BAKER HUGHES - Drilling Fluids Reference Manual

BOREHOLE PROBLEMS
Preventive Measures
Prevention of lost circulation is very important due to the difficulties in curing the problem once
it occurs. Measures to combat loss of circulation should include the following:
• Use minimum drilling fluid densities consistent with formation pressures and borehole stability
requirements.
• Evaluate area well data and drilling parameters to aid in proper placement of casing strings. In
many areas, it is possible to estimate formation pressures with shale density, drilling rates (“d”
exponent), and log data. If pore pressure is known, it is generally possible to predict the
fracture gradient accurately.
• Loss of circulation problems to known porous or finely fractured zones may be pre-treated
with a fine bridging material such as MIL-PLUG ® , MILMICA ® , LC-LUBE, MIL-
CARB, and/or CHEK-LOSS ® . However, fine shaker screens will remove a large
percentage of these additives from the fluid system. Occasionally, shaker screens are bypassed
in order to keep these products in the system without having to continuously replace what the
shaker screens are discarding. This practice is detrimental to solids control but, at times, can be
justifiable. To prevent the build-up of solids in the fluid system, lost circulation pills are
spotted in the area of loss in an attempt to cure the problem. When circulated out of the hole,
the lost circulation material is removed from the system at the shale shakers. It should be
recognized that the addition of these and other LCM may contribute to increased
drilling fluid rheological properties, thereby increasing pressure loss and the aggravation
of lost circulation. The application of LCM under these conditions should be carefully
considered.
• Surge pressures, circulating pressure losses, and pressures occurring while breaking circulation
after trips could contribute to loss of circulation. The surge pressures should be calculated
using ADVANTAGE SM engineering software on critical operations to determine safe piperunning
speeds and to predict effects of fluid rheological properties. Annular friction losses
while circulating may require alteration of flow properties and circulating rate to prevent
losses.
On deep, hot holes, gelation of drilling fluid may cause loss of returns during attempts to break
circulation. Tests with the Fann Consistometer, Fann Model 70/75 Viscometer, high temperature
high pressure aging cells and shearometers can be utilized to predict the degree of gelation and
assist in determining proper corrective treatment. When breaking circulation, upward movement
and rotation of drillstring will reduce pressures. Standpipe chokes (hand adjustable choke
installed on standpipe with bypass line discharging into mud pit) if installed, would be useful to
aid in breaking circulation with a slow, gradual increase in standpipe pressure. Procedures for
predicting surge/swab pressures are outlined in the section dealing with basic hydraulics
equations.
Annular restrictions due to balled bits, collars, and stabilizers are not uncommon while drilling
gumbo shales. Reduction in annular size, regardless of the cause, can impose additional pressure
against the formation which may result in loss of circulation. Preventive measures to be
considered are;
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 7-22