This paper presents a new design model that will enable the drilling engineer to select the proper hydraulics for problem-free drilling in high-angle holes (from 55 to 90° from vertical). Empirical correlations have been developed after carrying out an extensive experimental study of cuttings transport in a 5-in. full-scale flowloop. The model predicts the required critical transport fluid velocity (CTFV), the average cuttings travel velocity (CTV), and the annular cuttings concentration under most given sets of drilling operating conditions.
The concept of an "Effective" diameter is introduced for the flow of drilling muds through annuli. This new diameter accounts for both annular geometry and the effects of a non-Newtonian fluid. It provides the link between Newtonian pipe flow and non-Newtonian flow through concentric annuli. The method is valid in any flow regime and can be used to determine whether a non-Newtonian flow is laminar, transitional, or turbulent. An analytical procedure is developed for computing frictional pressure gradients in all three flow regimes. The analysis also quantifies how flow transition is delayed by increasing the yield stress of a fluid. In addition, it is shown that transition in an annulus is delayed to higher pump rates as the ratio of inner to outer diameter increases. Furthermore, the method accounts for wall roughness and its affects on transitional and turbulent pressure gradients for non-Newtonian flow through pipes and concentric annuli. Finally, the method runs on a 386 PC in only a few seconds.
A numerical model has been developed to simulate laminar flow of Power-law and Yield-Power law fluids in conduits of arbitrary cross-section. The model is based on general, nonorthogonal, boundary-fitted, curvilinear coordinates, and represents a new approach to the solution of annular flow problems. The use of an effective viscosity in the governing equation of the flow allows the study of the flow behavior of any fluid for which the shear stress is a function of shear rate only. The model has been developed primarily to simulate annular flow of fluids used in drilling and completion operations of oil or gas wells. Predicted flow rates versus pressure gradient for laminar flow of Newtonian fluids in concentric and eccentric annuli, and Power-law fluids in concentric annuli compare very well with results derived from analytical expressions. Moreover, the predictions for laminar flow of Power-law and Yield-Power-law fluids in eccentric annuli are in excellent agreement with numerical and experimental data published in the literature. The model was also successfully applied to the case of laminar flow of Power-law fluids in an eccentric annulus containing a stationary bed of drilled cuttings and the results are presented herein.
In the ever growing directional and horizontal drilling, hole cleaning is a common and costly problem. It can become particularly critical in the case of extended reach drilling, where larger and longer wellbores are drilled. During the past two decades, especially in the recent years, many laboratory studies as well as field observations have been directed to address the cuttings transport problem. This has resulted in better understanding of the subject, and some remedies to overcome the problems. Several correlations/models have also been developed which give the field engineer a tool to better specify hydraulic requirements to clean the hole. However, due to the complexity of the subject, a comprehensive and proven model, not existing at this time, requires much more laboratory research and field studies. This will be the major topic addressed in this article. Introduction Inadequate hole cleaning in drilling horizontal wells may cause a number of costly problems such as premature bit wear, slow drilling rate, formation fracture, high torque and drag, and stuck pipe. If the situation is not handled properly, the problem can lead to side tracking or loss of the well. Several field studies have documented that hole cleaning is a frequently occurring problem that has to be watched carefully, otherwise it can and does develop to the serious problems mentioned above. Persisting cuttings transport difficulties in the field and many studies with large scale wellbore simulators have proven that hole cleaning in highly inclined wellbores is a complex problem. Over the last twenty years, many papers have been published addressing the subject. The majority of these publications provided qualitative studies and/or practical field guidelines, while some have presented modeling schemes that can be used to help the field engineer in optimization of drilling operations. In this paper developments in cuttings transport over the years, the shortcomings of its present status, and future research needs are addressed. Pioneering Experimental Studies Large scale cuttings transport studies in inclined wellbores were initiated at the Tulsa University Drilling Research Projects (TUDRP) about two decades ago with the support of a few major oil and service companies. A flow loop was built which consisted of a 40-ft long of 5-in. transparent annular test section and means to vary and control:angles of inclination between vertical and horizontal,mud pumping flow rate,drilling rate, anddrillpipe rotation and eccentricity. Past results using the facility have revealed the marked difference between the cuttings transport in inclined wellbores and that of vertical wellbores. A cuttings bed was observed to form at inclination angles beyond 35 degrees from vertical, and this bed could slide back down for angles up to 50 degrees. The results of these studies were published by Tomren, Iyoho, and Azar. Mud velocities in the range of 3 to 4 ft/s were found necessary for high angles with no pipe rotation as compared with 1 to 2 ft/s normally used for vertical drilling. Eccentricity, created by the drillpipe lying on the low side of the annulus, was found to worsen the situation. Analysis of annular fluid flow showed that eccentricity diverts most of the mud flow away from the low side of the annulus, where the cuttings tend to settle, to the more open area above the drillpipe. The above study at TUDRP was followed by another landmark work in cuttings transport by Okrajni and Azar, where they investigated the effect of mud rheology on hole cleaning. Their work confirmed the earlier findings and established that the cuttings transport mechanism and flow behavior in high angle wellbores is quite different than that of vertical wellbores. It was observed removing a cuttings bed using a high viscosity mud, a remedy for hole cleaning problems in vertical wells, may in fact be detrimental in high angle wellbores (assuming zero to low drillpipe rotation), and that a low viscosity mud which can promote turbulence is more helpful. P. 389
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