Coiled Tubing Drilling Mechanical Properties

Coiled tubing (CT) is used extensively in well workover, well drilling, well completion, well logging and well stimulation in oil-gas field. It plays a more and more important part in exploration and production. It will improve the efficiency of exploration and exploitation, studying, developing and using CT technology. The load prediction of CT, mechanics characteristic and Fluid mechanics characteristic of CT are studied in the paper. It covers the following contents.3-D stiff-string model and soft-string model are applied to analyze the axial load of coiled tubing, which considers the boundary condition of the model, solution of normal contacting force, obtainment of the drag coefficient, treatment of survey data, modification of the buckling hole etc. Stress of CT is calculated. The model setted up can analyzes & compares force of whole CT, and it can be used to judge the working capacity and situation.Mechanical characteristic of CT is studied. The model of bending, internal pressure strength and external collapse strength, diametric growth and stuck-point of CT is established. Results show that CT will bring alternative bending lastic deformation. The larger the ovality is, the smaller the collapse resistance is. Ovality should be inspected strictly before using, and we should prevent the CT beyond the ovality limit from using in the high-pressure well and ultra-deep well. After working repeatedly under certain internal pressure, the diameter of CT will increase and wall thickness of CT will become smaller. Longitudinal stretch of CT will happen during shift, which will bring additional force.Fluid mechanical characteristic of coiled tubing is discussed in this paper, pressure drop is calculated in detail, including pressure drop on the reel, pressure drop in the well. Especially, pressure drop model of CT with tapered diameter and eccentric annulus is established. Calculation results indicate that the pressure drop of CT is much larger than that of conventional well. Tapered CT can be used to reduce the pressure loss in deep well and slim hole operation.Stress is analyzed of CT in the paper. The model of evaluating the fatigue life of CT is established. The factors having effect on the CT fatigue life are analyzed (sensitivity analysis) and the methods of improving the CT life are given. Internal pressure, CT OD, CT wall thickness, reel diameter, gooseneck radius, young's modulus and tensile strength affect the fatigue life of CT severely. Increasing the diameters of the reel and...