The rheological property of VES self-diverting acid system
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Summary:

VES self-diverting acid diversion acidizing technology is the new way which was developed for large well section and multiple series of strata in the past few years.VES self-diverting acid system is the key to make this technology come true.The indoor synthetic oleic amide propyl betaine as the thickening agent,optimize it and research the rheological property of VES self-diverting acid systerm.The system test the effect from PH,environmental temperature and shearing rate to to apparent viscosity of reacted acid.The experimental result shows that apparent viscosity of VES self-diverting acid systerm can keep more than 100mPa.S in different environment.It can ensure the smooth process of diversion acidizing construction.The main additive of acid(corrosion inhibitor and ferric ion stabilizer) can reduce the apparent viscosity of reacted acid system.We must test the compatibility in the process of optimizing acid system.When the ferric ion mass concentration in VES self-diverting acid system more than 1g/L,phase separation and flocculation occur,then the systerm lose the ability of diversion.

Key words:reservoir stimulation,diversion acidizing,self-diverting acid,oleic amide propyl betaine,rheological property

Acidification is one of the most major measures to increase production for carbonate reservoir.This reservoir has strong heterogeneity and different damage degree.If only general acidification in whole well,it won’t remove the formation damage,but increase the difference of longitudinal permeability.According to the theory of minimum flow,acid will enter the highest permeability minimum damage zone first,acid can hardly enter those low permeability which need to broke down and serious damage zone.It is hard to get good acidification reconstruction result.As sandstone reservoir for acidification reconstruction,it will cause acid wormhole in the process of acid injection.Further increase the permeability difference among layers.It is more difficult to distribute acid uniformly.On this occasion,we have to consider diversion acidizing technology.VES self-diverting acid diversion acidizing technology is the new way which was developed for large well section and multiple series of strata in the past few years.Depend on the self-diverting acid system properties of become sticky,slow,filtration reducing and no damage,realize the purpose of longitudinal work once and homogeneous transformation in reservoir.This technology has been widely used at home and abroad.The writer take the infoor synthetic ampholytic surfactant---oleic amide propyl betaine as the thickening agent,optimize it and research the rheological property of VES self-diverting acid systerm.

Test Method

  Experimental Instrument and reagent including Haake-RS600 Rheometer,oleic amide propyl betaine(self-restraint),hydrochloric acid,MgCl2,CaCl2,CaCO3,analytical reagent,deionized water.

VES self-diverting acid has special property which will become sticky according to the change of PH.This give the acid systerm diversion ability.The writer mainly studies the rheological property of  residual acid system.The preparation for residual acid is complicated.Abdulwahab H Ghamdi gives the preparation method of residual acid:Put CaCl2 slowly in acid,adjust the solution to specified PH,add acid additive,stirring to make sure the CaCl2 and additive uniform mixing in solution,add VES at last.The advantage of this method is that it can use any concentration acid to prepare the residual acid PH we need.At the same time it rules out the carbon dioxide bubble from the neutralization reaction.Make the residual acid preparation quickly.

[if !supportLists]2.[endif]Influence factor of apparent viscosity about residual acid

2.1 PH value

   According to acid preparation method make the 15% initial mass fraction dydrochloric acid to different PH acid.Test the apparent viscosity along with the change of PH value.(Figure 1).It can be seen from figure 1,when the PH value more than 0,the apparent viscosity significantly increased;when the PH value reach around 1.3,the apparent viscosity arrive maximum.The mass fraction of thickening agent is 3%,the apparent viscosity of residual acid will come down according to the reduce of PH value.The apparent viscosity of residual acid is not sensitive to the other thickening agent mass fraction.The reason of decline may be the thickening mass fraction is low.Some changes of micelle structure with the change of PH.The corresponding PH when the residual acid reach maximum apparent viscosity decide the sticky time after the acid enter stratum.Because the reaction of hydrochloric acid and carbonate is severe.It is foreseeable after the acid enter stratum,it will have high viscosity and diverting ability soon.It is insensitive after the acid reach maximum viscosity.It overcome the boundedness which the in situ crosslinking has high viscosity in small PH range.Further improved the diverting ability of residual acid systerm.

2.2 Temperature

  After the VES self-diverting acid entered the reservior,oneside,it is in high reserviortemperature,on the other side,the acid salt reaction release much heat,it makes the temperature has heavy influence on the apparent viscosity of residual acid.By different temperature the apparent viscosity of residual acid changes with shearing rate(Figure 2).Under low temperature the residual acid system shows shear thinning non-Newtonian fluid property.The apparent viscosity of residual acid system in 60℃ is higher than 40℃.When the temperature more than 60℃,the apparent viscosity of residual acid system decrease gradually wieh the temperature increment.But even in 90℃,shearing rate 200/s,the viscosity is 200mPa.s.When the temperature reach or more than 110℃,the apparent viscosity of residual acid system reduce obviously,it shows that the three D reticulate structure of residual acid system has already broken.

   The viscosity of VES self-diverting acid systerm caused by micellarassociation.The viscosity of associating micellar can be simply indicated as the sum of structural viscosity and non-structural viscosity.The viscosity of structural viscisoty is given by the structure from surfactant molecule association. The viscosity of non-structural viscosity caused by self hydrodynamic volume of micelle cluster.When the temperature rise,the micelle cluster dimensional shrinkage.showed volume conformation state.The exchange between surfactant molecules become more frequent.This will reduce the hydrodynamic volume of micellar and non-constructuralviscosity.With the temperature increasing,the association of intermolecular associating group will enhance.Structural viscosity increase.The final system viscosity is decided by two factors:Under the condition of low temperature,the increase of structural viscosity is more than the reduction of non-structural viscosity.It shows the systerm apparent viscosity increase with the tempreturerising.But as the temperature continue to increase,in a certain temperature range(70℃-90℃),the contribution from influence of 2 factors to system apparent viscosity will cancelled out,which makes the system apparent viscosity change slowly.When the temperature continue to increase,the surfactant molecules have higher energy.The surfactant molecules which xist in wormlike micelles cylinder and semi-circular ends will move intensely.Molecular exchange more frequent(Semi-circular ends having an energy of surfactant molecules generally at a higher ratio in the cylinder).Therefore,at high temperatures,molecules tend to have a high energy in a semi-circular end portion,the the length of wormlike micelles is reduced,eventually leading the wormlike micelles size reduction and may even cause struccuraldamage.The residual acid systerm lose its apparent viscosity.

2.3 Shearing rate

   Generally,acid shearing in pore and natural fractures is much lower than ground pipeline and acidification column.So it is necessary to research the change of residual acid system apparent viscosity in low shearing rate,to confirm residual acid has the diverting ability or not.

   The residual acid system showed very high apparent viscosity in low shearing rate,even appear yield stress(Stress was applied to the gel state only after more than a certain value has fluidity).With increasing of shear rate, the apparent viscosity decreased.(Figure 3).The reason that apparent viscosity reduce is surfactant associate to form wormlike micelles,micelles overlap between each other, the integration of three-dimensional network structure formed by winding the gel system.This intertwined structure under shear occurs unwrapping,given to the arrangement occurs to the molecule.When you stop shearing system apparent viscosity can be restored.It can be shown the apparent viscosity changes with shearing rate basically meet power liquid formula from Figure 3.Because the system has high apparent viscosity in low shearing rate,it is foreseeable residual acid form sticky congested area can have a huge sticky skin,forcing the subsequent injection of fresh sour turn into the low permeability reservoir,to achieve the purpose of uniform acid.

2.4 Main additive

   Generally speaking,the addition of corrosion inhibitor reduce the apparent viscosity of residual acid.Because the corrosion inhibitor we used is Quaternized Manny dilute alkali,after positively charged molecules join residual acid,solubilizing hydrophobic end to contact with the alkane chain inside the micelle, while the hydrophilic end is to stay in the micelle surface in contact with water the system was kept to a minimum energy state.Under the residual acid environment of the surfactant molecule is electrically neutral or slightly negative charge,The "inner salt" form,Positively charged molecules into the making micelles with the increase in electricity,Reduce repulsion between molecules,Likely to increase the number of wormlike micelles molecular aggregates,Increase micelle length,Apparent viscosity increases system,However, due to corrosion is usually low molecular substances,Micellar structure will change to acid systems after solubilization,Apparent viscosity decreases resulting acid system,Final apparent viscosity system

Impact of changes by the combined effect of the two aspects.Furthermore inhibitor usually contain a small amount of alcohols will reduce the apparent viscosity of the system.But the amount of corrisoin inhibitor is little,So the apparent viscosity of the system will not cause much impact.

3 kinds of iron stabilizers on the apparent viscosity acid residues have caused different degrees of impact(Figure 4).The reasons from analysis,in fresh acid condition,Since acetic acid is a weak acid can not ionizing,Mainly in the presence of molecular states,as the reaction of acid and salt,PH value increase in acid system,Acetic acid was gradually ionic state.When the acid pH value of more than amphoteric surfactants isoelectric point,Surfactant molecules form an electrically neutral and slightly negatively charged molecules,After acetic acid ionization,CO 0 entered the room amphoteric surfactant molecules hydrophilic groups to increase the charge density colloidal systems,So that the electrostatic repulsion between micelles increases,the number of micellar aggregation reduce,system apparent viscosity reduce.The effect of citric acid on the system after the apparent viscosity of up to 3 until the pH value was fully reflected,And this coincided with the citric acid pH value pH value corresponding ionization,Citric acid is negatively charged after ionization,Also make the system increases the electron density,number of micellar aggregation reduce,acid system apparent viscosity reduce.EDTA is widely used ferric ion stabilizer,Due to its limited solubility in fresh acids,Therefore, the increase of people's quality score is low, approximately 0. 5%.It can be seen from Figure 4,in this mass fraction,Likewise the apparent viscosity of the residual acid and causing injury,The main reason is ED TA containing four acid groups make the system greater charge density,So that more low residual acid apparent viscosity.Moreover,Iron ion stabilizer can simultaneously complex  Ca2+,Due to reduction in the number of counter ions,Making the system further increase the charge repulsion,To further reduce residual acid apparent viscosity.

2.5 Pollutants

   In acidizing(acid fracturing )process,Almost every stage there will be a contaminant (Fe) exists,in the following four aspects,in acid preparation process,Reservoir and mixing vessel rust after being dissolved acid generation Fe2+ and Fe3+,Acid dissolved oxygen to oxidize ferrous ions to convert the iron,In the acid flows through the ground line and column acidification process,Dissolved iron column wall and corrosion produce Fe3+,Acid is dissolved in a small amount of iron mineral formations produce,Acidizing use of industrial hydrochloric acid itself contains a certain amount of F e3 +.

   When the PH value less than 0,the mass fraction of ferric ion less than 1g/L,with the increase of ferric ion mass fraction,system viscosity gradually increase.But when the mass fraction more than 1g/L,Continue to increase the concentration of iron ions will reduce the apparent viscosity of the system(Figure 5),analysis of reasons that,Fe 3+belongs to the high metal ion,when it exist in ion state(low PH environment),With quaternary amine nitrogen betaine surfactant molecule (一N +) and a carboxyl group (一c O O一) forming a coordination bond,Forming a coordination bond between the surfactant molecules so unstable crosslinked structure Unimoleculargrowth,The molecule to increase their viscosity and structural viscosity,system apparent viscosity increase.But when the ferric ion in systerm continue to increase,more molecules occur unstable crosslinking.Make the phase separation happen in residual acid system.,Even gelatinous precipitates.Phase separation occurs and the system effective mass fraction of surfactant reduced flocculation,then the apparent viscosity reduce.

   With the increase of PH value,system apparent viscosity reduce with ferric ion mass fraction increase.In different PH value,Fe3+ has different impact with system apparent viscosity.The major reason is ferric ion with different state in different PH.Ferric ion with ion state in low PH value.With the increase of PH value,when PH reach 1,Fe3+ start becoming sediment.When PH reach 2,Fe3+ almost completely precipitated.Fe3+ sediment will affect the structure of wormlike mecellar,So that the residual acid architecture was damaged and  lose viscosity.

  Through systematic study of acid pH value, temperature and shear rate effect on the apparent viscosity of the acid residues,shows that VES self-diverting acid system can keep high apparent viscosity in different environment.It can ensure the smooth progress of the construction of bypass acidification.

   Acid main additive(corrosion inhibitor and ferric ion inhibitor) has certain influence on residual acid apparent viscosity.So,In the preferred acid system process must be acid system compatibility test.

   When the system is too high iron concentration will cause residual acid phase separation or flocculation system,which has big influence on residual acid apparent viscosity.So before construction must be cleaned and acid fracturing string tank.

About L.E.A.D.

LEAD OILFIELD SOLUTIONS COMPANY is a professional manufacturer and supplier of Sodium Polyacrylate, Corrosion inhibitor, Demulsifierin China. We offer stimulation service and chemicals, acid fracturing engineering. We produce such as clay inhibitor, encapsulated gel breaker, defoameretc. Please visit the company's website at www.sloilfield.com. Connect with L.E.A.D. onGoogle+, and LinkedIn.

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