CT studies of brittle failure in castlegate sandstone

Vinegar, Harold J; Waal, J.A. de; Wellington, S.L.

doi:10.1016/0148-9062(91)90082-w

Cited by 54 publications

(11 citation statements)

References 2 publications

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“…CT mat is determined by averaging the single modes of all CT numbers at every pressure stage, assuming that the most frequent CT number dominates the matrix of the rock sample. Our results reveal a matrix number of CT mat = 1862 HU, which is in line with other CT-based sandstone studies (Akin and Kovscek, 2003;Huo et al, 2016;Vinegar et al, 1991). Careful calibration with different spacers (0.19, 0.29, 0.41, 0.52 mm) within the fracture indicates a slope of the linear calibration line of 5890 ± 38.3 HU mm −1 (Fig.…”

Section: Image Processingsupporting

confidence: 74%

“…In the past, in situ imaging of experiments based on CT technologies was applied to a wide range of studies including geomechanical behavior of fractures (Re and Scavia, 1999), evaluations of triaxial tests (Feng et al, 2004;Ge et al, 2001;Lenoir et al, 2007;Ren and Ge, 2004;Vinegar et al, 1991;Zhou et al, 2008), shear tests (Tatone and Grasselli, 2015), sand production experiments (Santos et al, 2010) as well as various single-/multi-phase core-flooding experiments in fractured and unfractured rocks (Huo and Benson, 2015;Krevor et al, 2012;Oh et al, 2013;Perrin and Benson, 2010;Pini and Benson, 2013;Pini et al, 2012;Rangel-German et al, 2006;Schembre and Kovscek, 2003;Shi et al, 2009;Watanabe et al, 2011). CT technology generally represents a 3-D non-destructive method to image material contrasts in high resolution and, therefore, is well suited to reproduce dynamic processes in situ and in "real time".…”

Section: Introductionmentioning

confidence: 99%

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Simulating stress-dependent fluid flow in a fractured core sample using real-time X-ray CT data

et al. 2016

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Abstract. Various geoscientific applications require a fast prediction of fracture permeability for an optimal workflow. Hence, the objective of the current study is to introduce and validate a practical method to characterize and approximate single flow in fractures under different stress conditions by using a core-flooding apparatus, in situ X-ray computed tomography (CT) scans and a finite-volume method solving the Navier-Stokes-Brinkman equations. The permeability of the fractured sandstone sample was measured stepwise during a loading-unloading cycle (0.7 to 22.1 MPa and back) to validate the numerical results. Simultaneously, the pressurized core sample was imaged with a medical X-ray CT scanner with a voxel dimension of 0.5 × 0.5 × 1.0 mm 3 . Fracture geometries were obtained by CT images based on a modification of the simplified missing attenuation (MSMA) approach. Simulation results revealed both qualitative plausibility and a quantitative approximation of the experimentally derived permeabilities. The qualitative results indicate flow channeling along several preferential flow paths with less pronounced tortuosity. Significant changes in permeability can be assigned to temporal and permanent changes within the fracture due to applied stresses. The deviations of the quantitative results appear to be mainly caused by both local underestimation of hydraulic properties due to compositional matrix heterogeneities and the low CT resolution affecting the accurate capturing of sub-grid-scale features. Both affect the proper reproduction of the actual connectivity and therefore also the depiction of the expected permeability hysteresis. Furthermore, the threshold value CT mat (1862.6 HU) depicting the matrix material represents the most sensitive input parameter of the simulations. Small variations of CT mat can cause enormous changes in simulated permeability by up to a factor of 2.6 ± 0.1 and, thus, have to be defined with caution. Nevertheless, comparison with further CT-based flow simulations indicates that the proposed method represents a valuable method to approximate actual permeabilities, particularly for smooth fractures (< 35 µm). However, further systematic investigations concerning the applicability of the method are essential for future studies. Thus, some recommendations are compiled by also including suggestions of comparable studies.

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Section: Image Processingsupporting

confidence: 74%

Section: Introductionmentioning

confidence: 99%

Simulating stress-dependent fluid flow in a fractured core sample using real-time X-ray CT data

et al. 2016

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“…If the spatial distribution of minerals in the solid matrix is relatively homogeneous, then the average CT-number in a given area can be approximated as linearly related to the porosity (VINEGAR et al, 1991;DESRUES et al, 1996). In a plot of the grey level versus density (Fig.…”

Section: X-ray-computed Tomography: Density and Porosity Contrastsmentioning

confidence: 99%

Microstructural Inhomogeneity and Mechanical Anisotropy Associated with Bedding in Rothbach Sandstone

Louis

Baud

Wong

2009

Pure appl. geophys.

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This study present the result of conventional triaxial tests conducted on samples of Rothbach sandstone cored parallel, oblique (at 45 degrees) and perpendicular to the bedding at effective pressures ranging from 5 to 250 MPa. Mechanical and microstructural data were used to determine the role of the bedding on mechanical strength and failure mode. We find that samples cored at 45 degrees to the bedding yield at intermediate level of differential stress between the ones for parallel and perpendicular samples at all effective pressures. Strain localization at high confining pressure (i.e., in the compactive domain) is observed in samples perpendicular and oblique to the bedding but not in samples cored parallel to the bedding. However, porosity reduction is comparable whether compactive shear bands, compaction bands or homogeneous cataclastic flow develop. Microstructural data suggest that (1) mechanical anisotropy is controlled by a preferred intergranular contact alignment parallel to the bedding and that (2) localization of compaction is controlled by bedding laminations and grain scale heterogeneity, which both prevent the development of well localized compaction features.

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“…This indicates a uniformly stable and deformable pore structure that could support additional mechanical strength with increased deformation. Although no pore volume compressibility test reported in this paper was conducted up to the pressure causing mechanical failure of the core plug, the ability for CT to easily detect fractures within cores can be very useful in identifying samples that failed mechanically during such a test (Vinegar et al 1991).…”

Section: Porosity Fractionmentioning

confidence: 98%

Use of X-Ray CT to Measure Pore Volume Compressibility of Shaybah Carbonates

Siddiqui

Funk

Al-Tahini

2010

SPE Reservoir Evaluation &Amp; Engineering

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The Lower Cretaceous Shu'aiba formation (Shaybah field) in southeastern Saudi Arabia is a heterogeneous carbonate formation with various facies because of diagenetic alteration of the original rock fabric. Preliminary laboratory rock-mechanics studies indicated that the Shu'aiba carbonates are mechanically weak, with the majority of the rocks tested yielding very low strength values (less than 3,000 psi) when compared to samples from other carbonate reservoirs (Salamy and Finkbeiner 2002). On the basis of laboratory-derived rock-strength data from triaxial tests, the formation appears to behave in a plastic manner that strengthens the wellbore. To understand the stress behavior of the Shaybah rocks better, a set of pore volume compressibility tests was conducted in which the changes taking place within the samples were observed and quantified using an X-ray computerized-tomography (CT) scanner. A new technique involving dual-energy CT scanning was used to obtain the pore volume compressibility values, which were compared against the conventional triaxial-cell-generated data and published results by Harari et al. (1995) on cores taken from the same reservoir. Although the magnitude of the pore volume compressibility was the same between different setups, visual data showed some interesting behavior of the Shaybah cores including movement of grains in an irreversible manner even for a relatively low imposed effective stress of 2,000 psi. Some of the advantages of the new CT-based analysis technique include the generation of multiple pore volume compressibility curves from the same plug (a band of data, each at a different slice location), the "visualization" of the changes, the possibility of using either hydrostatic or triaxial cells to make the test more case-specific, and the possibility of measuring permeability under different stress conditions before, during, and after the test.

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CT studies of brittle failure in castlegate sandstone

Cited by 54 publications

References 2 publications

Simulating stress-dependent fluid flow in a fractured core sample using real-time X-ray CT data

Simulating stress-dependent fluid flow in a fractured core sample using real-time X-ray CT data

Microstructural Inhomogeneity and Mechanical Anisotropy Associated with Bedding in Rothbach Sandstone

Use of X-Ray CT to Measure Pore Volume Compressibility of Shaybah Carbonates

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