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Summary When drilling in an arid region through heavily fractured formations, it can be very challenging to manage drilling-fluid losses and at the same time maintain a downhole-pressure gradient that is compatible with the very-low geopressure gradient windows that are typically encountered in those drilling conditions. Nitrogen-enriched drilling muds may provide a good solution to both problems; however, the properties—such as density, rheology, specific-heat capacity, and thermal conductivity—of this type of drilling fluid are highly dependent on temperature and pressure, and in most cases those characteristics cannot be measured in situ, making it difficult to estimate the actual downhole-pressure conditions. The approach described in this paper consists of the reconstruction of the drilling-fluid-mix properties from the characteristics of its components and the incorporation of the resulting pressure- and temperature-dependent constitutive laws into a real-time multiphase- and multicomponent-drilling hydraulic model to estimate the downhole pressures along the drillstring and borehole as a function of the drilling parameters. Because of the uncertainty of some of the characteristics of the components of the drilling fluid as well as their actual proportion in the mix, the modeled values are only valid within a certain accuracy. Stochastic simulations are made during the estimation of the downhole pressures to ascertain the precision of the calculations. As a consequence, by comparing the obtained interval of confidence on the estimations with actual measurements, it is possible to evaluate whether the drilling conditions are normal or deteriorating. The validity and performance of the derived fluid-model extension are tested by use of a real-time data set recorded during the drilling of a well in the Erbril area of the Kurdish region of Iraq, by use of the wellsite information transfer standard markup language drilling-data-exchange protocol. The model results are reviewed and compared with the actual measurements recorded during the drilling operations. The potential sources of limitation, discrepancy, or error between the modeled and observed well and fluid behavior are discussed, along with potential explanations for the observed wellbore physics seen in the recorded-data feed.
Abstract This case study involves a well drilled in the Erbil area of the Kurdish Region of Iraq, a region characterized by challenging geological conditions for drilling. To achieve the key drilling objectives, the drilling mud was made less dense by the addition of nitrogen into the mud column. In order to get a full understanding of the downhole conditions using this mud, the complete drilling process was modelled in real-time. The model was driven using a real-time WITSML data feed. This transient modeling software calculates downhole pressures, temperatures, torque and drag and cuttings density at all depths in the well bore in real- time, including the depths where there are no physical measurements. The transient model is continuously updated in real-time to reflect the drilling processes undertaken on the rig (e.g. pipe movement, mud pump activity, thermodynamics). Surface system variables including virtual mud pit levels are also calculated in real-time. The modelled data is then continuously compared to the sparse data points that are being recorded in real-time, allowing both a continuous calibration of the model with the "as drilled" well operation. The calculation of important drilling parameters such as sliding friction, rotational friction, and hydraulic friction is performed in real-time. The paper will present the key observations upon the matches between the modelled data and the "as drilled" data and summarise the key lessons learned during the well operations and the real-time modelling processes.