U.S. patent number 3,817,039 [Application Number 05/086,755] was granted by the patent office on 1974-06-18 for method of filling subterranean voids with a particulate material.
This patent grant is currently assigned to The Dow Chemical Company. Invention is credited to Milton E. Heslep, John D. Stewart.
United States Patent |
3,817,039 |
Stewart , et al. |
June 18, 1974 |
**Please see images for:
( Certificate of Correction ) ** |
METHOD OF FILLING SUBTERRANEAN VOIDS WITH A PARTICULATE
MATERIAL
Abstract
A method is provided for backfilling a subterranean void, e.g. a
mined out cavity such as a tunnel, etc. An aqueous suspension of
solid particles (fill material) is injected through a conduit which
connects the void with a suitable work surface (said conduit and
void consisting of a closed pressurized system during injection)
and into the void at a certain critical minimum rate.
Inventors: |
Stewart; John D. (Littleton,
CO), Heslep; Milton E. (Casper, WY) |
Assignee: |
The Dow Chemical Company
(Midland, MI)
|
Family
ID: |
22200695 |
Appl.
No.: |
05/086,755 |
Filed: |
November 4, 1970 |
Current U.S.
Class: |
405/263;
405/267 |
Current CPC
Class: |
E21C
41/00 (20130101); E21F 15/08 (20130101) |
Current International
Class: |
E21F
15/08 (20060101); E21F 15/00 (20060101); E21C
41/00 (20060101); E21f 015/08 () |
Field of
Search: |
;61/35,36 ;169/2 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Rimrodt; Louis K.
Assistant Examiner: Kannan; Philip C.
Attorney, Agent or Firm: Kanuch; Bruce M.
Claims
What is claimed is:
1. In the method of emplacing a layer of solid particles in a
subterranean void wherein the layer occupies at least a major
portion of the height of said void, comprising injecting a mixture
of a carrier liquid and particulate solids through a conduit
connecting a work surface and said void, the improvement which
comprises:
a. providing a closed system between injection equipment for said
mixture, said conduit and said void;
b. providing a suspension of said carrier liquid and said solids;
and
c. injecting said suspension into said void through said conduit at
an injection rate which is sufficiently low such that initially
upon entrance into said void from said conduit the velocity of the
suspension is below its minimum linear velocity and at least a
portion of said solid particles are deposited to form a mound which
decreases the cross-sectional area of said void, and sufficiently
high to propel the suspension over said mound at a velocity at
least equal to its minimum linear velocity to carry particles over
said mound whereby they are deposited to increase the length and
height thereof, to form a layer of solid particles in said
void.
2. The improved method as defined in claim 1 wherein the height of
the void is substantially completely filled with said particulate
solids.
3. The improved method as defined in claim 1 wherein said conduit
comprises a substantially vertical borehole connecting the void
with a work surface.
4. The improved method as defined in claim 3 wherein the suspension
is injected through the borehole at a minimum rate calculated by
the formula V = Xd .pi. Dv wherein V is the rate of injection in
cubic feet per minute, X is a number of 3 or greater, d is the
diameter in feet of the largest solid particles in said suspension,
D is the diameter of the base of a cone formed by said particles in
said carrier liquid when motionless, said cone having a height
equal to the height to which said void is to be filled with said
particles, and v is the minimum linear velocity of said
suspension.
5. The improved method as defined in claim 4 wherein the carrier
liquid is an aqueous based liquid and said solid particles range in
size from about minus 3 to about plus 300 mesh.
6. The improved method as defined in claim 4 wherein said
suspension comprises an aqueous carrier liquid containing said
solid particles in a weight ratio of up to about 1:1.
7. The improved method as defined in claim 6 wherein the suspension
comprises an aqueous carrier liquid and sand wherein said sand
ranges in size from about minus 3 to about plus 300 mesh.
8. The improved method as defined in claim 3 wherein the suspension
comprises an aqueous carrier liquid and a particulate solid ranging
in size from about minus 3 to about plus 300 mesh and said weight
ratio of said solids to said liquid ranges up to about 1:1.
Description
BACKGROUND OF THE INVENTION
The problem of surface subsidence due to the collapse of
underground voids is as old as the mining industry itself. The
magnitude of the problems associated with mine collapse, including
a digest of some of the methods which have been employed in an
attempt to alleviate the problem, and immediate apparent needs have
been recently reported by the United States Bureau of Mines in a
report entitled "Investigation of Subsidence in Rock Springs, Sweet
Water County, Wyoming" by Donner and Whaite.
Earth strata overlying mine voids are subjected to collapse at some
point in time following the actual rock or mineral removal. The
general understanding is that once the natural support is removed
by mining, the weight of the overburdened is redistributed. Pillars
of unmined material, if insufficiently strong, eventually
disintegrate and allow overlying strata to break and fall into the
voids. Excessive extraction widths between pillars can cause the
roof over the mine area to collapse even though the pillars may be
of sufficient strength to support the additional weight. The height
of the mine void is an important factor and influences the distance
above the void that breakage occurs. If this height is great enough
caving may extend upward through the total overburden and cause
subsidence at the surface.
The time which elapses between the creation of the void and the
subsidence at the surface may vary from a few days to several years
depending on such factors as the nature of the overburden, the
depth below the surface of the mining operation, and size of the
voids created.
If a sufficient height of the void is reduced by filling the void
with a filler, e.g. sand, gravel, cement, fly ash, crushed slag,
limestone, etc. then an equilibrium of the stresses in an overlying
strata can occur before breakage reaches the surface. Complete
filling of the voids can substantially eliminate surface
subsidence.
Several methods have been developed in an attempt to fill mine
voids. These methods are generally broken down into two general
classes. One is called controlled back filling. This method can
only be employed where the underground void is accessible to
workmen which in many instances is impossible or at least highly
impractical because of cave-ins, flooding and the like.
Blind flushing is the second general method employed. In this
method several techniques have been proposed to fill a void. The
most common method has been to drill an injection hole from the
surface of the ground to connect with the void and then sluicing a
slurry of particulate material into the void by gravity flow. By
sluicing, a conical shaped bed of material is emplaced directly
under the borehole and for a very limited distance therefrom. The
area of support generally depends on the natural angle of repose of
the material in air or water, the size of the void, and the depth
of the bed. Several materials are employed in these sluicing
methods. Generally sand, gravel and fly ash are chosen. A variation
of this method is disclosed in U.S. Pat. No. 1,404,112. Another
technique is disclosed in U.S. Pat. No. 3,421,587 wherein fly ash
or another equivalent very fine particulate material is blown into
the void. The particulate material is very fine, normally of a size
such that 90 percent will pass a 50 mesh screen and 75 percent will
pass a 325 mesh screen. Another technique for emplacing a
particulate material is disclosed in U.S. Pat. No. 3,440,824. In
this method a slurry of solid material and water is pumped
downwardly through a conduit inserted in a borehole and the slurry
is physically directed towards a second borehole by means of a
variable direction nozzle attached to the lower end of the conduit
and extending into the cavity. Excess slurrying liquid, e.g. water,
is pumped outwardly through the second borehole to create a current
between the two boreholes which it is alleged aids in distributing
the solid material in the void.
The above-described methods generally represent the known
techniques which are employed in an attempt to prevent subsidence
caused by underground cavities and voids. All of these methods,
however, suffer from some disadvantage. First, the radial distance
around the borehole which can be essentially completely filled is
relatively limited. Secondly, it is usually difficult to
substantially fill the void to the ceiling. Thirdly, many boreholes
must be provided when the void to be filled extends over a great
distance. This latter disadvantage is particularly troublesome when
the void is located beneath a populated area since structures,
streets and the like prevent the drilling of a necessary number of
boreholes. For example, it has been reported in the Bureau of Mines
Report, cited previously, that in Rock Springs, Wyoming that if a
blind sluicing method was employed as many as 3,000 boreholes would
be required (as many as 75 in a single square block area) to treat
200 acres of land. Even with this many boreholes the voids cannot
be completely filled and support is provided only under the
streets, alleys and other areas of public access. Only a very
limited amount of support for structures, e.g. dwellings and the
like, can be provided.
In practicing the principles of the present invention one injection
borehole can replace as many as 75 or more boreholes required when
employing a sluicing method. Moreover, a more complete filling of
the void is accomplished.
Applicants have discovered that a subsurface void can be
substantially completely filled to the ceiling thereof with a
particulate material for an extensive radial distance surrounding a
single injection conduit. This radial distance can vary anywhere
from 100 to more than 1,000 feet around injection conduit.
Moreover, obstructions such as remaining pillars and cave-ins will
not prevent or otherwise affect the filling of the void. The area
of the void located behind pillars (not in line-of-sight from the
injection borehole) are readily filled by practicing the principles
of the present invention.
SUMMARY OF THE INVENTION
As employed herein "minimum linear velocity" is the minimum
velocity at which a suspension of particles must be conducted
through a conduit so that any substantial deposition of particles
from the suspension onto the lower portion of the conduit to form
an essentially stable layer thereon is prevented. For any given
suspension a "minimum linear velocity" can be experimentally
determined. In turn suspension means a liquid medium having
dispersed therethrough solid particles, said suspension being
provided by physical means, e.g. turbulent mixing, as opposed to
the use of thickening or gelling agents.
In the practice of the present invention an aqueous suspension of
solid particles is injected in a closed pressurized system through
a conduit into a subterranean void at an injection rate such that
the initial velocity of the suspension in the void is below its
minimum linear velocity to deposit at least a portion of the solid
particles to form a mound and said rate being sufficiently great
that upon the restriction of the cross-sectional area of the void
by the deposited solids, the velocity of the suspension over the
deposited particles increases to a value at least as great as its
minimum linear velocity to carry additional particles over the
mound to an area of greater cross-sectional area whereupon
additional particles are deposited to increase the size of the
mound.
For a background of the various investigations and experiments
relating to the flow of suspensions, e.g. the determination of the
minimum linear velocity of particular suspensions, reference may be
had to the following articles found in the literature: "Prop-Packed
Fractures - A Reality On Which Productivity Increase Can be
Predicted," E. N. Alderman and C. L. Wendorff, The Journal of
Canadian Petroleum Technology, pp 45-51, January-March, 1970; "The
Mechanics of Sand Movement in Fracturing," L. R. Kern et al.,
Journal of Petroleum Technology, pages 35-57, July 1959; "Sand
Movement in Horizontal Fractures," Harry A. Wahl et al., Journal of
Petroleum Technology, Vol. XV, No. 11, pp 1,239-1246, November
1963; "How To Handle Slurries," Richard LeBaron Bowen Jr., Chemical
Engineering, Vol. 68, pp 129-132, Aug. 7, 1961; and "Design So
Solids Can't Settle Out," J. G. Lowenstein, Chemical Engineering
Vol. 66, pp 133-135, Jan. 12, 1959.
BRIEF DESCRIPTION OF THE DRAWING
The drawing illustrates the filling of an underground void
employing the principles of the present invention wherein the
suspension is injected through a substantially vertical borehole
into a mined out void.
PREFERRED EMBODIMENTS OF THE INVENTION
In the practice of the present invention a conduit is first
provided connecting a suitable work surface with the void to be
filled. The conduit can be made by drilling a substantially
verticle bore from the surface of the earth to connect with the
void (as shown in the FIGURE) or other suitable connections can be
made, e.g., above the roof of the void or the like. The conduit is,
however, connected to the void in such a manner that a closed
pressurized system is provided between the void and the injection
equipment, e.g. pump, when the suspension is injected therein. The
work surface can be the surface of the earth, the floor of an
accessible void, e.g., tunnel, located above the void to be filled
or the like.
A suspension of solids is then prepared in any suitable manner. For
example, a particulate material, e.g., sand, and a carrying liquid,
e.g., water, brine, etc., are mixed together, e.g., in a blender
such as employed in well fracturing operations with sufficient
turbulence to provide a suspension. The suspension is then
conducted through pipes or the like, connecting the blender with
the conduit connecting the work surface with the void at a velocity
at least equal to the minimum linear velocity of the
suspension.
The particulate material can be any solid having a density greater
than the carrying liquid. The particles can range in size from
about minus 3 to about 300 mesh U.S. Standard Sieve Series. For
example, fly ash, sand, crushed slag, limestone, gravel or other
similar materials can be used. The exact composition of the solids
is not critical to the practice of the invention. Usually a
particulate material which is most readily available to the work
site is used.
The carrying liquid is preferably an aqueous based liquid, e.g.,
locally available water or brines being preferred.
The concentration of the particulate solid in the suspension is not
critical to the practice of the present invention. Generally a
concentration of about 0.5 pounds of particulate solids per gallon
of aqueous solution, to about 10.0 pounds/gallon can be employed.
The amount of solids influences the rate at which the void can be
filled employing a certain injection rate. The maximum amount is
dependent on the equipment employed to pump the suspension. For
example, a ratio of particulate material to the carrying liquid (by
weight) should not exceed about 1 to 1 for practical handling.
Preferably a ratio in the range of 1:8 to about 5:8 (solid to
liquid) is employed.
The suspension having the characteristics hereinbefore defined is
conducted through the conduit connecting the work surface with the
void at a sufficient rate that upon being ejected from the conduit
and into the void at least a portion thereof is propelled through
the void at a velocity at least equal to the minimum linear
velocity of the suspension.
The minimum rate at which the suspension must be injected can be
readily determined by employing, for example, the following formula
which is adaptable to a filling operation wherein a substantially
vertically displaced cased injection conduit is employed. The
formula is V + Xd .pi. Dv wherein V is the injection rate of the
suspension (for illustrative purposes in cu. ft/min.); X is a
number of 3 or greater (this factor relates to known relations
between particle diameter and the tendency to bridge in a confined
space); d is the diameter (in feet) of the largest particles in the
suspension; D is the diameter of the base of a cone formed by a
mound of the particles employed in the suspension having a height
equal to H (wherein H is equal to the height of the fill desired in
the void) with a natural angle of response a; and v is equal to the
minimum linear velocity of the suspension. As applied to the
filling of a void through a substantially vertical conduit, as
shown in the FIGURE, D is the diameter of the inverted cone formed
by the crater shaped mound 17, wherein H is taken to be equal to
the height of the void 10. It is evident that the diameter of the
base of the inverted cone (shown in the drawing) is essentially
equal to the diameter of the base of the cone shaped mound formed
by particles in the non-moving carrying liquid employed to suspend
the particles.
The suspension is introduced into the void until a certain desired
area of the void has been filled with particulate material to a
desired height (preferably to the ceiling thereof).
It is theorized that the void is filled with the particulate
material in the following manner. The suspension is continuously
being introduced into the void. When the suspension is first
emitted from the injection conduit and strikes the floor of the
void, the initial velocity of the suspension will drop below its
minimum linear velocity (because of the difference in
cross-sectional area of the injection conduit and void) and a
certain amount of the particulate solids will settle out to form a
stable mound on the floor of the void. The mound will continue to
increase in height until the cross-sectional area of the passageway
between the top of the mound and ceiling of the void is decreased
sufficiently that the velocity of the suspension moving over the
rim of the mound increases to equal at least its minimum linear
velocity. When the suspension reaches such a velocity particles are
carried over the rim of the mound and are deposited on the outer
edge thereof as the velocity of the suspension again drops below
its minimum linear velocity. This is a continuous condition which
is controlled by the rate at which the suspension is being
introduced into the void. The mound will continually grow in a
direction(s) away from the outlet of the injection conduit until
the distance therefrom is such that the velocity of the suspension
cannot be maintained above its minimum linear velocity.
Thus, the distance that the void can be filled is dependent on the
rate at which the suspension is injected into the void.
The minimum linear velocity for any particular suspension can
readily be determined experimentally or by employing known formulae
developed by investigations such as those taught in the references
hereinbefore cited. The minimum linear velocity for any particular
suspension can be readily determined by simple laboratory
procedures, for example, as described hereinafter.
One embodiment employing the principles of the present invention is
shown in the FIGURE. It is desired to fill the mined-out void 10
substantially to the ceiling thereof with a particulate material,
e.g. sand. The height of the void designated by the letter H. An
injection conduit 11, e.g., ranging in size from about 16 to 20
inches in diameter is provided, e.g., by drilling, to connect the
void 10 with the working surface 12, in this instance the surface
of the ground. The borehole is also normally cased with inner pipe
11a, e.g., ranging in size from about 12 to 14 inches in diameter.
The size of the injection conduit 11 is only limited by the
equipment which is available to mix and inject the suspension. The
system is connected in such a manner that when the suspension is
being injected a closed pressurized system results. An aqueous
suspension of a particulate material is injected, e.g., by pumping
through the conduit 11 and into the void 10. The mixing, supply
sources, e.g., sand and water, and injection equipment are
schematically shown in the FIGURE as 13, 14, 15 and 16,
respectively. As the suspension is injected down the conduit 11 at
a certain predetermined critical injection rate a donut shape mound
17 of particulate material is formed. As the mound increases in
height the distance (h) (and cross sectional area) between the rim
of the mound 17 and the ceiling 18 of the cavity decreases causing
the linear velocity of the suspension to increase to a value above
its minimum linear velocity and particulate material will be
carried over the rim of the mound. Thereafter the linear velocity
of the suspension will again drop below its minimum linear velocity
causing a further settling of material with a subsequent growth in
the radial size and height of the mound. The mound progresses out
radially from the injection conduit 11.
The suspension is injected for any desired length of time, or until
a certain desired area of the void is filled with the particulate
material, or until the distance from the borehole is such that
there is insufficient available hydraulic horsepower to extend the
mound further.
As previously indicated the rate at which the suspension is
injected through the injection conduit 11 is critical to the
practice of the present invention and, for example, can be
calculated for any fill material employing the following formula: V
= Xd .pi. Dv.
As can be seen the critical value V is not dependent upon the size,
i.e., crossectional area, of the injection conduit 11. However,
from a practical standpoint the hole cannot be so large that there
is inadequate horsepower to inject the suspension at the required
rate or so small that sufficient pressure is built up to fracture
the formation around the hole or in the void.
For illustrative purposes, the application of the above-defined
formula employing a suspension of sand and water to fill a void
having a height (H) of 6 feet is defined hereafter. Injection rates
V for various size ranges of sand are calculated and set forth in
the following Table I. The angle of repose and minimum linear
velocity v of the suspension were experimentally determined. The
value for V is calculated for an X value of 3 and 10. The minimum
linear velocity of the suspension (v) was determined experimentally
by pumping a suspension of sand and water (in a concentration of
2.5 pounds of sand/1 gallon of water) through a 4 inch inside
diameter pipe held in a substantially horizontal position. The
velocity of the suspension was adjusted until a layer of sand
started to deposit on the bottom of the pipe. The velocity of the
suspension was then increased until no further sand was deposited
and the velocity at this point was taken to be equal to the minimum
linear velocity of the suspension. Since the minimum linear
velocity can vary depending on the size and density of the
particles, the density of the liquid and other similar parameters,
the minimum linear velocity of any particular suspension should be
determined prior to practicing the present invention. ##SPC1##
The equipment employed to practice the present invention should be
capable of blending and injecting a suspension of sand and water at
a rate of at least up to 300 barrels per minute. As indicated
blending and injection equipment of the type commonly employed in
hydraulic fracturing operations, e.g., in oil and gas wells can be
used. Centrifugal type pumps are preferred since they are the most
efficient presently known means for obtaining the necessary volumes
and pressures. However, positive displacement pumps, e.g. piston
pumps, can be employed.
* * * * *