As the inclination of a wellbore increases, cuttings start having a tendency to accumulate in the lower section of the wellbore, and develop a cuttings bed. This developed bed causes a reduction in the flow area, an increase in the friction between the drillstring and the wellbore, leading to an increase in torque, decrease in force transfer to the bit, and lose control on the bottomhole pressure. Finally, drilling rate decreases. Estimation of total concentration inside the wellbore is not an easy task. Moreover, moving cuttings and fluid dragging them to move have different relative velocities inside the wellbore, causing variations in pressure drop. Although there are many attempts to estimate the cuttings concentration and slip between the cuttings and the fluid using mechanistic models and empirical correlations, the performances are limited either with the strong assumptions made, or the experimental facility capabilities. This study aims to determine some of the very-difficult-toidentify data for estimating total pressure drop and total cuttings concentration inside the wellbore. Extensive experimental work has been conducted using a cuttings flow loop at horizontal and inclined wellbores. Tests have been conducted using water to simulate low viscosity fluids. Data has been collected for a wide range of flow rates, cuttings injection rates and pipe rotation speeds. All experiments have been recorded using a high-speed digital camera. Images have been processed using special algorithms, and volumetric distributions of cuttings and fluid can be identified very accurately. By comparing consecutive images, very valuable information has been collected about the accumulated cuttings amount, concentration of moving particles, their relative transport velocities, slip velocity between the phases, the friction factor on the stationary bed, etc. Since the images are digital, information collected is converted into numerical values, and semi-empirical equations are developed as a function of known drilling parameters. The obtained information is tested in simple mechanistic models for estimating pressure drop inside a wellbore with the presence of cuttings, and the performance of the model is tested by comparing the results with the measured ones. It is observed that, after supplying the very-difficult-to-identify information to the mechanistic model, the performance of the mechanistic model improved very significantly. The information provided will improve the design of long extended reach wells while estimating hydraulic requirements, and make it possible to have a better understanding of what is really happening inside the wellbore.
A proposed viscosity model for Yield Power‐Law fluid by Souza Mendes and Dutra (SMD), devoid of discontinuities at vanishing shear rate, is adopted in a 3D CFD simulation study. Radial distributions of axial, tangential, and coupled components of velocities are measured along concentric and eccentric narrow annuli. The flow is assumed to be fully‐developed, laminar, and steady state. Several parameters such as diameter ratios, eccentricities, and inner pipe angular speeds were varied to analyze their influence on the flow distributions and annular pressure losses. The simulated results of radial distributions of axial and tangential velocities show good agreement with experimental data and analytic solutions from previous studies. Annular flow instabilities in the form of Taylor vortices in the flow fields are briefly discussed. This study shows how reliable CFD simulations can replicate the actual, yet complex, oil and gas drilling operations.
This study aims to investigate the hole-cleaning process during the flow of a drilling fluid consisting of a gas and a liquid phase through a horizontal annulus. Experiments have been conducted using the Middle East Technical University (METU) multiphase flow loop under a wide range of air-and water-flow rates while introducing cuttings into the annulus for different amounts. Data have been collected for steady-state conditions (i.e., liquid, gas, and cuttings injection rates are stabilized). Collected data include flow rates of liquid and gas phases, frictional pressure drop inside the test section, local pressures at different locations in the flow loop, and high-speed digital images for identification of solid, liquid, and gas distribution inside the wellbore. Digital imageprocessing techniques are applied on the recorded images for volumetric phase distribution inside the test section, which are in dynamic condition. The effects of liquid and gas phases are investigated on cuttings-transport behavior under different flow conditions. Observations showed that the major contribution for carrying the cuttings along the wellbore is the liquid phase. However, as the gas-flow rate is increased, the flow area left for the liquid phase dramatically decreases, which leads to an increase in the local velocity of the liquid phase causing the cuttings to be dragged and moved, or a significant erosion on the cuttings bed. Therefore, increase in the flow rate of gas phase causes an improvement in the cuttings transport although the liquid-phase flow rate is kept constant. On the basis of the experimental observations, a mechanistic model that estimates the total cuttings concentration and frictional pressure loss inside the wellbore is introduced for gasified fluids flowing through a horizontal annulus. The model estimations are in good agreement with the measurements obtained from the experiments. By using the model, minimum liquid-and gas-flow rates can be identified for having an acceptable cuttings concentration inside the wellbore as well as a preferably low frictional pressure drop. Thus, the information obtained from this study is applicable to any underbalanced drilling operation conducted with gas/liquid mixtures, for optimization of flow rates for liquid and gas phases to transport the cuttings in the horizontal sections in an effective way with a reasonably low frictional pressure loss.
In spite of many recent technology improvements in drilling, hole cleaning remains a significant challenge, especially in deviated and horizontal wells. Inadequate hole cleaning can lead to a series of problems such as stuck pipe, fractured formation, high drag and torque, premature bit wear, decreased ROP, logging, and casing and cementing difficulties. Hole cleaning is a complex issue that is affected by many drilling parameters. The major approaches of hole cleaning evaluations include experimental correlations and mechanistic models. But these techniques frequently require complex computations or numerical iterations to get the solutions. In this paper, a set of charts is developed to allow the drilling engineer to quickly estimate the cuttings volumetric concentration in the wellbore. From previous cuttings transport studies, we know that well inclination angle, drilling fluid velocity, fluid rheology, fluid density and drill pipe rotation have the most important effects on hole cleaning. These parameters are divided into several ranges based on their sensitivity to cuttings concentration in the wellbore. The charts are obtained by running a hole cleaning simulator, which is based on a large number of experimental and modeling studies of cuttings transport. The results of the charts are verified by experimental data. The difference between the charts' prediction results and experimental data are within 20%. Instead of solving complex equations in cuttings transport models, drilling engineers are able to quickly estimate the cuttings volumetric concentration by looking up the charts and conducting very simple calculations. This paper is also helpful in guiding the driller to quickly choose proper operational parameters during drilling operations.
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