Coiled tubing (CT) is being pushed to the limits of its capability by the more complex and challenging nature of wells currently being drilled. Either the yield limits are being approached because of the extremely deep high pressure, high temperature (HPHT) wells, even with the use of 100,000-psi strength CT, or the reach capability is being surpassed by the long step out of horizontal wells. To accurately predict the results of such an intervention, the planning phase must include detailed modeling combined with previous experience.
This paper will describe the theoretical effect that static friction would have on CT well interventions and compare this with the actual values found through the analysis of data recorded real time on the same wells. Static friction will be quantified in a number of HPHT wells and the resulting effect on the force required to pull out of hole will be presented. Testing performed to evaluate the static friction through the stripper will also be presented. These values will be used to help ensure the CT well bore friction is accurately quantified.
Friction is typically characterized by a coefficient of friction, which is the ratio of the frictional resistance to the normal force that presses the surfaces together. In this case the normal force is the weight of the block. Typically there is a significant difference between the coefficients of static friction and kinetic or dynamic friction. Note that the static friction coefficient does not characterize static friction in general, but represents the conditions at the threshold of motion only.
Frictional resistance forces are typically proportional to the force that presses the surfaces together. This force, which will affect frictional resistance, is the component of applied force which acts perpendicular or "normal" to the surfaces that are in contact and is typically referred to as the normal force. In many common situations, the normal force is the weight of the object sitting on some surface, but if an object is on an incline, or has components of applied force perpendicular to the surface, then the normal force is not equal to the weight.
Frictional resistance to the relative motion of two solid objects is usually proportional to the force that presses the surfaces together, and the roughness of the surfaces. Since it is the force perpendicular or normal to the surfaces which affects the frictional resistance, this force is typically called "the normal force."1
The frictional force is also presumed to be proportional to the coefficient of friction. However, the amount of force required to move an object starting from rest is usually greater than the force required to keep it moving at constant velocity once it has been started. Therefore, two coefficients of friction are sometimes quoted for a given pair of surfaces: a coefficient of static friction and a coefficient of kinetic friction. The force expression above can be called the standard model of surface friction.
While this general description of friction has practical utility, it is by no means precise. Friction is, in fact, a very complex phenomenon that cannot be represented by a simple model. Almost every simple statement you make about friction can be countered with specific examples to the contrary. Saying that rougher surfaces experience more friction sounds safe enough. Two pieces of coarse sandpaper will obviously be harder to move relative to each other than two pieces of fine sandpaper, but if two pieces of flat metal are made progressively smoother, a point will be reached where the resistance to relative movement increases. If they are made very flat and smooth, and all surface contaminants are removed in a vacuum, the smooth flat surfaces will actually adhere to each other, making what is called a "cold weld".