Increased demand for energy resulted in the expansion of drilling activities worldwide. Any improvement in terms of well planning and drilling operation can result in significant savings for the operator. A good plan includes all information from offset wells as well as sound engineering principles and it relies on subsurface characteristics and operating conditions. While formation properties are not controlled by drilling, the operational parameters can be modified to improve the drilling operation on a real time basis. Thus it is important to study the operational factors affecting the performance of a rig. Particularly, understanding the parameters that control wellbore cleaning is beneficial in both horizontal and deviated wells.
In this study, a section of casing-drill pipe annulus section was simulated using Computational Fluid Dynamics (CFD) to determine the effects of different parameters such as fluid velocity, cutting size, rate of penetration, drill pipe rotation and inclination angle in deviated wells. The simulations were conducted at steady state and the results for maximum cutting concentrations in the annulus were analyzed. Formation of cutting beds is noticed near the entry section of the annulus and the transport of the cuttings in the annular section occurred in the form of stratified flow. Fluid flow rate, angle of inclination and rate of penetration have a major impact on cutting concentrations and proper prediction of these parameters are important to avoid formation of cutting beds. It is also noted that drillpipe rotation can enhance cutting transport but it generally has a greater effect on smaller sized particles.
As horizontal wellbores get longer and deeper, and as practices such as extended reach drilling become more common, hole cleaning can be an increasingly difficult and different challenge from vertical wells. In an inclined well, cuttings settle vertically, but the fluid velocity has a reduced vertical component. Particles settle quickly and have less distance to travel before they hit the borehole wall. Here, the velocities are negligible and particles tend to deposit in the annulus leading to the formation of contiguous beds. Significant reduction of transport capacity occurs in horizontal wells. Inefficient hole cleaning and formation of beds lead to problems such as premature bit wear, high torque and drag, stuck pipe and slow drilling rates which increase drill time and costs.
The major parameters which affect hole cleaning in an annulus can be divided into three different groups. The first group consists of the fluid parameters such as fluid density, fluid viscosity and fluid flow rate. The second group consists of cutting parameters which include cutting density, cutting shape and size and cutting concentration. The third group consists of the operational parameters i.e. the angle of inclination, pipe rotation speed and eccentricity in the hole. Research into these cutting transportation parameters and their effects can lead to prediction of carrying capacity of fluids and therefore facilitating the optimum design of directional wells.
Real time data from on site locations is impossible to collect and analyze with cutting transport parameters. Hence, researchers[1,2,3,4] started building large scale flow loops to conduct experiments to observe and investigate the variables that affect cutting transportation. The observations of these experiments suggested the existence of different flow patterns based on the hole cleaning parameters.
Cutting transport problems in horizontal conduits occur only when moving bed or stationary-moving bed patterns are observed. The parameter values that lead to bed formations are the ones that are of importance and the ones that are studied in the literature. These experimental works mostly give us general correlations and rules of thumbs (ranges of parameters) to avoid the formation of the bed.