Method of treating shale and clay in hydrocarbon formation drilling

Swelling and migration of subterranean clay is inhibited during drilling for and stimulation of the production of hydrocarbon fluids, and preparation therefor, by treating said formations with a copolymer of about 5% to about 50% of an anionic monomer such as acrylic acid, methacrylic acid, or 2-acrylamido-2-methyl propane sulfonic acid and the balance a cationic monomer selected from dimethyl diallyl ammonium chloride, or acryloxy or methacryloxy ethyl, propyl or 3-methyl butyl trimethyl ammonium chlorides or methosulfates. Permeability damage to the formation is reduced in the presence of the copolymer; it is particularly effective in spite of the presence of a foaming agent.

TECHNICAL FIELD 
This invention relates to the drilling of underground formations for the 
production of oil, gas, and other formation fluids, including water, and 
particularly to the stabilization of boreholes whether or not to be used 
for the recovery of formation fluids, as in the case of mining test holes. 
This invention includes the treatment of subterranean shale and clay to 
prevent swelling caused by the absorption of water from drilling fluids. 
It relates particularly to the use of certain polymeric agents for the 
prevention of swelling of shale and clay in situ by the absorption of 
water, the consequent adverse effects of the entrance of clay and shale 
into the drilling system, and the adverse effects of clay and shale 
sloughing on borehole stability. The polymeric agents we use contain both 
cationic and anionic monomers. The cationic monomers are typified by 
dimethyl diallyl ammonium chloride ("DMDAAC"), and the anionic monomers 
are typified by acrylic acid and 2-acrylamido-2-methyl propane sulfonic 
acid ("AMPS"). The copolymers are shown to be effective for the above 
described purposes and especially insensitive to shear, while remaining 
compatible with anionic compounds such as alcohol ether sulfate and alpha 
olefin sulfonate-based foaming agents. 
BACKGROUND OF THE INVENTION 
A good description of the problem which this invention addresses in the 
context of drilling may be found in an article by Thomas W. Beihoffer et 
al in the May 16, 1992 Oil & Gas Journal, page 47 et seq, entitled 
"Cationic Polymer Drilling Fluid Can Sometimes Replace Oil-based Mud." As 
stated therein, "(S)hales can become unstable when they react with water 
in the drilling fluid. These reactive shales contain clays that have been 
dehydrated over geologic time by overburden pressure. When the formation 
is exposed, the clays osmotically imbibe water from the drilling fluid. 
This leads to swelling of the shale, induced stresses, loss of mechanical 
strength, and shale failure." Shale crumbling into the borehole 
("sloughing") can ultimately place a burden on the drill bit which makes 
it impossible to retrieve. 
Salts such as potassium chloride have been widely used in drilling 
treatments to convert the formation material from the sodium form by ion 
exchange to, for example, the potassium form which is less vulnerable to 
swelling; also the use of high concentrations of such potassium salts 
affects the osmotic balance and tends to inhibit the flow of water away 
from the high potassium salt concentrations into the shale. However, it is 
difficult to maintain the required high concentrations of potassium salts 
in the drilling fluids. In addition, the physical introduction of such 
salts causes difficulties with the preparation of the viscosifying 
polymeric materials typically used for drilling. Inorganic salts can also 
have a harmful effect on the environment if released. While in the above 
cited Beihoffer et al paper, the use of cationic polymers is suggested as 
a supplement for the potassium salts in drilling fluids, the authors do 
not propose the particular polymers we use, which we have found to be 
especially effective because of their resistance to shear and their 
compatibility with anionic agents, as well as their advantageous charge 
density. 
The reader may be interested in "The Separation of Electrolyte Effects From 
Rheological Effects in Studies of Inhibition of Shales with Natural 
Moisture Contents" by Beihoffer et al, SPE Paper 18032, which also 
contains a complete description of the Roll Oven Test referred to below. 
It is incorporated by reference. 
Foaming agents commonly used in air-foam drilling generally tend to be 
anionic surfactants. Such foaming materials are well known and frequently 
are ethoxylated and sulfated, such as alcohol ether sulfates. They may be 
mixed with alpha olefin sulfonates, and commercially may be in solvents, 
including organic solvents added for freeze resistance. Polymers used in 
the presence of such surfactants must be compatible with them. Also, the 
shear forces in air drilling can be particularly high, and any polymeric 
additive should be able to withstand turbulent flow conditions, i.e. a 
Reynolds Number of up to about 500,000. Most of the contemporary 
technology uses acrylamide-based copolymers that shear easily when 
subjected to such turbulent conditions. 
SUMMARY OF THE INVENTION 
We have found that the permeability of subterranean formations may be 
maintained during drilling, and other contact with aqueous fluids--that 
is, shales and clays can be inhibited from swelling, sloughing and 
retarding the flow of fluids--by including in the drilling or other fluid 
an effective amount of a polymer including (1) about 50-95% of a cationic 
monomer selected from (A) dimethyl diallyl ammonium chloride having the 
formula [CH.sub.2 .dbd.CH--CH.sub.2 ].sub.2 N.sup.+ (CH.sub.3).sub.2 
Cl.sup.- ("DMDAAC"), (B) an acryloyloxy monomer of the formula CH.sub.2 
.dbd.CR--COR.sup.1 N(CH.sub.3).sub.3 or (C) an acrylamido monomer of the 
formula CH.sub.2 .dbd.CR--CONR.sup.1 N(CH.sub.3).sub.3, where R is 
hydrogen or a methyl group, and R.sup.1 is a connecting linear or branched 
saturated hydrocarbyl group having from one to about five carbon atoms, 
such cationic monomers copolymerized with (2) about 5-50% of acrylic acid, 
methacrylic acid, or a monomer of a formula 
##STR1## 
where R and R.sup.1 are as above, and A.sup.- is an anion selected from 
chloride and methosulfate. Among the anionic comonomers useful in our 
invention are acrylic acid, 2-acrylamido-2-methyl propyl sulfonic acid 
("AMPS"), and methacrylic acid. Useful cationic monomers in addition to 
DMDAAC include methylacryloxy ethyl trimethyl ammonium methosulfate 
("METAMS"), acryloxy ethyl trimethyl ammonium methosulfate ("AETAMS"), 
methylacryloxy ethyl trimethyl ammonium chloride ("METAC"), acryloxy ethyl 
trimethyl ammonium chloride ("AETAC"), methyl acrylamido propyl trimethyl 
ammoniium chloride ("MAPTAC"), and 3-acrylamido-3-butyl trimethyl ammonium 
chloride ("AMBTAC"). The copolymers we use are compatible with foaming 
agents and can withstand highly turbulent flow conditions. 
It is known that when DMDAAC polymerizes in an aqueous medium such as in 
the presence of a redox catalyst, it internally cyclizes; thus the DMDAAC 
copolymers we use are randomly copolymerized copolymers having the 
structural formula 
##STR2## 
where m is the molar equivalent of about 50% to about 95% by weight of the 
polymer, n is the molar equivalent of about 5% to about 50 % by weight of 
the polymer, R is selected from hydrogen and methyl, and R.sup.3 is the 
remainder of the (meth)acrylamido or (meth)acryloxy group of the anionic 
monomer described above, such as O.sup.- or CONHC(CH.sub.3).sub.2 CH.sub.2 
SO.sub.3, with an associated hydrogen or other common cation. These 
materials can stabilize shales and clays encountered during drilling, and 
they can do so under the effects of strong shear forces and in the 
presence of anionic, cationic or nonionic materials such as foaming 
agents. 
As has been documented many times in the literature, shales commonly 
include clays which may cause difficulties. Examples are clays of the 
montmorillonite (smectite) group such as saponite, nontronite, hectorite, 
and sauconite as well as montmorillonite itself; the kaolin group such as 
kaolinite, nacrite, dickite, and halloysite; the hydrousmica group such a 
hydrobiotite, glauconite, illite and bramalite; the chlorite group such as 
chlorite and chamosite, and in addition vermiculite, attapulgite and 
sepiolite and mixed-layer varieties of the above minerals and groups. The 
entire specification of Himes and Vinson U.S. Pat. No. 4,842,073 is 
incorporated herein by reference as it should be understood that the 
present invention is applicable in all respects to the conditions and 
environment described in the '073 patent. 
Our copolymers may have average molecular weights from about 1000 to about 
1,000,000 and preferably in the range of about 20,000 to about 1,000,000. 
Copolymers of acrylic acid and DMDAAC may be made by the methods described 
by Boothe et al in U.S. Pat. No. 4,772,462, and particularly as referenced 
to Butler and Angelo JACS v 29, p 3128 (1957) or the technique suggested 
in U.S. Pat. No. Re 28,543, and the other copolymers described herein may 
be made in a similar manner. They may be used in admixture with the 
stimulation fluid in an amount effective to stabilize the formation 
against permeability damage at least to some degree as a result of contact 
with the aqueous fracturing fluid. The copolymer is generally admixed with 
the aqueous fracturing fluid in an amount of at least about 50 parts (or 
at least about 0.05%) by weight per million parts by weight of the 
fracturing fluid. Preferably our copolymer (DMDAAC/acrylic acid copolymer 
or other copolymer as described herein) is present in an amount of from 
about 50 to about 50,000 parts per million of the aqueous fracturing 
fluid; most preferably about 1000 to about 8000 parts per million. In the 
case of DMDAAC/acrylic acid copolymers, we prefer about 80-95% DMDAAC to 
about 5-20% acrylic acid. Very small-amounts have at least some beneficial 
effects. An effective amount may be determined by estimates of the amount 
of clay in the formation using representative core samples in standard 
core flow testing as is known in the art or by roll oven testing as 
described below. 
The copolymer may be admixed with the drilling fluid at any time prior to 
contact of the fluid with the subterranean formation. It is readily mixed 
with the constituents of the aqueous phase of drilling or stimulation 
fluid both prior to and subsequent to hydration of the gelling agent. The 
most commonly used gelling agents presently are bentonite clays, 
polysaccharides and in particular natural guar, hydroxypropyl guar, 
polyacrylamide, carboxymethylcellulose, hydroxyethylcellulose, and xanthan 
gum, but our treating agents are compatible with any and all such 
materials as well as foaming agents and may be used without them. The 
polysaccharide or other gelling agent may be used in a hydrocarbon solvent 
to form a "liquid gel concentrate." Our copolymers are also compatible 
with such concentrates, and also, as is shown below, with commonly used 
foaming agents. The ratios of polymer to foaming agent may vary widely, 
but will typically be about 20:1 to about 1:20 by weight. 
The drilling or fracturing fluid may thus comprise, for example, a gelling 
agent, a foaming agent, KCl, and my copolymer.

DETAILED DESCRIPTION OF THE INVENTION 
Table I shows that the polymers we use are stable to shear. Roll oven tests 
were conducted with and without the samples having been subjected to 
shear. All polymers were at 400 ppm and contained 2000 ppm of a common 
anionic surfactant. Shearing was accomplished by preparing a 2.5 gallon 
sample of polymer and passing it through a ceramic choke with a 1000 psi 
pressure differential at 4 gallons per minute for 30 minutes. The water 
used was brine water. The shale samples were ground to -4 to +10 mesh. 
Seven grams of sized shale was placed with 50 ml of test fluid in an oven 
rolling jar. The jars were rolled in the oven for 16 hours at 125 degrees 
F., then removed and cooled. The granules were collected on a 30 mesh 
screen, washed gently with distilled water and dried at 95 degrees until 
no further change in weight was observed. The percent shale recovery was 
then calculated as the weight remaining on the 30 mesh screen divided by 
the original weight. 
TABLE I 
______________________________________ 
Sample Shear % Recovery 
______________________________________ 
83D/17MAA NO 87.50 
83D/17MAA YES 88.78 
90D/10MAA NO 85.77 
90D/10MAA YES 88.73 
85D/15K.sup.+ AA 
NO 87.78 
85D/15K.sup.+ AA 
YES 88.35 
85D/15NH.sub.3 AA 
NO 80.85 
85D/15NH.sub.3 AA 
YES 88.65 
95D/5MAA NO 88.87 
95D/5MAA YES 88.18 
85D/15NH.sub.3 AA 
NO 94.34 
85D/15NH.sub.3 AA 
YES 84.94 
85D/15NH.sub.3 AA 
NO 88.29 
85D/15NH.sub.3 AA 
YES 85.31 
90D/10NH.sub.3 AA 
NO 80.09 
90D/10NH.sub.3 AA 
YES 84.96 
______________________________________ 
The materials are seen to be little affected by shear. 
Additional Roll oven tests were performed on other polymers useful in my 
invention, and the results are reported in Table II: 
TABLE II 
______________________________________ 
Copolymer Percent Recovery 
______________________________________ 
88DMDAAC/12AMPS 68.1 
74AETAMS/26MAA 26.3 
91AETAC/9AA 70.4 
91METAMS/5AA 84.2 
______________________________________ 
In Table II, "DMDAAC" is, as above, dimethyl diallyl ammonium chloride, 
"AMPS" is 2-acrylamido-2-methyl propane sulfonic acid, "AETAMS" is 
acryloxy ethyl trimethyl ammonium chloride, "MAA" is methacrylic acid, 
"AETAC" is acryloxy ethyl trimethyl ammonium chloride, "AA" is acrylic 
acid, and "METAMS" is methacryloxy ethyl trimethyl ammonium chloride. The 
numbers such as 12 and 91 represent the percentages of the monomers by 
weight in the copolymer. 
That the DMDAAC/anionic copolymers are compatible with foaming agents is 
demonstrated in Table III. In Table III, the weight ratio of DMDAAC to 
acrylic acid in a test polymer is written, for example, as 85D/A15, 
meaning 85% by weight DMDAAC and 15% acrylic acid. The weight ratio of 
polymer to foaming agent is indicated by Poly/Fm; FW/Br means the salinity 
of the aqueous carrier in the proportion of fresh water to 15% brine. The 
test was conducted to determine the foam height and its stability in a 
procedure as follows: 100 cc of the test solution containing 0.32% of the 
polymer/foam combination was placed in a Waring blender and subjected to a 
standard agitation for 30 seconds, the material was poured into a 1000 ml 
cylinder and the foam height immediately measured, and the half-life of 
the foam was measured in minutes and seconds. The half-life is the amount 
of time it took for one-half of the original solution to settle out of the 
foam. 
TABLE III 
______________________________________ 
Exmpl Compos Foamr Poly/Fm 
FW/Br FmHt Hf lf 
______________________________________ 
1 85D/15A* p 90.9/9.1 
100/0 580 4:42 
2 49D/51A p 90.9/9.1 
100/0 580 5:22 
3 85D/15A p 90.9/9.1 
100/0 590 5:28 
4 85D/15A* p 90.9.9.1 
0/100 230 0:37 
5 85D/15A p 90.9/9.1 
0/100 230 0:42 
6 85D/15A* p 90.9/9.1 
50/50 400 2:45 
7 85D/15A p 90.9/9.1 
50/50 380 2:25 
8 85D/15A p 90.9.9.1 
100/0 570 4:40 
9 75D/25A q 10/90 0/100 320 1:55 
10 75D/25A q 5/95 100/0 510 5:10 
11 75D/25A q 10/90 100/0 350 2:12 
12 75D/25A q 5/95 20/80 360 2:18 
13 75D/25A q 5/95 40/60 360 2:17 
14 75D/25A q 5/95 60/40 470 3:18 
15 90D/10A q 10/90 40/60 400 2:35 
16 90D/10A q 10/90 60/40 450 2:47 
17 49D/51A r 90.9/9.1 
100/0 550 5:08 
18 85D/15A r 90.9/9.1 
100/0 550 4:53 
19 85D/15A* r 90.9/9.1 
100/0 580 4:04 
20 49D/51A q 7.4/92.6 
100/0 530 4:44 
21 49D/51A q 7.4/92.6 
75/25 540 4:40 
22 49D/51A q 7.4/92.6 
50/50 510 4:15 
______________________________________ 
*Very low molecular weight, i.e. about 1000 to about 10,000. 
Foamers: "p" is a solvent-containing mixture of common anionic surfactants, 
namely alcohol ether sulfates and alkyl sulfonates, chosen for their 
ability to foam in fresh water or brine having up to 5% salt; "q" is a 
commercially available mixture of anionic surfactants chosen for their 
ability to foam in brine having as much as 23% salt; "r" is "Neodol 
91-2.5" a mixture of linear alcohol ethoxylates having 2.5 moles EO per 
mole of alcohol. 
Table IV also demonstrates foam heights and lives for four additional 
copolymers in varied concentrations of brine. In Table IV, the column 
titled "Fluid" presents the weight ratio of fresh water to 23% brine used 
in the experiment. In each case, the surfactant was "p" as defined above. 
TABLE IV 
______________________________________ 
Fluid Foam Ht Half Life 
______________________________________ 
Control (no polymer) 
100/0 540 4:44 
75/25 540 4:43 
50/50 500 4:25 
25/75 400 4:07 
0/100 360 3:01 
88DMDAAC/12AMPS 
100/0 450 2:05 
75/25 480 3:55 
50/50 480 4:28 
25/75 420 4:00 
0/100 370 2:50 
74AETAMS/26MAA 
100/0 600 4:58 
75/25 560 4:02 
50/50 520 4:00 
25/75 430 4:05 
0/100 360 3:46 
91AETAC/9AA 100/0 460 2:22 
75/25 530 5:24 
50/50 480 4:03 
25/75 420 3:06 
0/100 370 3:02 
95METAMS/5AA 100/0 450 2:02 
75/25 550 4:14 
50/50 490 3:28 
25/75 410 3:20 
0/100 370 3:03 
______________________________________ 
The meanings of AETAMS, AETAC, AND METAMS are as indicated above.