Drilling problems Lost Circulation

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Lost circulation is the loss of mud or cement to the formation during drilling operations. Lost
:circulation causes
. increased well costs, due to lost rig time and loss of expensive drilling fluid
. loss of accurate hole monitoring - 
. well control problems - 

Mud losses can be experienced as a result of either natural losses, induced fractures during
.drilling operations or due to excessive overbalance

NATURAL LOSSES

Natural losses occur in rocks containing porosity and permeability or with natural fractures
Three types of formations can be recognised
 Coarse Sands and Gravel Beds : Usually occur near the surface where the formation is both
porous and highly permeable: permeability in excess of 10 to 25 Darcy

 Natural Fissures or Fractures
Natural fissures and fractures usually occur in limestones and chalks which have been
subjected to tectonic activities or to leaching by acids. Losses in the formations is usually
.severe

 Cavernous Formations
Caverns develop in limestone and dolomite formations ranging in size from fraction of an
inch to large tunnels. They form as a result of ground water percolating through the formation
and subsequent dissolving of the calcium. Total losses are usually experienced when drilling
cavernous formations, resulting in the use of a special drilling technique called blind drilling
In blind drilling, drilling is carried out without returns to surface, usually using sea water

INDUCED FRACTURES

In formations where the difference between pore pressure and formation fracture pressure is
low, fractures may be induced by either the drilling ECD or surge pressures, Figure below
Mud losses will occur through the induced fractures. The increased volume of cuttings in the
annulus can increase the ECD to beyond the formation fracture pressure. This is especially
true in surface holes

CLASSES OF LOST CIRCULATION 

Lost circulation can be grouped into four classes
Seepage losses: From 1-10 bbl/hr and lost while circulating at t he normal drilling circulate -
.ng rate
 Partial losses: From 10-50 bbl/hr and lost while circulating at the normal drilling circulating
rate
 Severe losses: Greater than 50 bbl/hr and lost while circulating at the normal drilling circul -
ating rate. In some cases, no losses may be seen if pumping stops indicating that the ECD is
the cause of lost circulation

 Total losses: When the mud level in the annulus can not be seen or the hole can not be filled
through the annulus. Total losses usually occur in cavernous formations

with my best wishes


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References


Petroleum_Engineering_Drilling_And_Well Completions By -
Carl Gatlin
 Drilling Data Handbook, API2b -
Amco Drilling Manual, AMOCO Company -
 Amoco  Drilling Fluid Manual -
 Applied Drilling Engineering, Adam T. Bourgoyne Jr. , Kieth -
K. Mellhiem, Martin E. Chenevert,  F. S. Young Jr -


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