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Practical Approaches For Solving Lost Circulation Problems While Drilling
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CHAPTER 1
FORMULATION OF PROBLEM
1.1 INTRODUCTION
Lost
circulation is a common drilling problem especially in highly permeable
formations, depleted reservoirs, and fractured or cavernous formations.
The range of lost circulation problems begin in the shallow,
unconsolidated formations and extend into the well-consolidated
formations that are fractured by the hydrostatic head imposed by the
drilling mud (Moore, 1986). It can then be defined as the reduced or
total absence of fluid flow up the formation-casing or casing-tubing
annulus when fluid is pumped down the drill pipe or casing. The industry
spends millions of dollars every year to combat lost circulation and
its associated detrimental effects such as loss of rig time, stuck pipe,
blow-outs, and less frequently, the abandonment of expensive wells. Two
conditions are both necessary for lost circulation to occur down hole:
1) the pressure in the well bore must exceed the pore pressure and 2)
there must be a flow pathway for the losses to occur (Osisanya, 2011).
Sub-surface pathways that cause, or lead to, lost circulation can be
broadly classified as follows:
Induced or created fractures (fast tripping or underground blow-outs) Cavernous formations (crevices and channels)
Unconsolidated or highly permeable formations
Natural fractures present in the rock formations (including non-sealing faults)
The
rate of losses is indicative of the lost pathways and can also give the
treatment method to be used to combat the losses. The severity of lost
circulation can be grouped into the following categories (Abbas et al.
2004):
Seepage losses: up to 10 bbl/hr lost while circulating
Partial losses: 10 – 500 bbl/hr lost while circulating
Severe losses: more than 500 bbl/hr lost while circulating
Total losses: no fluid comes out of the annulus
Circulation
may be lost even when fluid densities are within the customary
safety-margin; less dense than the fracture density of the formation.
Stopping circulation losses before they get out of control is crucial
for safe and economically rewarding operations (Abbas et al. 2004).
According to Ivan and Bruton (2003), “Deepwater drilling has brought
loss circulation control to a more critical level as it involves narrow
pore-pressure/fracture-gradient windows, cold drilling fluid
temperatures, high equivalent circulating densities (ECDs), high
cost-per-barrel of synthetic-based fluids (SBM) and a high cost for rig
time/non-productive time (NPT).†The reduction of the fracture pressure
gradient in the deeper water is mainly due to the low stress regime as a
result of the reduction in the overburden pressure gradient. Also,
drilling through sub-salt zones poses a challenge to the operator
because of the problem of lost circulation encountered in these zones.
These wells have shear zones above and below the salt formations and
also narrow margins between the pore and fracture pressure and hence
these wells tend to register severe losses in circulation.
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ABSRACT - [ Total Page(s): 1 ]As the demand for petroleum resources increases, drilling of oil and gas wells are often carried out in challenging and hostile environments. Among the top ten drilling challenges facing the oil and gas industry today is the problem of lost circulation. Major progress has been made to understand this problem and how to combat it. However, most of the products and guidelines available for combating lost circulation are often biased towards advertisement for a particular service company. The purpo ... Continue reading---
