influence on fault throw and thickness of the primary seal than on fault dip. In contrast, the overall assessment of the presence of a membrane seal and geomechanical integrity showed less sensitivity to the depth conversion process. Consequently, we suggest that a relatively robust fault-risk assessment can be made with a variety of velocity model designs and data input. Nevertheless, we found a mean difference of 2% in the shale gouge ratio, 4% in the slip tendency and 9% in the dilation tendency for the Vette Fault Zone, emphasizing the importance of accounting for the influence of depth conversion in optimizing structural assessments in potential CO 2 storage sites.

Abstract The Gohta discovery well 7120/1-3, drilled on the Loppa High in the SW Barents Shelf in northern Norway encountered a meteoric karst system hosted by the late Permian lower Røye Formation that formed during a period of sub-Triassic exposure. The karst system is preserved as a series of interbedded collapse breccias representing formerly open palaeocaves, matrix-rich breccias that represent palaeocave fills with in situ and reworked stalactites and stalagmites. Layers of ‘Swiss cheese’ texture are interpreted as zones of dissolution in marine-meteoric mixing zones that may have controlled the levels of palaeocave development within the Gohta structure. These other cavities and the matrix of karst breccias were infilled by layered fine doloarenite matrix interpreted as an internal fill generated by collapse. This was followed by a later matrix of dark grey argillaceous mudstone that is interpreted as an external fill that pre-dates the overlying Klappmyss Formation. The karst system has undergone complex early diagenesis that includes extensive dissolution, early leaching of silica, the precipitation of pyrite on internal surfaces and sediment and the precipitation of carbonate and silica as speleothems and vadose cement. A borehole imaging log (BIL) was run across the lower Røye and Triassic Klappmyss formations, including the cored interval that provides a record of inclination and azimuth of the karst fills and in situ Røye Formation. The core provides information about the origin and timing of the different karst fills, and the BIL provides information about their azimuth and inclination. This integrated use of core and BIL data allows the incremental tilting history of the Loppa High structure during exposure to be reconstructed. At present, the Gohta structure has a finite tilt of 5–10° to the SE. The plunge of in situ stalagmites and stalactites suggests that the karst system was initiated while the Røye Formation was slightly tilted to SE. The dip of karst components and the host stratigraphy indicate several phases of incremental tilting during deposition of the doloarenite and argillaceous mudstone karst matrix. There appears to have been a reversal of tilt of the Gohta structure prior to its onlap by the Klappmyss Formation.

The Loppa High Composite Tectono-Sedimentary Element is a mature exploration area in the southwestern Norwegian Barents Sea hosting Skrugard (in 2012) and the Late Paleozoic Gohta (in 2013) discoveries. The exploration of the Norwegian Barents Sea started in 1980 and the Loppa High in 1985. Today the Loppa High is one of the most studied areas of the Barents Shelf. Subsurface mapping is based on dense coverage of modern 3D and 2D seismic data and drilling results of over 40 exploration wells. This chapter summarizes the tectonosedimentary and subregional development, and the petroleum geology of the Loppa High.

The core cleaning approaches aim to remove native crude oil, mud filtrates and evaporated salts which leads to strongly water-wet cores. In the standard core cleaning method, the solvent(s) injection continues until no more oil is observed in effluent. This is generally confirmed by visual examination, rather than by analyzing the composition of effluent, simply because it is expensive. In this work, we selected two preserved rock sections from an oil reservoir in North Sea. 1.5-inch diameter core plugs were cleaned by two different flush cleaning programs (so-called Program-1 and 2). In Program-1, samples were flushed first with 75% toluene + 25% methanol and then followed by alternate of toluene-chloroform-methanol until the samples were deemed cleaned from visual examination. In Program-2, samples were flushed by alternating toluene-tetrahydrofuran-chloroform for 8 cycles. Oil effluents from Program-2 were collected for composition GC analyses. The efficiencies of two flush cleaning programs were compared by measuring the wettability of the samples by combined Amott-USBM wettability method. The USBM index shows samples after cleaning program-2 are more water wet. Samples were then aged in crude oil at reservoir conditions for 4 weeks. Drainage and imbibition capillary pressure using centrifuge and porous plate and the end-point liquid permeabilities were then measured on restored samples. End-point properties after aging process, showed wettability being restored to intermediate water-wet as is expected from field production history. This study shows that standard cleaning does not fully clean these samples. Meanwhile, the elaborate Program-2 is often impractical, takes too long, is expensive and limited by laboratory capacity.

The electric power system lies at the heart of modern society. To optimize the total cost of the system a trade-off between investment and reliability must be made. Power system reliability analysis (PSRA) typically quantifies the reliability of the system using fixed component failure rates. However, the probability of failure for a component increases with usage. Therefore, a health index model based on failure modes, effects, and criticality analysis (FMECA) is presented to facilitate in the calculation process. FMECA is used to identify the failure mechanisms and assign a risk priority number (RPN) based on the severity, occurrence, and detectability of each failure mechanism. The health index is then evaluated based on failure mechanisms rather than directly from condition data and each failure mechanism's contribution to the overall health index is weighted relative to the RPN. A case study is presented where the health indices for a fleet of high-voltage circuit breakers (HVCBs) are evaluated based on data obtained from the Icelandic transmission system. The trip coil current (TCC) is a readily available measurement that can detect electrical and mechanical issues within HVCBs and was used as a key assessment criterion in determining the health indices for the HVCB fleet.

Seismic data acquired at different times can provide insight into changes in an oil/gas reservoir. We apply time-lapse AVO inversion methodology to the Edvard Grieg field in the North Sea. The methodology we use inverts for discrete lithology-fluid classes (LFCs), where different fluid fillings and/or pressure effects in the monitor are represented by different LFCs. Our focus here is on fluid substitution. We only look at amplitude effects, and use monitor time-lapse data that is time aligned with the base seismic.

This paper presents a case study from the Edvard Grieg field in the North Sea. PP and PS ocean bottom cabled (OBC) seismic data have been used to build high-frequency Vp- and Vs- velocity models using full waveform inversion (FWI). Since the start of production from the field in November 2015, three vintages of time-lapse (4D) OBC data have been acquired, in 2016, 2018 and 2020. In all instances, both PP-and PS- data have been processed and imaged to extract as much 4D information as possible. However, the PS data have been challenging to use in an image sense, as the quality of the images have been low, especially when compared to the PP data. A new PS-FWI method including PS-reflection data has been proposed to update and build a new high-frequency Vs velocity model. This includes both the low-wavenumber background model, as well as the high- frequency perturbation model, to bring the Vs model all the way up to 30Hz. The new 30Hz Vs model combined with a 65Hz Vp model is demonstrated to produce superior images of the reservoir target, both using RTM and FWI derivative image algorithms (FWI Imaging). The method shown here, using processed radial PS data components, is a key milestone in ongoing work to utilize raw PS component data, which will ultimately lead to a full elastic FWI solution.