H. Sellami

How Bit Profile and Gauges Affect Well Trajectory

The importance of wellbore deviation is well recognized by the drilling industry. An analysis of a drilling systems directional behavior must include the directional characteristics of the drill bit. This paper presentsa 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 rock-bit interaction model was developed to reproduce the drilling test results.

PDC Bit Classification According to Steerability

With the emergence of rotary steerable systems (RSSs), the technical issue concerning bit design for a specific directional application has reappeared. Today, a bit must be specifically designed for use with a particular directional system: rotary bottomhole assembly (BHA), steerable mud motor, or RSS. The reason is that the bit must have the ability to respond properly and rapidly to a side force applied by the steering system to initiate a deviation. To do so, the bit must have a predetermined steerability compatible with the directional system to provide the optimum dogleg potential. The new generation of directional-drilling systems differentiates “pointing the bit” from “pushing the bit.” As a consequence, the bit’s directional response is a key factor that operators and directional drillers need to know to make a good adaptation between the bit and the BHA. However, at the moment there is no standard method for classifying bits by steerability and walking tendency. On the basis of a comprehensive analysis of the directional behavior of polycrystalline diamond compact (PDC) bits (numerical simulation and pilot and field tests), a simple methodology has been developed that defines and evaluates their steerability and walking tendency. This methodology is used to classify the PDC bits defined with their IADC bit profile codes. Because PDC bit steerability is mainly a function of the bit profile, the gauge cutters, and the gauge pad, some design recommendations are given concerning these three parts. For each IADC bit profile code, the bit steerability and walking tendency are estimated through some formulas linking only the heights and lengths of the cutting profile. Some guidelines are also given about the gauge-pad length and gauge-cutter characteristics to achieve improved steerability. This simple method based on geometrical criteria allows quick estimation of not only the PDC bit steerability but also the maximum dogleg potential achievable by the bit when it is coupled with the steering system.

Microstructure Effect on Hard Rock Damage and Fracture During Indentation Process

This paper presents the results of simple and double indentation tests conducted on three hard rocks: granite, limestone and sandstone. The main objective is to analyze the rock behavior under indentation test in relation with the physical and microstructural properties. First, the experimental set-up used during the tests is described as well as the sample preparation. Then, an image analysis based technique is applied on thick sections made from the tested samples in order to evaluate the size of indentation-induced cracked zone and to assess the crack types (vertical or sub-horizontal) below the indenter. The interpretation of the results was made on two levels. First, several physical parameters (energies and displacements) have been derived from the so obtained experimental indentation curves. Very high correlations were found between the loading, indentation and specific energies versus the plastic displacement. Second, cracked zone radius was estimated showing a very high correlation to the specific energy and governed by the physical and mineralogical properties of the tested rocks. Finally, the analysis of double indentation tests proved that changing the distance between two adjacent inserts allows the determination of the optimal spacing producing overlay of cracked zones and causing rock damage and large chip departure.

Fraise nouvelle génération

ABSTRACT 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.

Stick-slip en forage pétrolier

ABSTRACT Dynamics of a drilling structure is studied in torsion. Torsional instabilities, also known as stick-slip, are often present during drilling and reduce strongly the system efficiency. Measurements of bit parameters are obtained with an instrumented bit. Two types of behaviour are experimentally highlighted, comforted by a beam model. In the first one (stationary or desired functioning), the system is linear and modal analysis allows to understand the phenomenology. A perturbation of structural parameters (generally, operating parameters) makes converge the system on a periodical attractor, characteristic of stick-slip. In fact, a punctual non linearity appears in the second stage, justified by rock-bit interaction considerations. This bifurcation is theoretically described with linearized analysis and is experimentally exhibited.