Quantifying Tortuosities by Friction Factors in Torque and Drag Model

Abstract

Abstract Tortuosity occurs when a well deviates from a straight hole. The mostcommonly known tortuosity is the local dogleg severity variation associatedwith the use of steerable motors in "slide/drill/slide" drilling. However, there is a tortuosity that exists in many wells, which the authors will referto as "micro-tortuosity" in which the hole axis is a spiral instead of astraight line. This paper presents a study of micro tortuosity using field datafrom the North Sea wells. The friction factors and tortuosity index are used toquantify the effect of tortuosity on the torque and drag. The results show thathole spiraling, associated with the use of conventional short-gauge bits, is amajor contributor to today's friction factors used in the torque and dragmodel. Introduction This paper suggests a measure to quantify tortuosities. It concentrates onthe "micro-tortuosity" and the commonly known "tortuosity" which the doglegseverity varies due to the slide/rotate action in the use of the steerablemotors. The micro-tortuosity is caused by hole spiraling where the hole axis isa spiral instead of a straight line. The method discussed requires thecomparison of the values of torque and drag in between wellbores that arelargely free from micro-tortuosity and those that are not. It is based on thepremise that hole spiraling is the primary cause of poor hole quality, thatspiraling can easily be eliminated, and that desirable benefits can be clearlydemonstrated by eliminating spiraling and thus improving hole quality. This begs the question of whether or not hole spiraling actually exists, andwhat it is. This is not a new problem. Many people reading this will befamiliar with "Crooked Hole Problems" and will immediately associate it withArthur Lubinski. Many people will recall the picture of a basically verticalbut crooked hole, showing ledges and keyseats. It is however instructive to goback to what MacDonald and Lubinski actually wrote in 1951 1–2because they defined exactly what they meant by crooked hole, they did coverspiraling, and their definitions have proved to be correct. It is worth quoting them exactly because their definitions can scarcely beimproved on, and can be readily extended to cover any drilled hole whether"vertical" or deviated. They said: "An understandable confusion exists in the customary use of the terms"straight", "crooked" and "vertical" when applied to a drilled hole. A 3 degreehole might be rifle-bore straight if the deviation were all in one direction, but obviously far from vertical. Such a straight but non-vertical hole wouldpresent little drilling or producing problem. A 2-deg hole could be morecrooked if it swung sharply from 2-deg in one direction to 2-deg in theopposite direction several times throughout its depth. Although such a holemight be classed as vertical because the bottom of the hole would be close to apoint directly beneath the surface location, it might have sharp doglegs up to4-deg at several places and develop relatively serious drilling and producingproblems. It is apparent that angle alone is inadequate to the definition of acrooked hole; the factor of rate of change in angle must also beconsidered. A 2-deg hole following a tight spiral would be vertical but far fromstraight; and if it held steadily to 2-deg, there would be no objectionablerate of change in angle, yet the spiral hole might develop serious key-seatingdifficulties, drill pipe wear on intermediate casing, etc. To guard against the development of this type of hole, the considerationof direction must be added to those of angle and rate of change. Thiscomplicates the situation because, while angle and rate of change may bedetermined from the customarily employed slope-testing devices, and instrumentfor the recording of direction is not normally employed [in 1951] as a routinedrilling tool. For these practical reasons, the study committee has not yetbeen able to develop a universally acceptable definition of a crooked hole.This subject is high on the agenda of future work."