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CA 02377347 2011-05-17
CEMENTING SPACERS FOR IMPROVED WELL
CEMENTATION
Background of Invention
Field of the Invention
[0001) The invention relates generally to cementing spacers for use in
cementing
wellbores. More specifically, the invention relates to the use of Stokes Law
cementing spacers when cementing wells.
Background Art
[0002] When drilling an oil or gas well, drilling fluid having a prescribed
density is
used during the drilling operation for several purposes including, for
example,
balancing a formation fluid pressure (which generally increases as the depth
of a
well increases) present in geologic formations that are penetrated by the
wellbore.
The drilling fluid, or "drilling mud," is typically pumped down a drillstring,
through a drill bit, and is returned to the surface though an annulus formed
between the drillstring and a wall of the wellbore. This process is known as
"circulation" of the drilling fluid.
[0003] If the density of the drilling fluid is excessive, the hydrostatic
pressure
exerted by the drilling fluid on the formations can result in fractured
formations
and a resultant loss of drilling fluid into the "broken down" formations. Loss
of
drilling fluid into the formation typically results in "lost circulation"
(e.g., the loss
of a return fluid communication path to the surface through, for example, the
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.wellbore annulus) and eventually a pressure underbalance with -respect to
formation fluid pressure. Lost circulation can result in uncontrolled
discharge or
"blowouts" of pressurized formation fluids to the surface because pressure
control
of the well has been lost. For example, when drilling fluid is lost into the
formation, the wellbore pressure drops and permits higher pressure formation
fluids to flow into the wellbore in the form of a "kick." The kick may
propagate to
the surface and result in a blowout that can damage rig equipment and injure
or
kill rig personnel.
[0004] These conditions may generally be avoided by appropriate selection of
the
density of the drilling fluid used to drill the well. The density of the
drilling fluid
is usually controlled by the addition of "weighting agents" in the form of,
for
example, particulate solids of heavy earth materials, such as barite. The
weighting
agents are added to the drilling fluid in a known ratio with respect to the
fluid
volume in the wellbore to produce a carefully regulated drilling fluid with a
known density.
[0005] During the drilling process, it is often necessary to periodically
lower steel
casing or well liners into the wellbore to line the walls thereof in order to
maintain
stability of the wellbore. Moreover, the casing may be required to protect
shallower formations from the high wellbore pressures required to maintain
fluid
pressure balance or overbalance with respect to formations near the bottom of
the
wellbore. The casing, which is typically steel, must fit inside the wellbore
diameter.
[00061 After the casing is placed in the wellbore, an external casing annulus
is
formed between an outer surface of the casing and the wall of the wellbore. In
order to prevent fluid communication along the external casing annulus, oil
well
cement is typically pumped into the external casing annulus. Cementation of
the
casing in the wellbore is important because undesirable fluid communication
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between the bottom of the wellbore and the surface through the external casing
annulus can result in formation fluid leakage to the surface or to other
subsurface
formations, and can result in other types of well damage resulting in a loss
of
production potential. The oil well cement is placed in the external casing
annulus
by pumping a substantially fluid cement slurry down the casing, out of the
bottom
of the casing, and up into the external casing annulus.
[00071 During the cementing process, the cement slurry must completely
displace
the drilling fluid from the external casing annulus because drilling fluid
that is not
displaced may provide a path for the flow of formation fluids up the external
casing annulus after the cement has set. Moreover, slurries of oil well cement
are
often not chemically compatible with common drilling fluids. For example, if
the
cement slurry comes into direct contact with the drilling fluid during the
displacement process, the cement slurry and the drilling fluid may mix
together
and form a viscous material. When the cement slurry is pumped into the
external
casing annulus, the cement slurry may bypass the viscous material, thereby
leaving channels of viscous material that do not set up to form a solid,
impermeable cement barrier to formation fluids. Accordingly, a cement "spacer"
fluid is often pumped into the wellbore between the drilling fluid and the
cement
slurry to improve the displacement of the drilling fluid and to prevent direct
contact and mixing of the drilling fluid and the cement slurry.
[0008) Again, however, the density of such a cement spacer cannot exceed
certain
limits or the lost circulation condition will be encountered, and it cannot
fall below
other certain limits or an underbalanced condition will occur. Thus it is
necessary
to be able to control the density of the spacer fluid used in cementing
operations in
a manner similar to that used to control the density of drilling fluid during
drilling
operations.
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[0009] Prior art spacers are generally made by mixing a suitable liquid base
fluid
with a viscosifier which may be, for example, a soluble polymer or bentonite
clay
and a weighting agent including, for example, solid particles of barite or
calcium
carbonate. The weighting agent may also include low-density particles such as
hollow glass or ceramic spheres or foamed nitrogen. The most common spacer
base fluid is plain water. "Plain water" includes, for example, any source of
chemically suitable water that is readily available for such applications,
including
fresh water, seawater, saltwater, and brine. Alternatively, a suitable organic
solvent may be used as the spacer base fluid. Organic solvents are often
advantageous for use in displacing oil based drilling fluids. When used in
this
manner, organic solvents may also include viscosifiers and weighting agents.
[0010] In prior art cement spacer fluids, the viscosifier is used to support
the
particles of weighting agent so as to prevent settling of the weighting agent
during
the pumping operation. A performance objective of the viscosifier is often to
develop "gel strength" under static conditions to aid in the support of the
weighting agent particles. Particle settling in cement spacer fluids, is
usually
evaluated in laboratories with settling tests, similar to the API free water
test used
for cement slurries. In the test, the volume of free water, which accumulates
on
the top of the spacer under specified conditions, is determined. A common
practice is to require that the free water be below some maximum volume.
[0011] Therefore, it is desirable to have a cementing spacer that is designed
to
displace drilling fluid in a wellbore and serve as a buffer between the
drilling fluid
and a cement slurry used to cement, for example, casing in a wellbore.
Moreover,
it is desirable to have a cementing spacer that can be readily formed at a
well site.
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Summary of Invention
[00121 A method of cementing a well using a cementing spacer. The method
comprises pumping a drilling fluid into a well and pumping a cementing spacer
into the well to displace the drilling fluid, wherein the cementing spacer
comprises
substantially unviscosified water and a weighting agent. Cement is then pumped
into the well to displace the cementing spacer and the drilling fluid to
complete the
cementing of the well.
[00131 Other aspects and advantages of the invention will be apparent from the
following description and the appended claims.
Detailed Description
[0014] Embodiments of the invention have been developed from a study of
particle
settling calculations based on the Stokes-Einstein equation. The particle
settling
calculations show that the sedimentation rate, or particle settling velocity,
of
particles of a weighting agent in a base fluid is relatively slow when
compared to
the depth of a typical well. For example, a total sedimentation distance of
about
40 feet in a 4 hour period was calculated for particles of a calcium carbonate
weighting agent in a water base fluid.
[00151 Accordingly, if a controlled density cementing spacer comprising
calcium
carbonate and water (and substantially no viscosifier) is pumped into an
external
casing annulus in a substantially vertical wellbore, particles of calcium
carbonate
in the cementing spacer typically will settle no more than about 40 feet by
the time
the cement has set. This degree of settling will not cause any operational
problems with respect to cementing the well.
[0016] In an embodiment of the invention, the cementing spacer comprises a
weighting agent (such as, for example, calcium carbonate, barite, ferrite,
hematite,
etc.) and water. Note that as previously disclosed, "water" may include fresh
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water, salt water, seawater, brine, or any other chemically suitable source of
water
that will not adversely react with drilling mud or the cement in the wellbore.
The
cementing spacers described above are typically referred to as "Stokes Law"
mixtures. The resulting cementing spacers have numerous advantages discussed
below when compared to prior art spacers that use viscosifiers to support the
weighting agent.
[00171 One form of the Stokes-Einstein equation is shown below as Equation 1:
V2gr2(d,-df) (1)
9v
wherein V is a particle settling velocity (cm/sec), g is gravitational
acceleration
(980 cm/sec 2), r is a particle radius (cm), dp is a particle density (g/cm3),
df is a
fluid density (g/cm3), and v is a fluid viscosity (poise).
[0018] Numerical solutions of Equation 1 have been determined for different
types
of particulate weighting agents (such as; for example, calcium carbonate,
barite,
ferrite, hematite, etc.) and different particle diameters. Moreover, the
solutions
have been determined using water or organic solvents. The results show that,
compared to a depth of a well (e.g., the overall height of a cement annulus
from a
casing bottom to a well head) and to a length of a typical casing string, the
sedimentation velocity (or sedimentation rate) of particles of the weighting
agent
is substantially slow.
[0019] Therefore, based on calculations performed using Equation 1, it has
been
determined that cementing spacers comprising water and a weighting agent have
a
substantially slow particle settling velocity so that they may be pumped into
a well
using typical rig operating techniques and do not require the addition of a
viscosifier (such as bentonite or viscosifying polymers) to impede particle
settlement or otherwise affect the rheology of the cementing spacer. A small
amount of viscosifier may be present in the cementing spacer as long as the
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amount does not substantially affect the rheology of the cementing spacer
(e.g., as
long as the amount of viscosifier does not substantially affect the settling
properties of the weighting agent). These cementing spacers do not adversely
affect cement slurries used to cement wells, and avoidance of the use of
viscosifiers may have several advantages, including:
^ Cementing spacers are less expensive because they comprise fewer
components.
^ Cementing spacers have predictable properties resulting in less pilot
testing
and quality control requirements.
= The reduction or absence of gel strength development, combined with the
settling motion of the cementing spacer particles, maintains hydrostatic
pressure on the cement slurry as it sets and thereby provides a better seal
through producing zones.
= Cement bond well logs are improved.
= Cementing spacers have a substantially Newtonian rheology and experience
turbulent flow at lower pumping rates and thereby improve the
displacement of drilling fluid (in the external casing annulus) by the cement
slurry.
^ Less mixing occurs at the interface between the turbulent flow cementing
spacer and the drilling fluid, which also improves the displacement of the
drilling fluid.
[0020] While the cementing spacers comprise substantially unviscosified water
and a weighting agent, other non-viscosifying additives may be used as well.
For
example, friction reducing additives may be used with the invention. Friction
reducing additives may also serve to either minimize or enhance solid packing
of
particles of the weighting agent.
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[00211 Moreover, during extended settling conditions (e.g., settling
conditions that
continue for some time after the cement has set), particles of the weighting
agent
(which may comprise, for example, barite) in the cementing spacer settle and
may
form a "plug" (e.g., a "barite plug") proximate the top of a cement column.
The
plug forms an additional seal and further prevents fluid transmission from the
bottom of the wellbore to the surface. The additional sealing properties of
the
plug may be useful, for example, in meeting regulatory requirements associated
with, for example, external casing pressure and/or microannular gas leakage (a
condition that results from the formation of a small microannulus or gap
between
the set cement and the casing and/or the formation which may allow slow
leakage
of gas to the surface).
[00221 Stokes Law calculations also apply to the "particle rise" of particles
of low
density weighting agents that may be added to the cementing spacer. For
eieample, the use of hollow glass or ceramic spheres, foamed nitrogen, etc.,
to
lower or reduce the density of the cementing spacer may also be used in
embodiments of the invention. Further, the cementing spacers may be used to
recover expensive oil based drilling fluids from wells for future reuse.
[00231 While the invention has been described with respect to a limited number
of
embodiments, those skilled in the art, having benefit of this disclosure, will
appreciate that other embodiments can be devised which do not depart from the
scope of the invention as disclosed herein. Accordingly, the scope of the
invention should be limited only by the attached claims.
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