workover or completion fluids are fluids
placed across the producing zones before or
immediately after perforating, or any fluid placed
across the formation during reworking, undreaming,
drill-in, or gravel pack operations. These fluids help
ensure that production is steady with the expected potential of the well. The
primary functions of completion/workover fluids are to: provide pressure
control by preventing formation fluids from entering the borehole maintain
borehole stability, minimize formation damage and control fluid loss with
minimal solids invasion. These fluids also keep the borehole “clean” of
perforation debris, solids such as drill cuttings, sand, etc., or any other
providing a transportation medium that for circulating
loose material to the surface.
A completion or workover fluid have the
• enough density to control
• allow capable solids removal during
circulation through filtration systems
• stable, nontoxic, low corrosively,
and bacterial growth retardant
• non-reactive to other soluble
salts, minerals, cement, etc.
is any fluid that is used in a drilling
operation in which that fluid is circulated or pumped from the surface, down
the drill string, through the bit, and back to the surface via the annulus. There are several
different types of drilling fluids, based on both their composition and use. The
three key factors that drive decisions about the type of drilling fluid
selected for a specific well are:
of drilling fluid
are two primary types of drilling fluids: Water Based Fluids (WBFs) and
Non-Aqueous Drilling Fluids (NADFs) or Non-Aqueous
Water Based Fluids (W BFs): WBFs
consist of water mixed with bentonite, clay and barium sulphate (barite) to
control mud density and thus, hydrostatic head. Other substances are added to
gain the desired drilling properties. These additives include thinners (e.g.
lignosulphonate, or anionic polymers), filtration control agents (polymers such
as carboxymethyl cellulose or starch) and lubrication agents (e.g.,
polyglycols) and numerous other compounds for specific functions.
Non-Aqueous Drilling Fluids
(NADFs): NADFs are emulsions where the continuous phase is the
Non-Aqueous Base Fluid (NABF) with water and chemicals as the internal phase.
The NADFs comprise all non-water and non-water dispensable base fluids. Similar
additives are used to
control the properties of NADFs. Emulsifiers are used in NADFs to stabilize the
water-in oil emulsions. Non-aqueous drilling fluid can further be group into 3
Group I: high-aromatic content
fluids. This category includes crude oil, diesel and conventional mineral oils.
These fluids are refined fromcrude oil and contain levels of total aromatics
between 5 and 35%.
Group II: medium-aromatic
content fluids. This
category contains products produced from crude oil with levels of total
between 0.5 and 5%
and is often known as ‘low toxicity mineral oil’.
Group III: low/negligible-aromatic
content fluids. This
group includes fluids produced by chemical reactions and highly refined mineral
oil which contain levels of total aromatics below 0.5% and polycyclic aromatic
hydrocarbon(PAH) levels below 0.001 %, according to the OGP definition.
A solids-free liquid used to
“complete” an oil or gas well. This fluid is placed in the well to facilitate final
operations prior to initiation of production, such as setting screens production liners, packers,
downhole valves or shooting perforations into the producing zone. The fluid is
meant to control a well should downhole hardware fail, without damaging the producing formation or completion components. Completion fluids are typically brines
(chlorides, bromides and formats), but in theory could be any fluid of proper
density and flow characteristics. The fluid should be chemically compatible
with the reservoir formation and fluids, and is typically filtered to
a high degree to avoid introducing solids to the near-wellbore area. Seldom is
a regular drilling fluid suitable for completion operations due
to its solids content, pH and
ionic composition. Drill-in fluids can, in some cases, be suitable for both