La présente invention concerne un système et une méthode d'estimation du comportement effectif d'un outil de forage fixé à l'extrémité d'une garniture de forage et entraîné en rotation dans un puits par des moyens d'entraînement situés en surface, dans laquelle on utilise un modèle physique non linéaire du processus de forage fondé sur des équations générales de la mécanique. Dans la méthode, on effectue les étapes suivantes:

• on identifie et on calcule les paramètres dudit modèle en prenant en compte les paramètres dudit puits et de ladite garniture,
• on linéarise ledit modèle autour d'un point de fonctionnement,
• on réduit ledit modèle linéarisé en ne conservant que certains des modes propres de la matrice d'état dudit modèle,
• on calcule, en temps réel, le déplacement de l'outil de forage ou l'effort appliqué à l'outil, à l'aide du modèle réduit et d'au moins un paramètre mesuré en surface.

A cable length measuring device, comprising guide rollers for maintaining the cable in a straight line along a given segment of a line, and two encoding wheels disposed tangentially to the straight segment cable; additional guides, the ends of which bearing against the cable is made of a wear resistant material, are provided between the guide rollers and the tangential wheel/cable contact area.

Methods and apparatus for accurately determining drill bit depth are provided. A hook load is sampled at a rate of at least 4 Hz and is compared to a load threshold to establish a slips-in condition. A determination is made retroactively that the drill string stopped moving when the hook load passed through a high dynamic threshold. On the slips-out procedure, the identical high threshold is used, with movement established when the hook load exceeds the high threshold. The high dynamic threshold corresponds to the points at which the drill string actually stops and starts moving in the slips-in and slips-out procedures. The apparatus provided is a drawworks encoder mounting assembly which retrofits an auxiliary brake section of the drawworks or the rotary seal air coupler of a drawworks clutch. A split ring gear fits around and is secured to the rotating cylinder of the rotary seal air coupler. The split ring gear is part of a pulley having another gear and a drive belt, such that rotation of the drum and rotorseal air coupler cylinder is translated to a shaft of an encoder coupled to the second gear. The encoder thereby tracks the rotational movement of the drawworks drum. At desired times, also provided is a second encoder which is part of a calibrator which, via a calibrator wire, precisely measures the location of the travelling block relative to a known vertical location. The first and second encoder readings are compared continuously and are used to provide excellent calibrations between the drum rotation and the travelling block movement.

A method for automating a downhole milling process includes receiving an input stream from at least one sensor within a downhole milling system, and segmenting the input stream. A safe operating envelope is identified based on segments of the input stream and models for the segments. At least one parameter of the milling system is then automatically changed along a path of optimal rate of penetration while remaining within the safe operating envelope. In some embodiments, an input stream may include surface pressure and the safe operating envelope may include swarf transport conditions.