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.

TX 75083 -3836 U.S.A., fax 1 .972. 952.9435. AbstractTorque and drag (T&D) modeling is regarded as an invaluable process to assist in well planning and to predict and prevent drilling problems. Although T&D software has existed for over 20 years, some confusion still exists over the validity of the models used to characterize drilling and completion operations. This paper provides an assessment of current limitations of the various T&D models (soft-string and stiffstring) and appraises their validity. Field data from various operations is used to illustrate certain limitations. The paper defines future requirements for what is considered to be the next generation of T&D models . Probably the most important technical requirement is a more realistic stiff-string model to correctly account for the impact of tubular stiffness, hole clearance and tortuosity effects.

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.

A new downhole torsional vibration sensor built into a rotary steerable system has provided a new monitoring system for improving drilling efficiency and avoiding downhole failures related to torsional vibration. The strength of the new sensor is that it measures rotational changes right at the bit as compared to the usual 50–100 feet behind the bit with conventional MWD/LWD sensors. In addition, the sensor is always at the same location regardless of BHA configuration, which provides a higher level of consistency in torsional vibration measurement. The ability to evaluate bit performance objectively has consequently resulted in better bit design and rotary steerable tool performance. The new torsional efficiency monitor has been used in areas where torsional vibration was recognized as a problem. This paper will discuss the application of the real-time downhole monitor and the results from detailed analysis of the data from the initial field runs in these harsh environments. The ability to immediately detect the occurrence of torsional vibration and to take corrective action before failures occur has significantly improved the ability of the rotary steerable system to survive harsh conditions. Introduction Rotary steerable systems (RSS) are routinely used in the most expensive drilling operations, and operators who employ these systems have high expectations of significant cost savings. However, due to the greater number of moving parts in rotary steerable systems, the level of reliability has been below that of measurement-while-drilling (MWD) and logging-while-drilling (LWD) systems, particularly when operating in high vibration environments. In some cases, the high cost of trips and tool repairs due to tool failures has compromised the potential economic impact of the RSS technology. The area of downhole vibrations and in particular, torsional vibration (or stick-slip) has been studied extensively in the past. A few papers have addressed special aspects of this complex subject[1,2,3]. Downhole monitoring systems have shed light on the dynamic motions of the drill string. Bit rotation speeds as high as 300–400% over the surface RPM has been recorded. However, a complete picture of the BHA (Bottom Hole Assembly) behavior would require downhole vibration sensors that:Can detect and accurately measure the full range of dynamic motion, including axial, lateral, and torsional vibration.Can alert operators of dangerous levels in a timely fashion (i.e., in real-time) so changes can be made to mitigate vibration before severe damage occurs on the drillstring / bottom hole assembly.Can record higher density data for detailed post-run vibration analysis to pin-point the source of vibration, so that modifications to the bit or BHA can be made for future runs.Can monitor at different points in the drill string Currently, vibration sensors are often under-utilized, mainly due to limited drilling budgets or a reluctance to pay for these additional services. This obviously results in critical gaps in data required to investigate the cause of a tool failure when it occurs. Lack of vibration data also makes fair performance comparisons to offset wells very difficult. Low-frequency torsional vibration, usually below 5 Hz (stick-slip) is measurable on the surface from the oscillation in the rotary torque[4]. However, high-frequency torsional vibration (>50Hz) will not transmit to the surface[5]. The use of soft-torque rotary tables, while sometimes reducing the severity of stick-slip, has tended to mask the downhole situation to some extent.