Sensitivity Analysis of Hydraulic Parameters on Cuttings Transport Efficiency in Underbalanced Drilling Operations
Abstract
Underbalanced drilling (UBD) in depleted carbonate reservoirs employs multiphase aerated fluids whose compressible nature makes prediction of cuttings transport difficult. Inadequate hole cleaning is a leading cause of non-productive time, with poor hole cleaning responsible for one-third of all stuck-pipe events globally. This study presents a systematic sensitivity analysis of hydraulic parameters governing cuttings transport efficiency in vertical UBD wells, based on the minimum kinetic energy criterion of Guo and Ghalambor. A computational model was developed in Python and Microsoft Excel, integrating terminal settling velocity calculations, the Beggs-Brill multiphase flow correlation, and the kinetic energy-per-unit-volume framework. The model considers a synthetic 10,000-ft vertical carbonate well with an 8.5-in. hole and 9.0-ppg aerated mud, with liquid flow rate varied from 200 to 600 gpm and gas injection rate from 500 to 3,000 scfm. Results demonstrate a non-linear relationship between gas injection rate and cuttings transport efficiency: increasing gas rate improves transport up to approximately 1,200 scfm by raising annular velocity, but beyond this threshold marginal gains diminish while equivalent circulating density (ECD) rises toward the fracture gradient. The safe operating window is determined as 350-450 gpm liquid flow and 800-1,400 scfm nitrogen injection, maintaining cuttings concentration below the 4% design limit and ECD within the underbalanced pressure window. Additional sensitivity analyses for cuttings particle size (0.125-0.50 in.) and rate of penetration (30-120 ft/hr) define how the safe window shifts under varying field conditions. Benchmarking against published Lahendong Geothermal Field operating data confirms the model correctly classifies adequate hole-cleaning conditions. Results provide field engineers with practical design charts for hydraulic optimization in depleted carbonate UBD operations.