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The effects
of clays may be subtle, causing loss in injectivity over time,
or extreme, causing the formation to “lock up” soon
after initiating water injection.
Many low permeability sandstone reservoirs contain clays that
swell and migrate upon flooding with fresh water. Montmorillonites
swell upon contact with fresh water, plugging pore throats and
reducing permeability to water injection. Migrating clays, including
kaolinites and illites, reduce permeability as they become trapped
in pore throat openings. When this happens, damage to the near
well-bore area occurs and the injection of the drive fluid is
restricted. Thus, waterflood efficiency is greatly reduced and
present worth value of the reservoir decreases because oil production
rate is proportional to water injection rate.
Most available technologies to stabilize clays are temporary.
The TIOR-KOH® process reacts concentrated potassium hydroxide
with the near well-bore rock of the injection well to permanently
stabilize the clays, rendering them immobile and non-swelling.
In effect, the treatment increases the injection well radius,
allowing higher injectivity to be maintained for a longer time.
The KOH-clay chemical reaction permanently alters the clay chemistry
so that the clay minerals are unaffected by changes in water composition.
KOH reacts chemically with clays, rendering them invulnerable
to destabilization through swelling or migration. While there
are many technologies available for stabilizing clays, KOH remains
the only means of stabilizing clays permanently a significant
distance into the formation.
Additionally, imbibition chemicals added
continuously to the injection water work synergistically to enhance
water penetration into the tightest oil bearing rock. TIORCO®
535 reduces the contact angle of injection water to the reservoir
rock, thus permitting water to enter pore throats that are normally
restricted to water intrusion. Thus, even more oil bearing rock
is swept for measurable recovery of incremental oil.
Computer modeling suggests an increase in injectivity due to a
clay stabilization treatment of 60% can result in an increase
in 50% of cumulative oil recovery over a five-year period in a
heterogeneous rock with a Dykstra Parsons permeability variation
of 0.7. A 60% increase in injectivity can increase present worth
value at a 10% discount rate by 25% or $300,000 for the modeled
injection well pattern of 1,200 acre-feet.
Case history data from treated and un-treated injection wells
in the same field attest to the long term effect of increased
water injection rates produced by the TIOR-KOH® process -
faster reservoir fill-up, re-establishment of reservoir pressure,
and increased fluid production.
Selecting
Candidates for Clay Stabilization
Candidates
for clay stabilization typically reveal themselves based on laboratory
data or past field case histories. Laboratory data can be helpful
not only in detecting a clay problem, but also in evaluating whether
the KOH process is a potential solution. Testing may involve (1)
Water Sensitivity Tests, where water movement through a core causes
a decrease in injectivity, (2) Relative Perm Curves, where relative
permeability to water may be extremely low relative to oil or
air permeability, and (3) Petrographic Analysis (SEM), which quantifies
and defines clay types. When testing cores from formations that
contain clay, special care must be taken to avoid damaging the
core prior to or early in the testing process.
One technique for evaluating injectivity of analogue wells in
field case histories is through the use of a Hall plot. Cumulative
pressure, pressure multiplied times days, is plotted against cumulative
injection volume. Once the formation is filled around the injection
well, the Hall plot should remain fairly straight. A concave upward
shape suggests a loss of injectivity over time, while a breaking
over the curve or sudden decrease in slope suggests formation
parting.
The performance of KOH treated wells can be evaluated with normalized
plots of injectivity versus time that account for differences
in rock properties. Plots for groups of wells can be averaged
to allow general numerical comparisons and performance projections.
Treatment
Application Variables
Once a clay
problem has been identified based on field performance and laboratory
data, additional considerations on a reservoir scale are important
in determining whether the KOH process is viable. One consideration
is injection water availability. Well cost is another.
Field applications of the KOH process consist of three basic steps
– pre-flush, KOH and post-flush. The purpose of the pre-flush
is to displace divalent ions from the near wellbore region prior
to KOH injection. Following the KOH, the post-flush must be applied
immediately as fluids must be kept moving away from the injection
well so the KOH is spent out in the formation and not near the
injection well.
Design issues in applying a KOH treatment are concentration and
sizing of the chemical stages, downhole conditions and field logistics,
including equipment. Proper metallurgy in surface and downhole
equipment, good downhole equipment integrity and clean tubulars
and wellbores are of paramount importance to good results with
KOH.
References
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