Development of a New Cuttings-Transport Model for High-Angle Wellbores Including Horizontal Wells
- T.I. Larsen (Unocal Corp.) | A.A. Pilehvari (Texas A&M U.) | J.J. Azar (U. of Tulsa)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- June 1997
- Document Type
- Journal Paper
- 129 - 136
- 1997. Society of Petroleum Engineers
- 4.2 Pipelines, Flowlines and Risers, 1.6.6 Directional Drilling, 4.1.9 Tanks and storage systems, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 4.3.4 Scale, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 1.6 Drilling Operations, 3 Production and Well Operations, 1.10 Drilling Equipment, 1.11 Drilling Fluids and Materials, 1.7.7 Cuttings Transport
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This paper presents a new design model that will enable the drilling engineer to select the proper hydraulics for problem-free drilling in high-angle holes (from 55 to 90° from vertical). Empirical correlations have been developed after carrying out an extensive experimental study of cuttings transport in a 5-in. full-scale flowloop. The model predicts the required critical transport fluid velocity (CTFV), the average cuttings travel velocity (CTV), and the annular cuttings concentration under most given sets of drilling operating conditions.
The majority of the previous studies on cuttings transport have been qualitative examinations of the effects of different variables.1-6 A few attempts to model the rather complex nature of cuttings transport have been made.6,7 This study focused on combining the experimental results with basic theoretical principles to develop an empirical predictive model for cuttings transport.
First, an extensive experimental test program investigated all variables believed to control annular hole cleaning for angles of inclination from 55 to 90°. The experimental part of this study focused on the annular fluid velocity needed to prevent cuttings from depositing in the wellbore, a concept that has been used in horizontal pipe flow in slurry transport. In addition, lower fluid velocities such that a cuttings bed would form in the annulus were also investigated.
Secondly, an empirical model based on the experimental results was developed to predict the minimum fluid velocity needed to keep all cuttings moving, the average CTV, and the cuttings concentration in the annulus for any fluid velocity lower than the minimum.
Critical Transport Fluid Velocity (CTFV).
The CTFV is defined as the minimum fluid velocity required to maintain a continuously upward movement of the cuttings. In other words, at CTFV and higher no cuttings will accumulate on the low side of the wellbore.
Subcritical Fluid Flow (SCFF).
If the annular fluid velocity is lower than the CTFV, cuttings will start to accumulate in the wellbore. Any flow rate corresponding to an annular velocity below the CTFV is referred to as SCFF.
The experimental testing was conducted in a 5-in. diameter annulus, which was 35 ft long with a 2.375-in. rotating inner (drill) pipe. The drillpipe eccentricity varied from negative (-62%) to positive (+62%). The cuttings were injected into an annulus through an auger system, while the fluid was pumped from a mud tank. After exiting the annulus, all cuttings were weighed on a scale.
Summary of Experimental Work
The experimental results used to develop the model were part of a study of more than 700 tests designed to investigate the CTFV and the SCFF while evaluating the effect of the following variables: flow rate, angle of inclination, mud rheology, mud density, cuttings size, drillpipe eccentricity, and rate of penetration (ROP).
The model presented in this paper was developed based on the data gathered at angles of inclination ranging from 55 to 90° from vertical. Table 1 shows the rheological properties for the three unweighted and two weighted water-based muds that were tested in addition to water.
The three cuttings sizes, by mean weight distribution, were 0.275 in. (large), 0.175 in. (medium), and 0.09 in. (small), and the cuttings-bed porosities (f) were 41, 36, and 39% respectively.
The CTFV and the SCFF were found experimentally using a positive (+62%) drillpipe eccentricity, corresponding to the pipe resting on the tool joints 0.5 in. above the low side of the annulus. This was done to simulate the worst condition in terms of cuttings transport.
Three cuttings injection rates of 10, 20, and 30 lbm/min, which correspond to a ROP of 27, 54, and 81 ft/hr in a 5.0-in. hole, were investigated. The results showed that the effect of rev/min was negligible for the experimental setup used in this study (drillpipe limited to only rotate around it's own axis by centralizers). The pipe was rotated at 50 rev/min throughout the experiments.
The effect of the above variables will be presented when comparing the model predictions and the experimental results in the "Results and Discussion" section of this paper.
Development of Equations To Predict the CTFV
The CTFV is found by adding the average CTV to the equivalent slip velocity (ESV), as shown in Eq. 17. The ESV is defined as the velocity difference between the cuttings and the drilling fluid.
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