Rock physics and geomechanics aspects of seismic reservoir monitoring

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ROCK PHYSICS FROM SMALL SAMPLES -SOMETIMES YOUR ONLY SOLUTION

ABSTRACT This paper illustrates the potential of using small shale samples, including cuttings, as a tool for determining rock physical parameters on field material. The measured static and dynamic mechanical results are basically consistent with those attained on larger, standard plugs in triaxial cells. Consequently, small samples can be used as a complementary tool to larger samples, providing faster and less expensive results with less material consumption, also allowing for larger test matrices. In some cases, use of small samples may actually be the only solution to obtain direct measurements on the shale material. INTRODUCTION To reach hydrocarbon reservoirs, one often has to drill through various shale sequences. Borehole stability problems may occur while penetrating these shale sections, potentially adding substantial costs to the drilling operations. In order to optimise the drilling procedures, this calls for a better rock physical characterization of the shales, as well as an understanding of how factors like mud composition affect the shale properties

Rock physics and geomechanics aspects of seismic reservoir monitoring

see Abstract VolumeIstituto Nazionale di Geofisica e Vulcanologia, Italy (INGV) Centre National de la Recherche Scientifique (CNRS) ExxonMobil Upstream Research CompanyUnpublishedErice, Italyope

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

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

Third-order elasticity of transversely isotropic field shales

The formations above a producing reservoir can exhibit large mechanical changes, creating a risk of significant subsidence and loss of rock integrity. These changes can be monitored by time-lapse seismic acquisition, which measures the corresponding velocity changes via time-shifts. Third-order elastic theory can be used to connect subsurface strains and stress changes to these seismic attribute changes. Existing models assume isotropic strain dependence of the dynamic stiffness in shales. It is important to re-evaluate this isotropic assumption considering the inherent anisotropy of shales and their abundance in the overburden. Thus, we instead propose a third-order elastic model with a transversely isotropic strain dependence of the dynamic stiffness. When calibrated, this new model satisfactorily predicted P-wave velocity changes determined in undrained laboratory experiments conducted on overburden field shales, covering a wide range of propagation directions and stress variations. The shales exhibit anisotropic dynamic strain sensitivity, resulting in a significantly higher strain sensitivity predicted for Thomsen's anisotropy parameters epsilon and delta subjected to a uniaxial strain parallel to the horizontal bedding plane compared to the vertical direction. Geomechanical modelling, considering a depleting disk-shaped reservoir surrounded by shales, was employed to predict the dynamic stiffness changes of the overburden using the laboratory-calibrated third-order elastic model. The overburden time-shifts increased with offset angle, peaking at about 45°, suggesting a strong influence of shear strains on the time-shifts. In contrast, a corresponding model with an isotropic third-order elastic tensor, calibrated to the same data, exhibited a significantly lower sensitivity to the shear strains. These results underscore the importance of considering the anisotropic strain dependence of the dynamic stiffness when studying shales. Interpreting offset-dependent trends in pre-stack time-lapse seismic data, along with geomechanical modelling and an appropriate strain-dependent rock physics model, can assist in quantifying subsurface strains and stress changes.</p