Hydrochloric Acid Applications to Improve Particle Gel Conformance Control Treatment

SPE-172352-MS

Hydrochloric Acid Applications to Improve Particle Gel ConformanceControl Treatment

Abdulmohsin Imqam, Hilary Elue, and Farag A. Muhammed, SPE; Baojun Bai, Missouri University of Scienceand Technology

Copyright 2014, Society of Petroleum Engineers

This paper was prepared for presentation at the SPE Nigeria Annual International Conference and Exhibition held in Lagos, Nigeria, 0507 August 2014.

This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contentsof the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflectany position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the writtenconsent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations maynot be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.

Abstract

Millimeter-sized (10 m~mm) particle gels have been used widely to control water flow throughsuper-high-permeability zones and fracture zones in mature oil fields. During particle gel extrusion intotarget zones, the gel can form a cake on the surface of low-permeability, unswept formations. This cakereduces the effectiveness of conformance control as well as the amount of oil that can be recovered fromunswept oil formations. Thus, we evaluated the effectiveness of using hydrochloric acid (HCL) to removegel cakes induced during conformance-control treatments.

The interactions between HCL and particle gels were evaluated to understand the swelling, deswelling,and the gel strength after adding acid. A Hassler core holder was then used to determine the corepermeability after gel and acid treatments. Gels swollen in brine concentrations of 0.05%, 1%, and 10%were injected into a sandstone core having a variety of permeabilities. Brine was then injected in cyclesthrough the gel into the core. The core permeability was measured after gel particle injection and after thecore surface with the gel cake was soaked in the acid solution for 12 hr. The results indicate that particlesswollen in brine concentrations of 0.05% caused more damage than those swollen in higher concentrationsof brine. The damage increased as the core permeability increased for all the swollen gels. The results alsoshow that HCL removed the gel cake effectively, and varying HCL concentrations did not exhibit asignificant difference in the gel cake-removal efficiency. The gel was found to swell much less in HCLsolutions than in brine. After it was deswollen in acid, the gel strengths were measured and found to behigher than those swollen in brine. This work concludes that HCL can be used effectively to mitigate thedamage induced by particle gels.

IntroductionWater production from a hydrocarbon reservoir is a major problem worldwide as more reservoirs becomemature. The American Petroleum Institute estimates that over 15 billion barrels of water are producedannually. This is about eight barrels of water produced for each barrel of oil (Environmental ProtectionAgency, 2000). Worldwide, averages of three barrels of water are produced for each barrel of oil (Baileyet al., 2000). Higher levels of water production result in higher levels of corrosion and scales, an increasedload on fluid handling facilities, more environmental concerns, and the shorter economic life of a well.

The annual cost of disposing of this water is estimated to be 50 billion dollars per year (Hill et al., 2012).Many factors are responsible for the excess water production from oil fields. Reservoir heterogeneity isone major reason. Fractures or channels (either natural or artificially induced) are examples of hetero-geneity existing in reservoirs. These fractures or channels often cause excess water production andreduced oil recovery efficiency.

Many methods and materials are proposed to tackle the water production problems during the oil fieldlifecycle. A chemical conformance/water-control technology is becoming more popular for correctingreservoir heterogeneity problems. Gel treatment is an example of the chemical technology that has beenproved to be a successful and inexpensive fluid diversion method. Particle gel of one millimeter in sizehas been widely used for conformance-control purposes. It is formed at the surface, then dried and crushedinto small particles to be injected into the reservoir (Bai et al., 2007b). Particle gel can significantly reducethe permeability of channels, and its plugging efficiency depends on the particle strength and thepore-opening sizes (Imqam et al., 2014). It is injected into these high-permeability formations to gainmore oil from unswept low-permeability zones.

In both cross flow and no cross flow strata, a small portion of gel still propagates into unsweptlow-permeability zones in spite of the millimeter-sized gel preferentially entering into fractures orfractures features channels. Gel penetrates into unswept zones and forms a cake on the surface oflow-permeability layers. This gel cake adversely affects oil production by reducing the permeability of thenear wellbore region. The extent of formation damage depends on the gel properties and the rockpermeability interactions (Elsharafi and Bai, 2012).

Numerous laboratory studies have been conducted using oxidizers and enzymes to understand andmitigate the damage caused by using crosslinked polymer fluids. Carr and Yang (1998) introduced flowback analysis to evaluate the polymer damage-removal efficiency. Crews and Huang (2010) proposed anew technique that uses nanoparticleassociated surfactant in brine that generates crosslinked-polymer-like fluid viscosity to enable the removal of residual polymer in hydraulic fractures. Sarwar et al. (2011)provide a guideline for gel degradation studies using oxidizers and enzymes to optimize the breaker typewhile also optimizing the concentration at specific temperatures. Reddy (2013) studied the filter cakecharacterization using zirconium-crosslinked fracture fluids and developed a non-oxidizer gel breaker thatcan actively decrosslink the crosslinked gel structure by reacting with the crosslinking agent rather thanby only breaking down the polymer chain. These works spend a large quantity of time and effort tooptimize the breaker system for the particular well conditions and fluid requirements. Most of this workwas conducted to optimize hydraulic fracturing fluid to obtain a better performance, but there were onlya few studies performed to evaluate this breaker during the conformance-control treatments.

This study is proposing a different method than the other applications by combining acidizing withconformance treatments to improve oil recovery from low-permeable zones and enhance the injectionprofile in mature oil fields. Some field applications reported promising results from combining watershutoff and stimulation technologies to improve oil recovery. Zhao et al. (2004) evaluated several acidsystems to be compatible with three kinds of plugging agents to use in different reservoirs. Their resultsobtained from Weicheng and Mazhai oilfields show a better injection profile and better oil increase inresponding wells. Turner and Zahner (2009) conducted a field study in Sockeye field, offshore California,on the applications of combining chromium crosslinked polyacrylamide gels and acid stimulation.Combining both treatments lowered water production and increased the oil rate in a manner that neithertechnique would yield on its own. Kosztin et al. (2012) presented a combine technology of water shut-offand acid stimulation in a mature field in North Oman. The results show a large increase in oil productionand a decrease in the average water cut.

To the best of our knowledge, no study has used particle gel as a shut-off material combined with acidstimulation. This paper examines the effectiveness of using hydrochloric acid to remove the damagecaused by particle gel penetrating into low-permeability zones. It examines first the interaction between

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particle gel and hydrochloric acid. The swelling ratio, deswelling ratio, effect of pH, and gel strength inacid is investigated initially. A core flooding test was then carried out to more fully understand the factorsaffecting gel cake formation on the surface of different low-permeability range cores. Various concen-trations of HCL along with variations in pH were used to obtain the optimum acidizing treatment toproduce the optimal retained core permeability.

Experimental Description

Materials

Particle Gel (PG) Superabsorbent polymer was used as the particle gel to conduct the experiments. Itsmain chemical component is potassium salt of crosslinked polyacrylic acid/polyacrylamide copolymer.Dry particle gel with a size of 30 meshes was selected to be swollen in different brine concentrations.

Hydrochloric Acid (HCL) HCL from Fisher Scientific was diluted with distilled water to obtainconcentrations of 5%, 10%, 15%, and 20%. A 10% HCL solution was diluted again with water to preparesolutions with pH values of 1.3, 3, and 5.5.

Berea Sandstone A variety of Berea sandstone having a diameter of 2.5 cm and length of 4.5 cm wasused for the experiments. The core was placed in the oven at 45C for an entire night before it wasvacuumed and then saturated with brine.

HAAKE RheoScope The storage modulus (G) for gel swollen in brine and acid was measured byusing a rheoscope. After being swelled in brine and deswelled in acid, gel strengths were measured andcompared to see if the gel strength in acid increased or decreased after acid treatment. The sensor usedfor measurements is PP335 TiPoLO2 016 with a gap of 0.8 mm between the sensor and the plate. Allmeasurements were performed at a room temperature of 25C.

Experimental SetupFig. 1 is a schematic model used to carry out the experiments. It is comprised of a syringe Isco-pump usedto inject brine concentrations and gel through the accumulator into a Hassler core holder. Berea sandstonewas placed inside the holder, and the confining pressure was adjusted to have a minimum of 500 psidifference above the injection pressure. Spacers 5 cm long were placed inside the core holder in front ofthe core to allow gel placement at the sand face of the core. An injection pressure gauge was installed atthe inlet of the core holder to measure the brine injection pressure during the experiment. Test tubes weremounted at the effluent to collect the brine produced during the injection processes.

Figure 1Experiment setup model.

SPE-172352-MS 3

Experimental Procedure

The experiment procedure was divided into two main steps. The first step was to investigate theinteraction between HCL and the particle gel. The second step was to evaluate the gel cake damageformed during the gel treatments and evaluate the gel cake removal-efficiency after the acid treatments.

Interaction between the Hydrochloric Acid and the Particle GelWe immersed 0.5 ml of 600 m dry gel in 49.5 ml of different brine concentrations (0.05%, 0.25%,

1%, and 10%) of NaCl at room temperature to determine the swelling capacity of the particle gel withtime. The swelling ratios of the particle gel in different brine solutions were obtained using this equation:

(1)

where V2 is the final volume of the gel sample after swelling and V1 is the initial volume of the gel samplebefore swelling.

To measure the swelling capacity of the same dry particle gel size (600 m) in relation to acidconcentration, solutions of 49.5 ml were prepared using different HCL concentrations (5%, 10%, 15%,and 20% by volume). In addition, the 10% HCL concentration was using to prepare varying pH valuesto examine the effect of pH on the swelling capacity measurement. The pH values of these solutions wereadjusted via addition of water and precisely checked using a pH meter. Samples of gel were collected afterswelling in brine and after deswelling in acid concentrations and were placed on the disc of the rheometerto measure their strength.

Samples of fully swollen gels from different brine solutions were collected and placed in test tubes tomeasure the gel deswelling in different acid concentrations and also to measure it in different pH.Deswelling capacity was measured against time, and the volume change was visibly monitored. Thedeswelling capacity of the particle gel can be calculated using this equation:

(2)

where Vi is the initial volume of the swellable gel sample and Vf is the final volume of the gel sample afterdeswelling.

Finally, after we measured the deswelling capacity of the gel in acid, the gel inside the tubes wasflushed with ten cycles of the same brine composition to test if the gel could be swelled again when itcontacted the same brine solution.

Evaluation of Gel Cake-Damage and HCL PerformanceCore flooding was carried out to evaluate the damage caused by the gel cake and to evaluate the

effectiveness of using HCL to remove this damage. The procedure used for the core flooding can bebriefly described as follows:

1) The Berea sandstone with a permeability range of 4 to 65 md was placed in the oven at 45C foran entire night before it was vacuumed and then saturated with 1% NaCl.

2) The core was put in the Hassler core holder and was subjected to a confining pressure. The averageabsolute permeability of the core was measured using flow rates of 0.5, 0.75, 1, 1.25, 1.5, 1.75, 2,and 3 ml/min.

3) A 60-ml solution of completely swellable PG in brine was injected through a 5-cm spacer andplaced facing the core. Saline water was injected again, with flow rates of 0.5, 0.75, 0.5, 1, 0.5,1.25, 0.5, 1.5, 0.5, 1.75, 0.5, 2, 0.5, 3, and 0.5 ml per min. The rationale for repeating the 0.5 mlper minute flow rates after each other flow rate was to determine whether or not the core isdamaged further when the flow rate increased.

4) The gel was removed from the core holder, and the permeability of the core was measured againto see the effect of the gel cake on the core permeability reduction.

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5) Finally, the core was soaked in 65 gm of HCL for 12 hrs and placed again into the holder tomeasure the permeability after the acid treatments.

Evaluation of the Gel Cake Formed and RemovedGel that was swelled in brine concentrations of 0.05%, 1%, and 10% was used to evaluate the gel cake

strength for each core permeability range. Different cycles of the same brine solution were flooded todemonstrate if the gel cake damages the core further when the flow rate increased. For each flow rate, thebrine produced as the effluent was collected each two minutes to monitor the gel cake-buildup during theinjection process.

Darcys law was applied to calculate the core permeability before and after treatments. The permea-bility can be obtained using Eq. 3.

(3)

where q is the flow rate (ml/sec), is the brine viscosity (cp), L is the length of the core (cm), A is thecross-sectional area (cm2), and op is the pressure drop across the core (atm).

The core permeability after the introduction of the gel can be expressed as the core permeabilityreduction, which is defined as the relationship between the initial permeability and the permeability afterthe introduction of the gel, and can be calculated using Eq. 4.

(4)

where kRD is the core permeability reduction (%), ki is the initial core permeability (md), and ka is the corepermeability after adding the gel (md).

The core sample was removed carefully from the holder, and just one centimeter of the core face wassubmersed within the HCL. The core permeability was measured to observe the change in permeabilityafter the acid treatments. Eq. 5 was used to calculate the retained permeability obtained after soaking forvarious times in acid.

(5)

where kRT is the core permeability retained (%), ki is the initial core permeability (md), and kf is the finalcore permeability after applying the acid (md).

Results and AnalysisTo investigate which factors caused more damage to the core and to make the HCL stimulation moreefficient, we first studied the interaction between the PG and the acid in terms of the swelling ratio, gelstrength, and deswelling ratio. The information obtained from this interaction would provide a deepunderstanding of the factors that caused more damage to the core and the factors that would help tomitigate such damage. Core flooding was then performed to confirm the results obtained from theinteraction study and to quantitatively evaluate the core damage and the HCL efficiency.

Interaction between HCL and PG Measurement Results

Swelling Capacity Measurement Dry particle gel was placed separately in test tubes filled with differ-ent brine concentrations and different HCL concentrations. The stable swelling ratio was computed foreach concentration. Fig. 2 shows the influence of the brine concentration and acid concentration on theswelling capacity. The particle gel showed normal swelling ratio behavior; its swelling capacity initiallyincreased with time and then attained equilibrium swelling capacity (ESC). The results indicate that theparticle swelled much more in brine compared to acid. Swelling ratio for PG swollen in brine could reachto 165 when it swollen in 0.05% brine concentrations. While, swelling ratio for PG swollen in acid couldjust only reach to 9 when it swollen in 5% HCL. The swelling ratio for the gel particles swollen in bothbrine and acid increased as the concentrations for both decreased. The concentration change had a very

SPE-172352-MS 5

clear effect on the brine swelling ratio compared to the acid. The swelling ratio increased by a factor oftwo (from 81 to 165) when the brine concentrations decreased from 0.25% to 0.05%. However, theswelling ratio increased 1.2 times (from 7 to 9) when the acid concentrations decreased from 20% to 5%.As the brine concentration decreased, the PG swelled more, it became weaker, and it began to soften. Thisdecrease in gel strength is likely the result of the gel adsorbing a large amount of water and alsopresumably due to the static electric repulsive force and charge balance. At low salt concentrations, theelectric repulsive force will separate the gel molecules and create more space for water to enter (Bai etal., 2007a).

The ESC data obtained from Fig. 2 is used in Fig. 3 to show how brine and HCL concentrationcorrelations can be used to predict the ESC values of both concentrations. Fig. 3 shows that the higher theconcentration, the smaller the ESC value. Eq. 6 is the correlation obtained to predict the ESC of the gelswollen in brine, while Eq. 7 is the correlation obtained to predict the ESC of gel swollen in HCL. Bothcorrelations were fitted with the power law model, with high R2 accuracy.

(6)

Figure 2Swelling ratio of gel in different brine and HCL concentrations.

Figure 3Effect of brine concentration and HCL concentration on the ESC.

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(7)

where ESC is the equilibrium swelling capacity, Cbrine is the sodium chloride concentration in wt. %, andCHCL is the HCL concentration in vol. %.

A 10% HCL concentration was diluted to get solutions of pH 1.3, pH 3, and pH 5.5. The swelling ratioof the particle gel composite in the different pH solutions was determined according to Eq. 1. Fig. 4 showsthat the swelling rate of the PG reached the highest value within 10 minutes, and later the swelling ratewas reduced and the curves became flatter. The solutions of varying pH had a pronounced effect on theswelling capacity. The results show that the swelling ratio of the PG decreased significantly when the pHdecreased to 1.3. This influence can be attributed to the release of proton ions in the acidic condition thatshield the electric repulsive force of the charged group. The concomitant release of ions sharply decreasesthe internal osmotic pressure, thus reducing the water absorbency (Zheng et al., 2008).

Gel Strength Measurements To investigate the influence of acid on the particle gel strength, a rheom-eter was used to measure the strength of the particle gel before and after introducing the acid. Fig. 5 showsthe measurement of the PG-storage modulus for gels swollen in different brine concentrations andcompared with the same gels deswelled in 10% acid concentrations. The results show a significant

Figure 4Swelling ratio of the PG as a function of pH.

Figure 5Gel strength measurements before and after introducing HCL.

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increased in gel strength for all brine concentrations after acid treatment compared to the gel before it wastreated with acid. This implies that acid dissolved a large percentage of the aqueous aspect of the gelcontent. The gel strength measured for the 10% brine concentration had increased to five times more thanit was before acid treatments. In addition, the results indicate that the PG swollen in the higher saltconcentrations was much stronger than the PG swollen in the lower salt concentrations both before andafter applying acid.

Deswelling Capacity Measurement The deswelling of the PG is an important effect that should beconsidered in designing a gel breaker so that the gel cake formed on the low-permeability zones can bemitigated. Particle gel, after being fully swollen in different brine concentrations, was placed inside testtubes filled with acid so that we could observe the acids ability to deswell the gel. Fig. 6 illustrates thegel deswelling in four HCL concentrations for gels swollen in different brine concentrations. The resultssuggest that gel deswelling is highly dependent on the brine concentration. The PG swollen in the lowersalt concentrations deswelled more in the HCL than the PG swollen in the higher salt concentrations. Thedeswelling ability reached approximately 85% on average when the gel was swollen in 0.05%, 0.25% and1% salt concentrations. This overall percentage, however, decreased to around 60% when the gel wasswollen using a 10% salt concentration. This decrease is likely because the PG swollen in the lower saltconcentration has low gel strength while the PG swollen in the higher salt concentration has high gelstrength. Results also reveal that the gel deswelled in different HCL concentrations exhibited similardeswelling ability to the gel swollen in the same brine concentrations. It is likely that this occurred becausethe acid concentrations have almost the same pH. A couple of measurements were also performed for thesame brine concentration but using 10% acid with pH values of both 3 and 5. The results indicate that thePG could not deswell in a medium of pH 3 and above; instead, the PG swelled to more than its initialvolume.

Gel Swollen Capacity in Brine after the Deswelling Process After the gel deswelling in acid wascompleted, we further investigated to determine whether the gel would still adsorb water when it againcontacted water with the same salinity. After the acid application process, the deswellable particle gels inthe 10% HCL solution were placed again in test tubes filled with the same brine compositions as the gel.

Figure 6Particle gels deswelling as a function of acid and brine concentration.

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The PG was washed with a variety of brine cycles, and for each cycle, the pH was measured precisely.We continued to wash the PG with the same composition of brine until the PG reached the original pHof the brine, which is 5.5. After no change in volume was observed after the pH of the PG reached 5.5,the PG was left to swell overnight. The volume changed for each cycle; the equilibrium swelling capacityratio for each brine concentration is listed in Table 1. The results show that PG swelled slightly, and itsswelling ratio increased as the brine concentration decreased. For the purpose of swelling ratio calcula-

Table 1MEASURE OF THE SWELLING RATIO AFTER THE PG WAS IMMERSED IN 10% HCL

Brine concentrations,%

Initial volume ofswellable PG,

ml

Volume of PGafter deswellingin 10% acid, ml

PG after being immersedagain in the samebrine solution

Flushed cycles pH change Gel volume, ml Equilibrium swellingcapacity ratio

0.05 20 1 1st 1 1 3.1

2nd 3 1

3rd 5 1

4th 5 1.5

5th 5 1.5

6th 5.2 1.9

7th 5.5 3

8th 5.5 4

9th 5.5 4

10th, kept overnight 5.5 4.1

0.25 20 3 1st 1 3 0.53

2nd 3 3

3rd 3.5 3

4th 5 3

5th 5.3 3

6th 5.3 3.7

7th 5.5 4

8th 5.5 4.3

9th 5.5 4.5


1 20 5 1st 1 5 0.18

2nd 3 5

3rd 4 5

4th 4.5 5

5th 5 5

6th 5 5

7th 5.3 5.5

8th 5.5 5.7

9th 5.5 5.8


10 20 8 1st 1 8 0.08

2nd 2.5 8

3rd 3 8

4th 3.5 8

5th 4.5 8

6th 4.8 8.4

7th 5 8.5

8th 5.5 8.7

9th 5.5 8.7


SPE-172352-MS 9

tions, V1 will be the final gel volume after deswelling in 10% HCL. The PG swollen in the 0.05% solutionreached the ESC value of 3.1, compared to 0.08 for the PG swollen in a 10% brine concentration.

Evaluation of the Gel Cake Damage and HCL PerformanceThe results obtained from the interaction between the PG and the HCL show that the PG swelling ratiorose significantly as the brine concentration decreased. Consequently, we expect that the gel swollen inthe low brine concentrations (weak gel) might cause more damage to the rock due to its softness andability to penetrate into small pores. In terms of HCL performance, we observed from the firstinvestigations that the PG did not swell much in acid, and its strength increased as a result of thedeswelling mechanism that could reach to 95% for a gel swollen in a 0.05% solution. This result, whichwas obtained for the acid interaction with the gel, provides a clue about the ability of HCL to mitigate geldamage, especially as the PG could not swell again significantly, as shown in Table. 1 To have a betterunderstanding of the interaction between the gel and the acid, core flooding experiments were performedto ensure and validate the results obtained from the interaction process. Experiments were conducted toinvestigate the effect of brine concentration, injection flow rates, and core permeability on the gel cakeformed. The effect of acid concentrations and pH were investigated to evaluate the impact of these twofactors on the efficiency of acid in removing the gel cake. The two parameters that were used to evaluateboth effects were the permeability reduction as a result of the gel cake formation and the retainedpermeability as a result of the HCL acid treatment.

Effect of Brine Concentration and Core PermeabilityFiltration Measurement Results Filtration is defined as the relationship between the cumulative

water filtered from the core as a function of the time during which the injection took place. Fifteeninjection flow rates were used to create the filtration curves during all of the experiments. Filtration testswere performed to determine if it were possible to form an external or an internal gel cake or both on thesurface of the core during the brine-injection process. If the relationship between the cumulative waterfiltered and time is linear, it means that no gel cake formed.

Figure 7aGel swollen in 0.05% brine concentrations.

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The gel swollen in 0.05% and 10% brine concentrations was placed on the surface of cores withpermeabilities of 21.8 and 42 md, respectively, to study the effect of brine concentration on the gel cakeformed. Fig. 7a and Fig. 7b illustrate the data obtained about the cumulative brine produced with time asa function of the injection flow rates, using two concentrations of salt. Repeated injection flow rates beganto increase nonlinearly and then began to become linear after a certain injection rate and time. This linearshape indicated that the gel cake would not grow in size with increased injection flow rates and times. Fig.7a shows the results for the gel swollen in the 0.05% solution, where a nonlinear trend continues untilaround 20 minutes, and then the results become linear for all flow rates. This indicates that a severe gelcake formed during the injection process. All of the results for the flow rate of 0.5 ml/min are drawnseparately on the right side of the figure to visualize the nonlinear aspects clearly and to compare theseflow rates with the flow rate before the gel was placed. All of the repeated 0.5 ml/min flow rates showsa nonlinear relationship at the beginning of the filtration process. There was not a big discrepency between

Figure 7bGel swollen in 10% brine concentrations.

Figure 7cComparison of injection pressure for two salinities during the filtration test.

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the repeated flow rates, which means that the gel cake did not decrease in size with incresaed flow rates.Additionally, the difference between the injection flow rate (0.5 ml/min) before the gel was placed andthe repeated flow rates after the gel was introduced clearly indicates the damage percentage caused to thecore.

Fig. 7b shows the results for the gel swollen in the 10% solution, where a nonlinear relationshipbetween the cumulative volume and the injection time was not seen during all the filtration test exceptduring the first 0.5 ml/min after the gel was first introduced. The increased flow rates not only did notcause increased damage but also could mitigate the external gel cake strength. This trend can be noticedclearly when repeated flow rates are compared. The first flow rate shows that the gel cake formed at thebeginning of the gel process; after the flow rates increased, the curves changed to become linear. Thecumulative volume curves increased after the first flow rate, and then lay closely to each other near thecurve at which gel was not introduced.

Fig. 7c shows a comparison between the brine injection pressure during the filtration process for bothsalinities. The injection pressure for the gel swollen in the 0.05% concentration increased signifcantly withthe flow rates when compared to the gel swollen in the 10% concentration. This high injection pressure,which reached 2500 psi, indicates how the gel cake could create a large back pressure during thetreatments. For example, during the filteration test, this high injection pressure reached the upper limit ofthe pump pressure, which prevented us from getting the cumulative volume data for the injection rate of3 ml/min, as shown in Fig. 7a.

The filtration test results displayed in Figs. 7a through 7c show that a gel cake was formed and that itsstrength and damage percentage to the core varied and depended on the brine concentration range. Thegel swollen in the low brine concentrations exhibitted more of a tendency to damage the core comparedto the gel swollen in the high brine concentrations.

Effect of Injection Flow Rates Filtration measurement results show that damage was only witnessed inthe first few gel injection flow rates. Thus, increasing the gel injection flow rate did not cause furtherdamage to the core. It is believed that channels have been created that allow the brine to flow easilythrough the gel. Fig. 8 is a simple sketch illustrating four sequences of the effect of injecting brine throughthe particle gel: (1) The static sequence occurs when the gel is first placed on the surface of the coresample. The sorting of the particle gels will be controlled by the gel strength and the particle sizes. Gelparticles are not unlike other solid particles in terms of retaining uniformity of shape. (2) The particlecompresses and penetrates sequence occurs when the brine is first injected through the particles; at thistime, the particles compress by moving closer to each other, and some of the gel penetrates a little bit intothe cores. The degree of penetration is dependent on the ratio of the particle gel size to the pore throat size.If the gel penetrates into the core, an internal gel cake is formed in addition to an external gel cake. If theparticle gels do not penetrate, then only an external cake is formed. Depending on the strength of the gelcake, back pressure can occur as a result of restricting the fluid propagation. The back pressure is likelyto cause the gel to be more rigid and can lead to high injection pressure. (3) The initiate channel sequence

Figure 8Sketch showing the effect of increasing injection rate.

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occurs as the brine begins to form internal microchannels inside the particle gel network. The pressurerequired to create these microchannels depends on the gel strength. As the injection flow rates increase,the brine filtrated at the outlet also increases as a result of creating these microchannels. (4) The channelformed sequence occurs because as the injection flow rates continue to increase, the channel becomes alittle larger, and this explains why we had a linear relationship when the injection flow rates increased.The network inside the gel will reform, and the channel will close when the driving force becomes lessthan the bending force between the particles.

Permeability Reduction and Retained Measurements Results To quantitatively determine the geldamage caused by the gel cake, the permeability reduction was used to express the damage. The gelcake-removal efficiency caused by the acid stimulation was expressed as the permeability retained. Afterthe gel was taken out of the core holder, different cycles of water having the same composition as the gelwere injected through the cores, and the stabilized injection pressures were obtained for each flow rate.Two different ranges of core permeabilities were used to observe the permeability change on thepermeability reduction caused by the gel cake. Fig. 9 shows the injection stable pressure results obtainedfor the range of permeability from 3 md to 4.5 md. The brine injection pressure increased as the saltconcentrations decreased. The injection pressure increased approximately five times (from 50 to 250 psi)as the salt concentrations decreased from 10% to 0.05%. This increase is likely due to the formation ofclay minerals formed on the surface of the core as a result of the lower salt content used. Also, the softnessand deformability of the swollen PG in the lower salt concentration enabled the gel to invade a smallamount into the pore throat. Fig. 9b illustrates the results obtained for these brine concentrations in termsof permeability reductions. The permeability reduction increased as the brine concentration decreased.Almost a 90 percent permeability reduction was observed when the gel was placed with the lower brineconcentrations; however, only a 29.5 percent permeability reduction was observed for the high brineconcentrations. Results from these two figures suggest that gel swollen in high brine concentrationsexhibited less ability to damage the core than gel swollen in low brine concentrations.

Fig. 10a shows the injection stable pressure results obtained for the range of permeability from 21.8md to 27.2 md. The injection pressure increased significantly as the brine concentrations decreased.Higher injection pressure was noticed for this range of permeability compared to the permeability rangein Fig. 9a. This increase reveals that gel can penetrate into high core permeability more deeply than if it

Figure 9Injection stable pressure and permeability reduction for a permeability range of 3 md to 4.5 md.

SPE-172352-MS 13

is placed into low core permeability. As a result, Fig. 10b shows that the decreased in core permeabilityis more significant in high permeability cores than it is in low permeability cores.

Comparing the two different ranges of permeability shown in Fig. 9 and Fig. 10, the gel swollen in0.05% brine exhibits a significant damage as both ranges of permeability reached above 90%. When thepermeability increased to a range of 21.8 md to 27.2 md, the permeability reduction increased from 29.5%to 85% for the gel swollen in the 10% brine. Consequently, brine injection pressure increased significantlyas the gel cake caused more damage to the core. The injection stable pressure increased as the brineconcentration decreased and increased more significantly as the permeability of the core increased.

Permeability Retained Measurement Results Different samples of the damaged core were immersed in10% acid concentrations at room temperature to determine the permeability retained values. Table 2 liststhe results obtained for the effect of the brine concentration for ranges of the core permeabilities. Theresults indicate that permeability after the soaking time was retained with approximately greater than 94percent for all brine concentrations and permeability ranges. These results obtained from the core floodingprocess are consistent with results observed in the interaction process as mentioned in Table 1. Resultsfrom the interaction show that the gel did not swell again significantly after being flushed with the samebrine compositions; consequently, a higher percentage of retained permeability was expected to beachieved. These results suggest that acid can be used effectively to retain the low permeability formationsduring the conformance-control treatments.

Effect of Acid Concentrations and pH Two HCL concentrations were evaluated during the core flood-ing measurements to investigate how much the retained permeability could be increased after the acid

Figure 10Injection stable pressure and permeability reduction for the permeability range of 21.8 md to 27.2 md.

Table 2PERMEABILITY REDUCTION AND RETENTION FOR GEL SWOLLEN IN DIFFERENT BRINE CONCENTRATIONS

Absolute permeability(md)

Brine concentration(%)

Permeability aftergel (md)

Permeability reduction(%)

Permeability afteracid (md)

Permeability retained(%)

3.5 0.05 0.3 91.4 3.3 94.2

4.3 1 0.5 88.3 4.1 95.3

4.4 10 3.1 29.5 4.6 104.5

21.8 0.05 0.08 99.8 20.87 95.7

25.5 10 3.80 85.0 27.7 108.6

14 SPE-172352-MS

treatments. Table 3 lists the permeability reductions and retention obtained when the gel was swollen inthe 10% brine concentrations. The retained permeability that was gained after the acid treatments did notresult in a significant difference in gel-removal efficiency. For a permeability of 7.8 md, acid treatmentonly retained 98.7% when the acid concentration was 5%; it increased only slightly, to 104.5%, when theconcentration increased to 10% for the same range of permeability (4.4 md). This result suggests that bothconcentrations could be effectively used to mitigate gel cake formation. This is an advantage becauseengineers would not have to be concerned about corrosion and could add an inhibitor to prevent theproblems associated with the use of high acid concentrations.

Changes in pH were also investigated to observe the effect on the core-retained permeability. Table 4provides the results obtained for a pH of 1.3 and a pH of 5.5 for gel swollen in a 10% brine concentration.The results show that the pH had a pronounced effect on the core permeability retained. Solutions withlower pH values had a stronger effect on the permeability retained than those at a higher pH. At a pH of1.3, the retained permeability reached 108.6% compared to a very small retained permeability (0.5%) fora pH of 5.5. It can be inferred that the pH has a significant effect on removing damage and should becarefully selected during the process of treating with acid.

DiscussionThis work was designed to understand the interaction between particle gels and hydrochloric acid. We alsoconducted core flooding experiments to determine the effect of gel strength and core permeability on both

Table 4EFFECT OF A CHANGE IN pH ON THE RETAINED PERMEABILITY


pH Permeability aftergel (md)



Permeabilityretained (%)

25.5 1.3 3.80 85.0 27.7 108.6

42 5.5 0.24 99.4 0.23 0.5

Figure 11Schematic for non-cross flow experiment.

Table 3PERFORMANCE OF HCL CONCENTRATION CHANGE IN RELATION TO PERMEABILITY


HCL concentration(%)

Permeability aftergel (md)



Permeability retained(%)

7.8 5 0.7 91.0 7.7 98.7

4.4 10 3.1 29.5 4.6 104.5

25.5 10 3.80 85.0 27.7 108.6

SPE-172352-MS 15

the degree of damage and on the stimulation process. We observed that acid stimulation can be usedsuccessfully to mitigate the damage in unswept rich oil low-permeability formations. This finding cansignificantly assist in optimizing the design of particle gel treatments. Additional work is needed toinvestigate the performance of combining these two technologies to gain more oil from low-permeabilityformations. In future studies, a new model will be tested by investigating two parallel formations havinglow and high permeabilities to evaluate how much oil recovery would be obtained from combining watershutoff and acid treatments. The heterogeneity model will be developed as shown in Fig. 11 to emulatethe case when there is no cross flow between layers. It is proposed that the gel be pumped into largepermeability zones to reduce their permeability so as to obtain more oil from unswept low-permeablezones. Acid is pumped to remove the gel cake formed in unswept rich oil zones; thus, more oil can berecovered. The combined technologies will then increase oil production from both low and highpermeable formations.

ConclusionThe main objective of this study was to conduct a comprehensive evaluation of the effectiveness ofcombining the use of HCL and particle gel to gain a better conformance control. The interaction betweenthe particle gel and the hydrochloric acid was investigated in terms of the swelling ratio, deswelling ratio,and gel strength. Core flooding was conducted simultaneously to evaluate the gel cake formed and theperformance of HCL in mitigating this cake. The following conclusions can be drawn from this study:

The gel and acid interaction demonstrated that particle gel swelling capacity decreased as the brineconcentration and acid concentration increased. A change in acid concentration did not reveal adifference in the particle gel deswelling for the same brine concentrations. Solutions with a changein pH had a significant effect on gel deswelling, where a pH above 1 was correlated with anineffective deswelling ability of the gels.

The gel strength increased as both brine concentration and acid concentration increased. The gelstrength measured after the acid treatment was stronger than it was before the acid was introduced.

The PG did not swell significantly after the HCL treatment when it was flushed with differentcycles of brine. This low swelling ratio decreases the chance of the PG damaging the low-permeable cores.

The filtration test results indicated that the particle gel formed a permeable surface gel cake on thelow-permeability cores. The cake formed was strongly dependent on both the brine concentrationand the rock permeability. The formation of a gel cake reduced the permeability to a significantdegree if the brine concentration was low and the rock permeability was high.

The filtration test results indicate that no further damage would occur as the injection flow ratesincreased. Four sequences were observed during brine injection through the gel: static, compressand penetrate, initiate channel, and channel formed.

The amount of permeability retained was calculated after stimulation treatments and, on average,reached more than 95 percent of the original permeability for all the various brine concentrationsand rock permeability ranges. Additionally, core-damaged permeability was removed effectivelywhen the pH was around 1.

Hydrochloric acid shows promising results when joined with gel treatments as an effectivetechnique to remove the gel cake formed on low-permeability zones, and hence, to improve theconformance control objectives.

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Bai, B., Liu, Y., Cost, J., et al 2007b. Preformed Particle Gel for Conformance Control: TransportMechanism Through Porous Media. SPE Res Eval & Eng. 10(02): 176184

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Hydrochloric Acid Applications to Improve Particle Gel Conformance Control TreatmentIntroductionExperimental DescriptionMaterialsParticle Gel (PG)Hydrochloric Acid (HCL)Berea SandstoneHAAKE RheoScope

Experimental SetupExperimental ProcedureInteraction between the Hydrochloric Acid and the Particle GelEvaluation of Gel Cake-Damage and HCL PerformanceEvaluation of the Gel Cake Formed and Removed

Results and AnalysisInteraction between HCL and PG Measurement ResultsSwelling Capacity MeasurementGel Strength MeasurementsDeswelling Capacity MeasurementGel Swollen Capacity in Brine after the Deswelling Process

Evaluation of the Gel Cake Damage and HCL PerformanceEffect of Brine Concentration and Core PermeabilityFiltration Measurement Results

Effect of Injection Flow RatesPermeability Reduction and Retained Measurements ResultsPermeability Retained Measurement ResultsEffect of Acid Concentrations and pH

DiscussionConclusion

References

Documents

Hydrochloric Acid Applications to Improve Particle Gel Conformance Control Treatment