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Evaluation of conformance improvement gels
This page focuses on important formula parameters and on temperature effects as they relate to gelation rate and gel strength of conformance treatment polymer gels.
Effect of varying formula parameters
Figs. 1 through 4 relate to gel formula parameters and the effect of temperature for a specific CC/AP gel formula. Other oilfield polymer-gel technologies tend to follow similar relationships. The gel formula of Figs. 1 through 5 is a fracture-problem fluid-shutoff gel that has a rigid and soft Buna rubbery consistency. The gel was formulated in fresh water and contained 2.0 wt% active polyacrylamide (PAM) polymer possessing 11 million MW and 2% hydrolysis. The polymer was chemically crosslinked together to form the gel using chromic triacetate (CrAc3) as the crosslinking agent.
In Figs. 1 through 6, dynamic oscillatory viscometry was used to measure the elastic strength of the gel sample as a function of aging time during the gel maturation process following the addition of the crosslinking agent to the gelant solution. The dynamic oscillatory viscosities were measured at an oscillation frequency of 0.1 radians per second.
These plots of dynamic oscillatory viscosity vs. gel aging time can be used to discern two important properties of any given gel sample:
- Equilibrium dynamic-oscillatory viscosity value that is eventually obtained for a given gel sample is a measure of the elastic strength of the gel
- Rate at which the dynamic oscillatory viscosity of the gel sample reaches the equilibrium viscosity value is a reflection of the gelation rate of the sample
As Fig. 1 shows, gel strength increases with polymer concentration when all other gel formula parameters are held constant. For the gels of this figure, the crosslinker loading was held constant at a 20:1 weight ratio of weight active polymer to weight active CrAc3 crosslinking agent. The gel samples were aged at 140°F. The trend shown is a near universal relationship for all gel technologies. As the concentration is increased of the polymer, monomer, or fundamental chemical building block of the gel’s solid-like structure (up to its solubility limit), gel strength increases.
Polymer molecular weight
When all other gel formula parameters are held constant, polymer-gel strength increases as the polymer molecular weight (MW) is increased. For Fig. 2, which depicts this trend, the crosslinking agent concentration within the gel samples was 1,000 ppm CrAc3. The trend shown explains why a gel formulated with a low-MW polymer needs a higher concentration of polymer to obtain the same strength of a comparable fracture-problem gel formulated with high-MW polymer. Comparable gels formulated with higher-MW polymers tend to be more elastic in nature.
As Fig. 3 shows, when all other polymer-gel formula parameters are held constant, increasing the crosslinking agent concentration increases gel strength. Although not readily apparent in this figure, in general, increasing the crosslinking agent concentration also increases the gel’s rate of gelation.
It should be emphasized that, for a given polymer gel, there usually is an optimum concentration for the crosslinking agent. For the samples of Fig. 3, that concentration is approximately 1,000 ppm CrAc3 (or alternatively stated as a weight ratio of 20:1 active polymer to active crosslinking agent). The optimum crosslinking agent concentration is often specified in terms of the ratio of weight of active polymer to weight of active crosslinking agent (or, alternatively, in terms of the weight of the active polymer to the weight of metal ion in the crosslinking agent). Above a gel’s optimum crosslinking-agent concentration, syneresis (expulsion of water from the gel) will occur. Below the optimum concentration, the gel is underoptimum in terms of gel strength. For most oilfield applications, the petroleum engineer wants to maximize effective gel strength to maximize economic performance per pound of polymer used in a given gel formula.
Syneresis results from shrinkage of the gel volume. Syneresis, a thermodynamic equilibrium phenomenon, results from excessive chemical attractive forces within the gel structure. Syneresis of a given polymer gel formula usually results from one of two causes. First, syneresis for a given polymer-gel formula (fixed salinity and pH) can result from excessive crosslinking agent being incorporated into the gel formula. The second cause results from additional and excessive crosslinking chemical sites developing on the polymer over time, as exemplified by autohydrolysis of acrylamide polymers of CC/AP gels at high temperatures.
Syneresis is usually considered undesirable and unacceptable for gels to be used in near-wellbore treatments that are applied in matrix-rock reservoirs for total-fluid-shutoff purposes or in gels for sealing fluid flow in fractures. However, for gels, especially microgels, that are to be used in deeply placed gel treatments of matrix-rock reservoirs and to function where the microgel particles act as check valves in pore throats, gel syneresis may not be of significant concern. Gel syneresis and gel degradation/degelation can be, and often are, distinctly different phenomena. Both of these phenomena can result in expulsion of water from a gel.
The optimum concentration of the crosslinking agent for a given CC/AP polymer gel formula decreases as the MW of the polymer increases, decreases proportionally with increasing polymer concentration (i.e., less pounds of crosslinker per pound of polymer), and increases with increasing temperature.
Fig. 4 shows how the rate of gelation substantially increases with increasing temperature. For the CC/AP gel technology, the rate of gelation for a given gel formula approximately doubles with every 10°C increase in temperature.
Gelation rate acceleration
There are instances when applying conformance-improvement gel treatments that the optimum treatment design will call for the acceleration of the gelation rate of a given gel formula. This is especially true for gel treatments applied to low-temperature (e.g., subambient temperature) reservoirs. For example, for the CC/AP gel technology, a chemical gelation-rate-acceleration additive package involving the use as chromic trichloride as the accelerating agent has been developed. Fig. 5 illustrates the use of this acceleration package with the CC/AP gel technology.
Gelation rate retardation
There are instances when applying oilfield gel treatments that the optimum treatment design will call for the retardation of the gelation onset time of a given gel formula. This is especially true for gel treatments that are to be applied to high-temperature reservoirs.
For example, for the CC/AP gel technology, a chemical gelation rate retardation additive package has been developed involving the use of strong carboxylate ligands as the gelation rate retardation agent. Fig. 6 illustrates the use of this chemical gelation rate retardation additive package with the CC/AP gel technology. In the study of Fig. 6, the gel being investigated was a gel formula that was intended to impart total near-wellbore fluid shutoff in matrix-rock reservoirs at elevated temperatures. The gel was formulated in fresh water and contained 5.0 wt% active polyacrylamide (PAM) having a MW of ~250,000 daltons. Two versions of the PAM polymer were used in the study. The first PAM was 1.9 mole% hydrolyzed. The second PAM was ultra-low-hydrolysis PAM that had a hydrolysis level of < 0.1 mole%. The gelation rate retardation agent used in the study of Fig. 6 was sodium lactate (NaLac). As the figure shows, the gelation rate was nearly instantaneous at 248°F when the gel was formulated with normal polyacrylamide (1.9% hydrolyzed) and no gelation-rate retardation agent was used. By using various combinations of gelation rate chemical retarder addition and ultra-low-hydrolysis PAM in appropriately formulated gel recipes, the gelation onset delay time at 248°F could be extended in increments out to 23 hours. As the figure also shows, if the near-wellbore reservoir rock is cooled to 212°F (as can normally be easily done near wellbore through the use of ambient-temperature water injection), the gelation onset time could be extended to 68 hours. Appropriately formulated lactate-retarded CC/AP gels usually contain somewhat higher loadings of polymer and/or crosslinking agent. These increased gel chemical loadings are incorporated to counteract the slight weakening and destabilization of the gel that is imparted by the lactate addition.
There are several different measurements of polymer-gel strength. These gel strengths have at least some analogous correlations in most other gel technologies.
- Elastic strength. As discussed previously, dynamic oscillatory viscosity can be used to measure the elastic strength of a gel. In practical oilfield terms, the measure of the elastic strength of a gel relates to the resistance to physical deformation that a gel will exhibit while extruding through a constriction in its flow path, such as a constriction in a fracture flow path.
- Yield or failure strength. When this strength is exceeded, portions of the chemical gel structure are broken. An example of exceeding the gel yield or failure strength is the rupturing of the chemical bonds of the polymer’s backbone structure of a shear-non-rehealable gel. This gel strength is measured by placing a mature gel sample in a large container and increasing container pressure until there is flow through a small orifice in the testing container. The yield or failure strength of a total-fluid-flow-shutoff and shear-non-rehealing gel formed and residing in a sandpack is measured by the differential pressure required to make the gel flow from the sandpack. The yield or failure strength of a polymer gel is often much larger than its elastic strength.
- Compressive strength. Polymer gels do not possess large compressive strengths compared to Portland cement. Thus, polymer gels should not be applied as plugging material where substantial compressive strength is required. Adding solids to a polymer gel can greatly increase its compressive strength, especially at high solids loading.
The relative effectiveness of gels for plugging fluid flow through flow channels and flow paths, as measured by the differential pressure that causes the gel to breakdown and flow, increases as the size of the flow channel decreases. Gels that are placed as clear-fluid gelant solutions are particularly effective and strong plugging agents for use when plugging or reducing the fluid-flow capacity in microflow paths, such as is pores of matrix reservoir rock. Gels are especially effective at plugging small fluid-flow paths where solids-containing plugging agents, such as cement or even “microfine” cement, cannot be readily placed.
Gel onset time
There have been many and, at times, apparently conflicting means proposed to define and measure gelation onset times. There are two distinctly different and important times in the gel maturation process. The first is the initial onset of gel formation. The second is the time required to reach full gel strength. Because the time required to attain full gel strength is often reached asymptotically, pragmatically it is often better to measure the attainment of “near full gelation,” where “near full gel strength” is quantitatively defined.
When determining the time of initial onset of gelation for a classical bulk polymer gel to be used to treat matrix reservoir rock, the investigator needs to determine when the first onset of microgel particle formation occurs. It is at this point that the gelant usually ceases to be readily injectable into sandstone of less than 1,000 md. Filtration testing, core flooding injectivity experiments, bottle testing, and dynamic oscillatory viscosity techniques have all been used successfully to determine the first onset of such gelation. For a bulk polymer gel, microgel formation usually occurs at a time just before the onset of first being able to visually detect any gel structure in the gelant solution. For classical bulk polymer gels, initial microgel formation constitutes the formation on a colloidal scale of the first discreet crosslinked polymer aggregates that are the precursor to the formation of the continuous macro-scale crosslinked polymer network of the bulk gel.
For crosslinked polymer gels, there are several different modes of the gelation onset delay and different ways these modes manifest themselves. First, there can be a substantial induction time before any gel formation, including microgel formation, can be detected. Second, and not mutually exclusive of the first mode, there can be “right angle gelation onsets.” Here, once the gelation process starts, the final gel strength is reached very rapidly (almost instantaneously). Third, the gelation begins as soon as the crosslinking agent is added and proceeds over a protracted period of time. CC/AP gels widely applied to treat fracture conformance problems in reservoirs with temperatures < 150°F are of the third gelation-delay type mode.
Gel bottle testing
Although not a highly sophisticated and exacting quantitative technique, bottle testing (ampoule testing at high temperatures) has been used widely in both the laboratory R&D setting and the field quality-control setting to test and evaluate gels, especially polymer gels.
Bottle testing provides a highly cost-effective and straightforward technique to obtain a semiquantitative measure of gel strength and a semiquantitative measure of gelation rate. It is also a convenient means to evaluate the long-term stability of gels at a given test temperature. Bottle testing is often relatively easy to conduct in the field setting. Bottle testing in the laboratory provides a means to screen rapidly the performance of a large number of gel samples before selecting a few of the gels samples for more costly and rigorous testing, such as dynamic-oscillatory-viscosity and core flooding experimental testing.
A polymer gel bottle-test gel strength code has been developed and is now widely used (at times in various forms and with various modifications) for visually determining and evaluating, in a semiquantitative manner, the strength of polymer gels. Table 1 shows the gel strength code. The gel strength code is set up such that two observers, who view the same gel sample, could possibly assign to the sample a gel strength code that differs by one letter code. However, the gel strength code is designed such that it is “virtually not possible” for two reasonable observers to view the same gel sample and assign the gel sample a gel strength code that differs by two letter codes. The comparison of the strength of different gels using this bottle-testing gel-strength-code scheme should only be made when the same volume of gel sample is placed in a bottle or ampoule with the same size and geometric shape. When quoting a gel strength code, the size of the gel sample and the size of the bottle should be provided. Bottle testing at low to intermediate temperatures is often conducted with a 50 cm3 gel sample placed in a 120 cm3 (4 oz) wide-mouth bottle.
When conducting polymer gel bottle testing, there are several pitfalls to avoid. When performing field quality control work involving polymer gels containing H2S, a nitrogen blanket should be applied to prevent the H2S from reacting with atmospheric oxygen and forming intermediate free-radical chemical species that attack and degrade the polymer gel. This deleterious reaction with free oxygen does not occur in anaerobic oil reservoirs (most oil reservoirs are anaerobic). Free oxygen in the injected water of the gel formula tends to be chemically reduced and deactivated quickly in the chemically reducing environment of most oil reservoirs. Consumption of free oxygen in the chemically reducing reservoir environment is generally much faster than the oxygen-induced gel degradation reaction.
When conducting high-temperature ampoule testing of this bottle-testing scheme, the gelant solutions need to be scrupulously deoxygenated to less than 10 ppb free O2. There are two recommended procedures to deoxygenate the gelant solution of polymer-gel samples for high-temperature testing in the laboratory setting. The first procedure involves vacuum degassing to the point that the aqueous solution boils (ebulates) for a minute or so and then glassblowing the ampoule shut under vacuum. It is highly recommended that properly tempered heavy-walled (~4 mm thick) glass ampoules be used for this purpose and that the glass ampoules be placed in appropriate transparent safety containers when the gel samples are aged in an oven at the test temperature. The second effective procedure for effectively deoxygenating gelant samples (to < 10 ppb O2) for high-temperature laboratory testing is to bubble high purity argon gas through the gelant solution. The need to deoxygenate gel samples during high-temperature testing is a laboratory artifact because gels existing in a reservoir will normally be in an anaerobic environment.
- Sydansk, R.D. 1988. New Conformance-Improvement-Treatment Chromium(III) Gel Technology. Presented at the SPE Enhanced Oil Recovery Symposium, Tulsa, Oklahoma, 16–21 April. SPE-17329-MS. http://dx.doi.org/10.2118/17329-MS
- Bryant, S.L., Rabaioli, M.R., and Lockhart, T.P. 1996. Influence of Syneresis on Permeability Reduction by Polymer Gels. SPE Prod & Fac 11 (4): 209–215. SPE-35446-PA. http://dx.doi.org/10.2118/35446-PA
- Sydansk, R.D. 1990. A Newly Developed Chromium(III) Gel Technology. SPE Res Eng 5 (3): 346-352. SPE-19308-PA. http://dx.doi.org/10.2118/19308-PA
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