Chilingarian G.V. et al. Surface Operations in Petroleum Production, II
Подождите немного. Документ загружается.
75
NORMAL TWO INNER OUTSIDE MULTIPLE
METHOD CEMENTING CEMENTING CEMENTING
DISPLACEMENT STAGE STRING CEMENTING STRING
Fig.
3-3.
Types
of
cement displacement operations. (Courtesy
of
the Halliburton Services.)
through the ruptured rubber plug, float collar, guide shoe and up the annular space
between the wellbore and casing. Pumping of cement slurry is stopped when the
solid top plug seats on top of the .float collar. The pumps are shut-in and the cement
slurry is permitted to set-up. Below the float collar, the casing
is
left full of cement.
If required, however, it can be drilled out later.
There are numerous variations of primary cementing techniques in use (Fig. 3-3).
Cementation of large-diameter casing, when tubing (or drillpipe) is used in order to
permit the use of smaller-diameter plugs and other equipment, is called
inner string
cementing. This reduces the cementing time, volume of cement necessary to pump
the plug, and amount of cement that must be drilled out. This technique utilizes a
modified float collar together with sealing adapters on the drillpipe, and cementing
is done through the drillpipe, which has been “stabbed” into modified float collar.
Cement mixing is continued until cement returns to the surface (Shryock, 1982, p.
323). Thus, when the drillpipe plug is released, only the cement present in the
drillpipe is released to the sump.
In
normal displacement
techniques, the cement is displaced through the tubing
and/or casing. If a primary cementing job is performed in two or more stages, it is
called
stage
cementing. The latter
is
used in wells requiring long columns
of
cement
and in the presence of weak formations which cannot support the hydrostatic head
of the cement column while cementing.
A
combination
of
plugs and baffles enables
the stage collar to be manipulated hydraulically, shutting off access to the casing
below this collar and opening ports in the collar
so
that fluid can be circulated from
this point to the surface (see Shryock, 1962, p. 322). After allowing sufficient time
for the first-stage cement to set, cement circulation is performed through these ports
back to the next stage or to the surface.
Liner
cementing is similar to the casing cementing method; however, the tech-
nique used to get the cement in place is different.
As
soon as the liner is in place, the
cement slurry is circulated down the drillpipe out of the liner and, then, up the
outside of the liner. Just as in the casing string, plugs are used to prevent mixing of
76
the drilling fluid and cement in the liner.
As
pointed out by Shryock (1982, p. 323),
the liner plug, whch is installed prior to running the liner in the hole, has an
opening that allows passage of the cement slurry.
A
drillpipe plug is released from
the surface at the proper time. It displaces the cement slurry from the drillpipe and
upon seating in the liner plug forces the latter down the liner displacing the slurry
into the annulus. After completion of the cementing job, the drillpipe is picked up
two to three stands above the liner top and the excess cement is reversed out
of
the
hole. The remaining cement on the liner top is later drilled out (Shryock, 1982, p.
323).
When seal is not achieved during the primary casing
or
liner job, cement is
squeezed out into the voids-squeeze cementing. This involves pumping cement
through the drillpipe, which is attached to a squeeze packer set in the casing above
the liner lap, for example.
If cement is pumped through a small-diameter tubing between casing and open
hole or between casing strings, which is commonly used on conductor or surface
casing where the top of the cement must reach the surface, it is called outside
cementing. Or cement can be pumped simply down the annulus.
In reverse circulation cementing, the slurry is pumped down the annulus, dis-
placing the mud in the wellbore through the casing. It can be used when pumping
the cement slurry in turbulent flow conditions is not possible without breakmg
down the weak zones above the casing shoe.
In order to obtain a more uniform sheath
of
cement around the casing, whch
may be impossible by using conventional displacement methods, one can use
delayed setting cementing.
The downhole and surface cementing equipment vary in type and design.
Cementing technique chosen depends on depth, pressure, formation temperature,
lithology, and the type of casing string being cemented.
Waiting-on-cement time (WOC)
Waiting-on-cement time (WOC) is commonly determined by the local, state, and
federal regulations, which should be checked. In the absence
of
rules, the WOC time
selected should permit enough time for the cement to gain sufficient strength to: (1)
anchor the casing and withstand the shock of subsequent operations, (2) seal
permeable and thef zones, and (3) confine fracture pressures. The WOC time can
be as short as 4-6 hrs under warm conditions, using densified cements (API Class
A,
G
or
H)
and a 2-3% CaCl,. It depends upon the (1) class of cement,
(2)
additives in the cement,
(3)
required time for placement,
(4)
temperature, and
(5)
pressure. It is usually desirable to have a minimum compressive strength of
500
psi
before drilling cement out of the casing is initiated. It is commonly recommended to
(1) run a cement bond log, (2) perforate, or (3) stimulate the well 24-72 hrs after
placement of the cement (or upon reaching a compressive strength of 2000 psi). A
pressure test of the casing is required in some areas, e.g., in Texas,
U.S.A.,
whereas
77
in others, eg, California,
U.S.A.,
a production test is required to determine whether
the cementing job was successful or not.
Slurry
density
Slurry density must be continually monitored in order to control the relative
amounts of water, cement, and additives used and to insure maintenance
of
the
correct water/solids ratio. Cement slurry density must be about the same as that of
the drilling fluid in the wellbore. As pointed out by Gatlin (1960), this will minimize
the chances for lost circulation or blowouts, which could occur during the placement
of the cement slurry.
As
mentioned before, a common material for decreasing the
cement slurry density is montmorillonite clay, commonly referred to by petroleum
engineers as gel. This reduction in density is mainly due to the high degree of
swelling of montmorillonite (smectite) clays in water. The content of a particular
additive in a cement slurry is expressed as a weight percent
of
the dry cement.
Gatlin (1960) presented the practical units whch are used in cement calculations.
The cement volume used is expressed in terms
of
sacks
of
cement, even though in
most cases bulk cement is delivered to the wellsite:
1
sack
=
94 lb
=
1
ft3 bulk
volume. Inasmuch as the specific gravity of portland cement is equal to 3.14, a sack
of cement contains 0.048 ft3
[
=
94/(3.14
X
62.4)]
of
cement. Cement porosity is
equal
to
52%. In computing the cement volume, the volumes and weights of various
components are assumed to be additive. Gallons per sack are used to express
water/cement ratios. For example,
if
6 gallons
of
water per sack of cement are used,
the slurry volume is equal to:
(6/7.48)
+
0.48
=
1.28 ft3/sack of cement (where 7.48 gal
=
1
ft3)
The specific weight of this slurry can be calculated as follows:
Y,V,
+
YCK
=
YSK
(3-1)
where
yw
=
specific weight of water,
V,
=
volume of water,
yc
=
specific weight of
cement,
V,
=
volume of cement,
y,
=
specific weight of slurry, and
V,
=
volume of
slurry. Upon rearranging:
Ys
=
(
YWVW
+
Ycv,
I/
v,
(3-2)
Thus,
y,
=
[(62.4
x
6/7.48)
+
(3.14
x
62.4
X
0.48)]/[0.48
+
(6/7.48)]
=
112.39 lb/cu
ft or 13.49 lb/gal. On adding 6% bentonite (by weight of cement), having a sp.gr. of
2.7, to the slurry and increasing the water/cement ratio to
8.0
gal/sack, the new
slurry volume can be calculated as follows:
V,+V,+v,=v,
(3-3)
where
V,
=
volume of bentonite.
Assuming additive weights:
wt,
+
wt,
+
wt,
=
wt,
(3-4)
78
where
wt,
=
weight of bentonite,
wt,
=
weight
of
water,
wtc
=
weight
of
cement,
and
wt,
=
weight of slurry.
Thus:
=
[(0.06
x
94)/(2.7
x
62.4)]
+
(8.0/7.48)
+
0.48
=
1.58
ft3/sack.
The new specific weight of cement slurry is equal to:
[(0.06
x
94)
+
(1
x
94)
+
(8
X
8.33)]/(1.58
X
7.48)
=
14.07 lb/ft3 or 1.7 lb/gal
Due to the fact that aeration affects the density measurements, samples of
cement slurry should be obtained from a special manifold on the discharge side of
the pump and not from the mixing container. Inasmuch as the last portion
of
cement slurry is placed around the casing shoe, great care must be exercised to
insure that it meets required weight specifications. In order to provide higher
strength around the casing shoe (point), cement slurries are often mixed with a
lower amount of water toward the end of the job.
SURFACE AND SUBSURFACE CEMENTING EQUIPMENT
Cement pumping equipment
Cement pumping units, which can be mounted on a truck, trailer, skid, or ship,
are operated at varying rates and intermittently at high pressure. These units must
(1)
have the lowest practical weight/horsepower ratio, (2) be capable of high
horsepower input and output over wide torque limits, and
(3)
lend themselves to be
easily transported from one site to another. The cement pumping units usually are
powered by either internal-combustion engines
or
electric motors (see Figs. 3-4 and
3-5). They may be manifolded with two
or
three pumps. In the case of low-pressure
systems, a centrifugal pump can be used for mixing and two positive displacement
pumps can be used for displacing. High-pressure systems may use one pump for
mixing, while the other is used for displacing (Halliburton, 1970).
Although cementing pumps commonly have to operate at pressures as
high
as
20,000 psi, most cementing work requires maximum pressure lower than
5000
psi.
The volume of cement to be pumped, anticipated pressures, and well depth
determine the number of required trucks for mixing the cement and displacing it
downhole. Low-pressure units are required for surface casing and conductor ce-
menting, whereas two
or
more trucks are required
for
primary cementation of
intermediate
or
production casing. Cement slurries are generally pumped into the
79
Standard Twin HT-400 Cementing Unit.
Standard Twin HT-400 Cementing Trailer.
Fig.
3-4.
Truck
and trailer mounted cementing units. (Courtesy
of
the Halliburton Services.)
casing at the highest possible rates dictated by the capacity of the mixing unit
(20-50
sacks per minute) (Halliburton, 1970; also see
API,
1974).
Cement mixing equipment
The mixing system
of
any cementing unit proportions and blends the dry cement
with the water (carrier fluid). The jet mixer shown in
Fig.
3-6
is one of the most
widely used mixers.
As
shown in this figure, the mixer consists of a funnel-shaped
hopper, a mixer bowl, discharge line, mixing tub, and water supply lines. The mixer
operates by forcing a stream
of
water through a jet into a bowl, in which cement
80
SCHEMATIC
OF
JET MIXER
Fig.
3-5.
Schematic
of
the jet cement mixer showing the working parts. (Courtesy
of
the Halliburton
Services.)
from the hopper mixes with water. Then the resulting slurry is discharged into a
discharge line and, finally, into a sump tub from which the slurry is pumped into the
well by the cementing pumps. The volume
of
water forced through the jet and the
u
WATER
n
DRY
CEMENT
CEMENT SLURRY
CEMENT HOPPER
JELL BREAKER1
OTARY JET TUB SCREEN
DR” CEMENT
CEMENT
SLUIIR”
DISPLACEMENT
PUMP SUCTION
Fig.
3-6.
Jet cement
mixing
operation. (Courtesy
of
the Halliburton Services.)
81
amount
of
cement introduced into the hopper while mixing, control the mixing
speed.
A
bypass line can supply extra water into the bowl discharge line for
lowering the specific weight of slurry by increasing the water/cement ratio. Depend-
ing on the rate
of
feeding of cement and water/cement ratio, mixing pressures may
range from
150
psi for low-pressure systems to
500
psi for high-pressure systems
(Halliburton, 1970). For an excellent discussion
of
the subject see Smith
(1976).
Recirculating
systems
The recirculating mixer (Fig.
3-7)
is designed for
mixing
hgh-density slurries.
Fig.
3-7.
A
recirculating mixer. (Courtesy
of
the Halliburton Services.)
82
WATER
Fig.
3-8.
Batch mixing system. (Courtesy
of
the Halliburton Services.)
Basically it is a pressurized jet mixer with a larger mixing tub capacity. As pointed
out by Smith
(1976,
p.
61),
the system uses recirculated slurry and water to partially
mix and discharge the slurry into the tub. The in-tub eductor adds additional energy
and improves mixing, whereas additional shear is provided by the recirculating
pump and agitation jets. It is used basically to provide uniform cement slurries
having densities as high
as
22
lb/gal, pumping as slowly as 0.5 bbl/min.
Batch
mixing
The pneumatic and mechanical types
of
batch mixers were often used to blend a
large volume
of
cement slurry at the surface conditions before it is moved into the
well (Fig.
3-8).
The pneumatic systems, however, are no longer being used in the
U.S.A.
The mechanical mixers are relatively simple to use and enable an engineer to
83
TABLE
3-111
Mixing rates and densities for various slurries using jet mixers and recirculating mixers (After Smith,
1976, table 7.6, p. 62; courtesy
of
the Society
of
Petroleum Engineers
of
AIME)
Jet mixer
Recirculating mixer
Mixing Slurry Mixing Slurry
rate density rate density
(bbl/min) (lb/g4 (bbl/min) (lb/gal)
Densified and 2-5 16-20 0-6
Neat slurries 2-8 15-17 0-8
High-water-ratio
weighted slurries
slurries
2-14
11-15
0-12
16-22
14-18
11-15
prepare a specific volume
of
slurry having the exact physical requirements before it
is displaced downhole (Halliburton, 1970). The ranges
of
mixing rates and slurry
densities
for
two different types of mixers are presented in Table
3-111.
Subsurface cementing equipment
Many types of subsurface equipment are used today in oilwell cementing. Table
3-IV
illustrates the wide variety of equipment available to aid cementing operations.
This
equipment has been developed through the years by operators and manufac-
turers to assist the operator in cementing operations. Specifications covering such
equipment are quite variable and are limited.
A
general description
of
these
Fig.
3-9.
Guide shoes: (a) with no ports; (b) with
down
jet type ports; (c)
Type
“M
Texas Patter (short)
shoe-open end. (Courtesy of the Halliburton Services.)
00
P
TABLE 3-IV
Summary
of
cementing equipment and mechanical aids (After Smith,
1976,
table 6.2, p.
56;
courtesy of the Society
of
Petroleum Engineers
of
AIME)
Equipment Function or Application Placement
Floating equipment
Guide shoes
Float collars
To
guide casing into well
To
minimize derrick
strain
To
prevent cement flowback
To
create pressure differentials to improve
To
catch cementing plugs
First joint of casing
One joint above shoe in wells less than
6000
ft
deep; two to three joints above shoe
in
wells
bond deeper than
6000
ft
Automatic fillup equipment
Automatic fillup float shoes
Differential fillup float shoes
Same
as
those
of
ordinary float shoes and
collars; also to control hydrostatic pressure
in annulus while casing is being
run
Same as for ordinary float shoes and collars
and collars
and collars
Formation
packer tools
Formation packer shoes
Formation packer collars
Stage-cementing tools
Two-stage tools
Three-stage tools
To
protect lower zones by expanding during
cementing
To
cement two
or
more sections in separate
stages
First joint
of
casing
As hole requirements dictate
Based
on
critical zones and formation fracture
gradients