Tortuosity versus Micro-Tortuosity - Why Little Things Mean a Lot

Abstract

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.

Influence of Rotary-Steerable-System Design on Borehole Spiraling

“…For the latter, the characteristic roots of Eq. 14 (Hale 1977;Michiels and Niculescu 2007) must be computed. The terms related to !…”

Analysis of Spiraled-Borehole Data by Use of a Novel Directional-Drilling Model

“…Assessing the stability of a linear DDE with constant coefficients and delays consists in computing the roots of its characteristic equation, obtained by Laplace transform or by assuming a solution of the form H ¼ e an , where a is a complex number (Hale 1977;Michiels and Niculescu 2007). Because of the delays, the characteristic equation is not polynomial, as for ordinary differential equations, but transcendental, meaning it has an infinite number of complex roots that have to be computed numerically.…”

Integrated Approach to Rotary-Steerable Drilling Optimization Using Concurrent Real-time Measurement of Near-bit Borehole Caliper and Near-bit Vibration

“…A spiraled hole is commonly observed with many drilling assemblies, depending upon a bit type, BHA tendency and formation [5,6,7]. When borehole spiraling occurs, the evidence can be found in wireline logs and LWD logs.…”