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The exploration of fractured-vuggy carbonate reservoirs usually involves hydraulic fracturing to maximize recovery. At present, effectively communicating natural discontinuities is a technical challenge. In this article, we investigated the origin and propagation of cracks in fractured-vuggy reservoirs using discrete element hydraulic fracturing simulations that included poromechanical effects. A particular focus on the microscopic force-displacement symmetry of adjacent pore pressures is introduced. Our results demonstrate that the poromechanical effect significantly increases the strength of overpressurized reservoir formations. Moreover, the effect of injected fluid viscosity on the hydraulic fracturing effectiveness was studied through two simulation tests. The outcomes highlight the critical influence of fluid viscosity on the propagation of micro-cracks in overpressure fractured-vuggy reservoirs. [ABSTRACT FROM AUTHOR]

This paper deals with the leader-following consensus control for a class of parametric output feedback non-linear multi-agent systems. To design distributed control laws without using agent's own out information, non-linear functions of agent's own output are transformed into non-linear functions of relative output information using mean value theorem and variable separation technique. By introducing an input-driven filter and employing adaptive backstepping method, distributed adaptive non-linear control laws are designed using only relative output measurements. The proposed control law is also independent of the Laplacian matrix. Therefore, it can solve the leader-following consensus problem for any directed communication graph that contains a spanning tree with the root node being the leader agent. A numerical example is given to illustrate the effectiveness of the proposed scheme. [ABSTRACT FROM AUTHOR]

Several experimental methods are used to investigate the swelling capacity and transport properties of re-sealed macro-cracked Callovo–Oxfordian (COx) claystone, particularly in the absence of confining pressure. Six COx claystone samples from four different geological locations of the Bure basin (France) are tested, five of which are macro-cracked and one remains intact. Sample swelling occurs, during re-hydration with liquid water, leading to the measurement of an apparent swelling pressure. The latter is continuously recorded with a dedicated device. The asymptotic apparent swelling pressure of macro-cracked UT (transitional unit) COx is approximately 1 MPa, while it varies from 3 to 5 MPa for macro-cracked UA (clayey unit) COx. Quantitative X-ray diffraction (QXRD) analysis demonstrates that the amount of smectite, which is a swelling clay, is weakly correlated with apparent swelling pressure. Surprisingly, the interstratified illite/smectite with lower smectite content is highly correlated to apparent swelling pressure. Nitrogen isotherms data imply that the Gurvich total pore volume (VGurvich) and specific surface area (SSA) are highly linearly correlated with the low smectite content interstratified phase. This means that the distribution of smectite strongly affects the swelling capacity of COx. Moreover, nitrogen sorption is an easier and more effective technique than QXRD for assessing COx swelling capacity, since both VGurvich and SSA have been proven as effective indicators. For both UT and UA COx, self-sealing can cause significant reductions in water permeability (Kw). In particular, UA COx shows higher sealing efficiency and faster kinetics compared to UT COx. After sealing, the equivalent crack aperture (calculated from Poiseuille's law) decreases from tens of microns to less than 1 micron. According to the gas breakthrough tests, the gas breakthrough pressure (GBP) of re-sealed macro-cracked COx is of the same order of magnitude as the equivalent capillary pressure of residual crack. This indicates that the gas migration in the re-sealed cracked COx claystone mainly occurs through the residual crack and is 'a priori' controlled by capillary processes. Highlights: A dedicated device was designed to study the transport and swelling properties of a sealed COx claystone under low confinement. The apparent swelling pressure and water permeability were continuously recorded during the sealing test due to water injection in fractures. Water permeability of samples decreased by orders of magnitude to almost meet the intact material permeability. Gas breakthrough experiments performed on sealed samples evidenced gas flow through the residual cracks. [ABSTRACT FROM AUTHOR]

As an elementary mesh quality improvement technique, smoothing has been widely used in finite element (FE) analysis. Heuristic smoothing methods and optimization-based smoothing methods are the two main smoothing types. The former is efficient. However, it operates heuristically and may create low-quality elements. In contrast, optimization-based smoothing is very effective at improving mesh quality. However, it suffers from high computational cost since it calculates the optimal position of a free node iteratively. In this paper, we present a new smoothing method. The proposed method imitates the optimization-based smoothing based on a neural network, but it calculates the optimal position of a free node straightforwardly. Hence, the proposed method is more efficient than these optimization-based smoothing methods while being comparable in terms of mesh quality. We present various testing results to illustrate the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

This paper studies the distributed output feedback consensus control for a class of uncertain nonlinear multi-agent systems (MASs). The dynamics of subsystems are heterogeneous and contain unknown control gains. A novel distributed fuzzy consensus control protocol is constructed by employing variable separation technology and adaptive backstepping technique. The main advantage of the protocol is that it is distributed only depending on the relative output information. Moreover, it does not require the observer information exchange, which can avoid network attacks. A numerical example is given to illustrate the effectiveness of the proposed scheme. [ABSTRACT FROM AUTHOR]

This paper deals with the ship course tracking control problem in a novel inverse optimal control approach. The inverse optimal stabilization problem and inverse optimal gain assignment problem are firstly extended to general systems affine in the control with unknown control gain. It is shown that a sufficient condition to solve the inverse optimal control problem is the existence of a stabilization control law in a special form for a corresponding auxiliary system. Then, by employing backstepping technique, control laws are designed which solve the inverse optimal stabilization, inverse optimal adaptive stabilization and inverse optimal adaptive gain assignment problem of ship course control system, respectively. Simulations are included to illustrate the effectiveness of the proposed control algorithms. [ABSTRACT FROM AUTHOR]

This paper deals with the leader-following consensus problem for a class of multi-agent systems with nonlinear dynamics and directed communication topology. The control input of the leader agent is assumed to be unknown to all follower agents. A distributed adaptive nonlinear control law is constructed using the relative state information between neighboring agents, which achieves leader-following consensus for any directed communication graph that contains a spanning tree with the root node being the leader agent. Compared with previous results, the nonlinear functions are not required to satisfy the globally Lipschitz or Lipschitz-like condition and the adaptive consensus protocol is in a distributed fashion. A numerical example is given to verify our proposed protocol. [ABSTRACT FROM AUTHOR]

This paper deals with the robust consensus tracking problem for a class of heterogeneous second-order nonlinear multi-agent systems with bounded external disturbances. First, a distributed adaptive control law is proposed based on the relative position and velocity information. It is shown that for any connected undirected communication graph, the proposed control law solves the robust consensus tracking problem. Then, by introducing a novel distributed observer and employing backstepping design techniques, a distributed adaptive control law is constructed based only on the relative position information. Compared with the existing results, the proposed adaptive consensus protocols are in a distributed fashion, and the nonlinear functions are not required to satisfy any globally Lipschitz or Lipschitz-like condition. Numerical examples are given to verify our proposed protocols. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

This paper deals with the leader-following consensus problem for a class of multi-agent systems with nonlinear dynamics and directed communication topology. By introducing a novel distributed observer and employing the backstepping methodology, a distributed adaptive nonlinear control law is constructed using the relative output information between neighbouring agents. For any directed communication graph that contains a spanning tree with the root node being the leader agent, the proposed control law solves the leader-following consensus problem. Compared with the existing results, our proposed adaptive consensus protocol is in a distributed fashion and the nonlinear functions are not required to satisfy any globally Lipschitz growth or Lipschitz-like growth condition. A numerical example is given to illustrate the theoretical results. [ABSTRACT FROM AUTHOR]

This paper focuses on Biot's coefficient measurement of saturated COx claystone. The tests were systematically conducted to provide clarification on its value as the available literature indicates quite erratic results from 0.26 to 1. The difficulty of this work mainly lies in the time that is necessary to stabilize each loading step (confining pressure and/or pore pressure) and also in sample preparation. Two main results were obtained: 1) Whatever the initial conditions were (confining pressure P c amplitude, hydric state, damaged or not...), all samples exhibited a significant anisotropic swelling during the first water injection; 2) even being a transversely isotropic rock, its Biot's tensor is (quasi) isotropic thus leading to a unique coefficient " b " which is very close to 1. Such results are consistent with what was observed with gas as being the fluid to control pore pressure in partially saturated samples. • Poro-mechanical tests on COx claystone samples, focused on Biot's tensor component measurements, were performed with water. • Significant anisotropic swelling was observed on every sample after the first water injection. • The measured Biot's tensor with water is isotropic with a unique component ' b '. • COx claystone still behaves as a Terzaghi's material (b ≈1) at a saturated state. • Consistency of these results was observed with results obtained with gas being the pore fluid. [ABSTRACT FROM AUTHOR]

This study addresses the output‐feedback control problem for a class of Markovian jump non‐linear systems under a quadratic risk‐sensitive cost function criterion. The transition probabilities of the Markov process are assumed to be completely unknown. By employing the high‐gain scaling technique, common Lyapunov function method and backstepping technique, a control law is constructed that guarantees any arbitrary small risk‐sensitive cost for a given risk‐sensitive parameter. Moreover, the resulted closed‐loop system solutions are bounded in probability. Compared with some previous results, this study does not require the uniform boundedness of the gain functions of the system noise, and the control law further achieves a zero risk‐sensitive cost and asymptotically stable in the large for the closed‐loop system solutions when the vector field of the disturbance vanishes at the origin. A numerical example is given to illustrate the proposed protocol. [ABSTRACT FROM AUTHOR]

Wetting–drying cycles are likely to induce damage of claystone. To evaluate and to prevent micro-cracking impacts on Callovo-Oxfordian (COx) argillite, which is, in France, the potential host rock of structures containing nuclear wastes, 18 months of tests were carried out on different series of samples. These tests consisted of gas permeability and Young's modulus measurements. They were carried out on dry material, as a reference state, previously submitted to successive cycles of wetting–drying. The results show without any ambiguity that repeated hydric cycles lead to damage i.e. additional micro-cracking that induces an increase in gas permeability (possibly by 2–3 orders of magnitude compared with the intact one) and decrease in Young's modulus by 5–15%. This means that strong precautions have to be followed to keep stable the initial material water content prior to the tests and to study a material representative of its in-situ state. [ABSTRACT FROM AUTHOR]

A correction to this paper has been published: https://doi.org/10.1007/s00603-021-02419-w [ABSTRACT FROM AUTHOR]