A Multi-Well Review Of Coiled Tubing Force Matching

Abstract

Abstract To increase the predictability of successful coiled tubing operations, a review of recorded versus predicted weights was undertaken for several Statoil operated Norwegian platforms. Results from thirty-three wells were analysed and showed that a coefficient of friction of 0.24 was applicable in most wells. Additionally, it was confirmed that deviation complexity, production rates and direction of travel had no effect on the friction coefficient. Discussion Ever increasing demands are being placed on coiled tubing operations around the world. It is of critical importance that operations, which may have taken months of engineering and logistical planning, are successfully completed at the wellsite. All operations, no matter the complexity, rely on the coiled tubing service provider being able to convey the coiled tubing through the wellbore to the target depth with sufficient forces remaining available to perform the operation1,2,3. An in house computer model was used to assist in planning coiled tubing operations. This model has been under continuous development for over 20 years. Other service providers have either developed their own software or use third party packages. While different models may exhibit reasonable agreement on simple wells, there is a potential for variation in more extreme well designs. In highly deviated wellbores three main factors affect coiled tubing penetration: the ratio of the sizes of the coiled tubing to the completion, coiled tubing wall thickness and the contact friction coefficient. All affect the resistance to buckling. As the coiled tubing is pushed horizontally, compression loads become greater as the mass of material being pushed in the horizontal section increases. The amount of this compression is a result of the force that is required to push the coiled tubing. This is dependent upon the contact friction coefficient between the coiled tubing and the wellbore, the weight of the bottomhole assembly, buoyancy and the drag from produced and circulated fluids on the external surfaces of the coiled tubing. The amount of compression force that can be withstood depends upon the ratio of the coiled tubing outer diameter to the completion inner diameter. The smaller this ratio, the greater the horizontal penetration will be prior to a frictional lock up occurring. Frictional lock up (sometimes known as helical limit) occurs when the coiled tubing has been pushed into a tight helical pattern and all axial loads are passed radially to the wellbore. When this occurs, pushing additional coiled tubing into the well only reduces the pitch of the helix with no additional forces being transmitted to the end of the coiled tubing. To accurately model the forces and operating limits that coiled tubing will experience in a well, certain critical information is required:Full three dimensional well deviation profileWell completion sizes and depthsCoiled tubing size, wall thickness and material propertiesWellbore contents, pressures, flowrate and phase volumesCoiled tubing contents and flowrateBottomhole assembly dimensions, weights and materialsProduced and circulated fluid propertiesAccurate contact friction coefficientsExternal bottomhole assembly drag forces All of these input parameters are easily measured, except for the contact friction coefficients. The engineers performing the analysis, based on their experience, input the friction coefficient value into the computer model. The Coulomb model is the contact friction calculation method used within the software. Coulomb friction is based on the fact that the resultant force used to move an object along a surface is directly proportional to the applied force from that object onto the contact interface. The ratio of the resultant to applied forces is the contact friction coefficient. In the case of coiled tubing operations, the applied forces would be the weight of the coiled tubing and the resultant force at the axial push (or pull) required to move the coiled tubingA.