| dc.description.abstract | World demand for oil and natural gas are increasing, at the same time as most easyaccessible oil are being drained. Therefore, more complex and longer extendedreachwells (ERD) are required by the petroleum industry. The complexity of mostwells drilled today, together with many new field developments being economicallymarginal, increases the demand for reduced drilling costs. Rig rate is normallyconsidered the most significant cost for offshore field developments, and if non-productive time (NPT) can be reduced, the drilling costs will simultaneously belowered. A significant amount of lost time on drilling rigs are spend cleaning dirtyholes and fishing for stuck equipment. If such events can be prevented, the overalldrilling costs may be reduced significantly.When tripping in and out of wells, the surface hook load (HKL), representing thepulling force, is monitored at all times. Measured HKL is the total sum of allforces acting on the drillstring: Dead weight at surface (travelling block etc.) Buoyant weight of the submerged drillstring Hydrodynamic viscous forces Drillstring elasticity Downhole friction (wall contact, side forces, cuttings, ledges etc.)Abnormal wellbore geometry, and cuttings in the lower side of the well, may causerestrictions when running in or out of hole. Such restrictions may cause abnormalHKL response, and can lead to requirement for mud- and hole cleaning, stuckpipe, or in worse case, lost hole.Pulling operations in six different wells (four without registered downhole restrictions, and two with registered overpulls) have been chosen from Statoil s database. These have been studied in detail, and causes of overpulls have been suggested, based on wellbore profile, shape of overpull signals, and other downhole factors. It was evaluated and concluded that one overpull most likely was caused by cuttingsaccumulation in the wellbore, due to dedicated inclination angle, and building HKL tendencies. The other overpull was most likely caused by ledges, which wasidentified by rapid increase of HKL, and correspondence with hard calcite stringersdetected while drilling.The goal of present study was to develop a method for mathematical descriptionof the expected HKL signal while pulling, for simplifying detection of abnormalities. This was done by combining existing mathematical models with simplifying assumptions. The models were transferred into several Matlab®-codes, describing HKL signals in vertical wells, and in build-up sections, the latter by applying two different methods for estimating side forces (analytical and discrete). All models applied a mass-spring model for estimation of drillstring stretch, treating stretch as a wave propagating downwards through the drillstring. Input variables were physical downhole properties (known or assumed), and the block position at all times while pulling.Simulations of pulling operations in vertical sections produced generally lowerHKL signals than real measured signals. This was caused by assuming too lowdownhole friction, compared to what is true. The deviations increased as the depthof the well increased, and simulated HKL signals showed tendencies of too muchdrillstring oscillations. This effect was reduced as the drillstring weight increased. HKL simulations of pulling out of deviated sections provided HKL response quite equal to the true measurements, with 8-14% deviation from the true average HKLvalue. Drillstring oscillations were not a severe issue in the deviated sections.The main conclusions obtained from the present study are: Simulations of HKL signals in deviated sections provided acceptable simulations,close to true measurements Side forces in curved sections were successfully estimated by both models(analytical and discrete) HKL signals in vertical wells were successfully modelled, but were hamperedby too much drillstring oscillations The mass-spring model should be developed further to better describe drillstringstretch, e.g. by applying a more complex wave equation More detailed description of parameters affecting downhole friction is required | nb_NO |
