The effect of fluid flow on coiled-tubing reach

A critical parameter for the success of many coiled-tubing (CT) operations in highly deviated or horizontal wells is the depth penetration that can be attained before the CT buckles and locks up. Achieving a desired depth is always critical in CT operations, and attaining an additional reach of a few hundred feet can be crucial. This paper addresses the effect of fluid flow in the CT and in the CT/wellbore annulus on the state of force and stress in the CT, and thereby predicts its effect on the reach attainable by the CT. The flow of fluid through the CT and the annulus between the CT and the borehole modifies the pressures and the effective force that govern the mechanical stability of the CT. The net force per unit length caused by fluid flow in the CT and annulus between the CT and casing/well is calculated in terms of the shear stress, and its effect on the onset of buckling and lockup is determined. A model is then implemented in a full-tubing-forces calculation, and the effect of flowing fluids and producing fluids on reach is analyzed. The new model is used in desiging commercial jobs. The exact analytic model shows that fluid flow inside the CT has zero impact on reach, that downward flow in the annulus has a favorable impact, and that upward flow in the annulus reduces the maximum attainable reach. A CT job can be designed using the full-tubing-forces model, taking into account the flow of a fluid with a specified rheology, density, and flow rate. Thus, the feasibility of attaining a given reach can be more accurately determined. Results are presented in the form of the surface weight for commercial wells and are compared to field jobs.