Design Techniques In Air And Gas Drilling: Cleaning Criteria And Minimum Flowing Pressure Gradients
Authors: Johnson P.W. 
Published: March 2010

Cleaning criteria for air and gas drilling are reviewed. The kinetic energy and terminal velocity criteria are shown to be equivalent, and are re-expressed in a succinct form that gives the actual minimum gas velocity (or pressure) as a function of gas flux, cuttings size, and drag coefficient. The homogeneous flow model of cuttings transport is also reviewed and the mixture density error therein is quantitatively examined. This model is shown to be useful in determining the lower bound on volume requirements, if cutting size information is included. Flowing pressure gradient charts are developed and demonstrated to be a highly versatile design tool, and their use is illustrated with examples.

Introduction

All techniques for selecting volume requirements in air and gas drilling require both specification of a cleaning criterion and a method for evaluating whether that criterion is met. There is no universal agreement on either of these elements.

Cleaning criteria fall into three categories: gas energy, terminal velocity, and minimum bottomhole pressure. Angel(1) was first to propose the minimum energy criteria. From limestone quarrying operations it had been observed that air at standard conditions travelling at 15.2 mls (50 ft.ls) would satisfactorily tram port the cuttings. Angel assumed that air or gas with this same specific kinetic energy would be sufficient to transport drilled cuttings regardless of depth, temperature, or pressure. What he didn't account for was cuttings size and shape, which can vary radically with drilling parameters and lithology. By using limestone quarrying as his reference he implicitly chose a size and shape, and consequently his cleaning criteria should actually apply only to limestone cuttings within a limited size range. For smaller cuttings this criteria can result in excess air and for larger cuttings a fluidized bed can begin to accumulate at the top of the collars(2).

Gray(3) investigated the effects of cuttings size and shape and suggested that successful cleaning occurs whenever the air velocity exceeds the cuttings terminal velocity. Following this approach, he developed a semi-empirical equation for the terminal velocity that includes the effects of the diameter and shape of the cuttings. For sandstone he found that the cuttings are subrounded in shape, with an average drag coefficient of 0.81 (8 samples. standard deviation 0.17), and for limestone and shale he found that the cuttings are flake shaped with an average drag coefficient of 1.4 (9 samples, standard deviation 0.24). Gray did not provide a method for determining what air volume was required to meet his cleaning criteria. Furthermore. while this criteria can be applied in order to achieve hole cleaning, it does not result in the minimum bottomhole pressure or the maximum rate of penetration.

The foregoing two criteria are directed toward finding the least amount of air required to clean the hole, but Ikoku et al. (2,4) have pointed out that the minimum amount of air does not correspond to minimum bottomhole pressure. If maximizing the rate of penetration is the aim then minimum bottomhole pressure should be the goal.