Experimental Determination of Solids Fraction and Minimum Volumetric Requirements in Air and Gas Drilling

Abstract

Abstract There are so many variables involved in air or gas drilling that one general correlation has not been developed to predict appropriate volumetric requirements in the field. The results of previous attempts to calculate minimum volumetric requirements are valid over limited ranges of drilling conditions. This research has developed new empirical correlations for solids friction factor for sandstone, limestone, and shale. A new and more realistic technique for predicting minimum volumetric requirements is presented. This method includes a complete analysis of pressures throughout the system. It also offers flexibility in the selection of standpipe pressure, air flow rate, and, thus, compressor specification. The method is primarily graphical, so it can be used at the well site without access to a computer. Our results confirm that existing methods may predict a volumetric flow rate too low or too high. Introduction In the late 1940's, air or gas drilling began, which can be defined as a technique that utilizes air or gas as the circulating medium. However, the injection of compressed air as a circulating medium has been in common use since the late 1800's. Air (gas) is the ultimate low-density drilling fluid. Optimal results and greatest economy from air or gas drilling depend on several factors. Hard formations that are dry or produce relatively few formation liquids provide the best results. In air and gas drilling, the particles transported in the annular flow stream are of various sizes, shapes, and densities; therefore, each particle has a different terminal velocity in a determined medium. The particles are disintegrated during removal by the turbulent flow in the annulus and the grinding action of the drill stream; thus, the distribution of particle sizes varies at different levels in the annulus. Equations governing the settling of rock particles and volume requirements for air drilling have been given by many researchers. The important relationship between the control of fluid properties and adequate hole cleaning was addressed first by Piggot in 1941. He applied modified forms of Stokes' law for viscous flow and Rittinger's formula for turbulent flow to predict terminal settling velocity. JPT P. 2645^