Hershel-Bulkley modeli kullanılarak surge basınçlarının tahmini ve sonuçların analitik ve yazılım karşılaştırılması

In drilling hydraulics, the estimation of surge and swab pressures is important due to the fact that, the excessive surge pressures generated by pipe movement in tripping operations may result in serious consequences such as blowout and formation damage. Tripping operation can be performed due to several reasons such as adding pipe stand to drillstring, changing worn bit, removing fallen parts from borehole (fishing) or running logging tools. The time spent in tripping operation is a significant portion of the total time spent during overall drilling operations. During the drilling of offshore wells in deep waters, extended reach drilling (ERD) wells and highly deviated wells, small margins are encountered between pore pressure and formation fracture pressure. In such cases, the proper prediction of surge and swab pressure may exhibit paramount importance in order to ensure trouble free and cost effective drilling operations. In this work, the method for estimating surge pressures by analytical calculations using Herschel-Bulkley rheological model is presented and the results are compared with those obtained from Paradigm Sysdrill software, version 10. In calculating the frictional pressure losses for Herschel-Bulkley model the equations introduced by Merlo, et al. (1995) are employed. The barite-weighted and unweighted fresh water sepiolite muds are used in this study. The results estimated with Sysdrill software shows discrepancy, and the surge pressures are lower as compare to the results taken from analytical model. The surge pressures are also estimated using Bingham Plastic and Power Law model, and results are compared with Herschel-Bulkley rheological model. Additionally, a sensitivity analysis is performed to investigate the major parameters that affect surge pressures. The effect of tripping speed and fluid density exhibits the direct relationship, while the effect of borehole clearance exhibits the inverse relationship to the surge pressures. The study shows that, upto the certain degree of temperature, the surge pressures decrease with the increase in formation temperature, and then afterwards the surge pressures display an increment. For better understanding of formation temperature effect on mud rheology and surge pressures, the rheological constants for the mentioned mud samples are taken, using both Fann35A rotational viscometer and Fann50SL high temperature and high pressure (HTHP) rheometer. It is also determined that at the formation temperature up to 350°F, the use of both weighted and unweighted sepiolite muds allow to fasten the tripping speed of the drillstring.