Transient Cuttings Transport with Foam in Horizontal Wells-A Numerical Simulation Study for Applications in Depleted Reservoirs

Abstract

Abstract A good knowledge of foam hydraulics and cuttings transport (hole cleaning) is essential for successful applications of foam drilling technology in horizontal wells. Estimating local fluid velocity in a partially blocked eccentric annulus (due to cuttings) is a challenge; so, a new analytical method of calculating local stress and velocity (near cuttings-bed in annulus) in an eccentric annular flow is derived which makes it possible to estimate the bed height for foam drilling in horizontal wells. Therefore, a new equation for local velocity (critical velocity) to initiate particle movement is used in order to predict the bed-height profile in a build-up or horizontal section of wellbore. This investigation also focuses on understanding the effects of drilling parameters on bed height and cuttings concentration. A new transient wellbore hydraulics and cuttings transport model has been developed using the finite difference method of continuity equations for foam and cuttings. In order to predict the cuttings bed formation in horizontal wells, a mechanistic hole-cleaning model consisting of two layers has been utilized. The model is based on torque balance for a particle on the surface of a bed. In addition, a new model has been formulated for the local shear stress by modeling the eccentric annular flow as the infinite number of concentric annuli with variable outer radius. Similarly, using the narrow slot-approximation technique, a local velocity profile has been determined analytically in the eccentric annulus to be applied in the torque balance equations. Subsequently, model predictions of bed height were compared with published experimental data and the model is fine-tuned to minimize discrepancies. Results show the cuttings bed front transitions through the annulus along the build-up and horizontal sections. Model predictions showed a good match with experimental results for concentric horizontal annulus except at higher polymer concentrations (greater than 0.25%). The simulation results show that bed height and cuttings concentration are quite sensitive to the changes in surface foam injection rates and back-pressure, thereby can be best optimized by properly adjusting these input parameters. The results also suggest that hole-cleaning is a function of inclination. The bed height increases with increase in inclination angle until a critical angle of 90°-φ (φ is the angle of repose) after which, it reduces.