Numerical Modeling of Radial Fracturing of Cement Sheath Caused by Pressure Tests

To achieve an acceptable level of zonal isolation, well integrity should be guaranteed in hydrocarbon production and geological CO2 sequestration. Well pressure test can cause different types of failures in the well system leading to leakages through these failures. Laboratory evidences have revealed that occurrence of radial tensile fractures is likely during pressure tests. In this paper, we use a numerical code call MDEM which was formulated based on discrete element method. The code can model discontinuum feature of fractures. A model of a lab-sized pressure test was built and compared to an experiment previously published. The model was tested under different confinement levels and effect of the tensile strength of rock on the radial fracture was investigated at the same lab-scale. Fracture opening profiles are also presented showing the leakage potential of these fractures under different pressure level.publishedVersio

Geomechanical analysis of the influence of CO2 injection location on fault stability

Large amounts of carbon dioxide (CO2) should be injected in deep saline formations to mitigate climate change, implying geomechanical challenges that require further understanding. Pressure build-up induced by CO2 injection will decrease the effective stresses and may affect fault stability. Geomechanical effects of overpressure induced by CO2 injection either in the hanging wall or in the foot wall on fault stability are investigated. CO2 injection in the presence of a low-permeable fault induces pressurization of the storage formation between the injection well and the fault. The low permeability of the fault hinders fluid flow across it and leads to smaller overpressure on the other side of the fault. This variability in the fluid pressure distribution gives rise to differential total stress changes around the fault that reduce its stability. Despite a significant pressure build-up induced by the fault, caprock stability around the injection well is not compromised and thus, CO2 leakage across the caprock is unlikely to happen. The decrease in fault stability is similar regardless of the side of the fault where CO2 is injected. Simulation results show that fault core permeability has a significant effect on fault stability, becoming less affected for high-permeable faults. An appropriate pressure management will allow storing large quantities of CO2 without inducing fault reactivation. © 2016 Institute of Rock and Soil Mechanics, Chinese Academy of SciencesThe first author acknowledges the support from the “EPFL Fellows” fellowship program co-funded by Marie Curie, FP7 (Grant No. 291771) and partial support from the “TRUST” project of the European Community's Seventh Framework Programme FP7/2007–2013 (Grant No. 309607) and the “FracRisk” project of the European Community's Horizon 2020 Framework Programme H2020-EU.3.3.2.3 (Grant No. 640979). Activities of the second author are sponsored by SCCER-SoE (Switzerland) (Grant No. KTI.2013.288) and Swiss Federal Office of Energy (SFOE) project CAPROCK (Grant No. 810008154). This publication has also been produced with partial support from the BIGCCS Centre (for the third author), performed under the Norwegian research program Centers for Environment-friendly Energy Research (FME). The third author acknowledges the following partners for their contributions: Gassco, Shell, Statoil, TOTAL, ENGIE, and the Research Council of Norway (193816/S60).Peer reviewe

Fracture Assessment of Quasi-brittle Rock Simulated by Modified Discrete Element Method

New developments of an in-house hybrid code, named Modified Discrete Element Method (MDEM) are presented in the paper. The new developments are on the treatment of pre-existing and propagating fractures in quasi-brittle materials. These developments are the embedment of Linear Elastic Fracture Mechanics (LEFM) and elastic-softening crack band model -based methodologies in the MDEM and their application in lab and reservoir scale. Using the first methodology, MDEM can calculate stress intensity factors, �I and �II using the internal contact forces of particles. �I and �II are calculated independent of boundary conditions and geometrical configuration with acceptable accuracy level. The methodology has been also used in reservoir scale to study the rupture likelihood of faults and fractures due to fluid injection. This methodology enables the code to model mode I and mode II failures and propagation direction based on the fracturing model proposed by Rao et al. (Int J Rock Mech Min Sci 40(3): 355–375, 2003). Using the second methodology, the MDEM can model nonlinear behavior of quasi-brittle materials including or excluding preexisting cracks based on fracture energy. A model was verified against an experiment of a three point bend test with a notch. The numerically obtained force-crack mouth opening curve was reasonably comparable to the experimental test. The analysis was repeated for three other mesh sizes and the results are less mesh size dependent. Finally, it was shown that MDEM has the potential in studying fracture mechanics of quasi-brittle materials both in lab and large-scale investigations

Modified discrete element method (MDEM) as a numerical tool for cement sheath integrity in wells

Cement sheaths undergo extreme loading conditions in wells during subsurface operations. A damaged cement sheath may lead to fluid communication between different formation layers and fluid migration up to the surface, which can cause environmental, technical, and economic problems. In this study, we numerically analyze cement sheath stability using the modified discrete element method (MDEM). MDEM considers the cement sheath and rock formation as porous media and can model discontinuous fracturing of the materials. Analyses are performed based on a small-scale cement sheath integrity test and field cement pressure data measured by Cooke et al. (1983). Oil well class H cement was used, and its poroelastic properties were estimated using a micromechanics model and a multi-scale homogenization technique. First, the evolution of the stress state was approximated in the cement sheath and rock formation. Then, radial fracturing, shear failure, and interface debonding formation were studied under pressure increase/decrease operations. The effects of several parameters, such as the casing size, rock elastic parameters, and loading time, were also investigated. During the hydration of cement, the compressive and shear stresses evolved in the cement sheath, and the stresses in the surrounding formation also changed. The simulation results showed that a decrease in the casing pressure can lead to debonding of the casing-cement interface, and the calculated interface opening was within 1–50 μm. A pressure increase in the casing can lead to progressive shear and tensile failures in the cement sheath. A further pressure increase did not extend those failures into the rock formation; rather, it increased the number of radial fractures in the cement sheath. The analyses showed that a lower decrease and increase in the casing pressure is required to generate debonding and radial fracture in the cement sheath, respectively, in the early stages of hydration after the cement is set. This is also the case when the casing has a larger diameter and smaller thickness. When the cement sheath is bonded to a softer rock formation, the pressure increase required to create a fracture in the cement is lower compared with the case in which the sheath is bonded to a stiffer formation. However, in the case of softer formation, debonding was not observed

Numerical Modeling of Radial Fracturing of Cement Sheath Caused by Pressure Tests

To achieve an acceptable level of zonal isolation, well integrity should be guaranteed in hydrocarbon production and geological CO2 sequestration. Well pressure test can cause different types of failures in the well system leading to leakages through these failures. Laboratory evidences have revealed that occurrence of radial tensile fractures is likely during pressure tests. In this paper, we use a numerical code call MDEM which was formulated based on discrete element method. The code can model discontinuum feature of fractures. A model of a lab-sized pressure test was built and compared to an experiment previously published. The model was tested under different confinement levels and effect of the tensile strength of rock on the radial fracture was investigated at the same lab-scale. Fracture opening profiles are also presented showing the leakage potential of these fractures under different pressure level

Numerical Modeling of Radial Fracturing of Cement Sheath Caused by Pressure Tests

To achieve an acceptable level of zonal isolation, well integrity should be guaranteed in hydrocarbon production and geological CO2 sequestration. Well pressure test can cause different types of failures in the well system leading to leakages through these failures. Laboratory evidences have revealed that occurrence of radial tensile fractures is likely during pressure tests. In this paper, we use a numerical code call MDEM which was formulated based on discrete element method. The code can model discontinuum feature of fractures. A model of a lab-sized pressure test was built and compared to an experiment previously published. The model was tested under different confinement levels and effect of the tensile strength of rock on the radial fracture was investigated at the same lab-scale. Fracture opening profiles are also presented showing the leakage potential of these fractures under different pressure level

Numerical analysis of mixed-mode rupture propagation of faults in reservoir-caprock system in CO2 storage

Injection-induced seismicity and caprock integrity are among the most important concerns in CO2 storage operations. Understanding and minimizing fault/fracture reactivation in the first place, and rupture growth/propagation beyond its surface afterwards, are fundamental to achieve a successful deployment of geologic carbon storage projects. Rock fracture mechanics provides useful concepts to study the propagation of discontinuities such as pre-existing faults and fractures. In this paper, we aim at developing a methodology to investigate the rupture propagation likelihood of faults/fractures inside a reservoir and its surrounding (including the caprock) as a result of reservoir pressurization. In this methodology, mode I (tensile) and mode II (shear) stress intensity factors of a given fault/fracture are calculated based on Linear Elastic Fracture Mechanics. A fault/fracture can propagate either in mode I, mode II or a combination of both (also called mixed-mode) based on the comparison of the stress intensity factors and fracture toughness. The proposed methodology, which has been embedded into a hybrid Finite Element Method-Discrete Element Method in-house code called MDEM, has the capability to obtain the direction of mode I and mode II rupture in front of a fault/fracture tip. Two coefficients are defined as stress intensity paths (κ) for a fault/fracture, as the change of stress intensity factors for the two failure modes of a given discontinuity per unit pore pressure change of the reservoir after injection. Based on these stress intensity path coefficients, a relationship is given to calculate the threshold pressure buildup above which the two propagation modes may occur. We use the proposed methodology to investigate the rupture growth likelihood of faults in and around a closed reservoir due to its pressurization. Simulation results indicate that mode I failure is likely to occur inside the reservoir for faults with low dip angle in compressional stress regimes. However, the initiated mode I failure may not have the chance to grow upwards because the minimum principal is in the vertical direction and thus, the initiated rupture tends to rotate and grow horizontally. In contrast, mode I rupture is likely to occur in the caprock for faults with high dip angle in extensional stress regimes. The initiated rupture may grow upwards if the newly created fracture becomes hydraulically connected with the reservoir. We find that mode II rupture is not likely to occur in any of the investigated scenarios. Simulation results show that the coefficients of the stress intensity factors depend on the faults location, dipping angle, fault length, presence of other faults, reservoir aspect ratio and reservoir and caprock stiffness

Numerical analysis of mixed-mode rupture propagation of faults in reservoir-caprock system in CO2 storage

Injection-induced seismicity and caprock integrity are among the most important concerns in CO2 storage operations. Understanding and minimizing fault/fracture reactivation in the first place, and rupture growth/propagation beyond its surface afterwards, are fundamental to achieve a successful deployment of geologic carbon storage projects. Rock fracture mechanics provides useful concepts to study the propagation of discontinuities such as pre-existing faults and fractures. In this paper, we aim at developing a methodology to investigate the rupture propagation likelihood of faults/fractures inside a reservoir and its surrounding (including the caprock) as a result of reservoir pressurization. In this methodology, mode I (tensile) and mode II (shear) stress intensity factors of a given fault/fracture are calculated based on Linear Elastic Fracture Mechanics. A fault/fracture can propagate either in mode I, mode II or a combination of both (also called mixed-mode) based on the comparison of the stress intensity factors and fracture toughness. The proposed methodology, which has been embedded into a hybrid Finite Element Method-Discrete Element Method in-house code called MDEM, has the capability to obtain the direction of mode I and mode II rupture in front of a fault/fracture tip. Two coefficients are defined as stress intensity paths (κ) for a fault/fracture, as the change of stress intensity factors for the two failure modes of a given discontinuity per unit pore pressure change of the reservoir after injection. Based on these stress intensity path coefficients, a relationship is given to calculate the threshold pressure buildup above which the two propagation modes may occur. We use the proposed methodology to investigate the rupture growth likelihood of faults in and around a closed reservoir due to its pressurization. Simulation results indicate that mode I failure is likely to occur inside the reservoir for faults with low dip angle in compressional stress regimes. However, the initiated mode I failure may not have the chance to grow upwards because the minimum principal is in the vertical direction and thus, the initiated rupture tends to rotate and grow horizontally. In contrast, mode I rupture is likely to occur in the caprock for faults with high dip angle in extensional stress regimes. The initiated rupture may grow upwards if the newly created fracture becomes hydraulically connected with the reservoir. We find that mode II rupture is not likely to occur in any of the investigated scenarios. Simulation results show that the coefficients of the stress intensity factors depend on the faults location, dipping angle, fault length, presence of other faults, reservoir aspect ratio and reservoir and caprock stiffness.This publication has been produced with partial support from the BIGCCS Centre, performed under the Norwegian research program Centres for Environment-friendly Energy Research (FME). The first two authors acknowledge the following partners for their contributions: Gassco, Shell, Statoil, TOTAL, ENGIE, and the Research Council of Norway (193816/S60). V.V. acknowledges financial support from the “TRUST” project (European Community's Seventh Framework Programme FP7/2007-2013 under grant agreement no. 309607) and from “FracRisk” project (European Community's Horizon 2020 Framework Programme H2020-EU.3.3.2.3 under grant agreement no. 636811). Authors are also grateful to SINTEF Petroleum Research providing the MDEM code.Peer reviewe

Effect of faults on stress path evolution during reservoir pressurization

Fluid injection operations, such as CO2 storage and enhanced oil recovery (EOR), imply reservoir pressurization, which changes the effective and total stresses due to poroelastic effects. These stress changes control the geomechanical stability of discontinuities like faults and fractures. Though the effect of these pre-existing discontinuities on stress path is sometimes neglected, the stress state is altered around them. We investigate the effect of a fault on the stress path evolution when pressurizing a reservoir using an in-house hybrid FEM-DEM code called “MDEM”. Simulation results indicate that the stress path is affected by the presence of faults considered to deform elastically, especially in the vicinity of the fault in the reservoir-caprock interfaces. The stress path perturbation is caused by the shear deformation of the fault plane, which is different in the reservoir and the caprock sections. Actually, the magnitude and the extension of the stress path perturbation around a fault become larger for faults with lower shear stiffness. The upper hanging wall and the lower footwall of the fault in the reservoir-caprock interface experience a higher stress path in the horizontal and the vertical directions. Furthermore, the stress paths decrease (negative in the vertical direction) in the upper footwall and the lower hanging wall in the reservoir-caprock interfaces. The fault effect on the stress path increases as the aspect ratio of the reservoir becomes lower. Moreover, the results indicated that both the caprock and the reservoir in the footwall experience a greater change for lower Poisson's ratio of the caprock. These stress changes are independent of the in situ stress regime as long as the fault deforms elastically. However, the impact of the stress path perturbation on the stability of the reservoir and the caprock is different in a compressional (reverse faulting) and an extensional (normal faulting) stress regimes. The stress state becomes less stable in the vicinity of the fault in the reservoir and in the caprock in a compressional stress regime than in an extensional stress regime. Therefore, a compressional stress regime leads to a less stable situation due to the fault effect on the evolution of the stress path. Overall, the presence of faults alters the stress state around them, which may lead to a stress state that is closer to failure conditions than predicted by models that do not explicitly include faults. © 2017 Elsevier LtdThis publication has been produced with partial support from the BIGCCS Centre, performed under the Norwegian research program Centres for Environment-friendly Energy Research (FME). The first two authors acknowledge the following partners for their contributions: Gassco, Shell, Statoil, TOTAL, ENGIE, and the Research Council of Norway (193816/S60). V.V. acknowledges financial support from the “TRUST” project (European Community's Seventh Framework Programme FP7/2007-2013 under grant agreement no. 309607) and from “FracRisk” project (European Community's Horizon 2020 Framework Programme H2020-EU.3.3.2.3 under grant agreement no. 640979). Authors are also grateful to SINTEF Petroleum Research provding the MDEM code.Peer reviewe