30
Geotechnical News • December 2017
THE GROUT LINE
for wetting and drying compacted soil-
cement mixtures”. This test method
calls for consolidated sand to be oven-
dried for 43 hours and then submerged
in water for 5 hours. Measurements
are taken over 12 cycles. Barco silica
sand was consolidated with a 4.5 ratio
sodium silicate diluted 1:1 with water
and then polymerized using citric acid
or triacetin. Table 2 summarizes the
loss of weight after each cycle. The
4.5 ratio set with triacetin showed only
a slightly loss of weight over the 12
cycles, while the sample set with citric
acid had good durability until the 6th
cycle: after that a crack developed in
the consolidated sand. No samples
formulated with conventional sodium
silicate were able to pass a 12 cycle
test. It should be noted that samples
formulated with citric acid passed
12 cycles when a small amount of
hardener (i.e. soluble calcium) was
included in the formulation.
Testing of sample permeability
was restricted to 4.5 ratio sodium
silicate using the same formulations
as durability testing using ASTM
D5084−16a, “Standard Test Meth-
ods for Measurement of Hydraulic
Conductivity of Saturated Porous
Materials Using a Flexible Wall Per-
meameter”. The test set-up is shown
in figure 1 and test results are summa-
rized in Table 3. Applying the Darcy’s
law, the permeability of the sand was
reduced by 4 to 5 orders of magnitude
and the final permeability of samples
would be in-line with silt.
Field trials
Upon completion of an oil or gas
well, cement is pumped and placed in
the annular space between the casing
the wellbore. In a perfect world, the
cement would form an impermeable
barrier that would isolate geological
zones and prevent the flow of fluids
or gas to other zones or the surface.
In reality, a significant percentage of
wells have a nuisance level of methane
leaking to the surface. It is open to
debate on the exact number of wells
in Western Canada that are leaking
gas but it is agreed that it is 10, 000’s.
Western Canada and globally there is
increasing regulatory and public pres-
sure for the oil industry to remediate
wells leaking nuisance levels of gas.
Figure 2 is a commonly referenced
diagram taken from the Alberta
Energy Regulator that shows the dif-
ferent gas pathways. These pathways
can develop during the cementing
process and/or over time as the cement
is subject to thermal cycling, geologi-
cal movement and production.
As repairs are non-revenue generating,
operators are looking for long term
solutions that are simple, environmen-
tally acceptable and cost effective.
The default method for blocking gas
migration is to squeeze cement. (Note
of the Editor: I didn’t change the
term “squeeze” here and below, that I
learned is the equivalent of grouting
in the Oil Industry). Class G cement
and micro fine cement are effective at
filling and blocking medium to large
micro annuli however, as the diameter
of micro annuli and fractures gets
tighter, physical limits of viscosity
and particle size make it progressively
more difficult to squeeze cement. This
leads to limited penetration and bridg-
ing in channels. Conventional sodium
silicates as well as the 4.5 ratio sodium
silicate have the advantage of being a
low viscosity, solids free solution of
smaller molecular weight molecules.
This allows the silicate to be squeezed
closer to the gas source and therefore
provide a more effective, long term
seal. Over the last 7 years conven-
tional sodium silicate has had good
success in Western Canada remediat-
ing wells venting gas. The interest in
evaluating the 4.5 ratio material was
driven by a few factors:
• achieve a higher success rate on the
1st squeeze (vs. multiple squeezes)
• avoid the use of setting agents
Table 2. Durability of consolidated sand after curing 14 days with 4.5 ratio
silicate using citric acid and triacetin
4.5 ratio -
50%
Setting agent
Citric acid
Triacetin
Test cycle Remaining
weight, g
Cumulative
weight loss
Remaining
weight, g
Cumulative
weight loss
Original
oven dry
mass
325.6
N/A
321.8
N/A
1
324.5
0.3%
320.0
0.6%
2
323.8
0.6%
319.1
0.8%
3
323.7
0.6%
318.9
0.9%
4
322.7
0.9%
318.7
1.0%
5
316.8
2.7%
318.9
0.9%
6
238.6
(Cracked)
26.7%
318.5
1.0%
7
218.7
32.8%
318.0
1.2%
8
215.3
33.9%
317.7
1.3%
9
210.7
35.3%
317.6
1.3%
10
200.9
38.3%
317.3
1.4%
11
200.6
38.4%
317.6
1.3%
12
194.3
40.3%
314.4
2.3%
