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 Such models are no longer valid with the introduction of PDC cutters with chamfer and special shape.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.

15 pagesInternational audienceDrill pipe in a curved section of the drilled well is considered as a rotating hollow cylinder subjected to bending and tension loads. So in this paper, the stress intensity factors and the fatigue growth of a circumferential semi-elliptical surface crack in a hollow cylinder subjected to rotary bending and tension are studied. A stress intensity factor database for three loading cases is build for numerous configurations using 3D Finite Element Models. The crack propagation model employs the Walker fatigue growth rate law. Using this model, we can study the evolution of parameters characterizing the process of crack propagation

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.

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 associated with 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 weights-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 design.

The importance of wellbore deviation is well recognized by the drilling industry. An analysis of a drilling system's directional behavior must include the directional characteristics of the drill bit. This paper presents a comprehensive analysis of the directional behavior of polycrystalline diamond compact (PDC) bits, including the effect of bit profile, gauge cutters, and gauge length. Numerical simulations as well as laboratory tests have been carried out to better understand the mechanisms of PDC bit deviation and to evaluate the most important parameters affecting the directional behavior of PDC bits. The analysis presented in this paper shows that each part of the PDC bit (profile and active and passive gauges) plays a major role in its walking tendency and steerability. A quantitative evaluation of how these factors contribute to well trajectory (inclination and azimuth) is given. For the first time, a full-scale directional-drilling bench was built to measure the walking tendency and steerability of PDC bits. The results obtained demonstrate that the bit profile, gauge cutters, and gauge length have a significant effect. A 3D theoretical rockbit interaction model was developed to reproduce the drilling test results.