DELAYED IN SITU CROSSLINKING OF ACRYLAMIDE POLYMERS FOR OIL RECOVERY APPLICATIONS IN HIGH-TEMPERATURE FORMATIONS

This is a continuation-in-part application of copending appli- cation Serial No. 939,660 filed on December 9, 1986, which is a continuation-in-part application of Serial No. 822,709 filed on January 27, 1986, and issued as U.S. Patent 4,683,949 on August 4, 1987, which is a continuation-in-part application of application Serial No.807,416 filed on December 10, 1985, now abandoned.

Background of the Invention

Technical Fiel :

The invention relates to an oil recovery process and more particularly to a process of placing an acryla ide polymer gel in a high-temperature formation for oil recovery applications.

Description of Related Art:

Polymer gels have potential application to a number of pro¬ cesses designed to- improve oil recovery formations. Such processes include conformance improvement, cementing, and fracturing. Con¬ formance improvement can be necessary when one is flooding or pro- ducing a formation exhibiting poor vertical or area! conformance.

Poor vertical conformance results from the vertical juxtaposition of relatively high permeability geologic regions to relatively low per¬ meability regions within a subterranean formation. Poor area! con¬ formance results from the presence of high permeability streaks and high permeability anomalies within the formation matrix, such as vertical fractures and networks of the same, which have very high permeability relative to the formation matrix.

Fluids generally exhibit poor flow profiles and sweep effi¬ ciencies in subterranean formations having poor vertical or area! conformance. Poor conformance is particularly a problem where vertical heterogeneity, fracture networks or other structural anom¬ alies are in fluid communication with a subterranean wellbore across which flui s are injected or produced.

A number of attempts to remedy conformance problems exist. U.S. Patents 3,762,476; 3,981,363; 4,018,286; and 4,039,029 to Gall or Gall et al describe various processes wherein cross!inked polymer gel compositions are formed from gelation systems in high perme-

Q5 ability regions of subterranean formations to reduce the perme¬ ability therein. However, in practice, prior art conformance treat¬ ments employing in situ gelation have proven unsatisfactory because the gelation systems are extremely difficult to regulate once the system is injected into a formation.

IQ Controlling the gelation reaction is a particularly acute problem in hi h-temperature formations because the high temperatures can induce excessive gelation of the system as soon as it contacts the formation. As a result, the gel sets up before it can suffi¬ ciently penetrate the treatment region and the region does -not

15 achieve uniform permeability reduction. This effect diminishes the degree of conformance improvement which one can realize with conven¬ tional gel treatments in high-temperature formations.

A need exists for a gelation process wherein a gelation system gels in situ at a slow, controlled rate, even at high formation

2Q temperatures. A need exists for a process wherein a gelation system substantially delays gelation until it has effectively penetrated the desired treatment region of a subterranean hydrocarbon-bearing formation at which time the system sets up as a uniform immobile gel.

Z5 Summary of the Invention

The present invention satisfies the above-described needs which are neither recognized nor satisfied by the prior art. The present invention provides a process for improving hydrocarbon recovery from a h gh-temperature subterranean hydrocarbon-bearing formation pene-

30 trated by a production or injection well. The process improves vertical and area! conformance in the formation and correspondingly improves flow profiles and sweep efficiencies of injected or pro¬ duced fluids in the formation. The process also has general utility to wellbore cementing applications.

These objectives and others are achieved by the polymer gela¬ tion process of the present invention. The process comprises pre¬ paring a single aqueous gelation system at the surface containing an aqueous solvent, a water-soluble unhydrolyzed aery 1 amide polymer, and a polyvalent metal cross! inking agent. The resulting gelation system is injected into a high-temperature subterranean region in which one desires to reduce pemeability. The polymer is hydrolyzed in situ and then crosslinked in situ to produce a viscous continuous single-phase gel . Once the gel sets up, fluids may be injected into or produced from the hydrocarbon-bearing regions of the formation in fluid com¬ munication with the weϋbore. The gel in place is immobile, i.e., it is substantially incapable of flowing from the treatment region and is substantially permanent and resistant to in situ degradation. An integral part of the present invention is the discovery that an unhydrolyzed acrylamide polymer does not substantially gel (i.e., crosslink) unless a significant number of amide constituents in the polymer are hydrolyzed to carbox late consti uents. Since heat promotes the hydrolysis reaction, one can utilize high formation temperatures to hydrolyze the ^' polymer in situ. Gelation of the polymer is delayed until the hydrolysis reaction has proceeded to a sufficient degree. The extent of the delay is a function of the rate of the hydrolysis reaction.

Delayed gelation enables one to penetrate a high-temperature treatment region with a gelation system before the gel sets up.

Heretofore, it has been difficult, if not impossible, to adequately place conventional acrylamide polymer gelation systems in many high- temperature regions because the heat promotes rapid crosslinked gelation of the polymer and causes the gel to set up before it can sufficiently penetrate the treatment region.

Brief Description of Drawings

Figure 1 plots the hydrolysis rate of unhydrolyzed polyacryl- a ide as described in Example IV.

Figure 2 plots the functional relationship between temperature, gelation rate and the degree of hydrolysis in the pol acr lamide as described in Example V.

Figure 3 plots the gelation rate .of an unhydrolyzed polyacryl- amide as described in Example VI.

Figure 4 plots the injection pressure drop of a gelation system as a function of time as described in Example VII.

Description of Preferred Embodiments

The gelation system utilized in the present invention is a solution comprising a water-soluble unhydrolyzed acrylamide polymer. The term "acrylamide polymer" refers to any polymer in which one or more of the linked monomeric groups are acrylamide groups. Thus, the term acrylamide polymer comprises polymers which contain only linked acrylamide monomeric groups (i.e., polyacryl- amide) as well as polymers which contain one or more other types of linked monomeric groups in addition to acrylamide groups (i.e., copolymers, terpolymers, etc. of acrylamide).

"Unhydrolyzed acrylamide polymer" as used herein is defined as an acrylamide polymer which has less than about 1.0 mole percent of the total monomeric groups in the polymer hydrolyzed. Hydrolysis is a reaction which converts the amide constituent contained within a monomeric acrylamide group to a carboxylate constituent. "Par¬ tially hydrolyzed acrylamide polymer" as defined herein is an acrylamide polymer which has at least 1.0 mole percent of the total monomeric groups in the polymer hydrolyzed.

In most conventional acrylamide polymerization processes, about 2 to 3 mole percent hydrolysis is considered an acceptable level of impurity in the unhydrolyzed polymer product. In the past, such product was commonly termed unhydrolyzed poly acrylami e or simply polyacryl amide because this level of hydrolyzed impurity was gen¬ erally not believed to significantly impact the utility of the polymer for oil recovery applications. However, the present inven¬ tion recognizes that acrylamide polymer gelation processes in

high-te perature environments have a much higher probability of success if the level of hydrolyzed impurity in the unhydrolyzed acrylamide polymer as initially added to the gelation system is strictly limited to a level below about 1.0 mole percent and prefer- ably below about 0.1 mole percent.

In addition to the above-recited limitation, the present un¬ hydrolyzed acrylamide polymer has an average molecular weight in the range of about 10,000 to about 50 million and preferably about 100,000 to about 20 million, and most preferably about 200,000 to about 15 million. The polymer concentration in the gelation system can be about 500 ppm up to the solubility limit of the polymer in the solvent or the rheological constraints of the system. The pre¬ ferred polymer concentration is about 10,000 ppm to about 80,000 ppm. The gelation system further comprises a polyvalent metal cross- linking agent. The polyvalent metal cross! inking agent of the pres¬ ent invention is defined as a salt or a complex of a trivalent or quatravalent metal cation in an aqueous solution wherein the metal cation is capable of crosslinking a partially hydrolyzed acrylamide polymer. Exemplary polyvalent metal crosslinking agents useful in the practice of the present invention are salts or complexes of Al ³⁺ , Fe ³⁺ , Cr ³⁺ , Ti ⁴⁺ , and Zr ⁺ . Preferred crosslinking agents of the present invention are salts or complexes of Al ³⁺ or Cr ³⁺ , including AICI3, CrCl3 and CrAC3 (chromic triacetate). The solvent of the gelation system is an aqueous liquid, such as deionized water, potable water, fresh water, or brines having a total dissolved solids concentration up to the solubility limit of the solids in water. Inert fillers known in the art, although not preferred, may also be added to the gelation system to reinforce the subsequent gel if desired. Such fillers include crushed or natur¬ ally fine rock material or glass beads.

The gelation system is formed by admixing the polymer, cross¬ linking agent, solvent and any optional inert fillers at the surface to form a single injectable gelation solution. Surface admixing broadly encompasses inter alia mixing the system in bulk at the

surface prior to injection or simultaneously mixing the system at or near the wellhead by in-line mixing means while injecting the system into a wellbore for the present gel .treatment. The weight ratio of polymer to crosslinking agent in the system is about 1:1 to about 500:1, preferably about 2.5:1 to about 200:1, and most preferably about 4:1 to about 50:1.

The practitioner of the invention injects the premixed gelation system as a single uniform slug into a wellbore in fluid communica¬ tion with a desired subterranean treatment region. The gelation system is displaced into the treatment region. The treatment region is defined as either a "matrix" or an "anomaly." An "anomaly" is a volume or void space in a formation which has very high permeability relative to the matrix. As used herein, the term, anomaly, may encompass wellbores. An anomaly further encompasses terms such as high permeability streaks, fractures, fracture networks, vugs, solu¬ tion channels, caverns, washouts, cavities. The "matrix" is sub¬ stantially the remainder of the formation volume characterized as essentially homogeneous, continuous, sedimentary reservoir material free of anomalies and often competent. It has been found that acrylamide polymers do not form detect¬ able gels in the presence of a crosslinking agent if the polymer remains less than about 1.0 mole percent and preferably less than about 0.1 mole percent hydrolyzed. It is believed that the carboxy- late constituents are the crosslinking sites in the polymer and that the polymer cannot gel if there are too few crosslinking sites in the polymer, i.e., less than about 1.0 mole percent and preferably less than about 0.1 mole percent based on the total number of mono¬ meric groups in the polymer. If the polymer is partially hydrolyzed above this level, the polymer gels at predictable rates. U.S. Patent 4,683,949 shows gelation rates for a number of different polymers and conditions and is incorporated herein by reference.

The practitioner of the present invention utilizes this infor¬ mation to prevent the complete gelation of a gelation system until it has uniformly penetrated a treatment region. Thus, the present process is particularly applicable to high-temperature formations

where it is extremely difficult to prepare a gelation system having a sufficiently slow gelation rate to enable placement of the system in the treatment region.

High temperature formations as defined herein are formations having temperatures above about 60°C, preferably above about 80°C and most preferably above about 100°C at the depth of the treatment region. Such temperatures typically cause premature gelation of known polymer gelation systems and subsequent unwanted plugging of portions of the wellbore, wellbore face or formation where it is desirable to maintain permeability.

According to the present invention, the gelation system is ungelled or at most only partially gelled when it reaches the desired treatment region. A "partially gelled" gelation system as defined herein is at least somewhat more viscous than a polymer solution which has the same polymer concentration as the gelation system, but does not contain a crosslinking agent. A partially gelled gelation system resists entering a less permeable region where it is desired to maintain permeability, but the system is suf¬ ficiently fluid such that it readily enters a desired treatment region.

The crosslinking agent of the partially gelled system has reacted incompletely with the polymer with the result that neither all of the polymer nor all of the crosslinking agent in the gelation system is totally consumed by the crosslinking reaction. The par- tially gelled system is capable of further crosslinking to comple¬ tion resulting in the desired gel without the addition of more crosslinking agent.

Once the gelation system is in place in a desired high- temperature treatment region, the heat of the treatment region pro- motes in situ hydrolysis of the amide constituents in the acrylamide groups of the polymer. After a sufficient number of amide constit¬ uents have been hydrolyzed to carboxylate constituents, crosslinking of the gelation system can proceed to completion at an orderly rate. "Crosslinked to completion" means that the gelation system is incapable of further crosslinking because one or both of the

required reactants in the initial system are consumed. Further crosslinking is only possible if either polymer, crosslinking agent, or both are added to the gelation system.

Thus, gelation of the unhydrolyzed acrylamide polymer gelation system is believed to be a two-step mechanism. The first step is the heat promoted polymer hydrolysis reaction and the second step is the polymer crosslinking reaction. The practitioner delays complete crosslinking of the gelation system until the system has fully pene¬ trated the desired treatment region by performing the hydrolysis reaction in situ. Significant gelation of the system due to cross¬ linking generally only occurs after at least more than about 1.0 mole percent of the monomeric groups in the polymer hydrolyze. Gelation may also be a function of the particular distribution of carboxylate groups along the polymer chain. The gelation rate can generally be delayed such that complete gelation does not occur for up to a week or more from the time the gelation system is formulated and injected into the desired treat¬ ment region. Even under extremely unfavorable in situ conditions, the gelation rate can be delayed such that complete gelation does not occur for at least 4 hours, preferably at least 12 hours, and more preferably at least 24 hours or more from the time of formula¬ tion. ^•

The final completed "gel" as defined herein is a continuous three-dimensional crosslinked polymeric network having an ultra-high molecular weight. The gel confines the liquid solvent within the solid polymeric network. The fusion of a liquid and a solid com¬ ponent into a single-phase system provides the gel with a unique phase behavior.

Gels employed by the present invention are immobile once in place, i.e., the gels have sufficient structure so as not to prop¬ agate from the confines of a plugged region into a less permeable region in the formation adjoining the plugged region once in place. "Plugging" is defined as a substantial reduction permeability in a region of a formation. Although some of the gels employed herein may qualitatively appear to flow under the force of gravity when

unconfined on the surface at ambient atmospheric conditions, all of the gels employed herein must have sufficient structure to be im¬ mobile within the confines of the treatment region.

The present process is applicable to a number of hydrocarbon recovery applications. According to one embodiment, the process is applicable to conformance treatment of formations which are in fluid communication with an injection or production well. The gel plugs anomalies such as streaks of relatively .high permeability, fractures or fracture networks in direct communication via the anomaly with an injection well or a production well. The gel is also applicable to the plugging of high permeability zones of the matrix. Conformance treatment of regions in direct communication with a production well by the process of the present invention can effectively improve the hydrocarbon productivity of the well or decrease the water to hydro- carbon ratio of the produced fluids.

According to another embodiment, the present process is applic¬ able to cement jobs. A cement job can be analogous to a conformance treatment in cases where both processes are designed to plug anom¬ alies. A cement job generally plugs anomalies in the wellbore or near wellbore of a formation while a conformance treatment generally plugs anomalies in a formation away from the wellbore.

The cement of the present embodiment is a gelation system as prepared in the manner described above. .The cement is applied according to conventional cementing methods known in the art. A rigid gel is the preferred final form for the cement composition. The present process is particularly applicable to remedial squeeze- cementing jobs which can also effectively improve the hydrocarbon productivity of a production well or decrease the water to hydro¬ carbon ratio of the produced fluids. The process is also applicable to plugging abandoned wells.

The following examples demonstrate the practice and utility of the present invention but are not to be construed as limiting the scope thereof.

^• EXAMPLES The polymer solutions of the following examples are prepared by diluting aqueous polyacry!amide solutions with a fresh water solvent, i.e., Denver, Colorado, U.S.A. tap water. In Examples I, II, and III, the dilute polymer solution is then combined with a crosslinking agent solution in a 0.12 liter widemouth bottle to form a 0.05 liter gelation system. The system is gelled in the capped bottle and the qualitative gel strength is determined by period¬ ically inverting the bottle. Examples I, II, and III are formatted as tables of data which describe the formulation and maturation of different gels. Each gel is represented in a table by a single experimental run. Data include the conditions for producing the gel and the qualitative strength of the produced gel. The table displays data in a three- tier format. The first tier is the values of the fixed gelation conditions which are constant and common to every run in the table. The second tier is values of the gelation conditions which vary among the different runs in the table but are constant for any given run-. The third tier is the qualitative gel strength which varies as a function of time within each run and is expressed in alphabetic code.

The following gel strength code and nomenclature are useful for interpreting the tables.

I

-11-

Gel Strength Code

A No detectable continuous gel formed: the bulk of the gelation system appears to have the same viscosity as a polymer solution of the same polymer concentration-, but absent the crosslinking agent. However, in some cases isolated highly viscous gel balls may be present in the system.

B Highly flowing gel : the gel appears to be only slightly more viscous than a polymer solution having the same polymer concen¬ tration, but absent the crosslinking agent. C Flowing gel: most of the gel flows to the bottle cap by gravity upon inversion.

D Moderately flowing gel: only a small portion (^5-10%) of the gel does not readily flow to the bottle cap by gravity upon inversion (usually characterized as a tonguing gel). E Barely flowing gel: the gel can barely flow to the bottle cap and/or a significant portion (>15%) of the gel does not flow by gravity upon inversion.

F Highly deformable nonflowing gel: the gel does not flow to the bottle cap by gravity upon inversion. G Moderately deformable nonflowing gel: the gel deforms about half way down the bottle by gravity upon inversion.

H Slightly deformable nonflowing gel: only the gel surface slightly deforms by gravity upon inversion.

I Rigid geT: there is no gel surface deformation by gravity upon inversion.

- Ringing rigid gel : a tuning fork-like mechanical vibration can be felt upon tapping the bottle.

7

-12-

Nomenclature

Polymer: type of acrylamide polymer Crosslinking Agent: polyvalent metal salt used in prepara¬ tion of ionized or co plexed cross¬ linking agent

% Hydrolysis: mole % of carboxylate groups in the acrylamide polymer molecule based on the total number of monomeric groups in the molecule

Polymer MW: average molecular weight of the acryl¬ amide polymer

Polymer Cone: polymer concentration in the initial polymer solution (ppm)

Polymer pH: pH of the polymer solution

Weight Ratio Polymer:Ions: weight ratio of polymer to crosslink¬ ing agent ions in the gelation system

Metal Ion Cone: polyvalent metal cation concentration in the gelation system

Temp: gelation temperature CO rt = room temperature

Time: gelation time (hr) Gel Code: gel strength code

Exam le I

No gelation whatsoever is evident in Runs 1 and 2, not even localized gel ball formation. Gelation in Run 3 proceeds at a rapid controlled rate.

Example II

Polymer: partially hydrolyzed polyacryl amide

% Hydrolysis: 30

Polymer MW: 5,000,000

Polymer Cone: 8350 ^•

Polymer Solvent: 5,000 ppm NaCl in aqueous solution

Temp: rt

Run Number

Crosslinking Agent Cr ₃ Cl ₃ Cr( 03)3

Metal Ion Cone 52.5 52.5

Total Ion Cone 270 405

Weight Ratio PHPA:Ions 30.1 20.6

Gel Code

Time THT A A 4.0 A A 9.0 A A 24 A A 48 A A 96 A A 336 A A 672 A A

Crosslinking in both runs occurs so rapidly that local gel balls form around the crosslinking agent solutions as they are added to the polymer solution preventing effective mixing and continuous gel formation.

Crosslinking of the unhydrolyzed polyacryl amide gelation system does not occur at 43°C as shown in Run 1. In comparison, the gela¬ tion systems of Runs 3-5 produce gels having utility for many sub¬ terranean high- temperature applications, i.e., >60°C. Furthermore, gelation of these systems can be delayed for 24 hours or more at high temperatures which enables one to place the gelation systems of Runs 3-5 into high-temperature formations. Run 2 indicates that the utility of the present process approaches a lower temperature limit around 60°C.

Examples I and II indicate that gelation of partially hydro¬ lyzed polyacrylamide gelation systems proceeds at room temperature, but. often at rapid controlled rates- or even excessive uncontrolled rates. For this reason, partially hydrolyzed acrylamide polymer gelation systems can be unsuitable for treatments of high- temperature formations where slower gelation rates are required.

In contrast, Examples I and III indicate that gelation of unhy¬ drolyzed polyacry!amide gelation systems does not proceed at room temperature, but proceeds at an orderly controlled rate at high temperatures. Thus, unhydrolyzed acrylamide polymer gelation systems are particularly suitable for treatments of high-temperature formations.

Example IV

An unhydrolyzed polyacryl amide having a molecular weight of

11,000,000 is maintained in solution at 82°C for 80 hours. Figure 1 shows the rate of the hydrolysis reaction under these conditions.

Significant hydrolysis does not occur for about 4 hours. Hydrolysis does not exceed 1.0 mole percent until about 15 hours have elapsed.

Example V Two separate gelation solutions are prepared by mixing 5,000,000 molecular weight acrylamide polymers in Denver, Colorado,

U.S.A. tap water at a concentration of 20,000 ppm. A chromic tri¬ acetate crosslinking agent is added to the gelation solution at a polymer to crosslinking agent ion weight ratio of 20:1. The first gelation solution (Curve 1) contains partially hydrolyzed polyacryl - amide which is 1.5 mole percent hydrolyzed and the second gelation solution (Curve 2) contains unhydrolyzed polyacr l amide which is <0.1 mole percent hydrolyzed. Both gelation solutions are main¬ tained at identical reaction conditions of 110°C and 3445 kPa.

Figure 2 shows the gelation rate of the two solutions as a function of time. Apparent viscosity at 0.1 radians per second and

30% strain is the measure of gelation rate. As Curve 2 of Figure 2 shows, the unhydrolyzed polyacryl amide substantially delays gelation

of the gel ation system for two hours at high temperature . Thi s del ayed gel ation time would enable one to pl ace the gelation system contai ni ng unhydrolyzed polyacryl amide in many treatment regions according to the present i nvention . In contrast, it woul d be extremely difficul t, if not impossibl e , to pl ace the gel ation system contai ni ng the parti al ly hydrolyzed polyacryl ami de of Curve 1 in a treatment region under the same condi tions .

Exampl e VI

A polymer gelation system is prepared containing an unhydro- lyzed polyacrylami e and a chromic triacetate crosslinking agent in solution. The polymer concentration in the solution is 40,000 ppm and the weight ratio of polymer to crosslinking agent is 6:1. The polymer has a molecular weight of 180,000 and initially has less than 0.1 mole percent of the amide constituents hydrolyzed to carboxylate constituents.

The system is maintained over time at a constant temperature of

104°C. The gelation rate of the system (as indicated by apparent viscosity) is recorded as a function of time. The results are plotted in Figure 3. Apparent viscosity is determined at 0.1 radians per second and 100% strain.

Figure 3 indicates that gelation of the system is delayed for more than 20 hours according to the process of the present inven¬ tion. For many treatment applications, this is sufficient time to inject an adequate volume of a gelation system into a desired high- temperature formation before the gel sets up.

Example VII A clean sandstone plug at residual oil saturation with normal decane is 2.54 cm long, 7.62 cm in diameter, and has a permeability of 290 md. An acrylamide polymer gelation system is initially injected into the plug at a high rate to saturate the plug. There¬ after, the system is continuously injected into the plug at a con¬ stant temperature of 104°C and a constant injection rate of 0.5 cm ³ /hr. Injection proceeds for 16 hours until a total of 1.6

pore volumes of the gelation system are injected into the plug under these conditions.

The gelation system contains a chromic triacetate crosslinking agent and an unhydrolyzed polyacryl amide in solution. The polymer concentration in the solution is 60,000 ppm and the weight ratio of polymer to crosslinking agent is 7.5:1. The polymer has a molecular weight of 180,000 and initially has less than 0.1 mole percent of the amide constituents hydrolyzed to carbox late constituents.

The injection time is recorded as a function of the injection pressure drop. The results are shown in Figure 4. The results indicate that the present process substantially delays gelation of the injected gelation system for about 10 to 12 hours. This delay allows the practitioner adequate time to place the system in a high- temperature formation before experiencing unacceptably high injection pressure drops.

While the foregoing preferred embodiments of the invention have been described and shown, it is understood that alternatives and modifications, such as those suggested and others, may be made thereto and follow in the scope of the invention.