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DNA nanotechnology offers a fine control over biochemistry by programming chemical reactions in DNA templates. Coupled to microfluidics, it has enabled DNA-based reaction-diffusion microsystems with advanced spatio-temporal dynamics such as traveling waves. The Finite Element Method (FEM) is a standard tool to simulate the physics of such systems where boundary conditions play a crucial role. However, a fine discretization in time and space is required for complex geometries (like sharp corners) and highly nonlinear chemistry. Graphical Processing Units (GPUs) are increasingly used to speed up scientific computing, but their application to accelerate simulations of reaction-diffusion in DNA nanotechnology has been little investigated. Here we study reaction-diffusion equations (a DNA-based predator-prey system) in a tortuous geometry (a maze), which was shown experimentally to generate subtle geometric effects. We solve the partial differential equations on a GPU, demonstrating a speedup of &sim, 100 over the same resolution on a 20 cores CPU.

The scope of this paper is to investigate rate effects on rock cutting process at bottomhole pressure . The solution of the cutting problem is derived in the framework of the theory of poroelastoplasticity. The problem is firstly solved analytically using some assumptions derived from the literature review. Then, a numerical resolution of the fully coupled problem is performed based on the finite element method . These two different approaches show that pore pressure in shear-dilatant rocks decreases as a function of the cutting velocity depending on rock permeability and interstitial fluid properties. Pore pressure change has a hardening effect leading to an increase of the rock drilling resistance.

International audience

International audience; The purpose of this study is to examine cutting speed and bottomhole pressure effects on cutting forces acting on PDC cutters during the oil and gas drilling process. A set of single cutter tests was performed in three sedimentary rocks of different permeabilities. Dry tests were carried out at atmospheric pressure and at different cutting speeds. As previously observed in the literature, these tests show that cutting forces increase with the cutting speed, especially the normal component. The same tests were performed at 20MPa bottomhole pressure. It appears that the cutting speed effect on cutting forces in the medium and low permeability rocks is moderate and weak, respectively. By contrast, rate effects in the highly permeable rock are significant and overcome rate effects observed in dry experiments by an order of magnitude. Interestingly, these observations are similar to those observed in submarines oil ploughing at high hydrostatic pressure. These results are believed to represent an important step into a deeper understanding of the oil and gas drilling process.

International audience

Summary First, this paper presents the new developments integrated in a recently advanced model for drillstring mechanics and takes into account the buckling phenomenon in any actual well trajectory. Second, this paper shows the influence of tortuosity and friction on the buckling phenomenon for some practical and critical cases met in the drilling industry. These tortuosity and friction effects are demonstrated with an experimental setup that confirms theoretical features. Finally, we compare results obtained from existant models with results obtained from our new model to evaluate the tortuosity and friction effects on the critical buckling load found in the literature.

The process of foundation works such as diaphragm walls or panels is sometimes penalised by the presence of very hard soil in the anchoring layer. Today, the Hydrofraise allows reasonable drilling rates for soils with compressive strength up to 100 MPa. The Versatile Fraise will be a new tool to extend this limit up to 200 MPa. The study begins with the analysis of the rock crushing phenomenon under carbide bits, is followed by the interaction of a group of bits, then by small scale tests with a roller. Scale 1 tests follow up using the definitive geometry of the drums on full size granite blocks. The definitive device still needs some engineering, but will soon be fully operational.

Access to reservoirs in ultra-deep and extended reach drilling projects leads the industry to develop new resistant and higher-strength materials. Drilling equipments manufacturers and drilling contractors have to meet higher requirements of operators to manufacture and operate quality tubulars in these severe environments. However, with the increasing complexity (deep, long, high-pressure/high-temperature, sour service) of drilled wells, steel may become a limiting drill pipe material. A non-steel alternative material may exist such as aluminum, titanium or composite materials. Buckling of steel, aluminum and titanium tubulars inside wellbore has been the subject of researches and articles in the past. According to some authors, the light Young Modulus of lightweight drill pipes does not affect their buckling response: the relative low stiffness of the pipes would be balanced by reduced contact forces with the wellbore. This paper presents a full study discussing clearly the buckling response of steel and lightweight drill pipes in both experimental bench and real field conditions. This paper shows a full comparative study between steel, aluminum and titanium drillstrings in Extended Reach Drilling (ERD) wells and studies the opportunity to use aluminum and titanium drillstring in ERD wells while the buckling, drilling efficiency and drillstring integrity stay acceptable. Critical buckling loads for each drill pipe material and dimension is reviewed and presented, using our numerical model and in the form of charts which could be used by drilling engineers. Along this comparison, we also focus on drillstring rotation, tool-joints and dimensions effects on buckling loads. Finally, this paper should give a better comprehension of the mechanical behavior of lightweight material for drill pipe, which can improve significantly well planning to drill wells with an increasing depth, length and complexity.

The giant Chicontepec field contains oil from 18 to 45 °API in laminated sandstones of 0.1 to 10 mD at a depth of around 2500 meters (8202 ft). Original Oil in Place (OOIP) is estimated to be 140, 900 MMSTB. The complex geology (complicated structural and stratigraphic nature of the reservoirs), lack of reservoir information and lack of technology availability caused a gap between discovery and development. Throughout a period of several decades some exploration wells were drilled based on 2D seismic and log correlations of the reservoirs.The exploitation of the Paleonchannel was postponed because most of the wells showed poor productivity. The reasons for the low recovery (around 3%) have never been thoroughly understood. Various hypotheses have been proposed to explain the deficient performance such as partial closing of the fractures with declining reservoir pressure (bubble-point pressure is near initial pressure), inadequate comprehension of the geological model, deficiency in the fracturing technology, oil-wetted or intermediate-wetted reservoirs, applicability of unconventional wells (horizontal wells, casing drilling technology), etc. Today, the Chicontepec Paleochannel is an intermediate stage.Due to the experience of different fields with similar characteristics, this paper describes an analysis of alternatives that may be considered to resolve the problems of exploitation at the Chicontepec field. Advanced technologies, hydraulic fractures, artificial lift systems, all of them combined with secondary and enhanced oil recovery, may be feasible to sustain or increase production. A number of hurdles will have to be overcome. This field, the second most important oil field in Mexico, should take advantage of the experience learned from these analogous reservoirs.

Axial force transfer is an issue in deviated wells where friction and buckling phenomenon take place. The general perception of the industry is that once drill pipe exceeds conventional buckling criteria, such as Paslay-Dawson, axial force cannot be transferred down-hole anymore. This paper shows that, even though buckling criteria are exceeded, axial force transfer could be still good if drill pipe is in rotation. On the contrary, there exists sliding operations where lockup is observed, due to buckling, even though standard buckling criteria are not exceeded. This paper is intended to show and explain how axial force is transferred down-hole in many simulated field conditions: sliding, rotating, with or without dog legs. These new results have been obtained from an advanced model dedicated to drill string mechanics successfully validated with laboratory tests. This paper will show applicable results for practical well operations where axial force transfer is an issue. These results will enable to give some guidelines to help the drilling engineer to select cases where conventional buckling criteria should be used cautiously. Indeed, simultaneous torque-drag-buckling calculations show that tubular can tolerate significant levels of compression, enabling to provide weight transfer to the drill bit, even though drill pipe is buckled. Others examples, in contrast, show that standard buckling criteria cannot predict the occurrence of buckling that may cause tubular lockup while tripping in the hole. The applications of these results are numerous for all deviated wells such as horizontal or extended reach drilling wells. This paper should contribute to reduce unpredictable lock-up situations and improve axial load transfer performance.

Abstract In the today high-cost and complex drilling environment, the importance of drillstring failure issue has dramatically reappeared, in spite of many manufacturing and materials improvements. Most drillstring failures are due to fatigue, resulting from repeated cyclic bending loads and stresses in tensile or buckled drill pipes. Fatigue prediction is usually based on the cumulative fatigue damage model from Hansford and Lubinski as defined in API RP7G. This model, based on S-N curves, a failure criterion and a damage accumulation rule, initially requires a calculation of the drillpipe stress caused by bending when rotated in a dog leg. This bending stress calculation, key point of the cumulative fatigue damage model, is usually made by assuming that the curvature of the drill pipe is the same as the dog leg. However, this paper shows that this strong hypothesis may lead to major under-estimation of the cumulative fatigue damage. Moreover, the stress distribution within a drill pipe may be completely different depending mainly on the position of the drill pipes along the drillstring and the wellbore architecture and tortuosity. The cumulative fatigue damage model as defined in API RP7G has been implemented in an advanced torque and drag model, which enables to track any given point of the drill pipe while drilling, such as the transition zone, the tool joints and the drill pipe body. For the first time, it has also been possible to fully track variation of stresses at a given point in the drill pipe. Based on drill pipes S-N curve available in the literature and actual drilling data, this paper shows and compares results of fatigue damage calculations as obtained from the conventional way (strong hypothesis on the contact) with results obtained from advanced torque and drag model that make no assumptions about the contact. This extensive study as presented in this paper has never been done in the past. This advancement should probably lead to minimize the risk of drillstring failures in complex wells by a better monitoring of stresses in drill pipe. Introduction Drill pipe failures are still responsible for rising costs in drilling industry. In spite of many research studies carried out to mitigate this issue, it continues to occur with high frequency. Many drill pipe failures analyses have shown that fatigue accounts for the majority of drillstring failures. Fatigue is a cumulative and non-reversible phenomenon, resulting from repeated cyclic bending loads and stresses in tensile or buckled drill pipe. Fatigue occurs even if the cyclic stress is much lower than the static strength limit of the drillpipe material. Fatigue prediction is based usually on the Cumulative Fatigue Damage model (CFD), introduced by Hansford and Lubinski in API RP7G1. This model allows determining the life duration of the drill pipe based on S-N curves, damage accumulation rules and a failure criterion. However, it requires calculation of the drill pipe stress in the wellbore, which is the key point in fatigue prediction. Bending stress calculation has first been performed by Lubinski in case of rotated drill pipe subjected to tension, and Paslay suggested improvement by studying the case of compression. Lubinski/Paslay model is based on strong hypotheses on the contact between drill pipe and wellbore, and assumes the axial load to be known. The aim of this paper is to address a new approach to assess the bending stress in the drillpipe. This methodology allows implementing the Cumulative Fatigue Damage model as defined in API RP7G, in an advanced torque/drag and buckling model 2,3,4, which allows tracking any given drill pipe while drilling.

Buckling of tubulars inside wellbores has been the subject of many researches and articles in the past. However, these conservative theories have always followed the same assumptions: the wellbore has a perfect and unrealistic geometry (vertical, horizontal, deviated, curved), the friction and rotation effects are ignored, conditions relatively far from actual field conditions. How do tubulars buckle in actual field conditions, that is, in a naturally tortuous wellbore with friction and rotation ? Can we apply theories developed for perfect well conditions (no tortuosity, no friction, no rotation) to actual well conditions ? For the first time, this paper presents how the drillstring rotation affects the critical buckling load in actual field conditions. These new results have been obtained from an advanced model dedicated to drillstring mechanics successfully validated with laboratory tests. Firstly, this paper presents the new developments integrated in a recently advanced model for drillstring mechanics that enables to take into account the buckling phenomenon in any actual well trajectory. Indeed, some simultaneous torque-drag-buckling calculations are presented and allow to properly take into account the additional contact force generated in a post-buckling configuration, and as a consequence the additional torque at surface. Secondly, this paper shows the influence of friction and rotation on buckling loads for some practical and critical cases met in the drilling industry. These friction and rotation effects are demonstrated with an experimental set up that enables to confirm theoretical features. Lastly, this paper shows that using standard buckling criteria may lead to too conservative solutions, and that under specific circumstances, the drilling and completion engineer could safely operate in a buckling mode for a given time. These new results presented in this paper should improve significantly well planning and operational procedures to drill and operate more and more complex wells.

Buckling of tubulars inside wellbore has been the subject of many researches and articles in the past. However, these theories have always followed the same assumptions: the wellbore has a perfect and unrealistic geometry (vertical, horizontal, deviated, curved) and the friction is ignored, conditions relatively far from actual field conditions. How do tubulars buckle in actual field conditions, that is, in a naturally tortuous wellbore with friction and rotation ? Can we apply conservatively theories developped for perfect well conditions (no tortuosity, no friction) to actual well conditions ? This paper gives answers to these questions in comparing results obtained from existant models with results obtained from an advanced model dedicated to drillstring mechanics. Firslty, this paper presents the new developements integrated in a recently advanced model for drillstring mechanics and enabling to take into account the buckling phenomenom in any actual well trajectory. Secondly, this paper shows the influence of tortuosity and friction on the buckling phenomenom for some practical and critical case met in the drilling industry. These tortuosity and friction effects are demonstrated with an experimental set up that enables to confirm theoretical features. Lastly, we compare results obtained from existant models with results obtained from our new model to evaluate the tortuosity and friction effects on the critical buckling load found in the litterature.

Interbedded formations hard/soft or soft/hard are a major cause of borehole tortuosity. Cases history have demonstrated that this tortuosity induces a higher torque and drag, running tubular problems, stabilizers wear, pipe damage and trajectory controlling problems. In some fields, shale formations have a tendency to cause wellbore deviations to undesired directions. The formation anisotropy modifies the rock-bit and the bit-string interactions. Those interactions have to be fully understood in order to eliminate the phenomena of tortuosity. To do so, an experimental drilling program has been carried out on a full-scale bench using various polycrystalline diamond compact (PDC) drilling bits (different gauges, profiles) in different formations (hard/soft, soft/hard with different interface angles). Some directional tests have been also carried out in anisotropic shale at several dip angles. Coupling a 3D bit-rock model with a 3D bottomhole assembly (BHA) model enables to predict the occurrence of tortuosity (due to the anisotropy) at small and large scales (micro and macro-tortuosity) and to evaluate feet by feet the response of all directional drilling systems (rotary, steerable motors or rotary steerable systems). Deviations caused by formation anisotropy are separated into two phenomena. An initial deviation is caused by the rock-bit interaction. The experimental and theoretical results explain how the gauge length, bit profile, and dip angle, can affect the amplitude of this deviation. A second deviation is generated when the different stabilizers are sliding through the initial one. They can be amplified or attenuated according to the drilling system used. The number of stabilizers and their positions, BHA characteristics and bit steerability have a considerable effect on these deviations. A post analysis of some real drilling cases showed that, for a given drilling conditions including the formations anisotropies, it is possible to select the best drilling system minimizing the well tortuosity.

PDC drill bit performances in hard rock has been greatly improved during the last decades by innovations in PDC wear, impact resistance and better vibrations understanding. The bit design is generally done by balancing the bit, distributing uniform wear along the profile and achieving high drillability and steerability. To obtain required drilling performances, drill bit designer adjust features such as profile shape, gage and mainly cutter characteristics (shape, type and orientation). Cutter rock interaction model became a critical feature in the design process. But previously used models considered only three forces on a cutter based on the cutter-rock contact area: drag force, normal force and side force. Such models are no longer valid with the introduction of PDC cutters with chamfer and special shape. This paper presents a new cutter rock interaction model including some several improvements. It is based on the presence of a build-up edge of crushed materials on the cutting face often described in the literature. In addition, the chamfer, which significantly affects bit Rate Of Penetration (ROP), is taken into account (shape and size). Forces applied on the back of the cutter and due to the rock deformation and back flow of crushed materials are considered in the model. Finally, results of numerous single cutter tests (under atmospheric and confining pressure) are presented and compared to the new cutter rock interaction model predictions. An analysis of the influence of the PDC characteristics (shape, size, chamfer, back and side rake angles, ….) is presented. The model has been applied to optimize the cutting efficiency and bit steerability and some design rules are given to minimize the specific energy and maximize the rate of penetration. Finally, full scale laboratory drilling tests and field results indicate that the use of accurate cutter rock interaction model can help the drill bit designer to find the best drill bit for a specific application. Standard laboratory full scale drilling procedures have been developed. The tests have shown that drillability, stability, steerability and wear can be improved and controlled by acting on the cutter characteristics, cutter setup, trimmer characteristics and gage type.

The well trajectories becoming more and more complex (Extended Reach Drilling, Horizontal or Multilateral Drilling, Ultra-Deep Drilling, 3D wells….), the associated drillstring composition being unconventional and the material being used to its operating limit, the necessity to have a tool that predicts realistically forces, bending moment and contact loads along the wellbore is essential. To simulate the mechanical behavior of the drillstring, the industry generally uses a stiff-string model for the bottom bole assembly (BHA) to predict drilling trajectories, and a soft-string model for the entire drillstring to calculate torque and drag. The conventional soft-string model gives a good approximation of forces and contact loads in the drillstring for very smooth well trajectories, but is inappropriate when the trajectory becomes tortuous (micro or macro-tortuosity) or complex, as the drillstring no longer contacts the low side of the borehole. An advanced numerical method has been applied to address the 3D mechanical problem of a complete drillstring moving and freely rotating inside a wellbore: computation of the unknown contacts between the drillstring and the wellbore. As this new model does not use the time consuming finite element analysis, it can be used in real-time drilling operation at the rig to monitor torque and drag. For the first time, it is even possible to perform simultaneously a torque and drag, buckling and directional analysis while drilling. The power of a 3D visualization of the drillstring deformed inside the wellbore enables to localize easily contact loads on any drillstring component, from the drilling bit (including side force at the bit and tilt) to the top drive or rotary table (hook load and rotary torque at surface), facilitating drilling problems analysis. This paper will describe the fundamentals of the model developed and shows the differences between the conventional soft-string model and the advanced stiff-string model in terms of tension, torque and contact points between the drillstring and the wellbore, for many actual wells. With the help of the 3D visualization software, this paper will show the cases where the drill pipe contacts the high side of the borehole, or goes up to the right or the left side of the borehole depending on the right or left turn rate of the well trajectory. This new model should improve significantly the torque-drag and buckling calculations and the understanding of the drillstring contributions to overall drilling performance.

International audience

The purpose of this paper is to analysis bit stick-slip motion by using rock mechanics considerations coupled with field bottomhole data. The data was collected using an instrumented PDC bit. This new tool enables to measure accelerations, Weight-On-Bit (WOB) and Torque-On-Bit (TOB) directly above the bit. Tests were conducted in Montrose (Scotland) at about 700 m depth in a consolidated Devonien sandstone. During several hours of testing, the sensitivity to operating and different structural parameters was evaluated. The effect of stick-slip on Rate Of Penetration (ROP) was quantified: ROP is really affected by this dynamical phenomenon which appears above a certain Weight-On-Bit threshold. Two stages rule the drillstring behaviour. The stationary behaviour (small oscillation of the rotation bit speed around the mean value) is described by modal analysis. Experimental frequencies are in agreement with the theoretical natural ones. A perturbation of the operating parameters (generally an increasing of the WOB) makes the system bifurc towards stick-slip. The dynamical system is now governed by a non linear autonomous boundary condition, linked to the rock/bit interaction. Rock mechanics considerations justifie such a law. Numerical simulations converge on stick-slip, typical limit cycle experimentally exhibited. The last analysis is focused on an analysis of "drill-off test". In this operating stage, the rock/bit interaction function changes. Numerical simulations, justified by clearly experimental evidence, show the benefit effect of such a test, which stabilizes the system. A discussion on the effect of the stabilizers and bit type follows as well as how to attenuate this non linear phenomenon.

Recent developments in PDC bit modelling and single cutter analysis have enabled to design a new generation of cutting structures – based on the optimization of a specific interactive cutting mode – that generates, by the mere drilling action of the cutters, an axial force oriented in the drilling direction: a negative weight on bit. This paper presents the application of this new PDC bit design and shows that WOB can be significantly reduced in a wide range of formations. A monobloc 8″1/2 PDC bit was designed, tested in laboratory, and self penetration was clearly achieved in field conditions. A full PDC bit design methodology using a new PDC cutters geometry optimized for a maximum thrust effect, along with specific manufacturing procedures for high-level precision in cutter positioning, were developed for this specific new bit generation. Laboratory tests showed that it was possible to drill without WOB at a high rate of penetration and with a small increase in specific energy. Preliminary field tests in cement demonstrated that self penetration – drilling with the neutral point just above the bit – was possible with specific surface control, and a first operational application was conducted by Total E&P Angola. This new bit technology has the potential to increase bit performance, specifically in operating conditions where power is available but WOB is limited due to friction, buckling or reduced vertical depth. The ability to drill with reduced WOB – or even without any WOB – should extend the potential of existing drilling technologies such as Extended Reach Drilling, Coiled Tubing or Steerable Motor in sliding mode and reduce axial vibration, thus increasing the MTBF of downhole tools.

International audience; Deep-hole drillstring vibrations are an important cause of premature failure of drillstring components and drilling inefficiency. PDC bits are more susceptible to the stick-slip phenomenon characterized by intense RPM fluctuations of the drill bit. Based on full scale laboratory drilling tests and numerical simulations, this paper aims at understanding the dynamic behaviour of the bit / rock interaction and assessing how stick-slip depends on bit design (bit profile and diameter; cutters geometry and set up). It is generally assumed in the scientific and technical literature that forces acting on PDC cutters do not depend on cutter velocity. However, an extensive single cutter experimental program shows that these forces are rate-dependent for four tested carbonate rocks. This rate effect is mainly due to the dynamic shearing of a layer of crushed rock carried away beneath the moving cutter. Based on these experiments, a semi-empirical rate-dependent cutter-rock interaction model is developed, implemented in a bit design software and used to predict the velocity signature of various PDC bit designs. These predictions fit well with numerous experimental results obtained using a full scale drilling bench. In particular, the model accounts for the negative damping effect, considered as the main source of stick-slip, and for important operational trends like an increasing risk of stick-slip in harder rocks, at higher weight-on-bit or for advanced cutter wear. The bit-rock interaction model is used as the boundary condition of a drillstring vibration software designed to perform a time-domain analysis of lumped systems. Simulations show that the bit design significantly impacts the risk of stick-slip. As a result, general optimization guidelines are suggested in order to improve PDC bit stability and ultimately to design an "anti-stick-slip" drill bit.

The optimization of the drilling practice requires a better understanding of drillstring harmful vibrations such as stick-slip. This form of torsional vibrations is a typical problem of PDC (Polycristalline Diamond Compact) drillbits. It can reduce the rate of penetration drastically and can raise fatigue of the drilling devices. Many attempts were carried out in the last years in order to determine the causes of stick-slip phenomenon. Field observations show that torque on bit decreases as a function of bit velocity during stick-slip oscillations. Hence, it is widely believed that this decreasing relationship is the root cause of stick-slip.The purpose of this work is to examine cutting speed influence on rock drillability as a function of operating conditions and hydromechanical properties of the drilled formation. For this, a set of drilling tests was performed in three sedimentary rocks of different permeability using a full scale PDC drillbit and a single PDC cutter. In the first step, dry tests were carried out at atmospheric pressure. As previously observed in literature, single-cutter tests showed that drilling forces increase with cutting velocity. In a second step, we performed the same experiments at 20 MPa bottom-hole pressure. It appears that rate effect on cutting forces in the medium and low-permeability rocks is relatively low. By contrast, rate effect in the highly permeable rock increases by one order of magnitude in comparison with dry experiments.In order to understand this phenomenon, a steady state solution of the cutting model is derived in the framework of the theory of poroelastoplasticity. The problem is firstly solved analytically using some assumptions derived from previous works. Then, a numerical resolution based on finite element method is presented to solve the fully coupled problem ensuring the satisfaction of poro-material physics basic equations. Using these two different approaches, we show that pore pressure in shear-dilatant rocks decreases as a function of cutting velocity depending on rock permeability and interstitial fluid properties. This change has a hardening effect resulting in an increase of rock drilling resistance. Comparison between theory and experience shows good agreements.; L'optimisation des systèmes de forage nécessite une meilleure compréhension des vibrations indésirables comme le stick-slip. Ce phénomène vibratoire, qui affecte principalement les outils PDC (Polycristalline Diamond Compact), met en péril l'intégrité des équipements de forage et réduit considérablement la vitesse de pénétration de l'outil. Plusieurs travaux ont été menés ces dernières années pour déterminer ses origines. Les observations réalisées en fond de puits montrent que ces oscillations s'accompagnent systématiquement d'une baisse du couple à l'outil en fonction de sa vitesse de rotation. De nombreux groupes de recherche attribuent cette baisse de performance à l'occurrence du stick-slip.L'objectif de ce travail est de développer un modèle élémentaire de coupe qui permet d'analyser l'effet de la vitesse de coupe sur la forabilité des roches dans des conditions opératoires réalistes. Dans le cadre de cette thèse, nous avons réalisé une série d'essais de coupe en utilisant des taillants et des outils à échelle réelle dans trois roches de propriétés hydromécaniques différentes, et ceci à pression atmosphérique et sous pression de fluide. Les essais réalisés à pression atmosphérique montrent que les efforts élémentaires de forage augmentent avec la vitesse de coupe. Sous pression de boue, cet effet dépend largement de la perméabilité de la roche. En effet, nous avons observé que l'effet de la vitesse est relativement faible dans les formations de faible et de moyenne perméabilité sous pression de boue de 20 MPa. En revanche, cet effet augmente d'un ordre de grandeur dans les roches très perméables.Afin de comprendre ces observations, nous avons développé un modèle hydromécanique d'interaction taillant-roche construit à partir de la théorie de la poroélastoplasticité. D'abord, le problème est résolu analytiquement en s'inspirant des travaux existants. Par la suite, nous avons apporté une résolution numérique aux éléments finis des équations de la promécanique appliquées à la coupe des roches sous pression de boue. Les deux modèles montrent que le phénomène de dilatance génère une baisse de la pression de pore qui augmente la résistance de la roche au forage. Cette chute de pression dépend de la vitesse de coupe ainsi que des caractéristiques hydrodynamiques de la roche. Les résultats théoriques ont été comparés aux nombreux résultats expérimentaux obtenus dans le cadre de ce travail.

The optimization of the drilling practice requires a better understanding of drillstring harmful vibrations such as stick-slip. This form of torsional vibrations is a typical problem of PDC (Polycristalline Diamond Compact) drillbits. It can reduce the rate of penetration drastically and can raise fatigue of the drilling devices. Many attempts were carried out in the last years in order to determine the causes of stick-slip phenomenon. Field observations show that torque on bit decreases as a function of bit velocity during stick-slip oscillations. Hence, it is widely believed that this decreasing relationship is the root cause of stick-slip.The purpose of this work is to examine cutting speed influence on rock drillability as a function of operating conditions and hydromechanical properties of the drilled formation. For this, a set of drilling tests was performed in three sedimentary rocks of different permeability using a full scale PDC drillbit and a single PDC cutter. In the first step, dry tests were carried out at atmospheric pressure. As previously observed in literature, single-cutter tests showed that drilling forces increase with cutting velocity. In a second step, we performed the same experiments at 20 MPa bottom-hole pressure. It appears that rate effect on cutting forces in the medium and low-permeability rocks is relatively low. By contrast, rate effect in the highly permeable rock increases by one order of magnitude in comparison with dry experiments.In order to understand this phenomenon, a steady state solution of the cutting model is derived in the framework of the theory of poroelastoplasticity. The problem is firstly solved analytically using some assumptions derived from previous works. Then, a numerical resolution based on finite element method is presented to solve the fully coupled problem ensuring the satisfaction of poro-material physics basic equations. Using these two different approaches, we show that pore pressure in shear-dilatant rocks decreases as a function of cutting velocity depending on rock permeability and interstitial fluid properties. This change has a hardening effect resulting in an increase of rock drilling resistance. Comparison between theory and experience shows good agreements.; L'optimisation des systèmes de forage nécessite une meilleure compréhension des vibrations indésirables comme le stick-slip. Ce phénomène vibratoire, qui affecte principalement les outils PDC (Polycristalline Diamond Compact), met en péril l'intégrité des équipements de forage et réduit considérablement la vitesse de pénétration de l'outil. Plusieurs travaux ont été menés ces dernières années pour déterminer ses origines. Les observations réalisées en fond de puits montrent que ces oscillations s'accompagnent systématiquement d'une baisse du couple à l'outil en fonction de sa vitesse de rotation. De nombreux groupes de recherche attribuent cette baisse de performance à l'occurrence du stick-slip.L'objectif de ce travail est de développer un modèle élémentaire de coupe qui permet d'analyser l'effet de la vitesse de coupe sur la forabilité des roches dans des conditions opératoires réalistes. Dans le cadre de cette thèse, nous avons réalisé une série d'essais de coupe en utilisant des taillants et des outils à échelle réelle dans trois roches de propriétés hydromécaniques différentes, et ceci à pression atmosphérique et sous pression de fluide. Les essais réalisés à pression atmosphérique montrent que les efforts élémentaires de forage augmentent avec la vitesse de coupe. Sous pression de boue, cet effet dépend largement de la perméabilité de la roche. En effet, nous avons observé que l'effet de la vitesse est relativement faible dans les formations de faible et de moyenne perméabilité sous pression de boue de 20 MPa. En revanche, cet effet augmente d'un ordre de grandeur dans les roches très perméables.Afin de comprendre ces observations, nous avons développé un modèle hydromécanique d'interaction taillant-roche construit à partir de la théorie de la poroélastoplasticité. D'abord, le problème est résolu analytiquement en s'inspirant des travaux existants. Par la suite, nous avons apporté une résolution numérique aux éléments finis des équations de la promécanique appliquées à la coupe des roches sous pression de boue. Les deux modèles montrent que le phénomène de dilatance génère une baisse de la pression de pore qui augmente la résistance de la roche au forage. Cette chute de pression dépend de la vitesse de coupe ainsi que des caractéristiques hydrodynamiques de la roche. Les résultats théoriques ont été comparés aux nombreux résultats expérimentaux obtenus dans le cadre de ce travail.

Maître de thèse : Laurent Marc Gerbaud; PDC drillbits can experience intense rotary speed fluctuations which hamper drilling operations. This phenomenon, known as stick-slip, is self-sustained and occurs in many common drilling contexts. It is generally assumed that stick-slip instabilities result from the decrease of the torque-on-bit with the drillbit rotary speed. Many solutions have been introduced to mitigate stick-slip, but neither the root cause of the torque decrease, nor the effective role of the drillbit have been clearly identified yet. This thesis aims at studying the influence of the rotary speed on the mechanical response of the PDC drillbit, both experimentally and theoretically. An extensive program of drilling tests has shown that forces acting on PDC drillbits, as well as on their individual cutters, strongly depend on the cutting velocity. This rate-effect has been attributed to the dynamic shearing of a dense layer of crushed rock trapped at the tip of the cutter. A new semi-empirical rate-dependent bit-rock interaction model has been calibrated on these experiments to predict the dynamic response of real PDC drillbits. It is in good agreement with drilling tests performed with full scale PDC drillbits. The model has been coupled with a drillstring torsional dynamics software to compute the corresponding risk of stick-slip. The dynamic bit-rock interaction model explains why the torque decreases with the rotary speed and shows that the risk of stick-slip can be significantly reduced by selecting the appropriate PDC drillbit design.; Les outils de forage de type PDC peuvent subir d'intenses variations de leur vitesse de rotation, qui perturbent le déroulement des opérations de forage. Ce phénomène auto-entretenu, appelé stick-slip, se produit dans une variété de contextes de forage actuels et on admet généralement que l'instabilité est due à la décroissance du couple à l'outil suivant la vitesse de rotation. De nombreux dispositifs ont été introduits pour limiter son apparition, mais ni la cause physique de cette décroissance, ni le rôle joué par l'outil n'ont été clairement identifiés jusqu'à présent. Cette thèse vise à étudier, expérimentalement et théoriquement, la réponse mécanique des outils PDC lorsqu'ils sont soumis à des variations de leur vitesse de rotation. Une campagne d'essais de forabilité des roches a montré que les efforts qui s'exercent autant sur les outils PDC, que sur les taillants qui les composent, dépendent significativement de la vitesse de rotation. On a attribué ce phénomène au cisaillement dynamique d'une couche de roche broyée, compactée à l'interface entre le taillant et la saignée. Un modèle semi-empirique d'interaction dynamique outil-roche a été ajusté sur ces expériences pour prédire la réponse dynamique d'outils réels, puis validé à partir des essais sur outils de forage à l'échelle 1. Il a été couplé à un algorithme décrivant la dynamique en torsion des garnitures de forage pour calculer le risque de stick-slip associé. Le modèle développé explique non seulement, pourquoi le couple à l'outil diminue avec la vitesse de rotation mais aussi montre qu'il est possible de réduire le risque de stick-slip en sélectionnant la conception d'outil PDC appropriée.

Maître de thèse : Laurent Marc Gerbaud; PDC drillbits can experience intense rotary speed fluctuations which hamper drilling operations. This phenomenon, known as stick-slip, is self-sustained and occurs in many common drilling contexts. It is generally assumed that stick-slip instabilities result from the decrease of the torque-on-bit with the drillbit rotary speed. Many solutions have been introduced to mitigate stick-slip, but neither the root cause of the torque decrease, nor the effective role of the drillbit have been clearly identified yet. This thesis aims at studying the influence of the rotary speed on the mechanical response of the PDC drillbit, both experimentally and theoretically. An extensive program of drilling tests has shown that forces acting on PDC drillbits, as well as on their individual cutters, strongly depend on the cutting velocity. This rate-effect has been attributed to the dynamic shearing of a dense layer of crushed rock trapped at the tip of the cutter. A new semi-empirical rate-dependent bit-rock interaction model has been calibrated on these experiments to predict the dynamic response of real PDC drillbits. It is in good agreement with drilling tests performed with full scale PDC drillbits. The model has been coupled with a drillstring torsional dynamics software to compute the corresponding risk of stick-slip. The dynamic bit-rock interaction model explains why the torque decreases with the rotary speed and shows that the risk of stick-slip can be significantly reduced by selecting the appropriate PDC drillbit design.; Les outils de forage de type PDC peuvent subir d'intenses variations de leur vitesse de rotation, qui perturbent le déroulement des opérations de forage. Ce phénomène auto-entretenu, appelé stick-slip, se produit dans une variété de contextes de forage actuels et on admet généralement que l'instabilité est due à la décroissance du couple à l'outil suivant la vitesse de rotation. De nombreux dispositifs ont été introduits pour limiter son apparition, mais ni la cause physique de cette décroissance, ni le rôle joué par l'outil n'ont été clairement identifiés jusqu'à présent. Cette thèse vise à étudier, expérimentalement et théoriquement, la réponse mécanique des outils PDC lorsqu'ils sont soumis à des variations de leur vitesse de rotation. Une campagne d'essais de forabilité des roches a montré que les efforts qui s'exercent autant sur les outils PDC, que sur les taillants qui les composent, dépendent significativement de la vitesse de rotation. On a attribué ce phénomène au cisaillement dynamique d'une couche de roche broyée, compactée à l'interface entre le taillant et la saignée. Un modèle semi-empirique d'interaction dynamique outil-roche a été ajusté sur ces expériences pour prédire la réponse dynamique d'outils réels, puis validé à partir des essais sur outils de forage à l'échelle 1. Il a été couplé à un algorithme décrivant la dynamique en torsion des garnitures de forage pour calculer le risque de stick-slip associé. Le modèle développé explique non seulement, pourquoi le couple à l'outil diminue avec la vitesse de rotation mais aussi montre qu'il est possible de réduire le risque de stick-slip en sélectionnant la conception d'outil PDC appropriée.