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 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.
With the increasing number of horizontal and highly inclined wells drilled through unconsolidated or poorly consolidated reservoirs, smaller sand-sized solids transport is becoming a main concern during drilling operations. The solids transport studied in this paper, however, is not limited to drilled cuttings transport. It is also of critical significance for gravel pack displacement operations, propping agent transport in hydraulic fracturing, and sand control during production operations. All the solids involved in the above applications are small sand-sized particles ranging from microns to millimeters. In an extended reach well with a highly inclined section of more than 20,000 feet, larger drilled cuttings can be ground to finer sand while being transported out of the hole, especially when rotary drilling is used. Because of excessive drag and torque caused by small cuttings settled at the lower side of the horizontal or inclined section, casing may not be able to run into place even if we manage to drill to the target depth.
While many studies have been directed towards addressing cuttings transport problem over the past 30 years, very limited information is available for small sand-sized solids transport during drilling applications. Field experience and experimental observations showed that sand or smaller cuttings may be more difficult to clean under certain conditions.1,2,9 To quantitatively describe cuttings transport efficiency, two types of parameters have been used as target variables in the previous studies. The first type indicates the amount of cuttings in a well under a given drilling condition. Cuttings concentration,3 equilibrium bed height4 and annular bed area5 are typical examples. The second type shows the required drilling condition to keep a minimum amount of cuttings in a well, normally known as critical velocity or critical flow rate. There are a number of different names for the second type of parameters, such as Critical Deposition Velocity (CDV),6 Critical Transport Fluid Velocity (CTFV),2 Minimum Transport Velocity (MTV),7 and more recently, Critical Foam Velocity (CFV).8 All of these names indicate the same condition, which is defined as the minimum average fluid velocity required to prevent bed formation. In this study, the term CDV is used to represent this critical condition.