Chapter 7 - Fluid Flow in Pipes and Boreholes

Abstract

The third and final phase of fluid flow from coal seams is the flow in pipes and cased boreholes. Unlike the diffusional flow from coal particles and darcy flow in coal matrices, the size of the conduits can be designed so that they do not restrict the flow. The four most pertinent cases of fluid flow are discussed: (1) flow of water (or other liquids) in pipes, (2) flow of gases in pipes, (3) flow of sand/coal slurry in pipes, and (4) flow of gases in vertical wells. In each case, pressure losses are calculated to determine the horsepower needed for the job or alternatively to design the size of the conduit to minimize the horsepower required. Dimensional analysis of multiple variables is done to express friction as a function of the Reynolds number and the roughness of the pipe for laminar flow. For turbulent flow only the roughness of the pipe is significant. Various equations are provided to calculate the friction factors. The minimum velocity to transport sand or coal cuttings in pipes or an annulus is calculated for various pipe sizes. The friction coefficient for slurries can be derived if the velocity, volumetric concentration, and drag coefficients of particles are known. Horsepower to drive a water pump that can enable drilling 3000ft long boreholes is calculated. Finally, compressor horsepower for surface transport of gases is calculated. Effective diameters of noncircular pipes are also defined.