Lavrov, A. (SINTEF Petroleum Research) | Torsæter, M. (SINTEF Petroleum Research) | Albawi, A. (Norwegian University of Science and Technology) | Todorovic, J. (SINTEF Petroleum Research) | Opedal, N. (SINTEF Petroleum Research) | Cerasi, P. (SINTEF Petroleum Research)
Integrity of the near-well area is crucial for preventing leakage between geological horizons and towards the surface during CO2 storage, hydrocarbon production and well stimulation. The paper consists of two parts. In the first part, a finite-element model of earlier laboratory tests on thermal cycling of a casing/cement/rock assemblage is set up. It is demonstrated that radial tensile stresses contributing to annular cement debonding are likely to develop during cooling of such an assemblage. The results of the modeling are in agreement with the results of the earlier laboratory experiments, with regard to the temperature histories, CT data, and location of acoustic emission sources. In the second part of the paper, a computational procedure is developed for upscaling of data about rock damage obtained from CT, to a finite-element model of flow in porous media around a well. The damaged zone is shown to dominate the flow along the axis of a compound specimen (a hollow cylinder of sandstone filled with cement). Implications for leakage along an interface between cement and rock in-situ are discussed.
This paper investigates the effects of inflating a reservoir via carbon sequestration or water flooding on the state of stresses in surrounding ductile formations. A better control is needed on the assessment of stresses in the caprock and how would they change due to the inflation process in order to avoid geomechanical problems such as seal integrity and induced seismicity. An important factor in analyzing the reservoir-caprock interaction is the compression-inflation model used to estimate reservoir behavior. A finite elements model of a simple circular disc is created to study the stress redistribution around an inflating reservoir using the commercial software ABAQUS. The paper analyzes the results using well known methods, namely; the Ellipsoidal Inclusion Approach and the Deformation Analysis in Reservoir Space Approach. The analysis verified the results of the Finite Elements Simulation and gave insight to generic redistribution of stresses around an inflating reservoir. The paper also compared two different compression-inflation models; the Modified Cam Clay Model (MCC) and the MIT-E3 model. The MIT-E3 model utilizes a non-linear elastic inflation model and estimates larger redistribution of stresses around the inflating reservoir.
Strain localization in the form of compactive shear bands or compaction bands is often observed in high porosity rocks such as sandstones or limestones. In the present study, we theoretically investigate the possibility of strain localization in a high-porosity carbonate rock (calcarenite) by means of a continuum mechanics approach. A critical state elasto-plastic constitutive model has been employed for this purpose. We examine the constitutive and structural response by solving boundary value problems (BVPs) for calcarenite specimens subjected to axisymmetric loading conditions. In order to perform the numerical simulation in the post localization regime, the model is enhanced with a rate dependent regularization scheme. The results demonstrate that material heterogeneity, kinematic constraints and boundary effects govern the formation of various modes of localized deformation in the transitional regime between brittle fracture and ductile faulting. Indeed, the predicted macroscopic response is found to be in good agreement with observations available in the literature.
Underground storage is currently being considered by numerous countries as a long term solution for the disposal of high level nuclear waste. Research and design within each national program is generally tailored to a specific rock type, such as stable granitic plutons, bedded salt formations, clay, and sedimentary rocks ranging from limestone to shale. One important technical aspect of these designs is the accommodation of the mechanical impacts of thermal inputs (heating) from the fuel as it goes through the remainder of its life cycle. The results of experiments completed in a variety of different geological settings, including FEBEX by ENRESA in Grimsel, Switzerland, the Drift Scale Test (DST) at Yucca Mountain, the Äspö Pillar Stability Experiment (ASPE) from the Hard Rock Laboratory (HRL) in Sweden, and the Mine-By Experiment (MBE) by Atomic Energy Canada Limited’s (AECL) Underground Research Laboratory (URL), are analysed and compared to examine how thermal loading and the modelling process varies between different rock types.
Experimental study of a two cutters PDC bit-rock interaction shows that increase in the confining pressure reduces ROP through two mechanisms. The negative influences of confining pressure on the bit performance in addition to the rock strengthening under elevated borehole pressure (BHP), is the accumulation of crushed cuttings material in between the face of bit cutters and rock surface in the zone of penetration. The flow of crushed cuttings material on the surface of the PDC cutter causes additional confinement of the rock surface in the zone of the penetration due to friction.
On the other hand, utilizing a bit with an appropriate jet flow significantly improves ROP. In order to maximize the positive effect of bottom hole cleaning on ROP, the jet velocity has been raised to achieve cavitation condition by reducing the pressure with a high flow velocity ahead of the nozzles. However, the experimental results indicate that there are optimal conditions for applying a high velocity jet flow ahead of the bit, which yields maximum ROP when the cutters penetrate the rock mainly under the chamfer. After the effective cuttings removal condition, further increase in jet velocity no longer assists the bit to penetrate the rock faster.
In tensile and low confinement conditions, the tensile strength plays a critical role in crack initiation and propagation. Being able to accurately determine the intact tensile strength as well as develop an appropriate failure criterion across the tensile and compressive (shear failure) regions, are critical components of any rock mechanics project. This paper presents a summary of both indirect and direct testing approaches to determine the intact tensile strength. A review of the reliability of each method as well as calibration factors to refine the estimates provided by indirect testing methods is presented. In order to more accurately capture the behaviour of the intact rock in the tensile region, a modified Hoek-Brown criterion consisting of a tension cutoff is introduced. Such an approach utilizes the Griffith theory of brittle failure and the generalized equation by Fairhurst to define the tensile strength, and can easily be incorporated with current curve fitting methods. Finally, a method for determining the tension cutoff for data sets without reliable tensile data is provided for preliminary assessment purposes.
The occurrence of roof falls in underground mines has been reduced, largely to improvements in roof bolt technology, but continue to be a common source of mining-related injuries and fatalities. Particle flow modeling is an effective tool for simulating rock processes and behaviors related to mining activities. In this study, particle flow modeling is used to simulate the deformation of a roof layer in a room-and-pillar mine. Shale and sandstone models are compared, as are different roof bolt configuration scenarios. As input parameters would predict, the shale roof models deform more than the sandstone roof models. Greater bolt spacing is also associated with greater maximum deformation of the roof layer. Similar, more detailed studies of this nature could be used for predictive modeling to mitigate the risk of catastrophic roof falls in underground mines and to optimize the bolt configuration for unique mine factors.
The Numerical Manifold Method (NMM) with a two-cover-meshing system is an ideal method to handle boundaries, considering its flexibility with no need to adjust mathematical nodes onto boundaries, the meshing efficiency and its integration precision. In this study, we derived different forms of Lagrange multiplier methods (LMMs) and jump function methods (JFMs) for boundary constraints in the frame of NMM. These approaches for boundary constraints make full use of the aforementioned advantages of NMM and are established based on a clear physical meaning of water flow by an energy-work seepage model. The LMM approaches are discontinuous approaches in which Lagrange Multipliers provide links between discontinuous physical covers cut by material interfaces, whereas the JFM approaches are continuous, in which the discontinuities of material interfaces are realized by introducing jump terms across a continuous medium. The boundary constraint approaches developed in this study were coded into an NMM water flow model. We compared simulation results involving Dirichlet boundary conditions and idealized faulted rock using LMM and JFM with analytical solutions, and prove that both methods provide accurate results, with additional degrees of freedom introduced (or eliminated by special physical meaning). Based on these results, we recommend the LMM considering its accuracy and efficiency, flexibility, especially when involving intense geometric change or fracture intersections. Last, we apply and demonstrate the LMM approach in a simplified model of flow through porous rock with a major fault and a tunnel and arrived at convincing results.
The direct and Brazilian tension tests are the most popular methods for measuring the tensile strength of rocks. While the values of tensile strength obtained from these test are theoretically expected to be identical, the accumulated experimental data show that the direct tensile strength is different from the Brazilian tensile strength in most cases. In an attempt to explore the potential reasons for such discrepancy, the direct and Brazilian tension tests are compared in the light of size effect and bimodularity. The problem of size effect is investigated using the two approaches of statistical theory of strength and linear elastic fracture mechanics. The effect of bimodularity of the material on the induced stresses in the tension tests is also examined. Based on the effect of size and bimodularity, equations have been derived to relate the direct and Brazilian tensile strength. The results of the direct, Brazilian and fracture toughness tests on Lac de Bonnet granite were used to validate the equations.
In this paper, a disposal cell for the high level radioactive wastes (HLW) is studied in order to identify and demonstrate the coupled phenomena evolving in HLW repository. In the disposal cell, the overpack and container, which represent important engineering barriers of disposal cell, may be corroded by the eventual arrival of underground water coming from host rock. The corrosion of metallic components induces a degradation of its useful properties including strength, permeability and volumetric expansions, etc. Based on the concept of disposal cell, different materials (i.e. clay, steel and air) are taken into account in numerical simulation. Their mechanical behaviour are characterised by different constitutive models respectively. In order to overcome the eventual numerical difficulties and quantitatively reproduce the closing process of spaces presented in the disposal cell, numerical methods are formulated to descript the long-term corrosion behaviour of steel and the progressive closure of spaces between the geological barrier and different engineering barriers. The numerical results show us that the spaces can be closed with the volumetric expansion created by the steel corrosion and allows us to obtain some quantitative results on the processes developed in the disposal cell with the consideration of the progressive corrosion of metallic components.