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.

Summary: The downhole pressure environment is one of the most important factors affecting the rate of penetration (ROP). It is believed that impermeable rocks experience high differential pressures because of shear dilatancy and become stronger and, thus, more difficult to drill. However, recently performed atmospheric and pressurized single-cutter experiments show that contrary to this belief, even at low pressures (100-200 psig) and even with permeable rocks, significant increase in mechanical specific energy (MSE) is observed compared to atmospheric tests. The experiments were carried out in a single-cutter high-pressure testing facility refurbished with high-precision sensors and a data acquisition system. In the experiments, a 13-mm polycrystalline-diamond-compact (PDC) cutter was used to cut Carthage marble and Indiana limestone samples with depths of cut ranging from 0.025 to 0.050 in. More than 70 high-precision tests were performed on these two rock types under confining pressures ranging from 0 to 1000 psig. The confining fluids were either water or mineral oil. Unexpectedly, analysis of the MSE consistently showed that increases in the confining pressure as small as 150 psig can increase the MSE of the cutting process significantly and reduce the cutting efficiency by half. These reductions in the cutting efficiency that were even more dramatic in the permeable and saturated Indiana limestone could not be explained by the strengthening of the rock under confining pressures. Upon analysis of the results of experiments (cutting forces, volume of cuts, and visual inspections of cuttings), a new theory was proposed to explain this unexpected behavior. This new theory, based on the frictional forces and the cutting mechanism under pressure, gives useful insights into the physics of cutter-rock interaction. Such insights are invaluable to the improvements of drilling practices selection [weight on bit (WOB), type of drilling fluid, and its properties] and the ROPs.

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.