PDC Bit Classification According to Steerability
- S. Menand (Armines/Ecole des Mines de Paris) | H. Sellami (Armines/Ecole des Mines de Paris) | C. Simon (DrillScan)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- March 2004
- Document Type
- Journal Paper
- 5 - 12
- 2004. Society of Petroleum Engineers
- 4.3.4 Scale, 3 Production and Well Operations, 1.6.2 Technical Limit Drilling, 1.6 Drilling Operations, 1.5.1 Bit Design, 4.6 Natural Gas, 1.6.6 Directional Drilling, 1.5 Drill Bits, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 5.3.4 Integration of geomechanics in models, 1.6.1 Drilling Operation Management
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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.
It is well recognized today that the directional behavior of a drilling system is a complex coupling of bit directional responsiveness and mechanical behavior of the directional system, but a possible rock-formation effect1 (anisotropy) must also be considered. This paper focuses on the directional behavior of PDC bits characterized by their walk tendency and steerability.
A previous paper2 noted that the bit steerability and walk tendency were mainly a function of the bit profile and gauge-cutter and -pad characteristics. In this paper, we propose a simple methodology to classify PDC bits defined by their IADC bit profile codes (shown in Fig. 1). This methology is based on a recent study of the directional behavior of PDC bits on the basis of theoretical models, numerical simulation, and pilot and field trials.2
The directional behavior of a PDC bit is generally characterized by its walk tendency and steerability. To quantify the walk tendency, Ho3 introduced the walk angle, which is the angle measured in a plane perpendicular to the bit axis, between the direction of the side force applied to the bit and that of the lateral displacement of the bit.2 The walk angle quantifies the intrinsic azimuthal behavior of the PDC bit.
Bit steerability (BS) corresponds to the ability of the bit, submitted to lateral and axial forces, to initiate a lateral deviation. The bit steerability can be defined as the ratio of lateral vs. axial drillability.
- Equation 1
The lateral drillability (Dlat) is defined as the lateral displacement per bit revolution vs. the side force. The axial drillability (Dax) is the axial penetration per bit revolution vs. the weight on bit (WOB). The BS (equivalent to the bit anisotropic index3,4) is generally in the range of 0.001 to 0.1 for most PDC bits, depending on the cutting profile, gauge cutters, and gauge-pad characteristics, as evaluated here. High steerability for a bit implies a strong propensity for lateral deviation, enabling maximum dogleg potential.
The PDC bit should have some stabilization and durability requirements as well as 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 steerability compatible with the directional system. The design of the bit should consider the three parts (see Fig. 2) that interact with the rock formation - the cutting structure (mainly the cutting profile and back-rake angle), the active gauge (gauge cutters or trimmers), and the passive gauge (conventionally called the gauge pad).
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