–When the drillstring is picked up to make a connection or trip out of the well, the mud in the annulus must fall to replace the volume of pipe pulled from the well.
–The hydrostatic pressure is momentarily reduced while the mud is falling in the annulus.
•Surging
–When the drillstring or casing is lowered or run into the well, mud is displaced from the well.
–The frictional pressure losses from the flow of mud in the annulus as it is displaced by the pipe causes pressures in excess of the hydrostatic pressure of the column of mud in the wellbore.
•Swab and surge pressures are related to the mud’s rheological properties:
–The mud’s gel strengths
–The speed at which the pipe is pulled from, or run into, the well
–The annular dimensions
–The length of drillstring in the well
•The rheological properties affect swab and surge pressures in the same manner as they affect annular pressure losses.
•Increases in either the plastic viscosity or the yield point will increase the swab and surge pressures.
•Since the maximum (not average) swab and surge pressures must be less than the pressures needed to swab the well in or break the formation down, swab and surge pressures must be calculated for the maximum drill string velocity when tripping.
•This is generally calculated as one-and-one-half times the average drill string velocity.
VMaxDrillstring(ft/min per stand) =(1.5 x stand length (ft) x 60 sec)/(min seconds per stand)
•The annular velocity must be calculated for each annular space.
•These annular velocities should be substituted into the API equations for the annular pressure losses for each interval.
•The swab and surge pressures are then calculated in the same manner as the ECD.
AVSwab-Surge(ft/min) =(VMaxDrillstring(ft/min) x drillstring displacement (bbl/ft))/(annular capacity (bbl/ft))
•The object of calculating swab and surge pressures is to determine safe pulling and running speeds and minimized trip times.
•This is done by changing the maximum or minimum time per stand and recalculating the swab and surge pressures until times per stand are found where the swab and surge pressures plus the hydrostatic pressure is approximately equal to the formation pressure and fracture pressure.
•This time per stand is only relevant for the present length of drillstring in the well.
•As pipe is removed from the hole, the drillstring length decreases and the bottom hole assembly will be pulled into large diameter casing.
•This will make it possible to pull each stand faster without risk of swabbing in the well.
•When tripping in to the well, the length of drillstring will be increasing and the annular spaces will decrease as the BHA is run into smaller diameters.
•This will require that the running time per stand be increased to avoid fracturing the formation.
•The swab and surge pressures should be calculated at either 500-or 1,000-ft intervals.
•Slip Velocity
–Free settling occurs when a single particle falls through a fluid without interference from other particles or container walls
•For Slip velocity we use stokes law
VS=( gC x DS*2(rS-rL))/(46.3μ)
–VS= Slip or settling velocity (ft/sec)
–gC= Gravitational constant (ft/sec2)
–DS= Diameter of the solid (ft)
–rS= Density of solid (lb/ft3)
–rL= Density of liquid (lb/ft3)
–μ= Viscosity of liquid (cP)
•This equation is a mathematical expression of events commonly observed:
–The larger the difference between the density of the cutting and the density of the liquid the faster the solid will settle.
–The larger the particle is the faster it settles
–The lower the liquid’s viscosity (1/μ), the faster the settling rate.