Elin Skurtveit

Experimental investigation of CO2 breakthrough and flow mechanisms in shale

Supercritical CO2 breakthrough and flow mechanisms in shale have been investigated in laboratory experiments using a high pressure flow cell and cylindrical samples of shale from the Draupne formation in the North Sea. The main objective is to study the basic mechanisms involved in the breakthrough process and define the controlling parameters for supercritical CO2 flow in a low permeable shale. Experimental testing provides new insight into the CO2 breakthrough process through simultaneous measurements of deformation and ultrasonic velocities in the sample. A marked sample dilation associated with the CO2 breakthrough is identified accompanied with a pronounced drop in ultrasonic velocities. X-ray images of the sample using a high resolution 3D computer tomography (CT) scanner provide information on macroscopic fracture distribution inside the sample before and after testing. The CO2 breakthrough pressure for the Draupne material seems to depend on confining pressure and effective pressure rather than pore pressure difference across the sample. After breakthrough the effective CO2 permeability was found to follow a simple model for permeability in fractured rock. The drop in ultrasonic velocity was associated with mechanical changes and possible micro fracturing inside the sample. Based on our observations we conclude that pressure-induced opening of micro-fractures during the breakthrough process is an important mechanism for flow in addition to capillary displacement. Our findings may have important consequences for later testing and estimation of CO2 breakthrough pressure and flow in shale.

Fault baffle to conduit developments: reactivation and calcite cementation of deformation band fault in aeolian sandstone

The damage zone of three small faults in the Navajo and Page formations, located on the NE side of San Rafael Swell, Utah, USA, are studied. Scanlines and microstructural analyses are used to document three distinct deformation events: (1) an early phase creating cataclastic deformation bands, during which most of the displacement on the fault took place; (2) a fracturing event with opening and shearing along fractures; and (3) fluid flow and local calcite precipitation along the NW-trending faults. Microstructural characterization of deformation structures shows complex interaction between deformation bands, fractures and the calcite precipitation. Calcite cement is observed as veins in the host rock and along cataclastic bands, with varying amount of cataclastic material floating within the veins. In addition to patchy calcite cement in the host rock, extensive poikilotopic cementation is observed to extend into cataclastic bands with a low degree of cataclasis. However, some cataclastic bands with a high degree of cataclasis show almost no cementation. Development of deformation bands and their link to fracturing affected the flow field, from a fault baffle to a conduit. Calcite cementation reveals the flow paths, before cementation recreated the fault baffle.

Deformation in a North Sea Jurassic trap analysed using a triaxial plane strain experiment

Abstract A classical Upper Jurassic fault block in the North Sea, the Fulla Structure, has Brent Group sandstones with good reservoir quality and apparently insignificant fault-related reservoir damage. Core data show high-porous sandstones that extend close to the main faults and there is no evidence of catalase, only of soft-sedimentary deformation. Shear bands are relatively thin with high offsets, and have a texture comparable to the wall rock. To investigate the deformation mechanism and products synthetic Brent Group sands are deformed in a triaxial plane strain box with pre-defined effective consolidation in the range of 100–8000 kPa, simulating a burial depth in the range of 10–800 m. This range covers the burial depth at the time of active faulting for most Jurassic traps in the North Sea, including the Fulla Structure. The experiments demonstrate that grain rolling and grain-boundary sliding are the dominant deformation mechanisms at all the simulated burial depths, and this deformation has no impact on the reservoir quality. The experiments concur with observations from the investigated wells and strengthen an interpretation of limited reservoir damage associated with the Late Jurassic fault activity.

Seismic Hazard Assessment

This paper presents the methodology and results of a probabilistic seismic hazard analysis (PSHA) for a site offshore Maheshkhali Island, Bangladesh. The tectonic setting of the area is complex, and the PSHA includes active crustal faults, megathrust and intraplate subduction faults, as well as a background gridded seismicity areal source zone based on historical and recorded seismicity. Previous studies only included areal source zones based on recorded seismicity, whereas this study takes advantage of recent publications to include faults sources in the PSHA. The peak ground acceleration values for return periods of 475 years and 2475 years are 0.33 g and 0.63 g, respectively. The deaggregation plots show that the main contributors to the hazard are a magnitude 6.5-7.1 earthquake 15-20 km from the site on the Maheshkhali fault and a magnitude 8.0-8.8 earthquake 120-250 km from the site on the Ramree megathrust.

Fault Zone Development with Temporal Impact on Fluid Flow - Example from Navajo Sandstone, Utah

This work presents a field study of deformation structures found in a fault within the Navajo Sandstone in the San Rafael Swell, Utah. The main objective of this work is to describe the structural characteristics of the fault and identify different deformation episodes and their impact on fluid flow in the fault zone. Field analogues can be a useful tool for describing the hydo-mechanical coupling within fault zones and provides important knowledge of fault architecture and structural characteristics within faults. The studied faults are found to contain cataclastic deformation band later overprinted by fracturing and with calcite cement indicating fluid flow along the fault.

Experimental Methods for Characterization of Cap Rock Properties for CO 2 Storage

This paper presents laboratory methods utilized at Norwegian Geotechnical Institute for characterizing cap rock for CO2 storage reservoirs. The focus is on the physico-mechanical characterization of shale using standard rock physical-mechanical testing and some special designed set-ups for advanced experimental conditions. The Brazilian, uniaxial and triaxial tests are explained along with some examples from North Sea, Barents Sea and Svalbard. The CO2 core flood tests have been designed and carried out in the NGI laboratory for investigating rock-CO2 interaction. Monitoring techniques such as CT-scanning, acoustic measurement, acoustic emission and resistivity are used and described in the paper. Brief description of development/upgrading of laboratory instruments for conducting more advanced experiments is also included.

Impact of Tensile Strength Anisotropy on Fracturing Pressure of Svalbard Sandstone and Shale Cap Rocks

This paper presents results of the laboratory tests for determining the tensile strength of anisotropic shale samples cored from Aagardhfjell Formation in Longyearbyen, Svalbard. This formation is considered as seal for the suggested CO2 storage reservoir. Therefore, it is essential to characterize and investigate behavior of the shale under storage conditions. This study includes results and analysis of Brazilian indirect tensile strength of cores parallel and perpendicular to bedding. The results are analyzed to incorporate in the calculation of fracture pressure. Because of significant burial and uplift of the Barents Sea region, the shale exhibits very high tensile strength and strong anisotropy. Calculation and analysis of fracture pressure, based on the laboratory data and some assumptions, show that the orientation of possible fractures can be dictated by the tensile strength instead of in-situ stresses. An example of the impact of tensile strength anisotropy on the fracture pressure of shale formation has been assessed. The results of this study suggest that the tensile strength of rock has significant impact on fracture pressure and the orientation of fractures.