Method and fluid for placing resin coated gravel or sand in a producing oil well

Disclosed are fluid and method for suspending resin coated sand in order to place the sand adjacent to a production well for the purpose of forming a permeable consolidated gravel pack. The fluid contains a viscosifying amount of hydroxyethylcellulose, sufficient fluorescent dye to increase the viscosity of the fluid, sodium chloride, and an acid forming component such as phthalic anhydride or succinic anhydride. As fluid containing the resin coated gravel particles is pumped down the injection string and positioned where it is desired to form the consolidated gravel pack, the acid forming material slowly reacts with water to form an acid, reducing the pH of the fluid, and thereby reducing the viscosity of the carrier fluid which facilitates the resin coated sand grains coming together in order to form the desired gravel pack.

FIELD OF THE INVENTION 
This invention relates to a novel fluid and a method using the fluid for 
suspending gravel or sand which is coated with a resin during the time it 
is formulated on the surface of the earth and injected into and 
transported down on an injection string in a producing wellbore for 
placing adjacent to the perforations. The improved carrier fluid exhibits 
sufficient viscosity to assist in suspending the resin coated sand grains 
and to prevent abrasive removal of the resin from the sand grains during 
the time the resin coated sand grains are mixed in the carrier fluid and 
pumped down an injection string into the site where consolidation of the 
coated sand grains is to be accomplished. The fluid includes a trigger 
mechanism which causes the viscosity of the suspending fluid to be reduced 
after the period of time required to locate the sand grains where they are 
to be consolidated in the production interval. 
BACKGROUND OF THE INVENTION 
One of the problems frequently encountered during the course of producing 
petroleum and other fluids from subterranean formations is the inadvertent 
flow of sand or other mineral particles from the petroleum formation along 
with the formation fluid being recovered from the producing well. This is 
a well recognized problem and many undesirable consequences result from 
this phenomena. The flow of sand into the wellbore causes the creation of 
a cavity or zone of greatly weakened formation around the perforation of 
the wellbore, which in severe instances will cause the formation to 
collapse and destroy the casing and cause loss of the production well. 
Also, continued production of fluid with simultaneous flow of sand or 
other abrasive mineral particles will cause the wellbore itself to become 
plugged which reduces the rate of production of oil from the well and in 
severe instances will completely terminate further production. Finally, 
the sand being produced and transported to the surface of the earth along 
with formation fluids causes abrasive wear of tubular goods and pumps 
utilized in producing wells, which cause early failure of these expensive 
components. 
The above described problems have long been recognized and many procedures 
have been disclosed in the prior art for forming barriers to the flow of 
consolidated sand from formations during the production of formation 
fluids. Most of these procedures have the common feature of forming a 
stable permeable zone around the production well which permits fluids to 
flow freely through the barrier, while restraining the flow of particulate 
matter such as sand or other particles. One particularly successful 
technique is referred to in the art as gravel packing, and it utilizes 
granular materials such as sand or gravel which is placed in a cavity or 
washed out zone adjacent to the production perforations of a producing 
well. The gravel placed in the well is restrained from itself moving along 
with the formation fluids either by a mechanical screen or by cementing 
the sand grains together using a polymerizable resin to form a solid 
permeable mass to completely fill the washed out zone in order to form the 
sand restraining, fluid permeable barrier around the production well. 
The use of thermal stimulation techniques has imposed even more severe 
requirements on sand control techniques utilized in wells completed in oil 
fields being stimulated by thermal means, e.g. by injection of steam or 
other hot aqueous fluids into the formation, because the passage of the 
hot aqueous and frequent by alkaline fluids through the resin-consolidated 
sand mass causes rapid destruction of many resin matrices used to bind the 
sand or gravel particles together. 
U.S. Pat. Nos. 4,427,069 and 4,428,427 introduced the concept of employing 
a durable resin formed by polymerizing furfuryl alcohol oligomer on the 
sand grains for the purpose of forming a stable consolidated sand mass for 
purposes of controlling sand flow into the producing well. In my pending 
application Ser. No. 07/135,162 filed Dec. 18, 1987, for "Consolidatable 
Gravel Pack Method," there is disclosed a method for suspending resin 
coated gravel or sand in an aqueous saline carrier fluid to prevent the 
unpolymerized resin coated particles from sticking together during the 
time they are being transported into the formation in order to form a 
consolidated gravel pack sand control barrier in the formation adjacent to 
a producing well. In this application, it was noted that during the time 
the resin coated sand particles are transported into the zone where the 
permeable mass is to be formed, abrasive removal of the resin from the 
sand particles is prevented by incorporation in the carrier fluid of a 
viscosifying amount of a hydrophilic polymers such as 
hydroxyethylcellulose. 
In applying the above feature, it has been found that the resin coated sand 
particles can indeed be transported from the mixing means on the surface 
of the earth via a tubing located in a producing well and placed where 
they are desired in a cavity or zone adjacent to the producing 
perforations in an oil well, without removal of a significant portion of 
the resin coating on the sand particles during the time of mixing and 
transportation. The viscosity of the carrier fluid becomes a hindrance 
once the fluid comprising the carrier fluid and the resin coated particles 
has been located in the well, in that it delays or restricts the proper 
packing of the resin coated particles in the producing well necessary for 
subsequent polymerization to produce the desired strong, durable, 
permeable sand controlling zone. 
In view of the foregoing discussion, it can be appreciated that there is a 
substantial, unfulfilled commercial need for a technique for suspending 
resin coated gravel or sand particles during the time they are mixed and 
transported via an injection string in a production well to the zone where 
it is desired to form a consolidated permeable gravel pack for the purpose 
of restraining the undesired flow of sand, while later allowing the 
resin-coated gravel or sand particles to pack closely together in a manner 
which enhances the formation of a dense, durable, strong permeable 
consolidated mass such as is necessary to control sand migration during 
production operations. Ideally, what is needed is a fluid which exhibits 
sufficient viscosity to prevent abrasive removal of polymer from the 
gravel particles during transport, which fluid automatically exhibits a 
significant reduction in viscosity after the resin coated granular 
material has been placed in the zone where the consolidated mass is to be 
formed. 
The present invention is directed to a novel fluid composition and methods 
employing this fluid whereby the above mentioned unfulfilled need and 
objective may be satisfied. The invention is directed toward formulating 
of a fluid which exhibits sufficient viscosity that it permits 
transportation of the resin coated gravel into the zone where the 
permeable mass is to be formed, after which the fluid viscosity is 
automatically reduced to a significant extent, permitting the resin coated 
sand grains to come together so they may form a uniform and continuous 
resin matrix bonding the sand or gravel particles together. 
SUMMARY OF THE INVENTION 
Applicants' invention pertains to fluid compositions and methods employing 
these fluid compositions whereby resin coated sand may be suspended in a 
viscous carrier fluid which prevents or reduces abrasive removal of resin 
from the sand or gravel during the time it is mixed on the surface of the 
earth and introduced into a subterranean zone where a permeable 
consolidated mass is to be formed. The fluid exhibits sufficient viscosity 
to protect the resin coated sand or gravel. The fluid contains an acid 
forming substance, such as an acid anhydride which reacts slowly with the 
water component of the carrier fluid causing a reduction in pH of the 
fluid. The viscosifying material is a mixture of hydroxyethylcellulose and 
a viscosity increasing additive which is a fluorescent dye. The presence 
of fluorescence dye increases the viscosity of the fluid containing 
hydroxyethylcellulose significantly, but the effect is eliminated entirely 
when the pH drops to a value of about 5 or less. A particularly preferred 
material for reducing the pH in the time desired, which is from two to 
four hours after mixing, is a slow reacting acid anhydride such as 
phthalic anhydride. Any other acid forming compound may be utilized, and 
the time for pH loss and hence viscosity reduction can be controlled by 
the choice of anhydride. Succinic anhydride is somewhat faster than 
phthalic anhydride. Higher molecular weight acyl halides may also be used. 
The concentration of acid anhydride should ordinarily be in the range of 
1.5.times.10.sup.-3 to 4.times.10.sup.-3 and preferably 
1.5.times.10.sup.-3 to 2.times.10.sup.-3 percent by weight. A preferred 
fluid for our purpose would contain from 0.17 to 0.22 percent by weight 
hydroxyethylcellulose, from 0.09 to 0.11 percent by weight fluorescent dye 
and from 1.5.times.10.sup.-3 to 2.times.10.sup.-3 percent by weight of 
phthalic anhydride or other acid anhydride.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Our invention is concerned with a fluid which contains a polymer to 
increase the fluid viscosity sufficiently to permit pumping a suspension 
of resin coated sand or gravel or other mineral particles in the fluid 
through an injection string located in a production wellbore without loss 
of significant amounts of resin from the surface of the resin coated 
particles. This fluid and the methods described herein may be utilized in 
connection with any type of resin coated fluid, but it is contemplated 
that it is especially suitable for use in connection with a sand control 
method employing the resin coated gravel described in our co-pending 
application 07/135,162 filed Dec. 18, 1987, the disclosure of which is 
incorporated herein by reference. In that disclosure a technique is 
described for coating sand or gravel with furfuryl alcohol which has been 
first mixed with an internal catalyst such as orthonitrobenzoic acid and a 
suitable solvent, preferably butyl acetate which assist in the subsequent 
polymerization reaction. These resin coated particles are suspended in an 
aqueous medium which contains sodium chloride, preferably saturated with 
sodium chloride. The sodium chloride carrier fluid also contains a 
viscosifying amount of a suitable hydrophilic polymer, and our preferred 
polymer is hydroxyethylcellulose, a non-ionic ether of cellulose which is 
soluble in hot or cold water, but insoluble in organic solvents. It is 
stable in concentrated salt solutions and nontoxic. The material may be 
obtained under the commercial name of NATROSOL.RTM. available from 
Hercules, Inc. from Wilmington, Del. The especially preferred product is 
NATROSOL.RTM. 250HHR. The 250 designation on this trademark indicates a 
hydroxyethyl molar substitution ratio of 2.5 and the HHR is an indication 
of the viscosity type. 
This fluid requires that the viscosity enhancing effect of the polymer be 
enhanced by incorporation of a fluorescent dye or other optical brightener 
in the fluid. For reasons that are not entirely understood, the viscosity 
of the solution of hydroxyethylcellulose is increased significantly if the 
fluid also contains a small amount of fluorescent dye material. Commercial 
products suitable for this purpose include UNITEX RSB or TINO CVS by 
Ciba-Geigy Corporation of Ardsley, N.Y. TINO CVS is a distyryl biphenyl 
derivative, specifically 2,2'-1,1'-biphenyl-4,4,-diyldi-2,1-ethenediyl 
bisbenzenesulfonic acid, disodium salt. These particular dyes are anionic 
and have a solubility in distilled water of 25.sup.x g/l at 25.degree. C., 
and 300 g/l at 95.degree. C. These and other commercially available 
fluorescent dyes increase the viscosity of solutions of 
hydroxyethylcellulose by a substantial degree, especially if the fluid pH 
is carefully controlled to the range of from 6 to 8. Another useful dye 
material which applicants have found to be suitable for this purpose is 
marketed under the trademark BLANCOPHOR SV by BASF-Wyandotte. 
A fluid suitable for use as the viscous carrier fluid of our invention 
comprises water having dissolved therein from 0.2 to 1 percent by weight 
of hydroxyethylcellulose and from 0.8 to 1.2 percent by weight fluorescent 
dye. Persons skilled in the art of using such fluids will of course 
recognize that more polymer is required to produce the necessary viscosity 
at relatively higher temperatures, and so adjustments either within or 
beyond the above range may be necessary. The objective of incorporating 
the viscosifying amount of hydrophilic polymer in the carrier fluid of our 
invention is to support the resin coated particles and to ensure that the 
minimum abrasive removal of resin from the gravel particles occurs during 
the time the fluids are mixed and pumped down the injection string of the 
producing well during their placement in the formation adjacent to 
perforations of the producing well. 
The above described aqueous fluid containing hydroxyethylcellulose and the 
fluorescent dye exhibits a most unique viscosity response to changes in 
pH, and it is this unusual response that is utilized to achieve the 
desired viscosity change of the fluid of the present invention. The 
viscosity of an aqueous fluid containing the above described amounts of 
hydroxyethylcellulose and fluorescent dye is in the range of 200 to 400 
centipoise at the original pH, which is usually from 6 to 8, but decrease 
viscosity as the pH falls to values less than 6 is very dramatic, as can 
be seen in the experimental section below. 
It is desired that the pH reducing additive which causes the reduction in 
viscosity at the appropriate time after the resin coated fluid has been 
pumped through the producing well and placed in the producing formation, 
cause the pH of the fluid to drop from a value in the range of from 6 to 8 
to a value of 5 or less, which is sufficient to cause the sharp reduction 
in fluid viscosity desired for this particular application. Ordinarily, 
the amount of time required to pump the slurry comprising the aqueous 
carrier medium in the resin coated particles down the production well and 
locate them in the desired position adjacent to the perforation for the 
producing well is in the range of from one to four hours, and usually 
around two to three hours. Accordingly, the pH reducing additive should be 
one which will react slowly to form an acid, thereby reducing the pH in a 
time interval in the range of two or 3 hours, such that after the fluid 
with the suspended resin coated particles has been pumped into the desired 
location, the fluid viscosity will be reduced which permits the resin 
coated particles to settle closer together which is desired in order to 
form a dense strong permeable mass around the producing interval. Since 
the resin utilized in the above described procedure is acid catalyzed, the 
increase in pH in the aqueous fluid surrounding the resin coated particles 
will enhance the polymerization reaction, but only after the reaction of 
the additive has proceeded to the extent that the pH drops into the acidic 
range. 
Any substance which reacts slowly with water to form acid may be utilized 
for the pH reduction necessary to cause drop in viscosity in order to 
achieve the desired effect in the fluid in the method of our invention. 
Acid anhydrides may be used for this purpose, although the anhydride 
chosen must be one which reacts sufficiently slowly that the pH reduction 
will not occur until after the carrier fluid containing the suspended 
resin coated particles has been placed in the well. Relatively low 
molecular weight acid anhydride such as acetic anhydride or propionic 
anhydride react much too quickly for our purpose. Higher molecular weight 
mono basic organic acid anhydrides can sometimes be used for this purpose. 
Polybasic acids, especially the high molecular weight anhydrides, such as 
succinic acid, glutaric acid or adipic acid may be used. Any polybasic 
acid having the following formula: HOOC-(CH.sub.2)n COOH where n is from 3 
to 7 may be reacted to form a cyclic anhydride which will be suitable for 
use in the process of our invention. 
An especially preferred anhydride for use in our process are the cyclic 
anhydrides of dicarbyoxylic aromatic acids. In particular, the cyclic 
anhydride of orthophthalic acid is the especially preferred anhydride. 
Since phthalic anhydride is relatively insoluble, even the low 
concentrations required in our application may involve some undissolved, 
dispersed phthalic anhydride. As the material hydrolyzes to form an acid, 
the undissolved anhydride will be dissolved in the aqueous fluid and so 
all of the phthalic anhydride will eventually be utilized after the fluid 
has been formulated and pumped into the location where the permeable mass 
is desired to be formed. We have found that the desired change in pH will 
be achieved if the concentration of the acid forming material, 
specifically, our preferred embodiment, phthalic anhydride is from 
1.0.times.10.sup.-3 to 4.0.times.10.sup.-3 and preferably 
1.5.times.10.sup.-3 to 2.times.10.sup.-3 percent based on the total weight 
of the fluid. 
EXPERIMENTAL 
For purpose of demonstrating the operability of our process and the results 
obtained by its use, the following laboratory tests have been performed. 
The following method was used to test whether the addition of a small 
amount of polymer would reduce the abrasive removal of resin from coated 
sand. A 50 g aliquot of &lt;40 mesh coated sand was placed in an Osterizer 
blender with 300 g of water. The sample was then mixed at "beat" speed for 
5 minutes. The sample was then rinsed, sieved, dried, and weighed. This 
procedure was repeated 3 times by placing the water with 300 g of each of 
the following:.sup.(1) 0.2% hydroxyethylcellulose (HEC) solution 
[Hercule's NATROSOL.RTM. 250], .sup.(2) 0.2% H.E.C. +0.1% dye [Geigy's 
UVITEX RSB], and (3) 0.3% H.E.C.+0.15% dye. The results are listed in 
Table 1. 
TABLE 1 
______________________________________ 
Abrasion Test Results 
Weight Weight 
Before After 
Fluid Viscosity Mixing Mixing % Lost 
______________________________________ 
Water 1.0 cps 50.01 g 47.02 g 5.98 
(1) 0.2% H.E.C. 30 cps 50.00 g 48.14 g 3.72 
(2) 0.2% H.E.C. 210 cps 49.97 g 48.44 g 3.06 
+ Dye 
(3) 0.3% H.E.C 400 cps 49.98 g 48.02 g 2.12 
+ Dye 
______________________________________ 
Samples of coated sand abraded in the kitchen blender with water could not 
be consolidated because no coating remained. Samples where the 5 minute 
abrasion test was made using the 0.3 H.E.C. with dye were consolidated 
along with samples which had never been abraded. The former broke on an 
average compression of 585 psi, while the latter withstood 1409 psi. In 
other words, the polymer mix alone succeeded in retaining 42% of the 
strength of an unabraded sample. 
An experiment was conducted to demonstrate the variation in viscosity of a 
fluid comprising water, 0.2% hydroxyethylcellulose, 0.1% dye and 3% salt. 
The pH of the solution was reduced in 0.5 pH unit increments with 
phosphoric acid and the viscosities were measured. The results are shown 
in Table II below: 
TABLE II 
______________________________________ 
Run Fluid pH Viscosity, Cps. 
______________________________________ 
12 pH 6 186.0 
13 pH 5.5 11.0 
14 pH 5.0 8.0 
15 pH 4.5 9.5 
16 pH 4.0 9.0 
______________________________________ 
PILOT FIELD EXAMPLE 
A producing well is completed in a subterranean petroleum containing 
formation from 8,520 to 8,588 feet in depth. Considerable sand has been 
produced along with the oil from this formation, and so it is known that a 
significant cavity has been produced around the perforations of the 
producing well. The well must be shut down periodically to remove the 
sand, and the frequency of such shut-in period is increasing as a 
consequence of the increasing rate of production of sand from the 
formation. In order to reduce the necessity for periodically shutting in 
the well and to avoid the possible risk of collapse of the formation 
around the producing well which might destroy the well, it is decided to 
form a consolidated gravel pack just outside the perforations of the 
producing well for the purpose of stabilizing the formation and 
restraining the flow of formation sand into the well while permitting free 
flow of formation fluids including petroleum from the formation. Based on 
the volume of sand that has been produced since the well has first been 
completed, it is estimated that the average diameter of the void space 
around the well is about 2 feet from the outer perforations of the 
producing well. Accordingly, the volume of washed-out zone in the 
formation to be treated around this well, whose outside casing diameter is 
10 inches, is given below: 
EQU (3.14)(2).sup.2 (68)-(3.14)(10/12).sup.2 (68)=705 cu.ft. 
In order to adequately fill the above described cavity, a total of 705 
cubic feet of resin coated gravel must be positioned in the formation. In 
order to accomplish this, a total of 26 cubic yards of gravel having an 
average particle size of 30 mesh is obtained, washed and dried. Several 
mixing tanks are positioned on the surface along with a 3 cubic yard 
concrete mixer which is utilized in the step of coating the sand grains 
with the resin. The resin employed in this procedure is Q01300 obtained 
from QO Chemicals Company. This is an oligomer of furfuryl alcohol, which 
applicants have found to be especially suitable in sand control processes 
where it is desired to form resin-coated gravel, position the resin coated 
gravel in the formation, and then cause polymerization of the resin to 
bond the gravel particles together. In order to properly coat the 705 
cubic feet of gravel employed in this process, a total of 18,330 pounds of 
resin are required. 
A first batch comprising 2,400 Gallons of the resin solution is formulated 
by mixing 1,833 gallons of the above described resin with 667 gallons of 
butyl acetate saturated with nitrobenzoic acid. The resin solution is 
prepared by first saturating the ester with nitrobenzoic acid and then 
mixing four parts of resin to this fluid mixture. 
The gravel particles are then mixed with the resin solution in the ratio of 
1 part by volume resin mixture to 10 parts by volume sand. Mixing required 
about 2-5 minutes per batch in order to ensure complete coating of the 
sand grains with the resin, after which each batch of resin coated sand is 
added to a tank containing saturated salt water which also contain 0.8 
percent by weight hydroxyethylcellulose and 0.1 percent by weight 
fluorescent dye which provides the needed viscosity for placement of the 
resin coated sand grains in the formation. To this aqueous carrier fluid 
is added 0.8 percent by weight of phthalic anhydride, which will cause the 
pH of the carrier fluid to drop significantly after several hours, which 
produces the reduction in viscosity desired after the resin coated sand 
grains have been positioned in the formation adjacent to the production 
well. The phthalic anhydride is added to the slurry immediately before the 
material is pumped into the well, so there will be no chance that the pH 
drop will occur before the suspension of resin coated sand in the viscous 
carrier fluid has reached the desired location, which might cause the sand 
particles to settle out within the injection string, and cause serious 
problems in the placement of the material in the well. This mixture, 
comprising the saturated salt carrier fluid containing phthalic anhydride 
and the suspended resin coated gravel are then pumped down a tubing string 
located in the producing well. The fluid mixture passes out through the 
perforations and into the washed-out cavity in the formation. 
The above procedure is continued on a batch process due to the limitations 
of the mixer volume until the total desired amount of resin coated gravel 
has been formed, suspended in the viscosified brine carrier fluid 
containing phthalic anhydride and injected into the well where the 
particles pass through the perforations and form a filter cake on the 
formation face, gradually filling up the washed-out cavity. During and 
immediately after the period when the particles are being packed in the 
washed out cavity is when it is desired for the viscosity of the carrier 
fluid to reduce as a consequence of reaction of phthalic anhydride with 
the water component of the aqueous carrier fluid, reducing the pH and 
causing a significant reduction in the viscosity of the suspending fluid. 
Less than one day is required for forming the resin-coated gravel and 
injecting the same into the formation. Since the resin coated sand and 
gravel are prepared in batches and suspended in the viscosified carrier 
fluid, the time for pumping each batch of carrier fluid and coated gravel 
into the formation is only about 1.6 hours. After the last batch of coated 
gravel has been injected, the well is shut-in for a period of seven days, 
which is more than sufficient for the resin material to polymerize, 
binding the sand grains together while still maintaining sufficient fluid 
permeability to allow passage of fluid there through. 
Although our invention has been described in terms of a series of specific 
preferred embodiments and illustrative examples which Applicants believe 
to include the best mode for applying their invention known to applicants 
at the time of this application, it will be recognized to those skilled in 
the art that various changes may be made in the composition and methods 
described herein without departing from the true spirit and scope of our 
invention which is defined more precisely in the claims appended 
immediately hereinafter below.