| dc.description.abstract | Directional drilling has been increased recently due to its multiple benefits when compared to conventional drilling. Among several advantages, the increase in the production of oil and gas through the use of multiple wells from a single vertical wellbore is the most significant one. However, there is a tendency of the drill pipe to rest on the low-side of the annulus due to gravity. Moreover, vertical component of annular fluid velocity is reduced resulting in accumulation of cuttings on the low side of the wellbore. With reduced vertical component, suspended cuttings in the annulus can settle and ultimately form a uniform cuttings bed, which partially blocks the flow and creates hydraulic resistance. Consequently, bottom hole pressure is affected by this phenomenon and drilling operation performance can be significantly reduced.
Numerous wellbore hydraulic studies (Haciislamoglu and Langlinais, 1989; Fang et al.,1999; and Escudier et al., 2002) have been conducted to predict annular pressure loss in eccentric annulus. However, very limited studies (Hussain and Sharif, 1998; and Azouz et al., 1993) have been conducted on fluid flow in partially blocked annular geometries. The aim of this research is to perform numerical simulation-based investigation to analyze the effect of cuttings bed formation on annular pressure loss in a partially blocked eccentric annulus under laminar flow condition. A Computational Fluid Dynamics (CFD) software (ANSYS FLUENT) is used to conduct numerical simulation studies through the use of a finite element algorithm for solving the governing equations of motion in such complex annular geometries with blockage. The simulation studies were conducted for power law fluid flowing in highly eccentric annulus (i.e. 90% eccentricity). Effects of fluid rheological properties (fluid behavior index and consistency index) and flow geometry (diameter ratio, and cuttings bed height) on velocity profile, frictional pressure loss and bed and wall shear stresses are investigated. Due to the presence of tool joints and wellbore irregularities, drillstring is expected to have approximately 90% eccentricity. In addition, for eccentricity more than 90%, simulation studies become very difficult and computationally intensive due to numerical instability.
Pressure losses predicted using CFD were evaluated by comparing them with results of published studies and experimental measurements obtained from a partially blocked eccentric annulus. A good agreement is obtained with CFD predictions and results of published studies and experimental measurements. For Newtonian fluids, CFD results for cases without cutting beds were validated using analytical solution.
After proper validation, simulation results were used to develop approximate correlations for friction factor and bed shear stress. The new friction factor correlation provides reasonable prediction with a maximum discrepancy of ± 5%. The new bed shear stress correlation exhibits slightly higher discrepancy (± 10%). As anticipated, the annular frictional pressure loss increased with cuttings bed at a constant flowrate. It was also observed that with greater shear thinning behavior, the lower is the impact of cuttings bed on the annular pressure loss. | en_US |
