Sunday, 5 November 2017

FACTORS AFFECTING DRILL STRING TORQUE AND DRAG

factors affect torque and drag
Torque and Drag

Factors Affecting Drill String Torque And Drag

When planning a 3D well trajectory, one of the most important considerations is torque and drag. If the torque and drag are not carefully considered, the drill string might fail. The torque and drag model used makes special assumptions that simplify the analysis and are used to model real drill strings. The most important factor influencing the torque and drag forces is the hole curvature. The well path should be redesigned with a smaller build-up rate if the drill string seems to fail when simulating these forces during the design stage. 

There are many causes for excessive torque and drag such as: sliding friction, tight hole, collapsing or swelling clay/shale, key seats, differential sticking and cuttings build-up. The minimum curvature method assumes the bending part in the equilibrium equation used to calculate torque and drag is discontinuous at survey stations. Some authors mean this is one of the main weaknesses of using the minimum curvature method. Due to the missing bending stresses, the method might not represent the real drill string configuration. 

Kick-Off Point

The kick-off point is often the major factor influencing Drill String Torque and Drag in a well. This is due to the fact that shallow doglegs combined with pipe tension will cause a high normal force, thus a high drag and torque at that point.

In deep directional wells, the kick-off point should be considered from a Torque and Drag standpoint, in addition to the other factors influencing the kick-off point.

Wellbore Tortuosity / Dogleg Severity

Tortuosity can be defined as: a measure of deviation from a straight line. It is the ratio of the actual distance traveled between two points, including any curves encountered, divided by the straight line distance. Tortuosity is used by drillers to describe wellbore trajectory, by log analysts to describe electrical current flow through rock and by geologists to describe pore systems in rock and the meander of rivers.

As directional wells become deeper and sophisticated, unwanted deviations from the pre-planned well trajectory occur frequently. Any unwanted deviation from the wellbore trajectory is regarded as wellbore tortuosity.

The change in inclination and azimuth influence the normal force acting on the Drill Pipe. The larger the change in either, meaning increasing dogleg severity, the larger the normal force and the larger the increase in drag due to that point. Smaller doglegs lead to less Torque and Drag.

Mud Lubricity

Mud lubricity is a term generated from a lab test (on a specific piece of equipment) that is intended to mimize the Drill Pipe/casing interaction. The lubricity is scaled opposite the friction factor, increasing lubricity reduces the friction factor.

Hole Cleaning

Hole cleaning is an important factor. A proper hole cleaning is a must during drilling operations to reduce the unwanted cuttings in the circulating system as these cuttings have potential to resist in drill string rotation and movement. In a directional wellbore, cuttings tend to accumulate up in the high angle build section; this is why removing the cuttings from the hole is difficult. Improper hole cleaning also brings the risk of damaging downhole tools such as MWD tools and mud motors. However, if an MWD tool is employed on the drill string, cuttings intensity in the mud can be monitored by following the parameter called Equivalent Circulating Density (ECD), which is the apparent drilling fluid density. An increasing trend of ECD will imply accumulation of cuttings in the wellbore. 
Cuttings beds will locally change the effective coefficient of friction, and can be modeled in that manner if the location, length and friction factor of the cuttings bed is known. Unfortunately, these parameters continually change in the presence of a cuttings bed.

The typical technique is to deduce the presence of a cuttings bed when unexplained trends in Torque and Drag develop in well. This would be seen very practically on trip out of the hole when pick-up weight begins increasing during the trip, rather than decreasing, then suddenly returns to the baseline trend.

Drilling Tools

It is not hard to imagine that the influence of a stabilizer on drag would be much greater than that of a similar length of Drill Pipe or Drill Collar and that the interaction of the stabilizer with the well-bore may include additional components than does the simple model we have been working with Drilling tools can greatly impact Torque and Drag are stabilizers, Drill Pipe protectors, Drilling Bits Type (roller cone vs. drag type), drill collars (flex vs. slick) and hard banding on Drill Pipe.

Hole Instability 

Hole instability is a displeasing case that occurs when the original hole size, shape and structural conditions of a borehole are not well maintained. It appears with the failure of balance between the rock strength and in-situ rock stress at some depth during drilling. It may also arise with the effects
of drilling fluid such as erosion in the borehole and chemical interactions of drilling fluid with drilled formation. Hole closure, which is the narrowing process of borehole, results in an increase of torque and drag.

Key Seating

Key seating is the condition when a tubular of a small diameter is worn into the side of a larger diameter borehole. It is generally a result of severe hole direction changes such as a high dogleg or a hard formation ledge left in soft formations which erodes and enlarges in time. In both cases, the diameter of the drilled hole is expected to be close to the diameter of the drill  pipe. The larger diameter tools such as stabilizers, drill collars and tool joints are not able to pass through the key seat and become stuck which will result in problems associated with high torque and drag. The preventive method is to enlarge the point of key seat so that the tools with larger diameters can pass through it
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