Crosslinked guar based blocking gel system for use at low to high temperatures

High temperature blocking gel is shown for temporary workover operations. The blocking gel is prepared by blending a guar or derivatized guar polymer with an aqueous fluid. The mixing method employed mixes and pumps the guar polymer essentially unhydrated. The resulting lower viscosity minimizes friction pressure during placement.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to the art of the production of hydrocarbons 
from subterranean formations and, more specifically, to blocking gels of 
the type used in well bore operations for the production of hydrocarbons. 
2. Description of the Prior Art 
Occasionally, production from well bore operations must cease temporarily 
to perform auxiliary procedures such as repairs at different depths of a 
subterranean formation. The repairs are called workover operations. 
Workover operations frequently use heavy brines and other fluids to 
maintain pressure control within the reservoir. the fluids can leak-off 
into the production zone, causing damage which interferes with the 
efficient operation of the well. 
Isolating the production zone, however, protects it from damage. Specific 
blocking materials, such as solid blocking agents or temporary blocking 
gels isolate the production formation. The solid blocking agents, for 
example Nylon or rubber balls, are injected into the fluid stream and seal 
the production formation by physically stopping up perforations in the 
formation. When the injection ends, the material is no longer held against 
the perforations and falls to the bottom of the well. 
The use of temporary blocking gels successfully protects the production 
zone. Blocking gels formed by gelation of suitable polymers, such as 
appropriate polysaccharides produce a relatively impermeable barrier 
across the production formation. The barrier cordons off the production 
zone from the area undergoing the workover operations. These areas must 
remain separated until production is ready to resume. 
Production resumes after removal of the blocking gel. The recovery of the 
blocking gel is accomplished by reducing the viscosity of the fluid to a 
low value such that it flows naturally from the formation under the 
influence of formation fluids and pressure. This viscosity reduction or 
conversion is referred to as "breaking" and is often accomplished by 
incorporating chemical agents, referred to as breakers, into the initial 
gel. Blocking gels have been used in low temperature zones for many years. 
However, certain problems have been associated with isolating high 
temperature subterranean zones which were mostly due to the insufficient 
or unstable rheological properties of the crosslinked blocking gels. 
The present invention has as its object to provide an improved blocking gel 
which is suitable for use from low to relatively high temperatures greater 
than about 250.degree. F. to enable temporary isolation of the producing 
zone for an extended period of time. 
Another object of the invention is to provide such a blocking gel which can 
be removed at any time without leaving damaging polymeric residue in the 
subterranean formation. 
Another object of the invention is to provide a high temperature blocking 
gel for temporary workover operations which utilizes a guar polymer which 
is more temperature stable and which provides improved rheological 
characteristics over previously known cellulose polymer blocking gel 
systems. 
SUMMARY OF THE INVENTION 
In the method of the present invention, a blocking gel is formed for use 
within a well bore within a subterranean formation. A gellable blocking 
fluid is formed by blending together an aqueous gelled fluid containing up 
to about 100 pounds per 1000 gallons of a hydrated polymer selected from 
the group consisting of guar and guar derivatives, the aqueous gelled 
fluid having added thereto from about 10 to 300 pounds most preferably 
from about 20 to 250 pounds, per 1000 gallons of aqueous fluid of said 
selected polymer in an unhydrated, particulate form. 
The gellable blocking fluid will also preferably have added thereto a 
suitable crosslinking agent for crosslinking a hydratable polymer to form 
a polymer gel and a gel breaker for producing a controlled break of the 
gel. 
The breaker can be an enzyme breaker which is provided as an original 
source within the formulated gellable blocking fluid or which is applied 
as an additional breaker to the previously crosslinked polymer gel. After 
applying the blocking gel to the desired formation interval, the original 
and/or additional enzyme breaker is allowed to degrade the crosslinked 
polymer, whereby the fluid can be removed from the subterranean formation 
to the well surface. Because the fluid is mixed and pumped partially 
hydrated, it has a low viscosity which minimizes friction pressures and 
which allows placement using coiled tubing. 
The blocking gel can also be used for blocking non-productive, thief zones, 
for example to prevent lost circulation during drilling operations. In 
such cases, the blocking fluid would not generally require the addition of 
an enzyme breaker, but would remain as a semi-permanent blocking material. 
Additional objects, features and advantages will be apparent in the written 
description which follows.

DETAILED DESCRIPTION OF THE INVENTION 
In order to practice the method of the invention, an aqueous blocking gel 
is first prepared by blending a hydratable polymer into an aqueous fluid. 
The aqueous fluid could be, for example, water, brine or water alcohol 
mixtures. A number of hydratable polymers are familiar to those in the 
well service industry. These polysaccharides are capable of gelling in the 
presence of a crosslinking agent to form a gelled base fluid. For example, 
hydratable polysaccharides include the galactomannan gums, guars, 
derivatized guars, cellulose and cellulose derivatives. Specific examples 
are guar gum, guar gum derivative, locust bean gum, caraya gum, 
carboxymethyl cellulose, cellulose, carboxymethylhydroxyethyl cellulose 
and hydroxyethyl cellulose. 
Traditionally, cellulose based polymers have been preferred for use as 
blocking gels due to their low residue content after degradation. See, for 
example, U.S. Pat. No. 5,224,544, issued Jul. 6, 1993, to Robert M. 
Tjon-Joe-Pin, et al., entitled "Enzyme Complex Used For Breaking 
Crosslinked Cellulose Based Blocking Gels At Low To Moderate 
Temperatures", the disclosure of which is incorporated herein by 
reference. 
Despite the fact that cellulose polymers have traditionally been preferred 
for blocking gel applications, guar polymers tend to be more temperature 
stable due to their structure. See Davidson, R. L., "Handbook Of Water 
Soluble Gums And Resins" McGraw-Hill Book Company, New York (1980), page 
64. Thus, utilizing guar polymers in blocking gel applications presents 
the opportunity to extend the practical temperature limit of the prior art 
cellulose based gels. Recent studies and testing by independent 
laboratories have also shown that guar based blocking gels can be 
effectively removed from sand packs and from formations using conventional 
and recently developed enzyme breaker technology. 
The preferred hydratable polymers for use in the present invention are thus 
guar gum and guar gum derivatives. The preferred gelling agents are guar 
gum, hydroxypropyl guar and carboxymethylhydroxypropyl guar. The most 
preferred hydratable polymer for the present invention is 
carboxymethylhydroxypropyl guar having a stability across a pH range from 
about 3.0 to 12.0 at temperatures in the range from 70.degree. F. to 
275.degree. F. and higher. The hydratable polymer is added to the aqueous 
fluid in the range from about 0.5 to about 1.5% by weight based on the 
total weight of aqueous fluid. 
In addition to the hydratable polymer, the blocking gel fluids of the 
invention can include a crosslinking agent. The crosslinking agent can be 
any of the conventionally used crosslinking agents which are known to 
those skilled in the art. For instance, in recent years, gelation of the 
hydratable polymer has been achieved by crosslinking these polymers with 
metal ions including aluminum, antimony, zirconium and titanium containing 
compounds including the so-called organotitanates. See, for example, U.S. 
Pat. No. 4,514,309. Transition metals are preferred. Zirconium 
crosslinking agents are most preferred. 
In the case of zirconium crosslinkers, the crosslinking agent is any 
material which supplies zirconium ions in solution. Thus, the crosslinking 
agent can be any convenient source of zirconium ions. A preferred 
crosslinking additive is a zirconium chelate such as sodium zirconium 
lactate. This crosslinking additive is selected from a group of zirconium 
compounds preferably present in the range from about 0.005 to about 1.0% 
by weight of the aqueous fluid. Preferably, the concentration of 
cross-linking agent is in the range from 0.015 to about 0.1% by weight 
based on the total weight of the aqueous fluid. 
Breakers commonly used in the industry for this particular application 
include chemical oxidizers such as persulfates, oxidizer-antioxidizer 
mixtures such as persulfates and triethanolamine and organic materials 
such as sucrose or polyglycolic acid. However, the present invention 
preferably utilizes an enzyme breaker system which is a mixture of highly 
specific enzymes which, for all practical purposes, completely degrade the 
polysaccharide backbone of the cross-linked blocking gel. The enzyme 
breakers can be internally incorporated within the gel, externally applied 
to the gel or a mixture of both. When the enzyme breakers are added 
depends upon the conditions of the procedure. The preferred method of 
application is a mixture of both of the above techniques. 
The preferred enzyme breakers of the invention are hydrolases that are 
stable in the pH range from about 2.0 to 11.0 and remain active at a pH 
above about 8.0. The same enzymes are active at low to high temperatures 
of about 50.degree. F. to 275.degree. F. and above. The preferred enzyme 
breakers are specific to hydrolyze greater than about 90% of the guar 
polysaccharide backbone. The enzymes attack the mannosidic and 
galactomannosidic linkages in the guar backbone, breaking the backbone 
into monosaccharide and disaccharide fragments. Under some conditions, the 
enzymes break the polysaccharide backbone completely into monosaccharide 
fragments. The preferred enzymes are gammanase hydrolases collectively 
called galactomannase and they specifically hydrolyze the 
1,6-.alpha.-D-galactomannosidic and the 1,4-.beta.-D-mannosidic linkages 
between the monosaccharide units in the guar backbone respectively. One 
preferred galactomannase is commercially available from Novo Nordisk of 
Norway as "Gammanase 1.5 L." The preferred concentration of galactomannase 
is 1:2 (weight/weight) solution of 1,6-.alpha.-D-galactosidase and mannan 
indo-1,4-.beta.-D-mannosidase, the galactomannase being present in the 
range from about 0.001 to 0.004% by weight, based on the total weight of 
aqueous fluid. 
The method of the invention utilizes guar technology in a novel manner for 
blocking gel applications. The blocking gel is mixed and pumped partially 
hydrated. This allows mixing and pumping at low viscosity which minimizes 
friction pressures. The lower friction pressures allow placement of the 
blocking gel system using coiled tubing, where desired. 
In formulating a typical blocking gel for use in low to high temperature 
formations, a guar or substituted guar polymer is added to an aqueous base 
fluid to provide a polymer loading in the range from about 0 to 100 pounds 
polymer per 1000 gallons of aqueous base fluid depending upon the required 
viscosity and other well bore conditions. As has been described and is 
customary in the industry, additional amounts of such additives as 
crosslinking agents comprising organometallic compounds or the like, pH 
control agents, crosslinking delay agents, antifoamers, surfactants and 
the like are added to the base fluid. Additionally, an alcohol such as 
methanol, ethanol, isopropanol may be added to the aqueous base fluid to 
retard hydration of the polymer, as is well known in the art. 
At a time prior to pumping the solution into the well bore, additional 
relatively large amounts of unhydrated polymer are added to the aqueous 
base fluid in the amounts ranging from about 10 to 300 pounds, preferably 
about 20 to 250 pounds of such additional, unhydrated polymer per 1000 
gallons of base fluid and the fluid is pumped through the well bore to the 
formation interval of interest. The blocking gel fluid is pumped at a rate 
sufficient to coat the formation interval. 
In the examples which follow, a rheometrics pressure rheometer was used to 
evaluate the stability of a blocking gel at 300.degree. F. The fluid was 
subjected to continuous sinusoidal oscillatory shear. The amplitude of 
oscillation was kept small to minimize shear degradation. This method is 
considered nondestructive, unlike steady shear measurements which 
typically degrade crosslinked gels, especially at higher temperatures. The 
stress component of oscillatory shear is composed of two parts, one 
in-phase with oscillation and the other out of phase. The in-phase 
component of stress is the energy storage modulus, G'. This modulus 
represents the elastic contributions of the fluid. The out-phase component 
is the energy loss modulus, G". It represents the viscosity contributions 
to the fluid. The dynamic viscosity is the quotient from the division of 
G' by frequency. A classical gel is defined as a semisolid that exhibits 
G' values that are both independent of frequency and exceed G" values. 
The preparation of the blocking gel was also evaluated. In the first 
example, 150 pounds of polymer per 1000 gallons was prehydrated prior to 
addition of crosslinker and heating. At temperature, the fluid was 
subjected to a dynamic rate sweep from 0.1 to 100 radians/second at 100% 
strain. The sweeps were conducted after 65, 220, 343 and 411 minutes of 
heating. The G' and G" values at 300.degree. F. suggests that, at this 
temperature, the crosslinked junctions are beginning to disassociate. The 
loss of G' and G" values over time suggest polymer decomposition. These 
results are shown in FIGS. 1 and 2. 
In the second example, only 20 pounds of polymer per 1000 gallons was 
hydrated. The pH of the fluid was then raised above about 9.0 by the 
addition of a suitable buffer and the additional 100 pounds of polymer was 
added. Suitable buffers for pH adjustment include calcium oxide, magnesium 
oxide, ammonium hydroxide, or other well known oxides, hydroxides or basic 
materials. It is generally desirable to raise the pH of the fluid above 
about 9.0, most preferably in the range from about 10.0-11.0 to limit the 
hydration of the additional polymer. In the example under consideration, 
the additional polymer in the alkaline fluid did not hydrate and the 
solution remained thin. On addition of crosslinking agent and heat, the 
fluid finally viscosified. Dynamic rate sweeps were made after 50, 145, 
256 and 398 minutes at 300.degree. F. The G' values are much lower than 
those of the first test. The values are also frequency dependent 
suggesting that the fluid is not a true gel. After the first sweep, the G" 
values initially decline but afterward, the rate of decline is 
significantly reduced. Although less polymer was used, the G" values are 
also larger than those of the first test. The results are shown in FIGS. 3 
and 4. 
It is apparent that the behavior of the fluids in the examples relates to 
the method of preparation. Specifically, the formulation of a blocking gel 
by adding additional unhydrated polymer to the base fluid results in 
improved rheological properties as well as temperature stability. It is 
theorized that the unhydrated polymer only swells in the alkaline water. 
Heating increases the degree of swelling. Before fully hydrating, however, 
crosslinking at the swollen particle surface occurs inhibiting the release 
of polymer from the particle surface to the water. This results in the 
formation of small, solvated polymer domains. The swelling causes the 
particles to become plastic like and deformable. In addition, the particle 
surfaces are sticky, allowing some association of these polymer domains. 
The particle's stickiness and the concentration of particles yield the 
higher viscosity fluid than that of the fluid made from fully hydrated 
polymer. A viscosity and temperature profile is shown in FIG. 5 for a guar 
based blocking gel formulated from 80 pounds per thousand gallons of 
aqueous fluid (ppt). 
Gel damage permeability tests were also performed using a computerized 
permeameter. These test assess the recovery of permeability of a core 
sample after degradation of the blocking gel fluid polymer. A test core is 
typically drilled from a sandstone formation sample. The initial 
permeability of the core sample was obtained using filtered 2% potassium 
chloride at 275.degree. F. A pore pressure of 500 psi was maintained using 
a back pressure regulator. The initial permeability was 7.86 md. The 
direction of flow was reversed and a flow of 2% potassium chloride was 
established. The blocking gel was injected using a volume equal to 
approximately 1 gallon per linear foot in a 6.75 inch hole. A differential 
pressure of 1000 psi was maintained during the test period of 6 hours. The 
removal treatment consisting of an enzyme breaker solution was injected in 
the same direction as the blocking gel and allowed to stand for 1 hour. 
The final permeability was obtained in the original direction. Core test 
results show an extremely low leakoff, a spurt loss of 0.0 gal/ft.sup.2 
and C.sub.iii of 0.00335 ft/min.sup.1/2. The core test shown in FIG. 6 was 
acidized prior to the blocking gel test. Regain permeability for this test 
was 7.90 md or 100% regain. 
An invention has been shown with several advantages. The guar based 
blocking gel of the invention enables the temporary isolation of producing 
zones for extended periods of time at temperatures greater than 
250.degree. F. The blocking gels of the invention can be removed at any 
time without leaving damaging polymeric residue. The guar based blocking 
gels of the invention are effective and have desirable cleanup 
characteristics in high temperature subterranean zones. 
The method of the invention allows higher polymer concentrations, above 150 
pounds per 1000 gallons of aqueous fluid, which can be mixed and still 
have a pumping viscosity of below 30 cps at 511 sec.sup.-1. The mixing 
procedure of the invention allows the blocking gel to reach deep 
formations with less friction pressure than the fully hydrated cellulose 
based blocking gels of the prior art. As the fluid is pumped, the higher 
temperatures encountered cause a transition from the delayed, partially 
hydrated state to a fully hydrated and crosslinked system. This technique 
yields more stability, lower fluid leakoff and less gel penetration into 
the formation matrix. 
The guar based blocking gels of the invention possess stable rheologic 
properties at higher temperatures for longer period of times than their 
cellulose counterparts. Temporary isolation of the production zone is 
achieved without leaving damaging polymeric residue. The lower viscosities 
achieved allow the use of coiled tubing placement of the blocking gel, if 
desired. 
The blocking gels can also be used to block non-productive, thief zones to 
prevent, for example, lost circulation during drilling operations. 
While the invention has been shown in only one of its forms, it is not thus 
limited but is susceptible to various changes and modifications without 
departing from the spirit thereof.