MicroBlock®
Gas tight cement
Gas migration
In gas and oil bearing formations there
is always a risk for gas migration during
drilling and production. At worst, gas migration can lead to surface leakage and pose a
threat of ignition and fire.
The critical phase of WOC is during the
transition period. The cement slurry
changes from a pumpable fluid to an
unpumpable gel with properties comparable
to those of a solid mass and the slurry loses
its ability to transmit hydrostatic pressure.
Minor shrinkage (or volume reduction)
occurs, as the cement sets. This volume
reduction, which can be increased by high
fluid losses, causes a drop in the pore pressure within the cement slurry. A reduction
in pore pressure and the decreasing ability
to transmit hydrostatic pressure can lead
to pressure imbalance and to an increased
risk of gas migration.
Additional benefits of using Microblock are:
• adsorption of additional water
• improved slurry stability
• reduction of volume shrinkage
• improved bonding
Use of Microblock
Microblock not only catalyzes rapid gelation,
but also enhances fluid loss control, lowers
slurry ‘permeability’ and increases early
compressive strength development.
Issue: 01/2002
250
1,8
Gas and pore pressure (bar)
The specific characteristics of Microblock,
e.g. small particle size (average size
0,15 µm), pozzolanic reactivity and large
surface area (21 m2/g), make the product
an effective anti-gas migration additive.
Gas tight testing
Gas tight 13,6 ppg (1,63 SG) cement slurry,
2,83 gals/sk (25 lhk) Microblock added
1,6
Pore pressure
200
1,4
150
1,2
1,0
100
0,8
Gas pressure
0,6
Gas flow
50
0,4
0
0,2
-50
0,0
0
100
200
300
Time in minutes
400
500
Gas flow (ml/min)
Secondary gas migration can occur due to
chemical attack of the cement sheath or by
strength retrogression. Impermeable set
cement with good retrogression properties
is an additional long-term benefit for gas
tight cements.
Gas tight testing
Standard 13,0 ppg (1,56 SG) cement slurry, no Microblock added
160
Pore pressure
1,4
140
120
1,2
100
1,0
80
0,8
60
0,6
Gas pressure
40
Gas flow
0,4
20
0,2
Gas flow (ml/min)
Gas and pore pressure (bar)
1,6
The 16,68 ppg (2,00 SG) cement slurry
was batch mixed to ensure a homogeneous
slurry prior to pumping it down-hole. All the
jobs executed so far show excellent results
and fulfill the client’s requirements.
Well information:
0
-20
0,0
0
100
200
300
400
500
Well inclination: Horizontal
600
Time in minutes
Dosage rates
The minimum Microblock dosage to control
gas migration depends on:
• cement slurry density
• BHCT / BHST
• expected reservoir gas pressure
Recommended dosage rates for Microblock
Cement weight (SG)
1,50
1,56
1,62
1,68
1,74
1,80
1,86
1,92
5
1,98
BHCT: 141 F (61 C)
BHST: 205 F (96 C)
Slurry components:
Components
Oilfield
Metric
Class G cement
Micromax
Microblock
Fluid loss additive
Dispersant
Sea water
94 lbs
9,4 lbs
0,5 gals
0,7 gals
0,5 gals
3,5 gals
100 kg
10 kg
4,44 litre
6,21 litre
4,44 litre
31,1 litre
40
30
GAS TIGHT REGION
3
20
2
Slurry properties:
10
1
0
12,5
Lhk
Gals / Sk
4
13,0
13,5
14,0
14,5
15,0
15,5
16,0
0
16,5
Cement weight (ppg)
Remark: Dosage of Microblock required at different
cement slurry densities to withstand a maximum
gas pressure of 125% of the water gradient
PV:
YP:
Free water:
Fluid loss:
TT at BHCT:
24 hr CS at BHST:
43 cps
6 lbs/100 ft2
0 %
30 cc/30 min
4 hours
4700 psi (324 bar)
Example slurry design
Well description:
An operator in the Middle East required to
cement a 7” liner from 12000 ft (3658 m)
to 9500 ft (2896 m) in an 8 1⁄2” Horizontal
Open Hole. As the liner is considered a production string, the set cement had to perform excellent sealing and bonding characteristics to both the liner and formation.
Furthermore the reservoir had to be isolated using a gas-tight cement slurry and the
set cement had to exhibit high compressive
strength (+ 4000 psi (+ 276 bar)).
For more information please visit our web-site
or contact our local sales representative