Underground Construction - January 2022 - 33

TABLE 1. Initial resistivity and curing model parameters for smart cement without and with SBR
SBR Content
(%)
1
3
Initial Resistivity
ρo
(Ω-m)
0.99 + 0.03
1.03 + 0.02
1.11 + 0.04
Materials, Methods
Class H smart well cement, with water-to-cement (w/c) ratio
of 0.38 was used. To improve its sensing properties and
piezoresistive behavior, the smart cement was modifi ed with
0.1 percent conductive fi llers (CF) by weight of cement.
Two other smart cement slurries were prepared with
commercially available SBR polymer at 1 percent and 3 percent.
Cement, water, and additives (0.1 percent CF and SBR polymer)
were mixed at the speed of 4000 rpm for 3 minutes and
35 seconds at 1200 rpm.
To monitor cement-setting and compressive stress tests,
50-millimeter (2-inch) diameter, 100-millimeter (4-inch) height
cylinders were prepared. For real-time monitoring, a two-probe
method was selected.
Electrical resistivity was measured by using an inductance,
capacitance, and resistance (LCR) digital meter during the cement
curing and compression test. To minimize contact resistances, the
resistance was measured at 300 kHz using the two-wire method.
Th e electrical resistivity (ρ) was related to the measured
electrical resistance (R), per Equation. (1) :
ρ = R/(K + GR)
where parameters K and G are based on the type of material,
such as conductor, semiconductor or insulator.
Experimental studies have shown that parameter G for
the smart cement was zero. Hence, the normalized change in
resistivity with the changing conditions (curing, stress) can be
represented as follows (Equation (2)):
Δρ
ρ
=
ΔR
R
(2)
In this study, the modifi ed smart cement is represented in
terms of resistivity (ρ) to the changes (composition, curing and
stress), since it has been shown to be a sensitive parameter.
Th e 2-inch-diameter, 4-inch-high cylindrical specimen was
capped and tested at a predetermined controlled displacement
rate. Compression tests were performed on cement samples after
one day of curing using a hydraulic compression testing machine.
Since oil well cement serves as the pressure-bearing part of
wells in real applications, the piezoresistivity (change in electrical
resistivity of a material under pressure) of modifi ed (1 percent
and 3 percent SBR) and unmodifi ed cement was investigated
under compressive loading.
(1)
FIGURE 1: HPHT gas flow study device
During compression testing, electrical resistance was
measured in the stress axis. To eliminate the polarization eff ect,
alternating current resistance measurements were made using
a LCR meter at a frequency of 300 kHz.
A high-pressure and high-temperature (HPHT) device was modifi
ed to measure the resistance real-time during the fl uid loss for 30
min (API 13A and API 13 B) and during gas leak study (FIG. 1). Th e
HPHT device has an area of 22.58 cm2
and can withstand a pressure
of around 2,000 psi. For this study, up to 700 psi was used with the
cement height of 4 inches.
Polymer modifi ed smart cement and smart cement were placed
in the device for gas migration tests, performed using nitrogen gas.
During the entire test, samples cured for 30 minutes and 24 hours,
and their vertical resistances were measured using the LCR device.
Th e change in resistance was used to determine the resistivity (Eqn.
(2)), which is a material property. Also, gas fl ow meter was used to
determine the gas leak with applied pressures.
Results, Discussion
Initial resistivity was measured immediately after mixing the
smart cement with and without the SBR polymer, 0.99 Ωm
(TABLE 1). With the addition of 1 percent SBR, it increased
4 percent, to 1.03 Ωm - four times the amount of SBR added.
With 3 percent SBR, the initial resistivity increased 12 percent,
UCONonline.com | JANUARY 2022
33
ρmin
(Ω-m)
0.97 + 0.01
1.00 + 0.03
1.05 + 0.04
tmin
(min)
58 + 5.0
65 + 4.5
80 + 3.5
ρ24 h
(Ω-m)
3.48 + 0.02
4.07 + 0.02
4.63 + 0.02
RI24
(%)
t0
p1
259
307
341
0.65
0.65
0.65
q1
0.298
0.325
0.337
(min)
142.8
173.4
183.3
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