Supervised By: Hazzaz Bin Yousuf, Assistant Professor, Department of PMRE, BUET
Supervisee: Khandaker Fahim Anjum, Nizar Mohammad Qiasi
Research Period: Jan 2019 - Dec 2019
Abstract
Replacing the requirements of several vertical wells, directional wells lead to technological advancement and improved operational efficiencies recently. Regardless of the favorable circumstances, such wells include a range of radius of curvatures and eccentric conditions of drill pipe inside casing where the cuttings generally store quickly in the annular bends causing stuck pipe issues. In this particular research, a parametric study had been carried out to observe the changes in annular pressure drop and optimize cutting accumulation against changes in the critical parameters of the drilling operation i.e. drilling mud (non-Newtonian fluid) density, drill pipe rotation, the eccentricity of annular trajectory. The Eulerian approach was used for modeling the multiphase flow of drilling fluid and spherical cuttings. A significant distinction in the cuttings' volume fraction due to the drill pipe's modified rotational speed with such smaller cuttings was observed in this study. However, the continuous deposition of larger particles in the annular bends still poses a challenge to the wellbore cleaning process despite this rheological upgrade and critical drilling parameter modification.
Keywords: Directional well; Radius of curvature; Annular bend; Stuck pipe; Non-Newtonian Fluid; Cuttings; CFD modeling; Eulerian-Eulerian method; Volume fraction
Objective
Predict the flow behavior of Herschel-Bulkley (HB) fluid
Understand the effect of bend angle and mud rheology with single-phase flow model using CFD
Understand the effect of the rotational speed and eccentricity of a drill string on a multiphase flow model using CFD
Summary of Literature Review
Previous investigation showed that several parameters like drill pipe rotation speed, drilling mud nature, flow pattern through the annulus, eccentricity, mud circulation rate etc. influence the pressure drop and the cutting accumulation in the annulus. Most of the studies are related to the vertical experimental setup and as the interest in directional drilling is increasing and is a popular operation throughout the world, there is still a lack of studies regarding directional drilling geometry. Thus, In this research paper, the effect of drill pipe rotation, eccentricity, and the effect of bend angle change for directional drilling geometry is investigated.
Methodology
To study the effect of drill pipe rotation and its eccentric position in terms of cuttings transport, the CFD method was adopted and simulated in ANSYS FLUENT. The following sequences were followed and analyzed to produce a scenario that was as realistic as possible while taking the limitations into account.
Initial Thinking
Geometry Creation (ANSYS Workbench)
Mesh Generation (Hexa-Hedral Mesh)
Flow Specifications (Initial and Boundary Conditions, Flow Parameters, Primary and Secondary Phase Parameters etc.)
Numerical Solution of Equations (SIMPLE Method)
Result Analysis (Microsoft Excel)
Modeling Domain: The length of the drill pipe being modeled is 2.34 m with various eccentricities as per requirement for the study. Two bends were obtained at different angles aiming to determine the pressure drop along the annular trajectory and the cuttings volume fraction that is accumulated at different sections of the annulus.
Assumptions
Particles and the non-Newtonian fluid (which is the conveying medium) are considered to be continuous.
No-slip condition between continuous phases and the walls (drill pipe and wellbore).
Particle shape factor is not included.
According to particle-particle interactions, there is no form or mass change.
The casing inner wall and the drill pipe outer wall is assumed to be smooth ( there is no roughness factor).
Cuttings' initial volume fraction is considered to be 0.6 unit.
For the multiphase flow implicit solution is considered for a steady condition.
Particle inlet velocity is assumed to be 0.5 m/s.
A velocity inlet and atmospheric pressure outlet is adopted for the simulations in this research work.
Limitations
The simulated dimension of the pipe is not realistic considering the directional wells in real life.
The fluid properties used in the simulation might be can differ from the actual drilling mud properties.
Initial cuttings' volume fraction in the drilling mud flowing inside the annulus is assumed to be 0.6 as the field data could not be managed.
Result and Discussion
Effect of Bend Angle and Mud Rheology in Single-Phase Flow Model
Mud Rheology Variation: Mud-1 (0.5% CMC Solution), Mud-2 (0.5% CMC Solution+ 4% Bentonite)
Bend Angle Variation: Bend-1 (15°,35°,55°)
For the case of increasing drill pipe rotation from 150 RPM to 200 RPM, the average percentage pressure drop increase for mud-2 is lesser than mud-1 which demonstrates that mud-2 is better for its added viscosity property and thus reliable for operation.
Change of bend angles for the directional drilling has a greater effect on pressure drop along the annular gap. As the bend angle of the upper bend (Bend-1) is increased from 15 degrees to 55 degrees percentage of average pressure drop is increased from 3.16-34.05% and 0.68-23.63% for Drilling Mud-1 and Drilling Mud-2 respectively.
Increased viscosity nature of mud-2 increases the annular pressure by an average of 69.15% and 66.89% for drill pipe rotation of 150 rpm and 200 rpm respectively which depicts that Mud-2 has reduced the effect of rpm increase. That is why this type of annular geometry operation can be optimized by 150 rpm with mud-2.
Figure-1: Simulation Conducted in 15° Bend
Figure-2: Simulation Conducted in 55° Bend
Effect of RPM and Eccentricity of drill pipe in Multiphase Flow Model
Drilling Mud: Mud-2 (as from the previous study, the mud-2 is better than mud-1)
Cuttings: Sand Particle (0.002 m)
RPM Variation: 0 RPM, 50 RPM, 100 RPM, 150 RPM
Eccentricity Variation: Concentric (0 unit), Eccentric (0.2, 0.4, 0.6 units)
The increasing rotational speed of the drill pipe aids the hole cleaning as it reduces the overall cutting concentration and even at bends (Figure-3 and Figure-4).
The impact of modification in rotational speed is observed to be more dominant in the eccentric condition of pipe compared to the concentric condition (Figure-4).
The annular pressure drop increases with pipe rotation at low flow rates of drilling mud to an extent. But for higher flow rates, the annular pressure drop significantly decreases with the improvement of pipe rotation (Figure-3).
Cutting particles concentrate more at bend-1 compared to bend-2 causing the increasing pressure drop at bend-1 in annuli (Figure-7 and Figure-8).
The hole eccentricity affects the cuttings' transport efficiency adversely. Cuttings accumulate more in eccentric conditions of drill pipes compared to concentric pipes (Figure-8).
To diminish the adverse effect of eccentricity, the rotational speed to be much higher aiding the hole-cleaning process.
Figure-3: Annular Pr Drop vs Rotational Speed of Pipe
Figure-4: Cuttings' Volume Fraction vs Rotational Speed of Pipe
Figure-5: Annular Pr Drop vs Eccentricity of Pipe
Figure-6: Cuttings' Volume Fraction vs Eccentricity of Pipe
Figure-7: Effect of Pipe Rotation in Concentric Condition (e=0)
Figure-8: Effect of Pipe Eccentricity in Stationary Condition (ω=0)