Well treatment with artificial matrix and gel composition

Hydrocarbon bearing earth formations are treated to reduce formation permeability and fill void spaces by injecting a polymer gel solution having an artificial matrix material dispersed therein and comprising beads of materials such as polystyrene and polycarbonate having densities substantially the same as the density of the polymer gel to improve distribution of the artificial matrix material into such void spaces.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention pertains to treating an earth formation in the 
vicinity of a wellbore by introducing an artificial matrix into void 
spaces in the formation together with a polymer gel to improve the 
consolidation or conformance of the formation and control the permeability 
thereof. 
2. Background 
In fluid production or injection wells, it is often necessary to reduce the 
permeability of certain portions of the earth formation in communication 
with the well to control the flow of fluids to or from the well. One 
technique which is employed to improve the conformance and reduce the 
permeability of earth formations in the vicinity of hydrocarbon fluid 
production wells is to inject a polymer type gel composition which forms a 
continuous single phase gel in the formation of sufficient strength and 
stability to effectively reduce the permeability of the formation to 
control flow of fluids in a desired direction or to prevent unwanted flow 
of fluids from certain portions of the formation. 
In many instances, the formation consists of a matrix and certain 
anomalies. The anomaly is generally a void space in the formation having, 
of course, very high permeability relative to the formation matrix. 
Anomalies may include fractures, vugs, channels, caverns, wash-outs or 
cavities whereas the formation matrix is substantially the remainder of 
the formation volume characterized as essentially homogenous, continuous 
sedimentary material. In attempting to reduce the permeability of certain 
formation zones, the anomalies may present problems when the permeability 
reduction technique involves the injection of certain cross-linked polymer 
type gels, for example. Conventional polymer gels usually do not have 
sufficient strength to occupy an anomaly and resist the incursion of 
fluids into and through the anomaly. Since void spaces or "voids" often 
occur in the vicinity of perforations in a fluid production well, in order 
to plug these perforations or reduce the permeability of the formation in 
the vicinity of these perforations, filling the void with the gel and 
causing the gel to permeate into the natural matrix around the void space 
is not sufficient to provide a competent permeability reducing or plugging 
operation. The present invention has been developed with a view to 
improving the use of polymer gels and similar formation plugging or 
permeability altering compositions and associated processes. 
SUMMARY OF THE INVENTION 
The present invention provides an improved method for plugging or reducing 
the permeability of certain portions of earth formations in the vicinity 
of fluid producing or fluid injection wells wherein void spaces in the 
formation in communication with the well are filled with an artificial 
matrix material which is carried into the formation entrained in a 
suitable fluid. 
In accordance with an important aspect of the invention, treatment of earth 
formations in the vicinity of a wellbore with polymer type gel 
compositions is improved by placing an artificial matrix of particles or 
"beads" of solid material into voids or spaces in the formation which are 
in fluid flow communication with the wellbore through which the polymer 
gel is injected. The artificial matrix material is preferably of a density 
substantially equal to the density of its carrier fluid so that the 
material will not settle nor float and leave unwanted void spaces occupied 
only by the gel material. 
In one embodiment of the invention, a polymer gel is injected into a 
formation zone of interest, which also includes a void in communication 
with a wellbore, and wherein the polymer gel is injected into the void 
through the wellbore with an artificial matrix material of small plastic 
spheres entrained in the gel solution. The artificial matrix within the 
gel provides additional strength and support for the gel composition when 
filling void spaces and cavities in the formation. The plastic sphere 
artificial matrix material may be selected to be included in a variety of 
different polymer gels to maintain the equal or near equal density 
requirement. 
The artificial matrix material can be added to the polymer gel and its 
cross linking fluid solution in a mixing tank and injected after a normal 
injection of polymer gel without matrix material has been performed or the 
matrix material can be injected into the void spaces or cavities with 
another carrier fluid prior to injection of the polymer gel solution. The 
packed matrix material will still allow the polymer gel solution to pass 
into and through the matrix material to penetrate into the formation 
matrix. 
The present invention further contemplates a process of modifying the 
permeability of or plugging an earth formation in the vicinity of a 
wellbore by conditioning the well to accommodate a particular polymer gel 
composition followed by establishing the injectivity characteristics of 
the formation, followed by introducing a slurry of the artificial matrix 
material in a carrier fluid into the formation including, in particular, 
any cavities or voids desired to be filled and then pumping into the 
formation a sufficient volume of polymer gel to fill the pore spaces 
required including at least the pore volume of the artificial matrix. 
Finally, the well is then "shut in" for a sufficient period of time to 
allow the gel to set before the well is returned to fluid production or 
injection processes. 
Those skilled in the art will recognize the above-described features and 
advantages of the invention together with other superior aspects thereof 
upon reading the detailed description which follows in conjunction with 
the drawing.

DESCRIPTION OF A PREFERRED EMBODIMENT 
Processes have been developed for modifying the permeability of relatively 
high permeability regions of subterranean hydrocarbon bearing formations 
to improve sweep efficiencies of fluids flowing through the formation and 
to prevent incursion of unwanted fluids into the production zone of the 
formation, for example. One method of treating formations comprises 
injecting a polymer solution which will form a gel-like substance upon 
penetrating the formation to decrease its permeability. Certain types of 
polymers may be mixed with a cross linking agent and pumped into the 
formation zone of interest through a wellbore whereupon, after a 
predetermined period of time, the reaction between the polymer and the 
cross linking agent will form a gelled substance which will permanently 
and effectively reduce the formation permeability in the desired area or 
zone. U.S. Pat. No. 4,683,949, issued Aug. 4, 1987 to Sydansk, et al, 
discloses and claims certain high molecular weight, water soluble polymer 
such as acrylamide polymers which are reactable with a chromium 
carboxylate complex capable of cross linking the polymer and an aqueous 
solvent. Other gel systems may be adapted for use with the present 
invention, such as sulfonated acrylamide copolymer-aluminum citrate and 
the like. The gel components may be combined at the surface and injected 
into the desired treatment zone to form a continuous single phase gel. The 
gel may be tailored to a specific subterranean application to provide the 
desired gelation rate, resultant gel strength and stability to meet the 
demands of the treated area. 
One problem with using polymer gels and other permeability and stability 
enhancing substances known in the prior art is the inadequate strength of 
these materials when occupying a void space in the formation. If formation 
fluid pressures are sufficient, the formation matrix bordering the void 
space will tend to collapse or allow fluids to penetrate into the void 
space to defeat the purpose of the gel treatment. A similar problem has 
been recognized and a solution presented therefor in application Ser. No. 
07/976,602, filed Nov. 16, 1992 in the name of Joseph H. Schmidt, et al, 
and assigned to the assignee of the present invention. In the Schmidt, et 
al application, formation void spaces are treated for plugging with 
Portland cement mixtures by filling the void space with resin coated 
hydraulic fracturing proppant comprising basically a resin coated sand of 
a predetermined grain size. 
However, the use of a material such as sand, cement particles and hydraulic 
fracturing proppants may not be suitable since these materials are usually 
more dense than the carrier fluid and have a tendency to settle out of the 
fluid stream, thereby rendering the filling of formation void spaces 
incomplete and tending to cause plugging of certain small cross sectional 
area flow paths before the void space filling process can be completed. 
Thus, the use of these types of solids materials is not considered 
appropriate for developing an artificial matrix for use with polymer gel 
type well treatment systems, in particular. 
In accordance with the present invention, it has been discovered that 
efforts to reduce the permeability of certain formation zones of interest 
that may include certain void spaces is enhanced by the use of particulate 
solids which have specific gravities or densities substantially the same 
as the carrier fluid which is used to place the artificial matrix in the 
void spaces which are to be filled. In particular, it has been determined 
that an improved treatment process is obtainable when using high molecular 
weight, water soluble acrylamide polymers which are cross linked with a 
chromium/carboxylate complex to form a gel which modifies the formation 
permeability in the zone of interest, particularly when the formation is 
highly permeable or includes one or more voids. In particular, it has been 
determined that particulate solids comprising spherical particles of 
polystyrene and polycarbonate materials having diameters in the range of 
about 400 microns to 2.0 millimeters may be mixed with certain carrier 
fluids, including solutions of polymer and a cross linking agent, and 
injected into wellbores to fill the void spaces in the vicinity of the 
wellbore to provide an artificial matrix in the void spaces. Polystyrene 
spheres or beads with a density of from about 1.04 g/ml to 1.05 g/ml are 
considered suitable candidates for use with polymer gel materials of the 
type described in U.S. Pat. No. 4,683,949 as well as the polymer gels 
described in U.S. Pat. Nos. 4,724,906, issued Feb. 16, 1988 to R.D. 
Sydansk, 4,730,675, issued Mar. 15, 1988 to Wygant, et al, 4,968,442, 
issued Nov. 6, 1990 and 5,010,954, issued Apr. 30, 1991 to D.O. Faulk, and 
5,082,057, issued Jan. 21, 1992 to Sydansk, all of which are incorporated 
herein by reference. The densities of certain ones of the aqueous polymer 
gel solutions described in these patents may vary from 1.0 g/ml to 1.5 
g/ml. Accordingly, the aforementioned polystyrene and polypropylene 
compositions are suitable for use with these aqueous polymer gel solutions 
at the low end of the density range (0.90 g/ml to 1.05 g/ml) and for 
higher density injection fluids of the same family, artificial matrix 
beads made of materials such as polycarbonate may be used wherein the 
density of the bead material is in the range of 1.35 g/ml to 1.45 g/ml. 
Other bead materials which may be suitable include polyethylene, 
polypropylene, nylon, polyvinyl chloride, fluorocarbons, polyurethanes and 
polybutadiene. This range of materials will permit adaptation to different 
density and bead strength requirements. The chemical inertness of the 
proposed bead materials is also advantageous. The concentration of the 
matrix material should be limited to about 50% by weight of the mixture of 
matrix material and gel composition to preserve flowability and 
pumpability. 
Polystyrene and polycarbonate beads are also advantageous for use as the 
artificial matrix composition in that these materials are somewhat in the 
category of commodities that may be purchased in bulk quantities and in an 
assortment of spherical bead sizes. For example, polystyrene particles 
having bead sizes ranging from 400 microns to 2.0 millimeters are used in 
making a wide variety of polystyrene foam products. Moreover, if the 
density of the artificial matrix bead material is not matched to the 
predetermined polymer gel solution and the density difference is large 
enough that significant settling or floating of the bead material may 
occur, the solution density may be varied somewhat without affecting the 
gelation properties. For example, it is possible to dilute the 
aforementioned types of polymer gel solutions with water to reduce 
density, or increase the solution density by adding more polymer to the 
solution and/or certain salts such as sodium chloride, potassium chloride 
and calcium chloride. 
The incorporation of the artificial matrix bead material into polymer gels 
may be easily applied in the field as part of a conventional polymer gel 
treatment process. Conventional polymer gel injection processes comprise 
mixing an aqueous solution of the polymer and the cross linking agent at 
the surface and then pumping the mixture down the wellbore through coiled 
tubing, drill pipe or other pipe strings disposed in the wellbore wherein 
the solution then flows into the formation matrix and/or void through 
perforations in the wellbore casing, for example. When it is known that 
certain void spaces exist in the vicinity of or in fluid flow 
communication with the wellbore, the treatment process may be modified by 
mixing into the polymer gel mixture the artificial matrix material such as 
the aforementioned plastic beads of the appropriate particle size and 
density. These matrix materials may be added to mixing equipment used to 
mix the polymer and its cross linking agent and then pumped into the 
wellbore. If settling or floating of the matrix material is observed, the 
polymer solution may be modified by lowering its density, such as by 
adding water, or increasing its density, such as by adding more polymer 
and/or salt. 
Preliminary laboratory experiments with polystyrene beads indicate that it 
is desirable to use beads which do not incorporate any "blowing agent" 
such as pentane and the like in the event that the beads are exposed to 
elevated temperatures. Moreover, pre-wetting polystyrene beads with a 
small amount of detergent solution, such as Alconox or a similar 
laboratory grade soap, before placing them in the polymer solution aids in 
the distribution of the beads into the polymer while preventing small air 
bubbles from attaching to the surface of the beads and causing fewer beads 
to float in the polymer solution. The addition of sodium chloride to the 
polymer solution also aids in sustaining suspension of polystyrene beads 
in the solution. Shear pressure tests and equilibrium displacement 
pressure tests on a polystyrene bead/polyacrylamide gel mixture having a 
1:1 weight ratio of beads to gel composition indicate acceptable shear 
pressures and displacement pressures compared with gel composition alone. 
Preliminary tests also indicate that the density of some polymer gel 
solutions is not uniform. Accordingly, it may be desirable to provide 
artificial matrix beads of different sizes or densities in the solution to 
assure that a sufficient quantity occupies the void space without 
settling. 
The present invention contemplates that artificial matrix materials of the 
type mentioned above may be delivered to voids or cavities in wellbore 
penetrated earth formations prior to injecting the polymer and cross 
linking agent solution which will eventually gel in the formation, 
including the artificial matrix portion. The beads may, for example, be 
injected with water, seawater or brine as a carrier liquid prior to 
injection of the polymer gel solution. In this regard, the beads may be 
incorporated in a wellbore conditioning treatment fluid, such as water, 
seawater or brine, to cool down the formation zone of interest prior to 
injecting the polymer gel so that the polymer gel solution will adequately 
penetrate the formation matrix in the zone of interest before gelation 
occurs. Moreover, the artificial matrix material may be injected into the 
formation zone of interest during injectivity tests using fluids such as 
water, brine or diesel fuel. Lastly, of course, the artificial matrix 
material may be mixed in with the polymer gel solution and pumped into the 
formation void spaces in sufficient volume to fill the spaces with the 
artificial matrix material. 
Referring now to the drawing figure, there is illustrated a schematic of a 
well 10 penetrating an earth formation 12 including a zone of interest 14 
which is to be treated in accordance with the present invention. The well 
10 extends from the earth's surface 13 and is provided with a conventional 
casing 15 and a wellhead 16 from which depends a tubing string 24. The 
tubing string 24 is connected to a conventional packer 25 and terminates 
in the vicinity of plural perforations 26 which penetrate casing 15 and 
open into a void space 28, 29 which is shown already subjected to 
treatment in accordance with the present invention. The void spaces 28, 29 
are shown filled with an artificial matrix material 30 comprising the 
aforementioned particles or beads of suitable plastic, for example, having 
a density similar to the density of the fluid which is used to carry the 
matrix material into the void spaces. Boundary lines 32, 33 are 
illustrated as indicative of the penetration of the polymer gel solution 
into the formation matrix beyond the boundary of the void spaces 28, 29. 
The drawing figure also illustrates a source of artificial matrix material 
comprising a hopper 40 which may be placed in communication with a 
selected one of mixing tanks 42 and 44, which, in turn, may be selectively 
placed in communication with a pump 34 for pumping a mixture of the 
artificial matrix material with a carrier liquid through the tubing string 
24, the wellbore space 46 and the perforations 26 into the void spaces 28, 
29. The carrier fluid may be any one of the liquids previously mentioned 
and with the particularly desired feature of having the same or 
approximately the same density as the matrix material 30. 
Accordingly, when a conformance or permeability modifying fluid is selected 
which will effectively reduce the permeability of the zone of interest 14 
within the boundaries of the boundary lines 32, 33, an artificial matrix 
material comprising, for example, polystyrene beads or polycarbonate 
beads, in the spherical size range mentioned above may be selected for the 
matrix material and having a density approximately the same as the 
selected polymer gel. A sufficient quantity of the polymer material in an 
aqueous solution, together with the cross linking agent, is then mixed in 
the tank 42, for example, and a quantity of matrix material 30 is also 
mixed in with the polymer gel carrier fluid. This mixture is then pumped 
into the spaces or voids 28, 29 under pressure. The polymer gel solution 
will permeate into the formation zone 14 at least to the exemplary 
boundary lines 32, 33 while the uniformly mixed and dispersed matrix 
material 30 substantially fills the void spaces 28, 29. When a 
predetermined sufficient quantity of polymer gel and matrix material has 
been injected into the formation, the well is then shut in a sufficient 
length of time to allow the cross linked polymer to set whereupon the 
quantity of material remaining in the tubing string 24 and the space 46 
may then be flushed from the well by a gel breaker and evacuation fluid 
which may be injected into the space 46 through coilable tubing, not 
shown, for example. 
The formation zone to be treated may require certain other conditioning 
operations. For example, if the formation temperature is one which would 
accelerate the setting of the polymer gel, it may be necessary to inject a 
cool-down treatment fluid into the formation, such as water, to lower the 
formation temperature in the area delimited by the boundary lines 32, 33 
to a temperature which will allow proper injection and filling of the void 
spaces 28, 29 before the gel sets. Moreover, it may be necessary to 
establish the injectivity rate and pressure required for injecting the 
polymer gel and/or the polymer gel matrix mixture before that step is 
carried out. In other words, a fluid such as water, seawater or diesel 
fuel may be injected into the formation to determine the injectivity rate 
that the formation will accept without exceeding the fracture pressure at 
any time during the treatment. 
Once the above-mentioned preliminary steps are carried out, if they are 
necessary, a slurry of the formation treatment material such as the 
aforementioned type of polymer gel with the entrained neutrally buoyant 
artificial matrix material is pumped into the formation through the 
perforations 26. Alternatively, a quantity of artificial matrix material 
30 may be pre-mixed with a suitable carrier liquid such as water or 
seawater in the tank 44 and injected into the void spaces 28, 29 prior to 
injection of the polymer gel until these spaces are full of matrix 
material. In this way, the polymer gel may then be injected in a 
conventional manner since the formation is now rid of any void spaces 
which might adversely affect the quality of the polymer gel treatment. 
Although a preferred embodiment of the present invention has been described 
in some detail herein, those skilled in the art will recognize that 
various substitutions and modifications may be made to the method 
described without departing from the scope and spirit of the invention as 
recited in the appended claims.