Sweeps-so low that the flow regime in the annulus changes from laminar to turbulent-can be effective. Indeed, above 200 ft/min, little improvement in hole cleaning is usually observed, and the primary effect of increasing AV above this level is to increase ECD. On the other hand, little is gained from very high AVs. The achievable flow rate is restricted by surface pressure constraints, nozzle selection, use of MWD (measurement while drilling) tools, and allowable ECD. In practice, the optimum theoretical flow rate may vary from the achievable flow rate. However, in extended-reach, high-angle wells (Zone III), AV places third in critical importance, though there is a critical velocity below which a cuttings bed will not form. Generally, in near vertical and moderately inclined hole intervals, annular velocity (AV) has the largest impact upon whether a hole can be cleaned of cuttings. Cutting size, shape, density, and integrityįor a given drilling-fluid density, which is generally determined by well bore stability requirements, the hole may be classified into three hole cleaning “zones”according to hole angle: Drilling Elements That Affect Hole CleaningĬritical elements that can affect hole cleaning include the following: Increased ECD when initiating drill string rotationĢ. With the advent of PWD (pressure while drilling) tools and accurate flow modeling, a number of other indicators have come to light that foreshadow poor hole cleaning and its attendant consequences. The nature of those cuttings, on the other hand, provides good clues: Good cuttings transport is indicated by sharp edges on the cuttings, whereas smooth and/or small cuttings can indicate poor hole cleaning and/or poor inhibition. When some cuttings are observed, however, the quantity of cuttings itself does not adequately reflect hole-cleaning efficiency. Historically, the combination of the necessity to pump or backream out of the hole and a notable absence of cuttings coming over the shale shaker prior to pulling out of the hole has been a reliable indicator of poor hole cleaning. Figure 2.9 shows a decision tree that can be useful in identifying and solving hole-cleaning problems. Thus, to minimize mud losses, drilled solids must be removed as early as possible. The particles become so small that they must be removed via the centrifuge overflow (which discards mud, too) and/or a combination of dilution and chemical treatment. In aqueous-based fluids, when drilled solids become too small to be removed by the solids-control equipment, they are recirculated downhole and dispersed further by a combination of high-pressure shear from the mud pumps, passing through the bit, and the additional exposure to the drilling fluid. To achieve this end, quick and efficient removal of cuttings is essential. One of the primary functions of the drilling fluid is to bring drilled cuttings to the surface in a state that enables the drilling-fluid processing equipment to remove them with ease. Good solids control begins with good hole cleaning.
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