Tortuosity is commonly defined as the amount by which the actual well bore deviates from the planned trajectory. Elimination of excessive tortuosity has been regarded as a critical success factor in extended reach drilling operations. In this paper the authors will refer to "micro-tortuosity", not measurable by survey data, in which the hole axis is a helix instead of a straight line. It is argued that this is Lubinski's"crooked hole" described in the early 1950's. The paper presents a study of micro-tortuosity using field data from hundreds of wells. The paper details how and why micro-tortuosity occurs and the negative impact micro-tortuosity can have on the entire drilling operation. The paper also presents a solution that eliminates or drastically reduces micro-tortuosity. Field results will be presented to demonstrate that micro-tortuosity is in fact the dominant component of the total tortuosity. Introduction Tortuosity has been recognized recently as one of the critical factors in extended-reach well operations1,2,3. The effects include high torque and drag, poor hole cleaning, drillstring buckling and loss of available drilled depth, etc. Conventional wisdom has always held that tortuosity is most often generated by steerable motors while attempting to correct the actual well trajectory back to the planned trajectory. However, in the early days of drilling in the mid-continent area of the United States, drillers observed a problem with running tubulars into wells. A vertical well drilled with a 12–1/4" bit would not drift 12–1/4". This led Lubinski et al.4,5 to develop a formula for determining the minimum drift size for a hole drilled with a given collar and bit combination (or the reverse). This became known as the "crooked hole country" formula. Thus there was early recognition of the potential for problems due to the fact that the wellbore was not straight. This recognition predated the first use of steerable motors by some 30 years. Today, several types of drilling tools are targeted at achieving reduced hole tortuosity as measured by survey data, with a view to reducing torque and drag. Obvious examples are adjustable gauge stabilizers and adjustable gauge motors, and, more recently, rotary steerable systems. In parallel, it is commonly suggested that bent-housing steerable motors increase tortuosity as measured by survey data by mixing high dogleg sliding footage and low dogleg rotating footage. In brief, low dogleg equals low torque equals "good", high dogleg equals high torque equals "bad". Recent evidence suggests that any torque and drag benefits derived from reducing dogleg as measured by survey data (macro-tortuosity) are likely to be completely overwhelmed by the torque and drag generated by poor wellbore quality (micro-tortuosity). In the last two years, over 200 wellbore sections have been drilled using long gauge bits, primarily in pursuit of drilling improvements broadly encompassed by the term "hole quality". Most of these bits have been run on steerable motors; some, on rotary steerable systems. Modeling, measuring, and comparing torque and drag values for sections drilled with long gauge bits and with short gauge bits immediately showed two surprising results. First, there is no dramatic difference between the resulting torque and drag values for steerable motors versus rotary steerables when both use similar bits. Secondly, there is a significant difference between torque and drag values for long gauge bit runs versus short gauge bit runs regardless of the method used to drive them. The use of long gauge bits also gives a clear improvement in activities that might be expected to benefit from improved hole quality or reduced micro-tortuosity. These include hole cleaning, logging operations, resultant log quality, casing runs, and cementing operations.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractTortuosity occurs when a well deviates from a straight hole. The most commo nly known tortuosity is the local dogleg severity variation associated with the use of steerable motors in "slide/drill/slide" drilling. However, there is a tortuosity that exists in many wells, which the authors will refer to as "micro-tortuosity" in which the hole axis is a spiral instead of a straight line. This paper presents a study of micro tortuosity using field data from the North Sea wells. The friction factors and tortuosity index are used to quantify the effect of tortuosity on the torque and drag. The results show that hole spiraling, associated with the use of conventional short-gauge bits, is a major contributor to today's friction factors used in the torque and drag model.
Summary Once a crack is initiated by cyclic stress in a drillstring component, it will grow under further service loading and will fail when insufficient uncracked material remains to carry the applied load. Because the levels of cyclic and subsequent stress in drilling may be unknown (or unknowable), the drilling equipment supplier's main defense against component fatigue failure (after design) is detection of the initial crack, conventionally by magnetic-particle or dye-penetrant inspection (respectively, MPI or DPI). A clean bill of health from MPI or DPI means only that no crack indications were found. The likely locations of cracks (e.g. thread roots in box connections) are often difficult to examine. Detection and interpretation are subjective and depend on the skill of the inspector. A crack and a surface defect may be indistinguishable from one another. No reviewable evidence of component inspection is left to allow an audit of inspection previously performed. Alternating current field measurement (ACFM) induces a current in the surface of a component. If ACFM detects a perturbation in the magnetic field created in the free space above the surface, a surface defect is present. ACFM is able to determine the length and depth of a defect. It does not require a clear line of sight between operator and crack location. All data are recorded electronically and the evidence for the existence or nonexistence of a crack can be revisited. The paper describes the theory of the technique, the equipment used, and practical results from the first application of ACFM to downhole motor components. Crack Detection Methods Magnetic particle inspection. The near-universal method of detecting cracks in the carbon steels used for downhole motor body components is commonly known as MPI or "black-light inspection." The component is magnetized and sprayed with a medium containing magnetic particles in suspension. These cling preferentially to surface defects. When the component is viewed under an ultraviolet light, the surface defects can be seen as bright lines. There are several drawbacks inherent in this system; the principal one is that the process does not identify cracks, but makes surface defects more easily visible. The inspector must have line of sight to the defect. Because, for example, a cracked box connection is most likely to crack in the root of the innermost thread, this may require a certain amount of physical contortion, and mirrors, to see. Further, the process allows measurement of the length of a defect, while what is important is its depth. The only practical way to establish whether a defect is a crack, and how deep it is, is to grind or polish it out and make a judgment. If the "crack" disappears, it was a surface defect. If it does not, further grinding may be attempted or the component scrapped. Essentially, the process is on-the-spot assessment. It relies on the quality of the complete operation from cleaning to defect identification. It is also subject to variations in reliability depending on the consistency of interpretation between different operators.1 No audit trail is possible because the evidence is not available as hard copy to be revisited. ACFM Inspection. ACFM is a crack-detection method which has been in use for offshore structure and aerospace inspection for several years. The technique works on magnetic and nonmagnetic, ferrous and nonferrous metals by detecting anomalies in a current flow along a metal surface. If the surface is defect-free, the current flow will be uniform. If the current flow is disturbed, the disturbance can be detected.2 ACFM creates a uniform "thin film" electrical field in a metal surface, and a corresponding magnetic field is created in the free space above it. A crack or defect on the metal surface may be thought of as causing the current to deflect around and beneath the crack. These current deflections cause anomalies in the magnetic field. By measuring perturbations in this magnetic field the cracks or surface defects which caused them can be detected. The value and direction of the perturbations in the magnetic field allow a close estimate of the length and (more importantly) the depth of the crack or defect, to be obtained. Because the values of the components of the magnetic field vary in a consistent pattern (Fig. 1) when a crack is detected, operator interpretation can be eliminated and the process automated. At a practical level, because it detects anomalies in the free space above the surface, ACFM detecting probes need not contact the parent metal. This simplifies the design, eliminates lift-off problems (losing electrical contact) and increases the life of probes. Further, cleaning to "bright metal" condition is not necessary, reducing the time required to complete an inspection. The principles are explained in more detail in Appendix 1. ACFM Inspection Station. The system used in the test consisted of a hand-held probe connected by an umbilical to a crack microgauge. The crack microgauge (a 41´31´16 cm box weighing 14 kg) contains an oscillator and amplifier which outputs to the magnetic field generator contained within the hand-held probe. The probe returns sensor readings to the microgauge, which amplifies the signal, digitises it, and transmits it to a standard laptop computer. The complete system can be hand-carried.
fax 01-972-952-9435. AbstractTorque and drag can be critical issues in drilling directional wells especially in extended reach drilling (ERD). During well planning, torque and drag must be projected to ensure the rig's rotating and hoisting equipment are adequately sized and to evaluate the limits for slide-oriented drilling motors. Depending upon formations, typical open hole friction factors (FF) used in simulation range from 0.22 in oil base mud to 0.35 in water based mud. These friction factors are scaled to a higher value than those measured in the field in order to account for tortuosity created by drilling assemblies.A new drilling technology has been developed with the objective to reduce torque and drag by drilling a smooth and straight wellbore. The technology involves extended-gauge bit design with a matched steerable motor system or a point-thebit rotary steerable system. Friction factor was studied for North Sea's wells drilled by the conventional motor systems and by new drilling systems. Significant reductions in the actual friction factors and the tortuosity index have been seen from the wells drilled by the new drilling systems.
A high-quality wellbore is generally considered to have (1) a gauge hole, (2) a smooth wellbore, and (3) a wellbore with minimum tortuosity. This paper will demonstrate that wellbore spiraling is the primary contributor to poor hole quality and that almost every well contains some degree of spiraling unless specific actions are taken to prevent it. Hole spiraling was first studied by Lubinski et al. in the 1950s, and they described it as a "crooked hole." Although the symptoms have been well recognized in the industry, only recently has a solution been proposed and tried specifically to cure hole spiraling. To implement the concept, two new drilling systems (a steerable motor and a rotary steerable) have been developed. Field data indicate that generating a straighter, high-quality wellbore has improved almost every aspect of drilling. These improvements include lower vibration, better bit life, fewer tool failures, faster drilling, better hole cleaning, lower torque and drag, better logging tool response, and better casing and cement jobs. Several case studies will be discussed to demonstrate the positive economic impact of producing a high-quality wellbore.
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