Simulation of Transient Cuttings Transportation and ECD in Wellbore Drilling

Abstract

Abstract Among the costs considered on an oil field exploration, the drilling process related ones constitute a significant share. Consequently, the focus on the removal of cuttings generated by the drilling process, or the hole cleaning operation, is essential. This operation remains a critical issue in the drilling of high inclination-wells, since the cuttings tend to deposit themselves due to gravity's action, forming a bed in the annular space formed between the drill string and the casing. When this bed takes a sizable fraction of the annular space, it becomes responsible for many problems that appear on the drilling stage, such as premature bit's exhaustion, low rates of penetration, formation fracture, stuck pipe, fluid flow interruption, etc. If this situation is not treated properly, it may cause the loss of the well. Once the phenomena affecting cuttings transportation, bottom hole pressures and ECD prediction are time dependents, a transient model for the whole trajectory of the wellbore was build. This model can predict the pressures developed in the annular due to the phenomenon that occurs in the drilling process, like bed height increase/decrease, drillpipe movement (surge and swab), increase in the amount of solids caused by rock instability problems and operational parameters changes (increase/decrease in ROP and fluid flow rates). This paper objective is to present a new transient methodology to simulate the cutting's carrying and annular pressure evaluation on a transient approach and to present some examples of this model. These examples will showcase bed height, cuttings concentration, pressure and ECD curves along time and over the whole trajectory, due to changes in the parameters mentioned above. Introduction Transport of cuttings has been one of the greatest concerns in the oil well drilling. An investigation undertook by Massie et al. (1995) showed that 70% of the time lost with unexpected events was associated with stuck pipe. A study done by Hopkins and Leicksenring (1995) presented that a third of the problems of stuck pipe were due to inadequate hole cleaning. The problem of drilled cuttings transported by fluid is a theme of interest of several industrial areas, such as: petroleum, mining, among others. Basically, the solids, since they are denser than the fluid, tend to be transported with a lower velocity than the medium velocity of the fluid. In the past, this type of analysis was accomplished without taking into account the transient effects that had happened during the drilling stage, as in the works of Iyoho (1980), Gavignet and Sobey (1986), Martins (1990), Campos (1994), Pilevari et al. (1996). A more realistic representation of the phenomena requires a detailed process simulation, including transient effects. Simulations in turn allow the introduction of disturbances in the variables at any instant of time, permitting the reproduction of field conditions, such as: change in the flow rate, alteration in the drilling fluid, increase in the amount of solids to be carried and progression in the hole cleaning process. Pressure variations taking place in the annular due to the interruption and the retaking of circulation and the drillstring trip and ECD variations along the annular can also be evaluated in a transient methodology.