Annular Velocity and Hole Cleaning

Annular velocity is flow over annular area, and because that area is a difference of SQUARES it changes fast with hole size: the same pump rate that cleans a small hole around a given pipe is nowhere near enough in a large one. That is why rate has to rise with every larger hole section, and why a washed-out interval is a cleaning problem as well as a cement problem. The number that matters is transport ratio rather than velocity alone -- what counts is how much faster the mud rises than the cuttings fall. Slip velocity depends on cutting size and density and on the mud's rheology, so a thin mud carries poorly at any rate, which is why hole cleaning is fixed with sweeps, rheology and pipe rotation as much as with flow. On a high-angle well none of this is sufficient: cuttings form a bed on the low side of the hole, and mechanical agitation from rotation is what removes them, so a horizontal section that looks clean by this arithmetic can still be packing off. That is the limit of the calculation and it is a real one. Bottoms-up time is the companion number and the one a crew actually uses: how long before what the bit is making reaches the shakers. The rate a target velocity would need is reported too, because in a large hole that rate is often more than the pumps or the motor will take, and that is the moment sweeps and rheology stop being optional. Slip velocity is ENTERED because it depends on the cuttings and the mud. This is a vertical-hole screen with a concentric annulus: it does not compute slip velocity, model cuttings beds or eccentricity on a deviated well, account for pipe rotation, evaluate equivalent circulating density or the pressure the rate costs, or address the effect of hole washout on the real annular area. The drilling program, the mud engineer, and the directional driller govern.

annular velocity ft/min = 24.5 x gpm / (D_hole^2 - D_pipe^2); annular capacity bbl/ft = (D_hole^2 - D_pipe^2) / 1029.4; transport ratio = (annular velocity - slip velocity) / annular velocity; bottoms up = annular volume / pump output; and the flow for a target velocity is the first relation inverted.

The oilfield annular relations as every drilling and well-control manual writes them. Slip velocity is ENTERED because it depends on cutting size and density and on mud rheology. A vertical-hole screen with a concentric annulus: it does not compute slip velocity, model cuttings beds or eccentricity on a deviated well, account for pipe rotation, evaluate equivalent circulating density or the pressure the rate costs, or address hole washout.

A velocity, a capacity, and a ratio.

Estimate. AHJ and licensed professional govern.

Field names used by the API: hole_dia_in, pipe_od_in, flow_gpm, slip_velocity_ft_min, measured_depth_ft, pump_output_bbl_stroke, pump_spm, target_velocity_ft_min, annular_velocity_ft_min, transport_ratio, annular_capacity_bbl_ft, annular_volume_bbl, bottoms_up_strokes, bottoms_up_min

• Slip velocity is entered it depends on cutting size and density and on mud rheologythe mud engineer
• Vertical hole, concentric annulus cuttings beds and eccentricity on a deviated well are not modelledthe directional driller
• No circulating pressure the equivalent circulating density the rate costs is separatethe drilling program