Friction Factors in Oil Wells through Analysis of Hook Load

Abstract

As the easy oil and gas is gone, there is today a need for more complex wells in orderto extract the remaining hydrocarbons. Over recent decades, both inclination andlength of drilled wells have increased significantly. Analysis of the mechanical dragduring tripping is important to detect abnormal conditions in the well and could helpreducing the operational costs. This is also the most important reason for the need ofdetermining an exact friction factor. One way of revealing these restrictions is tomonitor hook load (HKL) from real time drilling data (RTDD) while pulling andcompare it with modeled drag. If HKL from RTDD suddenly increases or decreasescompared to normal modeled drag, a problem in the well have occurred as a resultfrom restrictions in the well. Several models have therefore been developed todetermine the coefficient of friction (COF). Most of these models have in common touse measured HKL from RTDD. The COF is defined in terms of roughness betweenthe wellbore wall and drill string (DS), but other influencing factors are also included.This is also a reason why the friction factor is often called a fudge factor (Masonand Chen, 2007). Some of the influencing factors to friction in wells are related topore hole cleaning, key seat, and pipe stiffness. All forces acting on the DS will affectthe HKL. To calculate modeled drag, it is necessary to know the friction factor. Ashigh drag forces are a critical limitation in extended reach wells (ERW), frictioninterpretation is important for successful wells. History, regarding the COF, fromnearby wells can be used in planning to provide a better basis when nearby wells aregoing to be drilled.In present study, the friction in a specific wellbore was determined. Well C-47, an oilproducer and water injector from the Gullfaks field in the North Sea, was studied toget an indication of the overall COF in each section of the well. Because some vitaldata from the final-well-report were missing, several assumptions had to be maderegarding the drill string and wellbore trajectory. This turned the study and analysisfor finding the correct value of the COF, a larger challenger challenge then necessary.Althoug relevant data were missing, it was managed to find reasonable values of theCOF by comparing modeled friction to expected friction. From evaluations of the 12¼ section, the COF can be concluded to lie within a range of 0,25 and 0,35.