Abstract
Evolution of static and kinetic friction coefficients across the depth of a horizontal wellbore and field validation against the downhole parameters is presented in this paper. It has been observed in the literature that stick-slip oscillations exist in drill strings even with bit off-bottom and persist with the bit on bottom. In the past, various lumped mass models have failed to explain this phenomenon, as they have only considered bit-rock interaction to be the main reason for the stick-slip oscillations. The model considered in this study was used previously to estimate the friction factors at particular connections with bit off-bottom. For effective estimation of the downhole parameters, the model employs an adaptive soft sensor, robust to capture the disturbances occurring at the downhole. Using only the topside measurements (i.e., surface measurements), downhole RPM is estimated along with the evolution of friction coefficients (static (μ_s) and kinetic (μ_k) using the soft sensor. For obtaining the friction factors profile, those values of μ_s and μ_k that converge and remain stable for a specified time are considered. The model is first validated against the downhole RPM data obtained for a well in North America. It was observed that the estimates of the downhole RPM obtained using the model are in close agreement with that of the actual data. Next, the model was employed to obtain reasonable downhole RPM estimates for another well in North America where the downhole data is unavailable.
NSTL