Potassium hydroxide clay stabilization process

An aqueous solution having potassium hydroxide dissolved therein is injected into a subterranean sandstone formation containing water-sensitive fine particles, including clays. Potassium hydroxide stabilizes the fine particles for a substantial period of time thereby substantially preventing formation permeability damage caused by encroachment of aqueous solutions having a distinct ionic makeup into the treated formation.

DESCRIPTION 
1. Technical Field 
The invention relates to a process for stabilizing clays present in a 
water-sensitive, subterranean sandstone formation, and more particularly, 
to a process for stabilizing clays present in the environment near a well 
bore penetrating a water-sensitive, subterranean hydrocarbon-bearing 
sandstone formation for a substantial period of time. 
2. Background Art 
Encroachment of aqueous solutions having a distinct ionic makeup from 
connate water into subterranean sandstone formations containing clays 
often result in reduced fluid flow, and therefore, reduced oil production. 
Encroachment of ionically distinct fluid in a subterranean sandstone 
formation can occur in a variety of ways, such as, invasion by an 
underlying aquifer, invasion of a secondary or tertiary oil recovery 
flood, and invasion of treatment fluids utilized in the near well bore 
environment. Two distinct types of clay damage can result from 
encroachment of aqueous solutions having distinct ionic makeup. First, 
swellable clays, such as montmorillonite, have interstitial layers. Fresh 
water contact affects the ionic properties between these interstitial 
layers and swells these clays thereby impeding fluid flow therethrough. 
Secondly, migratable clays, such as poorly cemented kaolinite and illite 
clay particles, become detached from the subterranean sandstone formation 
during flow of fresh water therethrough. The resultant mobile clay 
particles can become trapped in the formation pore-throat openings, and 
thus, reduce permeability and fluid flow therethrough. The second type of 
permeability reduction is referred to as clay particle migration. Often, 
encroachment of aqueous solutions having a distinct ionic makeup, such as, 
fresh water, into a subterranean sandstone formation containing clays 
results in the occurrence of both types of permeability damage. 
Several prior art processes have been proposed to stabilize clays present 
in subterranean formations, and therefore, alleviate fresh water damage 
thereto. U.S. Pat. No. 3,640,343 to Darley discloses a method for 
stabilizing hard shaly earth formations (i.e., migratable clays) during 
drilling or fluid production by injecting into the hard shaly earth 
formation a dilute aqueous solution of alkali metal silicate containing 
SiO.sub.2 in an amount from about 2 to about 6 percent by weight and 
having a viscosity of less than 2 centipoise and a pH of from about 11 to 
about 12. It has been proposed to inject an aqueous solution of potassium 
chloride into a subterranean sandstone formation to stabilize clays. 
However, potassium chloride will stabilize clays only when the connate 
brine in contact with the clay has a high potassium-to-sodium ion ratio. 
Thus, clay stabilization resulting from treatment with potassium chloride 
has proven only temporary in that most formation and injection waters have 
high sodium-to-potassium ion ratios, and as such, potassium ions are 
rapidly exchanged from the clays resulting in the loss of any clay 
stabilization attributable to the potassium ions. Polyvalent cations 
containing solutions and other proposed clay-stabilization treatment 
fluids include a solution containing water soluble organic polymers, a 
hydroxy-aluminum acidic solution, such as set forth in U.S. Pat. No. 
3,603,399 to Reed, a calcium hydroxide solution, such as set forth in U.S. 
Pat. No. 4,031,959 to Henderson, and a dissolved zirconium salt solution. 
Utilizing sodium hydroxide to stabilize clays has proven relatively 
ineffective in that sodium hydroxide can promote significant formation 
permeability damage, and in some instances, actually increases the fresh 
water sensitivity of formation clays. The remaining treatment solutions, 
most of which do not contain hydroxide ions, have resulted in varying 
effectiveness, are relatively expensive to utilize, or result in adverse 
in situ side effects, such as, permeability reduction. Thus, a need exists 
for a process for stabilizing clays in a subterranean sandstone formation 
which not only effectively prevents fresh water permeability damage to 
subterranean formation containing clays, but also permanently stabilizes 
such clays. 
DISCLOSURE OF INVENTION 
The present invention provides a process for stabilizing water-sensitive 
fine particles, including clays, present in a subterranean sandstone 
formation for a substantial period of time. An aqueous solution having 
potassium hydroxide dissolved therein is injected into the sandstone 
formation. The potassium hydroxide stabilizes the fine particles for a 
substantial period of time thereby effectively reducing formation 
permeability damage caused by encroachment of aqueous solutions having a 
distinct ionic makeup into the formation. Potassium hydroxide 
concentration and/or the period of time over which potassium hydroxide is 
injected into the formation can be increased at relatively low formation 
temperatures to ensure effectiveness of the process. 
BEST MODE FOR CARRYING OUT THE INVENTION 
The present invention relates to a process for stabilizing clays present in 
a water-sensitive, subterranean sandstone formation for a substantial 
period of time. As utilized throughout this description, the term "clay 
stabilization" refers to treating a sandstone formation in such a manner 
as to substantially prevent permeability damage and fluid flow reduction 
caused by any variation in salt (ionic) makeup of injection and produced 
waters. Reduced fluid flow results from negative interaction between an 
aqueous solution having a distinct ionic makeup, such as, fresh water, and 
fine particles, including clays. Fine particles which are involved in 
fresh water permeability damage include all swelling and potentially 
mobile fine particles present within sandstone pore bodies. Fine particles 
are those particles which have diameters less than 37 micrometers. 
Examples of fine particles which can contribute to permeability damage are 
clays, high surface area silica, minerals, mica, feldspars, and barite. As 
utilized throughout this description, the term "clays" encompasses both 
swelling clays, such as, montmorillonite, vermiculite, swelling chlorite, 
and mixed layered swelling clays and migratable clays, such as, poorly 
cemented kaolinite and illite clay particles. "Mixed layered swelling 
clays" is inclusive of layered mixtures of swelling and non swelling clays 
which will swell when contacted with fresh water. The term "fresh water" 
refers to an aqueous solution which has a relatively low concentration of 
dissolved salts, including monovalent ions. 
The process of the present invention comprises the continuous, relatively 
slow injection of an aqueous solution containing potassium hydroxide 
dissolved therein into a water-sensitive, subterranean sandstone 
formation. The injected potassium hydroxide solution effectively 
stabilizes clays thereby substantially preventing fresh water permeability 
damage and reduced oil production. It has been discovered that the 
hydroxide ion interaction with formation clays in the presence of 
potassium ions unexpectedly results in clay stabilization for a 
substantial period of time. While it is not completely understood exactly 
why such stabilization results, it is believed that the hydroxide 
ion-sandstone interaction in the presence of potassium ions retains the 
beneficial potassium ion stabilization affect on clays by one of the 
following two mechanisms, both of which include clay dissolution to some 
degree. First, a fine, potassium aluminosilicate mineral (possibly a 
potassium zeolite) is precipitated over the clays in the subterranean 
sandstone formation. This potassium-aluminosilicate precipitate prevents 
fresh water from contacting the clays and also cements migratable clay 
particles to pore walls of the formation. Secondly, the irreversible 
hydroxide ion sandstone interaction partially dissolves formation clays 
resulting in the breaking of silicon-oxygen bonds which subsequently 
reform in a more stable manner. The stable rearrangement results in 
migratable clay particles being chemically bonded to the sandstone 
formation pore walls and the interstitial layers of swelling clays, being 
chemically bonded together. 
The process of the present invention is applicable to a wide range of 
subterranean sandstone formation temperatures and also to a wide range of 
subterranean sandstone formation mineralogies. The process of the present 
invention can be applied to subterranean sandstone formations having a 
temperature of about 22.degree. C. up to about 85.degree. C. and above. 
Injection of a potassium hydroxide solution in accordance with the present 
invention should be preferably conducted with a finite solution flow rate 
as static treatment may result in minor formation permeability damage. Any 
frontal advance rate greater than about 0.03 m/day will effectively 
prevent treatment permeability damage. Although significantly larger 
frontal advance rates can be utilized, such rates usually provide little 
additional benefits. Excessively large flow rates should be avoided due to 
high chemical cost. 
Preferably, the potassium hydroxide solution of the present invention is 
injected into the near well bore environment of a subterranean sandstone 
formation. As utilized throughout this description, the term "near well 
bore" denotes the area of a subterranean sandstone formation surrounding a 
well bore penetrating same which exhibits relatively homogeneous 
horizontal characteristics. As a general guide, the near well bore 
environment usually extends a radial distance into the formation of up to 
about 3 meters from the well bore and any extend up to about 9 meters or 
more. As the greatest fluid pressure drop, fluid velocity and quantity of 
fluid transported occurs in the near well bore environment of a 
subterranean formation, the near well bore environment is the area of a 
subterranean formation most susceptible to clay damage. Additionally, 
permeability damage in the near well bore environment has the greatest 
effect on fluid injection and production. While the process of the present 
invention is not inherently limited to the near well bore environment, far 
well bore applications are relatively expensive and of decreased value 
since clay stabilization problems are less acute in the far well bore 
region. 
As the effectiveness of process of the present invention is at least 
partially controlled by the kinetics of the hydroxide ion-sandstone 
interaction, the effectiveness is dependent on several kinetic parameters. 
Treatment effectiveness is dependent upon potassium hydroxide 
concentration. The concentration of potassium hydroxide utilized in the 
aqueous solution can range from about 1 wt.% up to the solubility of 
potassium hydroxide in solution. The concentration of potassium hydroxide 
utilized in the aqueous solution preferably can range from about 5 wt.% to 
about 30 wt.%, and more preferably is about 15 wt.% to about 25 wt.%. 
Treatment effectiveness is also dependent upon the treatment time 
employed. Treatment time can range from a lower limit which is dictated by 
the kinetics of the hydroxide ion-sandstone interaction to an upper limit 
which is dictated by the economics involved in unrealized hydrocarbon 
production due to shutting in a well bore during treatment. Treatment time 
is preferably from about 1 hour to about 48 hours, and more preferably, is 
about 24 hours. The sandstone formation temperature has a bearing on 
treatment effectiveness, and in part, dictates the potassium hydroxide 
solution concentration and/or the treatment time. Thus, although the 
process of the present invention can be utilized to stabilize clays for a 
substantial period of time over a wide range of formation temperatures, at 
relatively low formation temperatures, for example, from about 22.degree. 
C. to about 45.degree. C., potassium hydroxide concentration or treatment 
time must be increased to maintain the effectiveness of the treatment 
which occurs at higher formation temperatures. At such low formation 
temperatures, potassium hydroxide concentration is the preferred parameter 
to increase. In general, a variance in formation temperature will vary the 
preferred potassium hydroxide concentration and treatment time.

The following examples are illustrative of the application of the process 
of the present invention to stabilize clays in a water-sensitive, 
subterranean sandstone formation and are not to be construed as limiting 
the scope thereof. Three distinct indicia are utilized throughout the 
following examples to evaluate the effectiveness of clay stabilization 
treatments. All three indicia utilize the ratio k.sub.final /k.sub.initial 
(k.sub.f /k.sub.i) which is the ratio of the final fluid permeability 
measured after a given operation to the initial fluid permeability 
measured prior to application of any operation. The first indicia is the 
ratio (k.sub.f /k.sub.i).sub.t which is the ratio of fluid permeability of 
a subterranean sandstone formation core measured after application of a 
given treatment to the fluid permeability of the core measured prior to 
treatment application. This ratio indicates the permeability damage which 
is attributable to the treatment. Such damage may result from, for 
example, in situ precipitation of constituents of a treatment fluid. An 
ideal treatment should result in (k.sub.f /k.sub.i).sub.t equalling 1.0. 
The second indicia is the ratio (k.sub.f /k.sub.i).sub.cs which is the 
ratio of the fluid permeability of a subterranean sandstone formation core 
measured after application of a given clay stabilization treatment, and 
subsequent 3.0 wt.% NaCl and fresh water floods to the fluid permeability 
of the core measured prior to application of any operation. The fresh 
water floods utilized in the examples consist of distilled water floods 
and, unless noted to the contrary, consist of ten pore volumes of 
distilled water flooded at a frontal advance rate of approximately 30 
m/day. The ratio (k.sub.f /k.sub.i).sub.cs indicates the susceptibility of 
the plug to permeability damage due to encroachment of an aqueous solution 
having a distinct ionic makeup after application of a clay stabilization 
treatment. An effective clay stabilization treatment should result in a 
(k.sub.f /k.sub.i).sub.cs ratio which approximates (k.sub.f 
/k.sub.i).sub.t. The third indicia is the ratio (k.sub.f /k.sub.i).sub.c 
which is the ratio of the fluid permeability of a comparable untreated 
subterranean sandstone formation core measured after a fresh water flood 
to the fluid permeability of the core measured prior to application of any 
operation. The ratio (k.sub.f /k.sub.i).sub.c indicates susceptibility (or 
sensitivity) of the plug to permeability damage due to fresh water 
encroachment with the plug has not been previously treated in an attempt 
to stabilize clays. The fluid utilized to measure fluid permeability of 
the core before or after any operation may be crude oil or brine. Two 
distinct fluids can be utilized to measure the fluid permeability of one 
core. 
To evaluate the effectiveness of a clay stabilization treatment, the 
(k.sub.f /k.sub.i).sub.cs ratio must be evaluated with respect to the 
(k.sub.f /k.sub.i).sub.c ratio. Thus, when a subterranean sandstone 
formation plug is relatively sensitive to permeability damage due to fresh 
water, i.e. a relatively low (k.sub.f /k.sub.i).sub.c ratio, for example, 
0.01, a moderate (k.sub.f /k.sub.i).sub.cs ratio, for example, 0.5, would 
indicate an effective treatment. However, when a plug is relatively 
insensitive to permeability damage due to fresh water, for example, a 
(k.sub.f /k.sub.i).sub.c ratio is 0.4, the moderate (k.sub.f 
/k.sub.i).sub.cs ratio of 0.5 would indicate a relatively ineffective clay 
stabilization treatment. As a general guide, divergent (k.sub.f 
/k.sub.i).sub.cs and (k.sub.f /k.sub.i).sub.c ratios coupled with a 
(k.sub.f /k.sub.i).sub.cs ratio approaching (k.sub.f /k.sub.i).sub.t 
indicate an effective clay stabilization treatment. However, an otherwise 
effective treatment can actually be relatively ineffective if application 
of the treatment results in a high degree of formation permeability 
damage, i.e., a relatively low (k.sub.f /k.sub.i).sub.t ratio. 
Unless otherwise noted, the floods performed in the following examples are 
conducted at atmospheric pressure and are conducted in plugs previously 
flooded with brine solutions only. The presence of only brine within the 
plug renders the plug more susceptible to damage due to encroachment of 
aqueous solutions having a distinct ionic makeup. In each of the examples, 
comparable sandstone formation plugs are flooded to determine the (k.sub.f 
/k.sub.i).sub.c ratio at comparable conditions, i.e., temperature, volume 
injected, and frontal advance rate of distilled water. 
EXAMPLE 1 
A linear, unfired, homogeneous Berea standstone plug having an initial 
permeability of 260 md is injected with 17 pore volume of a 15 wt.% KOH 
treatment solution for 24 hours at a frontal advance rate of 0.6 m/day. 
The injected fluids and plug are maintained at 85.degree. C. by a heat 
exchanger. The resulting (k.sub.f /k.sub.i).sub.t ratio is 1.20 and the 
(k.sub.f /k.sub.i).sub.cs ratio is 1.03. The (k.sub.f /k.sub.i).sub.c 
ratio is determined on a comparable core to be less than 0.01. 
EXAMPLE 2 
A homogeneous sandstone plug from an Oligocene aged reservoir is injected 
with 26 pore volumes of a 10.0 wt.% KOH treatment solution for 24 hours at 
a frontal advance rate of 0.7 m/day. The injected fluids and plugs are 
maintained at 700 psig back pressure and 85.degree. C. by a heat 
exchanger. The results for both crude oil and brine permeability are set 
forth in Table 1. 
TABLE 1 
______________________________________ 
Measurement 
Fluid (k.sub.f /k.sub.i).sub.t 
(k.sub.f /k.sub.i).sub.cs 
(k.sub.f /k.sub.i).sub.c 
______________________________________ 
Crude .80 .74 .066 
Brine .81 .75 .064 
______________________________________ 
The treatment aforedescribed was repeated in core plugs from the same 
reservoir utilizing a 15.0 wt.% KOH treatment solution. The results are 
set forth in Table 2. 
TABLE 2 
______________________________________ 
Measurement 
Fluid (k.sub.f /k.sub.i).sub.t 
(k.sub.f /k.sub.i).sub.cs 
(k.sub.f /k.sub.i).sub.c 
______________________________________ 
Crude 1.05 .91 .20 
Brine .95 .90 .20 
______________________________________ 
This example indicates that clays present in an extremely water-sensitive, 
subterranean sandstone formation can be effectively stabilized by 
treatment with an aqueous solution containing KOH without significant 
damage to formation permeability. 
EXAMPLE 3 
Comparable, linear, unfired, homogeneous Berea sandstone plugs are each 
treated with an aqueous solution having KOH dissolved therein. The 
injected fluids and plugs are maintained at the temperature hereinafter 
indicated by a heat exchanger. The results are set forth in Table 3. 
TABLE 3 
______________________________________ 
Treatment #1 #2 #3 #4 
______________________________________ 
Parameters 
Temperature 85.degree. C. 
65.degree. C. 
45.degree. C. 
22.degree. C. 
Pore Volumes KOH 
Injected 17 16 8.2 22 
Wt % KOH 15.0 15.0 30.0 30.0 
Treatment Time (Days) 
1.0 1.0 4.0 1.0 
Flow Rate (m/day) 
0.6 0.58 0.08 0.6 
Results 
(k.sub.f /k.sub.i).sub.t 
1.20 1.09 .98 .80 
(k.sub.f /k.sub.i).sub.cs 
1.03 .85 .97 .80 
(k.sub.f /k.sub.i).sub.c 
&lt;.01 &lt;.01 &lt;.01 &lt;.01 
______________________________________ 
As indicated by these results, the process of the present invention can be 
employed to stabilize clays over a wide range of formation temperatures. 
The process should effectively stabilize clays at formation temperatures 
in excess of 85.degree. C. Based on these results and kinetic theory, the 
treatment should be more effective, at temperatures in excess of 
85.degree. C., i.e., less KOH and less treatment time are necessary to 
accomplish a desired result. As the process of the present invention is 
dependent on the kinetics of hydroxide ion-sandstone interaction, it is 
important to note that at lower formation temperatures, for example, the 
temperatures encountered in test nos. 3 and 4, KOH concentration in the 
treatment solution is preferably increased to achieve effective clay 
stabilization comparable to that achieved at higher temperatures. 
EXAMPLE 4 
Three comparable, linear, unfired, homogeneous Berea sandstone plugs having 
initial permeabilities of approximately 380 md are injected with 
approximately 8 pore volumes of an aqueous treatment solution having KOH 
dissolved therein for 24 hours at a frontal advance rate of approximately 
0.3 m/day. The KOH concentration of the injected solution is varied for 
each plug. The injected fluids and plugs are maintained at 85.degree. C. 
by heat exchangers. The results are set forth in Table 4. 
TABLE 4 
______________________________________ 
KOH 
Concentration 
(k.sub.f /k.sub.i).sub.t 
(k.sub.f /k.sub.i).sub.cs 
(k.sub.f /k.sub.i).sub.c 
______________________________________ 
15.0 wt. % KOH 
1.04 .88 &lt;.02 
10.0 wt. % KOH 
1.02 .58 &lt;.02 
5.0 wt. % KOH 
1.12 .07 &lt;0.2 
______________________________________ 
In addition, two comparable, linear, unfired, homogeneous Berea sandstone 
plugs having initial permeabilities of approximately 200 md are injected 
with approximately 20 pore volumes of an aqueous treatment solution having 
KOH dissolved therein for 24 hours at a frontal advance rate of 
approximately 0.6 m/day. The injected fluids and plugs are maintained at 
22.degree. C. by heat exchangers. The results are set forth in Table 5. 
TABLE 5 
______________________________________ 
KOH 
Concentration 
(k.sub.f /k.sub.i).sub.t 
(k.sub.f /k.sub.i).sub.cs 
(k.sub.f /k.sub.i).sub.c 
______________________________________ 
30.0 wt. % KOH 
.80 .80 &lt;.01 
15.0 wt. % KOH 
.87 .02 &lt;.01 
______________________________________ 
As indicated by the results in Tables 4 and 5, the concentration of 
potassium hydroxide in the injected treatment solution can be increased to 
assure treatment effectiveness at lower formation temperatures, as 
previously discussed. 
EXAMPLE 5 
Two comparable, linear, unfired, homogeneous Berea sandstone plugs having 
initial permeabilities of approximately 400 md are injected with 10 pore 
volumes of an aqueous treatment solution having KOH dissolved therein. The 
injected fluids and plugs are maintained at 85.degree. C. by heat 
exchangers. Ten pore volumes of 15.0 wt.% KOH solution are injected into 
one plug for 28 hours at a frontal advance rate of 0.55 m/day and into the 
other plug for 7 hours at a frontal advance rate of 2.0 m/day. The results 
are set forth in Table 6. 
TABLE 6 
______________________________________ 
Treatment 
Time (k.sub.f /k.sub.i).sub.t 
(k.sub.f /k.sub.i).sub.cs 
(k.sub.f /k.sub.i).sub.c 
______________________________________ 
28 hours 1.04 .92 .01 
7 hours 1.20 .36 .01 
______________________________________ 
As illustrated by this example, treatment time can be increased so as to 
correspondingly increase the effectiveness of the clay stabilization 
process of the present invention. As previously discussed, the preferred 
treatment time will vary with the temperature of the formation to be 
treated. 
EXAMPLE 6 
A linear, unfired, homogeneous Berea sandstone plug having an initial 
permeability of 260 md is sequentially injected as indicated in Table 7. 
The injected fluids and plug are maintained at 85.degree. C. by a heat 
exchanger. The results are set forth in Table 7. 
TABLE 7 
______________________________________ 
Treatment Sequence (k.sub.f /k.sub.i) 
______________________________________ 
21 pore volumes of 15.0 wt. % KOH 
for 24 hrs @ 0.6 m/day 1.17 (k.sub.f /k.sub.i).sub.t 
3.0 wt. % NaCl permeability, 8 pore 
volumes of 3.0 wt. % NaCl, 10 pore 
volumes of distilled water, 8 pore 
volumes of 3.0 wt. % NaCl 
0.89 (k.sub.f /k.sub.i).sub.cs 
10 pore volumes of 0.30 wt. % NaCl 
1.00 
88 pore volumes of 0.10 wt. % NaCl 
over 7.0 days, @ 8 and 10 pore 
volume increments at 23.5 m/day 
1.07 
10 pore volumes of 3.0 wt. % NaCl 
1.09 
10 pore volumes of distilled water, 
10 pore volumes of 3.0 wt. % NaCl 
0.80 
3.0 wt. % NaCl permeability, 10 pore 
volumes of distilled water, 8 pore 
volumes of 3.0 wt. % NaCl 
0.01 (k.sub.f /k.sub.i).sub.c 
______________________________________ 
As illustrated by this treatment, the potassium hydroxide clay 
stabilization treatment of the present invention is extremely effective as 
stabilizing clays over a prolonged period of time. The permanent nature of 
the treatment of the present invention eliminates costs associated with 
performing subsequent treatments to stabilize clays. 
Thus, it can be appreciated that the present invention provides a process 
for effectively stabilizing clays present in a water-sensitive, 
subterranean sandstone formation for a substantial period of time. The 
process can be applied to sandstone formations having varying mineralogies 
and temperatures by varying treatment, and therefore, kinetic parameters 
of the hydroxide ion-sandstone interaction to achieve preferred treatment 
effectiveness. 
While the foregoing preferred embodiments of the invention has been 
described and shown, it is understood that all alternatives and 
modifications, such as those suggested, and others may be made thereto, 
and fall within the scope of the invention.