Observed and Estimated Effects of Cuttings on Equivalent Circulating Density
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More extended reach wells, highly deviated and ultra deep are being drilled to meet the world s continuously increasing energy demand. Hole cleaning remains one of the greatest challenges throughout drilling operations, as material from the formation is generated during the entire drilling process. Depending on several parameters like rate of penetration, flow rate and mud rheology, the cuttings concentration will fluctuate throughout the wellbore. Insufficient hole cleaning may result in a number of drilling problems. Satisfactory cuttings transport is therefore essential to avoid problems associated with poor hole cleaning, and it is important to reduce non-productive time associated with downhole failures and wellbore integrity.The objective of the present study was to observe and estimate an effect from cuttings on equivalent circulating density, and to find the amount of cuttings in the vertical part of the wellbore at a certain time. This was attempted by calculating the length of displacement of generated cuttings from the horizontal part within each section, and the amount of time required for these cuttings to be transported upwards the annulus, to the vertical part of the wellbore. This has been done for both observed and theoretical values.The effects of cuttings on equivalent circulating density have been observed and estimated, based on data from the drilling operation of well NO-34/10-C-47 in the North Sea. Twenty different cases were chosen, and standpipe pressure information was gathered from real time drilling data. The cuttings concentrations were then calculated. The same procedure was then performed theoretically, where data from the Final Well Report were used. The results of observations and estimations were compared to evaluate the quality of the results. Also, extensive research of published knowledge has been made, along with interpretation of already existing cuttings transport models. The parameters affecting hole cleaning are presented to give an overview of the complexity of the subject.The main conclusions obtained in the present study are: Effects of cuttings on equivalent circulating density were successfully observed and estimated. Good consistency of model results when comparing the observed and estimated values for lengths of displacement. The field data provided relevant information. However, field data from several wells would have been preferable for increased reliability. The model calculations were not able to account for transient effects, which limits the results significantly. Simple theoretical models were made for clarity, but more complex and detailed descriptions are required, accounting for transient effects.