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Oilfield Technology
June
2015
Ageing, abandoned or deteriorating wells can be repaired using
remedial cementing techniques. Remedial cementing is a process
used to repair or properly abandon a wellbore using regular or
specially formulated cement slurry. The need for remedial cementing
can stem from a variety of issues such as:
Ì
Abandoned non‑productive or depleted zones.
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Casing leaks.
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Gas migrations or surface casing vent flows (SCVF).
There are other operations that fall under the remedial cementing
umbrella but the highest frequency of remedial cementing jobs is
referred to as a ‘cement squeeze’. As the name implies, a cement
squeeze typically forces or squeezes the cement slurry into
perforations and seals the wellbore to ensure well integrity.
In this low price oil environment it is imperative to evaluate
both producing and suspended wells. During peak activity, remedial
services are often prioritised towards well construction, but the
current market has created the opportunity to procure remedial
related services at a lower cost.
In Western Canada, SCVF resulting from shallow gas production
contribute to a large percentage of the total number of wells in
need of remediation. SCVF detection on the backside of the inner
casing is an indication of gas migration or fluid movement within the
annulus. When performing the initial cement job, good mud removal
practices are essential for proper isolation of gas producing zones,
with centralisation, pipe movement, fluid package design, and pump
rate affecting mud displacement efficiency. In the post set phase,
cycling stress fromwellbore completion, workovers, or production
can contribute to cement sheath damage. Additionally, shrinkage of
1 ‑ 3% is commonly observed for cementing systems in the absence of
bulk expansion additives. Shrinking cement pulls away from bonded
surfaces, creating a channel for gas to flow through. In some cases, the
tensile stress created by contraction may even cause cement cracking.
In Alberta and Saskatchewan, the first 400 m from the surface, the
formations frequently consist of gravel beds, silts, and undeveloped
shale with swelling clays, which provide challenging conditions for
establishing and maintaining
isolation. Unconsolidated
formations can make it
challenging to get a good bond
to the formation face. As the
cement develops gel strength
and begins supporting its own
weight, the hydrostatic pressure
of the column drops below the
exerted gas pressure, providing
an opportunity for gas to
migrate through the cement.
Low temperature conditions
typically contribute to long
transition times throughout
static gel strength development,
lengthening the period of
time over which the cement is
susceptible to fluid invasion. An
inventive solution to remediate
SCVF uses a combination of
abrasive jetting coupled with
various well specific squeeze
techniques to ensure the best
chance of remediating wells with
surface casing vent flows and
annular gas migration. Abrasive jetting provides complete radial access
to the zone of interest with minimised local stress on the wellbore and
casing, resulting in less cement sheath cracking and reduced formation
damage. Once zonal communication is achieved, the feed rate is
established so that a tailored solution can be created for the well. In
most applications a microfine cementing system is used to penetrate
formations with low feed rates, or improve penetration of cement
across extended gas producing zones. Using high quality microfine
cement in conjunction with a high pressure, slow rate continuous
squeeze ensures the best chance of isolation. This process has been
applied to hundreds of wells across Western Canada. For this method to
achieve maximum results a five step process has been developed:
Step1: Identifyingthegassourceandcaprock
The wellbore is first assessed using log analysis methods such as
noise logs and density neutron logs to identify the gas sources and the
shallowest cap‑rock formation. The best result is typically obtained
when the gas producing zone is squeezed off at the source; however,
where several gas producing zones are present, the cost of remediation
can often be reduced by cementing the voids between the casing and
formation wall across the cap‑rock. This technique can reduce the
number of treatments for the well, lowering the cost of services.
Step2: Interveningthegassource
Hydraulic abrasive jetting is used to cut slots in the annulus and
establish communication with the zone of interest. Gelled fluid is used
to carry abrasive sand as it is pumped through two opposing jets. The
jets are rotated an angle of 120˚ creating cuts that extend through the
casing, cement, and nearby formation. Two or more overlapping cuts
are performed to establish complete radial coverage, ensuring the
best chance of intercepting annular gas flow.
Step3: Formation injectivitytest
Next an injectivity test with water or compatible fluid must be
performed to assess the formation feed rate. In some cases
communication to surface may be established or formation fracture
Figure 1.
Camille LeRouge of Sanjel Corporationperforms laboratory analysis onaproposed cement squeeze slurry
to determine fluidmigrationpotential as the cement develops static gel strength.
