TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractField experience has shown that inefficient transport of small cuttings is a main factor for excessive drag and torque during extended reach drilling; however, very little is known about the transport behavior of small cuttings. In this study, extensive experiments with three sizes of cuttings (0.45 mm-3.3 mm) were conducted in a field-scale flow loop (8 in.×4.5 in., 100-ft long) to identify the main factors affecting small cuttings transport. The effects of cuttings size, drill pipe rotation, fluid rheology, flow rate and hole inclination were investigated.The results show significant differences in cuttings transport based on cuttings size. Smaller cuttings result in a higher cuttings concentration than larger cuttings in a horizontal annulus when tested with water. However, a lower concentration was achieved for smaller cuttings when 0.25 ppb Polyanionic Cellulose (PAC) solutions were used. Unlike the transport of large cuttings, which is mainly dominated by fluid flow rate, the key factors controlling small cuttings transport were found to be pipe rotation and fluid rheology. Improvement by pipe rotation in the transport efficiency of small cuttings is up to twice as large as the improvement in large cuttings transport. Compared with water, PAC solutions significantly improve smaller cuttings transport, while the transport of larger cuttings is only slightly enhanced.Mathematical modeling was performed to develop correlations for cuttings concentration and bed height in an annulus for field applications. Predictions from a three-layer model previously developed for larger cuttings were also compared with experimental results. Differences (up to 80%) indicate the need for improving the frequently used three-layer model by including correlations specifically developed for small cuttings to get a better design of extended reach drilling. This study is also useful for horizontal or high-angle well drilling and completion through sand reservoirs.
Although rotary drilling is being used in most wells, the impact of drillpipe rotation on hole cleaning during foam drilling has not been investigated. There has been no single predictive tool developed to address pipe rotation effects on foam drilling. This paper presents the first study of cuttings transport using foam with pipe rotation under simulated downhole conditions. A field-scale, high pressure high temperature wellbore simulator with a 73-ft long, 5.76" by 3.5" eccentric annular test section was used to investigate the effects of pipe rotation, foam quality and velocity, downhole pressure and temperature on cuttings transport and pressure losses in a horizontal wellbore. Experiments were conducted with backpressures from 100 to 400 psi and temperatures from 80 to 160 degrees F. Pipe rotary speeds were varied from 0 to 120 RPM with foam qualities ranging from 60% to 90% and foam velocities from 2 to 5 ft/s. It was found that pipe rotation not only significantly decreases cuttings concentration in a horizontal annulus but also results in a considerable reduction in frictional pressure loss. The reduction in cuttings concentration is up to 40% at a medium foam velocity (3 ft/s) when pipe is rotated up to 120 RPM. The decrease in frictional pressure loss is up to 50% at a medium foam velocity and is more than 60% at a low velocity. Using a higher foam velocity or quality also improves hole cleaning, however, pressure losses are significantly increased. A mechanistic model and an associated computer simulator were developed for practical design and field applications. It can be used to predict cuttings concentration, bed height and pressure drop during horizontal foam drilling with various pipe rotary speeds, eccentricities, foam qualities and velocities under different pressure and temperature conditions. Comparisons between model predictions and experimental results show that the difference is less than 15% in most of the cases. Introduction The study of cuttings transport using foam is important not only because foam has a high cuttings carrying capacity compared to many conventional fluids, but also foam has many applications in drilling that cannot be replaced by conventional fluids. The typical applications, in under-balanced or near-balanced drilling, have enabled the successful exploitation of low pressure, low permeability or naturally fractured reservoirs. An ever-increasing concern in today's drilling operations is a narrower operating window between the continuously changing pore pressure and facture pressure gradients. Previous studies (Chen et al. 2007b; Duan et al. 2008) show that it is possible to use foam to create a curved pressure profile within the narrow window, which is not possible with conventional fluids. After more than 20 years of practice in slide drilling with downhole motors for the purpose of well trajectory control, the industry realized the limitations of slide drilling and the irreplaceable advantages of rotary drilling in hole cleaning. It is believed that pipe rotation is a major factor that contributes to good hole cleaning in highly inclined wells. It is also a major factor that promotes the ever-increasing applications of the Rotary Steerable System. Despite the many applications of foam and the importance of pipe rotation, there has been no study on cuttings transport using foam with pipe rotation. No single model has been developed to predict cuttings concentration or pressure losses during foam drilling with pipe rotation, primarily because the complexity of flow caused by pipe rotation has been a bottleneck in the development of predictive tools for wellbore hydraulics and cuttings transport. Pipe rotation effects in foam drilling are more complex because of foam compressibility and the non-Newtonian behavior of foam.
Summary Field experience has shown that inefficient transport of small cuttings is a main factor for excessive drag and torque during extended-reach drilling; however, very little is known about the transport behavior of small cuttings. In this study, extensive experiments with three sizes of cuttings (0.45 to 3.3 mm) were conducted in a field-scale flow loop (8 in.×4.5 in., 100-ft long) to identify the main factors affecting small cuttings transport. The effects of cuttings size, drillpipe rotation, fluid rheology, flow rate, and hole inclination were investigated. The results show significant differences in cuttings transport based on cuttings size. Smaller cuttings result in a higher cuttings concentration than larger cuttings in a horizontal annulus when tested with water. However, a lower concentration was achieved for smaller cuttings when 0.25-ppb polyanionic cellulose (PAC) solutions were used. Unlike the transport of large cuttings, which is mainly dominated by fluid flow rate, the key factors controlling small cuttings transport were found to be pipe rotation and fluid rheology. Improvement by pipe rotation in the transport efficiency of small cuttings is up to twice as large as the improvement in large cuttings transport. Compared with water, PAC solutions significantly improve smaller cuttings transport, while the transport of larger cuttings is only slightly enhanced. Mathematical modeling was performed to develop correlations for cuttings concentration and bed height in an annulus for field applications. Predictions from a three-layer model previously developed for larger cuttings were also compared with experimental results. Differences (up to 80%) indicate the need for improving the frequently used three-layer model by including correlations specifically developed for small cuttings to get a better design of extended-reach drilling. This study is also useful for horizontal or high-angle well drilling and completion through sand reservoirs. Introduction Efficient cuttings transport is a major challenge when a long extended-reach well with a horizontal and highly inclined section of more than 20 thousand feet is drilled (Guild et al. 1995; Gao and Young 1995; Schamp et al. 2006). Cuttings can be ground to finer sand while being transported out of the hole, especially when rotary drilling is used. Drilling may not be able to proceed if cuttings transport remains a problem in such a hole. Because of excessive drag and torque caused by small cuttings settled at the lower side of the horizontal or inclined section, it may not be possible to run casing in place even if drilling to the target depth can be achieved. Similar problems exist in horizontal and highly inclined wells drilled through unconsolidated sand reservoirs. Field practice and experimental observations (Parker 1987; Larsen 1990; Gavignet and Sobey 1989; Ahmed 2001; Bassal 1995) show that smaller cuttings are more difficult to transport under certain conditions. Moreover, smaller particles tend to more easily stick a drillpipe because of their cohesive effects (Ahmed 2001; Gailani et al. 2001). It is even more difficult to release the pipe once it is stuck by small, sand-sized cuttings. An investigation into previous studies in the area of hole cleaning and sand transport shows that very limited information is available for small cuttings transport under drilling conditions. Though different cuttings have been tested by several investigators (Parker 1987; Larsen 1990; Ahmed 2001; Bassal 1995; Ford et al. 1990; Peden et al. 1990; Martins et al. 1996), no study has been conducted on small cuttings transport in horizontal or high-angle annuli involving drillpipe rotation. Previous conclusions about the cuttings size effects on cuttings transport are quite diverse, and even contradictory in some cases (Parker 1987; Larsen 1990; Gavignet and Sobey 1989; Ahmed 2001; Ford et al. 1990; Peden et al. 1990; Martins et al. 1996). Their experiments, upon which these conclusions are based, were conducted under incomparable conditions. It may not be correct, or at least not safe, to state explicitly that smaller cuttings are harder or easier to transport. The result may depend on various combinations of drilling parameters. This study was undertaken to understand why and under what conditions they are harder or easier to transport.
fax 01-972-952-9435. AbstractEffective removal of small sand-sized solids is critical for successful drilling and completion operations in sand reservoirs. Recent experience in extended reach drilling also indicates that inefficient transport of smaller cuttings is a main factor for excessive drag and torque. This study was undertaken to determine two critical conditions for efficient transport of small solids. The two conditions are represented by the Critical Re-Suspension Velocity (CRV), the minimum fluid velocity necessary to initiate solids bed erosion, and the Critical Deposition Velocity (CDV), the minimum fluid velocity required to prevent bed formation.Experiments were conducted in a field-scale flow loop (8×4.5 in., 100-ft long) to determine CRV and CDV, for 0.45 mm and 1.4 mm sands in different fluids over a range of bed heights and hole inclinations. The results show that, depending on sand size and fluid properties, CDV is approximately two to three times larger than CRV. Water is more effective than low concentration polymer solutions for bed erosion. However, polymer solutions are more helpful than water in preventing bed formation. This indicates the need for different drilling fluids for cleanout and drilling operations.A mechanistic model was developed to predict CRV for a solids bed. Both experimental and theoretical results indicate the importance of inter-particle forces that are incorporated into the model. The model accounts for drill pipe eccentricity in any direction in an annulus, which is consistent with experimental observations. The model predictions are in good agreement with experimental results. Existing CDV correlations developed for larger cuttings were verified by experimental data for sands. The differences are around 25%. Results in this study will be useful not only in drilling and completion through sand reservoirs, but also in extended reach drilling and sand control.
Summary Effective removal of small, sand-sized solids is critical for successful drilling and completion operations in sand reservoirs. Recent experience in extended-reach drilling also indicates that inefficient transport of smaller cuttings is a main factor for excessive drag and torque. This study was undertaken to determine two critical conditions for efficient transport of small solids. The two conditions are represented by the critical resuspension velocity (CRV), the minimum fluid velocity necessary to initiate solids-bed erosion, and the critical deposition velocity (CDV), the minimum fluid velocity required to prevent bed formation. Experiments were conducted in a field-scale flow loop (8 × 4.5 in., 100 ft long) to determine CRV and CDV for 0.45-mm and 1.4-mm sands in different fluids over a range of bed heights and hole inclinations. The results show that, depending on sand size and fluid properties, CDV is approximately two to three times larger than CRV. Water is more effective than low-concentration polymer solutions for bed erosion. However, polymer solutions are more helpful than water in preventing bed formation. This indicates the need for different drilling fluids for cleanout and drilling operations. A mechanistic model was developed to predict CRV for a solids bed. Both experimental and theoretical results indicate the importance of interparticle forces that are incorporated into the model. The model accounts for drillpipe eccentricity in any direction in an annulus, which is consistent with experimental observations. The model predictions are in good agreement with experimental results. Existing CDV correlations developed for larger cuttings were verified by experimental data for sands. The differences are approximately 25%. Results in this study will be useful not only in drilling and completion through sand reservoirs, but also in extended-reach drilling and sand control.
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