ABSTRACT
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
purpose of this study is to develop practical guidelines that are
general and not biased towards a particular service company product and
which will also serve as a quick reference guide for lost circulation
prevention and control at the well-site for drilling personnel.
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.
1.2 LITERATURE REVIEW
Lost circulation is a broad subject and several studies and measures
have been introduced in the industry to combat it. For example, Moore,
(1986) noted that in shallow, unconsolidated formations where the
drilling fluid may flow easily into the formation, the most common
method used to combat lost circulation is to thicken the mud. This may
be done in fresh water muds by adding flocculating agents such as lime
or cement. He also stated that in areas such as below surface casing in
normal-pressure formations where natural fractures are common, the most
common method used to combat lost circulation is to drill without fluid
returns to the surface. The purpose is to remove the generated cuttings
from the hole and deposit them at the lost circulation zone. However,
this practice requires large volumes of water and close supervision as
there is the possibility of encountering high drill-string torque and
drag.
Current research on lost circulation has been focused on the use of
Lost Circulation Materials (LCMs), especially chemical formulations
which have been proven to be more effective. Hamburger et al. (1983) of
Exxon Production Research Company developed a Shear-thickening Fluid
(STF) which was tested successfully in 10 different wells that
experienced severe lost circulation. A STF is a multi-component system
composed of water-swellable material (usually clay) dispersed in an
oil-external emulsion. The emulsion consists of liquid oil, an
oil-soluble surfactant, and aqueous-phase droplets containing dissolved
polymer. At the low shear rates encountered while it is being pumped
down the drill pipe, the fluid is a low-viscosity, pumpable liquid. Yet
as it passes through the drill-bit nozzles, the resulting high shear
rates cause the fluid to thicken irreversibly into a high strength
viscous paste.