A one dimensional transient mechanistic model of cuttings transport with conventional (incompressible) drilling fluids in horizontal wells has been developed.
The model is solved numerically to predict cuttings bet height as a function of drilling fluid flow rate and rheological characteristics (n, K), drilling rates, wellbore geometry and drillpipe eccentricity.
The results of the sensitivity analysis showing the effects of various drilling operational parameters on the efficiency of solids transport are presented.
The model developed in this study can be used to develop computer programs for practical design purposes to determine optimum drilling fluid rheology (n,K) and flow rates required for drilling horizontal wells.
Poor hole cleaning can lead to pipe sticking, higher drag and torque, slower rate of penetration, formation fractures and wellbore steering problem1. It has been estimated that one third of stuck pipe problems are due to inadequate hole cleaning2. Azar and Sanchez3 presented a comprehensive review of the factors that affect cuttings transport and listed drilling fluid rheology, flow rate, annular eccentricity, drillstring rotation and drilling rate as the most important ones.
Iyoho4 presented some extensive experimental results on the cuttings transport performances in directional wells. He found that the major factors affecting cuttings transport are drilling fluid velocity, inclination angle, drilling fluid viscosity and drilling rate. Specifically, it was observed that the increase of hole angle or drilling rate had negative effect on cuttings transport, while higher drilling fluid viscosity was more favorable than the lower drilling fluid viscosity for cuttings transport within the same flow regime, and annular eccentricity had moderate effect although the concentric annulus provided the best transport performance.
Larsen5 confirmed Iyoho's results by concluding that the angle of inclination and drilling fluid flow rate had the most significant effect on hole cleaning.
Tomren et al. 6 extended the Iyoho's work and found that concentric annulus yields maximum efficiency of cuttings transport for inclined holes. They also determined that, in inclined annulus, cuttings bed formation in high viscosity drilling fluid was slower than that of in low viscosity drilling fluid in laminar flow.
Okrajni and Azar7 particularly investigated the effects of drilling fluid rheology on hole cleaning in directional wells. The drilling fluids with yield values in the range of 0 to 9.6 Pa (0 to 20 lbf/100 ft2) and with three values of YP/PV ratio (0.5, 1.0 and 2) were used in the tests. Investigations revealed that drilling fluid rheological properties (yield value and YP/PV ratio) generally did not affect the cuttings transport under turbulent regime although the drilling fluid with higher yield value resulted in better cuttings transport under laminar flow regime. The trends observed were identical for both the bederosion (annulus-cleaning) and cuttings transport experiments.
An experimental study of drilled cuttings transport in inclined boreholes by Ford et al. 8 found that cuttings were removed by two distinct mechanisms including rolling/sliding and transport in suspension. Seven slurry flow patterns were observed including homogeneous suspension, heterogeneous suspension, suspension/saltation, sand cluster, separated moving bed (dunes), continuous moving bed and stationary bed.