Summary Eccentricity of the annulus can greatly affect the velocity profile, especially in extended-reach wells and slimholes. While frictional pressure losses in concentric and fully eccentric annuli have been studied before, this paper focuses on the effect of arbitrary eccentricities on velocity profile and corresponding influence on frictional pressure losses. In this study, axial flow of yield-power-law (YPL) fluids in eccentric annuli for a 2D steady-state flow has been investigated numerically and verified against experiments. A boundary-fitted coordinate system is used to discretize the flow equations and generate the mesh network. Fluid-flow equations were solved adopting an iterative method. The results of simulation include the effects of fluid rheology, flow rate, annulus dimensions, and eccentricity on velocity profile and frictional pressure losses in the annulus. Numerical results were compared with the available extensive experimental investigations on the flow of a variety of drilling fluids that have a strong shear-thinning property and high yield stress (e.g., polymer-based and bentonite fluids). The tests were implemented over a wide range of flow rates using a flow loop that is equipped with a pipe viscometer and several annular test sections with various sizes and eccentricities. Field observations, experimental data, and the analytical approach in this study indicate that increasing eccentricity lowers frictional pressure drop in the annulus. A good agreement was observed between the numerical-simulation results and experimental measurements. Comparison of the present and past studies with analytical solutions of Newtonian fluids in an eccentric annulus shows that the current study provides more-accurate results. Detailed numerical simulation and the equations that are presented in this study can be used by investigators. The application of the Cartesian and boundary-fitted coordinate systems and algebraic correlations for geometry transformations are presented. Moreover, the exact method of flow-rate calculation based on the velocities at each gridpoint is shown in the provided details. The application of findings in this study includes more-accurate predictions of velocity profile and frictional pressure loss in the annulus, which lead to better predictions of barite sag, cuttings transport, equivalent circulating density, and mud cement displacement in special cases of cementing operations.
Wellbore instability is one of the most consequential drilling operation risks. Planning a trajectory without knowing the wellbore instability risk can be costly during operations. It is critically important for engineers to be able to validate wellbore stability coherently when trajectories are planned and be able to adjust and optimize a trajectory to minimize the risk during the planning phase. The risks of wellbore collapse in the buildup sections, if the trajectory azimuth is not optimized with formation stress orientation, could be catastrophic. Due to shale heterogeneity, the horizontal section of the wellbore also has a high risk of wellbore instability. All the wellbore instability issues can lead to non-productive time and increase the cost of well construction. The solution presented in this paper is a cloud-based, coherent trajectory planning solution with wellbore stability validation using a Mechanical Earth Model (MEM). Detailed well planning is required to mitigate all the wellbore instability issues. This cloud-based, coherent wellbore stability validation provides an efficient way to improve trajectories by advising the best azimuth and hole inclinations to avoid wellbore instability risks. The MEM is automatically used to compute the wellbore stability (WBS) on the trajectory design. Mud weight can also be validated in the wellbore stability model based on the mud weight window provided by the MEM. In the cloud collaborative environment, all other well planning workflows, such as BHA, casing design, etc. will be validated with the designed trajectory. A case study of unconventional well planning will be presented to show how to avoid wellbore instability by choosing a different trajectory than original proposal. In the case study, WBS was computed from a MEM whose data are acquired from wireline logging. This study mitigated the risk of wellbore instability in the curve section by changing the dogleg of the trajectory. Simultaneously, mud design, BHA design, and casing design were concurrently validated to ensure safer and better well planning. Non-productive time was avoided because of a better trajectory design and wellbore stability. This new workflow can help operators optimize well trajectory with reduced effort and deliver high quality well planning.
For offshore wells, the regulatory agency requires the submission of a worst discharge analysis and relief well planning report. The ability to control the blow out under worst case blowout scenario shall be documented and is a requirement for the operators to successfully apply for a permit to drill in the US offshore fields. As the water depths of offshore drilling operations are getting deeper and deeper, due to the increased frictional pressure losses in kill lines and formation fracture strength, bringing the blow out well under control with worst case discharge becomes more challenging. Operational parameters need to be carefully controlled to avoid exceeding the operational limitations such as breaking the formation or exceeding available pump capacity. In this study, dynamic simulations of multiphase flow are carried out to evaluate the operational parameters during the kill process. The simulations account for transient changes including frictional pressure losses, U-tube effect and fluid density variations. By optimizing the operational sequence with regards to, kill mud density, pump flow rate, pump down staging, relief well drillstring and trajectory, blowout can be controlled without exceeding the operational window. The paper shows the required volumes of the kill mud, required pump capacity, optimal flow rate arrangement, and minimum time required to get full kill mud return to the sea floor during the well kill operation. Through the aid of advanced transient software models, assessment of the required capacity to kill a blowout enables development of realistic contingency plans to ensure that well control can be re-established in case of an ultra-deep water worst blowout scenario.
Under certain drilling conditions, the weighting material particles such as barite can settle out of the drilling fluid. This phenomenon, known as barite sag, can lead to a number of drilling problems including lost circulation, well control difficulties, poor cement job, and stuck pipe. This study investigates barite sag, both experimentally and numerically, in the annulus under flow conditions. Experimental work has been conducted on a large flow loop to investigate the effects of major drilling parameters on barite sag by measuring the circulating fluid density. Results of the tests indicate that the highest sag occurs at low annular velocities and rotational speed and also at high inclination angles. It was observed that at inclination angles less than 60°, for any annular velocity, barite sag is not significant. Eccentricity of a non-rotating inner pipe did not have a significant effect on barite sag. However, effects of inner pipe rotation on barite sag for an eccentric annulus are more significant than concentric case.The simulation part of this study is based on a proposed particle tracking method called "Particle Elimination Technique". The traveling path of each solid particle is assumed to be a function of size and shape of the particle, fluid velocity and rheology. Based on the estimated traveling path of particles, density of the fluid is updated considering the number of particles whose paths lead to the bottom of the annulus and become motionless. In order to capture the complexities associated with the solid-liquid flow, a lift force is assigned to the solid particles that enable adjustment of the model with experimental results.Comparing the results of numerical simulation to the experimental study on the effects of annular velocity on barite sag in a horizontal annulus shows a good agreement. The numerical simulation was modified from laboratory scale to real wellbore dimensions for practical drilling applications. Results of the simulation show prediction of the density of the drilling fluid in the horizontal section of a wellbore with various lengths and dimensions under different annular velocities.
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