W
ith the current low oil and gas prices, operators are looking
for ways to increase production fromexisting wells without
incurring the expense of large-scale intervention and
stimulation programmes. Many oil companies have wells that are
performing poorly or have stopped producing because of formation
damage or plugged perforations caused by drillingmud, or by scale in
the well near the production zone that impedes flow. Some wells drilled
as injectors have somuch formation damage that water cannot be
injected through them. Inmany cases, the root cause of these problems
is barite weightingmaterial from the drillingmud or barium sulfate,
strontium sulfate, magnesiumand calcium carbonate scales caused by
incompatible injected or connate water.
Barite and scales pose a difficult problembecause they are very hard
to remove. They are insoluble in hydrochloric acid (HCl) or hydrofluoric
acid (HF) treatments, whichmay in fact make the problemeven worse.
Mechanical intervention can scrape away scale and barite that remains
in the well, but cannot reach into perforation tunnels or into the
near-wellborematrix where barite damage seals off the formation and
prevents flow into the well.
Chelationchemistry
Beginning in themid-1990s, operators have found that the right
chelation chemicals, when applied properly, can dissolve barite
deposited by drillingmud and remove acid insolublematerials in well
screens and tubing. High density converters (HDC) have achieved
consistently favourable results in these applications.
Barite (barium sulfate) is highly insoluble in water or acid, but can be
removed through chelation using high density converters. Chelation is a
chemical reaction in which a chelating agent forms multiple bonds with
metal ions to removemetal, in this case barium, from larger molecules.
Barite particles as large as 70microns can be reduced to particles
1 or 2microns in diameter, which stay in suspension and easily flow out
of the well without obstructing pore throats in the formation.
HDC is a blend of salts and acids with a pH higher than 12.5. It is not
corrosive and has similar corrosion behaviour to seawater. It does not
damage the formation, as proven by independent laboratory regained
permeability tests. The chemical is a straight chemical, spotted ‘neat’
without water or additives, and left to soak for several hours to remove
barite or scale. HDC treatments clean up perforated liners, wire wrap
screens in open hole, and expandable screens clogged with barite left by
drillingmud or scale.
The chemical has proven effective in removing field-grade barite,
which is not pure barium sulfate, but can have up to 30%hematite. It
removes drilling fluid solids and LCMweighted with barite, and cleans
near-wellbore scales, whichmay be pure barium sulfate, pure strontium
sulfate or magnesium sulfate.
RESTORING WELL
PRODUCTIVITY
PAUL W. BRADLEY
AND TOM SHERWIN,
WELL FLOW INTERNATIONAL,
BAHRAIN, EXPLAIN HOW
HIGH DENSITY CONVERTERS
REMOVE BARITE AND
SCALE TO RESTORE WELL
PRODUCTIVITY.
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