AkzoNobel Surface Chemistry. Armovis EHS A novel high-temperature viscoelastic surfactant for carbonate reservoir acidizing.

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1 AkzoNobel Surface Chemistry Armovis EHS A novel high-temperature viscoelastic surfactant for carbonate reservoir acidizing.

2 Armovis EHS 2 Summary Low permeability carbonate reservoirs (limestone and chalk) are a major source of hydrocarbon around the world, most significantly in the Middle East, but with most continents holding some such reservoirs. Commercial exploitation of these resources can be achieved through acidizing and/or fracturing of these low permeability reservoirs to increase the contact of the wellbore with the reservoir, which improves hydrocarbon productivity. Acidizing of reservoirs via bullhead treatment will lead to preferential wormholing in the highest permeability zones of the well, often leading to only incremental hydrocarbon productivity increase. Two options can be considered to more productively focus acid treatment placement of the acid in the target zone (using coiled tubing) or using chemical diversion techniques, such as viscoelastic surfactants (VES). As well as providing diversion, VES systems are also used to reduce the rate of reaction of the acid with the reservoir. This has the affect of creating predominantly long, deep-penetrating wormholes. Experimental modeling has shown these to provide the best performance in terms of incremental productivity gain. Also, if the acidizing is performed to remove skin effects (such as scale, drilling damage etc), that this type of treatment is the most efficient method of circumventing. Such VES systems have been used extensively with great effect, significantly improving the incremental hydrocarbon recovery in acidizing treatments. The importance of these advantages have been compounded by modern well completion techniques, where long horizontal perforations are the most efficient well geometry. Bullheading is no longer effective, and coiled tubing expensive. VES containing diverting acids or damage removal treatments provide excellent cost/performance to the operator. To date, a number of limitations of VES technology in-use have limited the widespread adoption of this for fluids in the field. These include: Thermal limits to the viscosifying properties of the depleted acid to about 120 C/250 F Reduction of viscosification upon addition of necessary corrosion inhibitors into the field applied solution Loss of elastic properties (which enhance diversion) of the depleted fluid at low temperatures above about 100 C/210 F Intolerance of the acid/ves to Iron (III) picked up from dissolution of corrosion products, leading to phase separation and potential damage upon injection into the reservoir Requirement of a high concentration of VES (5-8%) in acid to develop diversion, making the solution expensive High toxicity of the viscoelastic surfactant, eliminating products from consideration in some parts of the world, and causing a significant environmental burden where fluids are disposed of in marine environments. A novel technology available from AkzoNobel Surface Chemistry Armovis EHS has been developed that addresses many of these concerns.

3 Armovis EHS 3 Theory of viscoelasticity Surfactant Microscale Mesoscale Macroscale Figure 1. Viscoelastic surfactant, associative behavior and macroscopic effects Viscoelastic surfactant spontaneously forms worklike micelles in saline solutions. This enhanced viscosification as salinity increases is unusual in nature, but can find many uses in the oilfield, including: Fracturing Completion Acidizing Water shut-off The enhanced viscosity with salinity results from spontaneous salting-out of surfactant from free monomers in solution, into the desired wormlike micelles, with the micelles increasing their length and hydrolytic diameter with increased concentration. These worm-like micelles inhibit fluid flow through volume exclusion micellar entanglement The micellar entanglement provides the desirable elastic properties in the fluids, through a process of micellar recovery once the shear-forces are removed (assuming the shear forces have not fragmented the micelles). Even if the shear forces have fragmented the micelles, the micelles will spontaneously re-associate, recovering their form, offering advantage over ultra-high molecular weight polymers (>10MM MW), such as polyacrylamide, which will not recover from high shear degradation, which is important in high temperature (HT) fracturing operations. VES in acidizing deep penetration and diversion In acidizing applications, a concentrated strong acid (typically 20wt% HCl) is injected into the reservoir. The VES is fully miscible in the neat acid, and because the system is non-saline, it does not have significant viscosity during pumping (leading to low pump pressures). The viscosification results directly from dissolution of the carbonate or dolomite reservoir matrix rock, leading to generation of up to 30wt% CaCl 2 or mixed Mg/Ca chloride brine as the acid depletes. Initial acid contact with the reservoir starts to form narrow channels into the matrix rock - called wormholes. Acid depletion occurs first in the near-wellbore, which creates an elastic gel which temporarily protects the near-wellbore which minimizes facial depletion. The non-depleted acid is channelled further into the part-formed wormhole network, penetrating deeper into the matrix before depleting and gelling. This deep penetrating wormhole network creates better contact between wellbore and reservoir postjob. Further, as wormholes initially formed in high permeability zones deplete out and form stiff elastic gels, further acid ingress is inhibited, which is diverted further down the wellbore into lower permeability zones, thus creating a more evenly spread wormhole network and creating improved access to less-easy to stimulate zones. Gelling of Armovis EHS containing acids with depletion 6wt% Armovis EHS in 20% HCl at 200 F low viscosity Breaking of viscosified fluids Diversion and deep penetration of acids are favorable characteristics but these viscous fluids are only desirable in the short term. To be effective in promoting hydrocarbon recovery, the viscous fluids need to be broken upon return of the well to production. A number of breaking mechanisms are known: Residual hydrocarbon breaking. Hydrocarbon that remains in the near-wellbore will attract viscoelastic surfactants to its oil/water interface. This attraction to the interface is higher than to itself, thus disrupting the wormlike micelles and reducing viscosity of the fluid in-situ. Dissolution of the VES in water. Through exposure to saline water, eventually the surfactants in the worms will be lost as monomers to the brines and the viscosity will be lost. This will not work where the formation water is low salinity. Hydrolytic or thermal instability of the VES. Not desirable in acidizing applications if the viscosity half-life is less than the required time of operation of the VES Internal breakers. There is significant IP in chemistries that are incorporated in the VES system, which upon exposure to various triggers will change character and destabilize the wormlike micelles. These can be used in low salinity and/or gas wells 6wt% Armovis EHS in brine resulting from full depletion of limestone at 200 F high viscosity Figure 2. Image of viscosfication of a VES solution with acid depletion at elevated temperature

4 Armovis EHS 4 Ecotoxicity of VES and limitations on use in the North Sea As depleted acid from acidizing operations is usually disposed of directly in the sea, the high concentration (approx. 6-10%) of surfactant used in acidizing fluids and poor toxicity of these surfactants (typical EC 50 <2mg/l), regulatory concerns can be a major factor in use in environmentally sensitive areas, such as the European continental shelf. The novel viscoelastic surfactant in Armovis EHS was designed primarily for optimized performance, but our structure/function understanding of the product also indicated, and molecular modeling confirmed, that the new viscoelastic surfactant should be more environmentally benign. Whilst the below ecotoxicity testing performed does not conform exactly with OSPAR-accepted test methodologies, the stated tests were all performed to GLP standard according to internationally recognized test methodologies using both marine and freshwater test methods. Fish (carp) LC 50 = >100 mg/l (Nominal) (14 mg/l Mean Measured) Daphnia EL 50 = >100mg/L (Nominal) (1.08 mg/l Mean Measured) Brine shrimp- EC 50 = >100 mg/l (Measured) Freshwater algae ErC 50 = >1,048.5 mg/l (Measured) Marine algae - EC 50 = > 1, m g / L ( N o m i n a l ) Biodegradation 61% 28 days OECD 301D

5 Armovis EHS 5 Thermal stability of new VES in use acidizing applications Traditionally used viscoelastic surfactants are only able to viscosify spent acid up to a temperature of around 250 F/120 C. This limits the scope of the application of this product as many carbonate reservoirs have bottom hole temperatures exceeding this temperature. Apparent Viscosity (cp) New VES in 30 wt % CaCl 2 at 350 F s -1 and 400 PSI % EHS 4% EHS 2% EHS Temperature Temperature ( F) Methods such as solvent cooling of the near wellbore can be applied, but these are timeintensive, prolonging the job and costing extra deferred oil and manpower time. They can also damage the reservoir if fines are mobilized and injected. The new VES offers a much broader window of temperatures suitable for application. Figure 3 shows the viscosification profile of different concentrations of the surfactant heated to 350 F/175 C. For effective diversion of fluids, it is believed that a viscosity of 100cP at 100s -1 is required, and this data suggests a dose of about 5% is required to achieve this at this temperature. Further studies to see where the temperature limit of viscosification is with this VES were conducted at 375 F/190 C and 400 F/205 C. The data showed that the viscosity of the fluid was notably impacted at the higher temperatures, resulting in viscosity degradation over time. The effective life of the viscosity of these fluids is 40 minutes and 20 minutes respectively with 6% VES. However, these performance windows may be sufficient to meet application need Time (min) Figure 3. Viscosity of a depleted acid (30wt% CaCl 2 ) with 6% VES at 350 F/175 C

6 Armovis EHS 6 Elastic behavior of the novel VES Uniquely among clear brine viscosifiers at high temperature, VES systems provide fluids with elastic as well as viscous behavior. The elasticity in acidizing helps with diversion of unreacted acids further down the well, optimizing treatment efficiency. Figure 4 shows that the rheological behavior of the depleted VES solution (30wt% CaCl 2 ) as shear is applied to thermally equilibrated 6% solutions of VES in depleted acid. This shows that the fluid has more Newtonian characteristics at low temperature (<100 F/40 C) but that from 150 F/67 C to 350 F/175 C the behavior is strongly shear thinning/pseudoplastic. Above 375 F/190 C, the fluid viscosity was not stable enough to be able to fully map the shear behavior of the fluid. The fluid rheological behavior can be converted into a power law model. The k and n numbers are given for the fluid at different temperatures. Comparable VES systems show significant reduction in pseudoplastic behavior at temperatures much below 250 F/120 C. This shear data conforms to the power law model where shear stress, equation: п = K ( u ) y is given by the following Where: K is the flow consistency index (SI units Pa s n ), u/ y is the shear rate or the velocity gradient perpendicular to the plane of shear (SI unit here is Pascal second (Pa s), and n is the flow behavior index (dimensionless) The effective viscosity of a fluid is a function of shear rate. п 1 Apparent Viscosity (cp) µ eff = K Fluids with an n value of 1 are Newtonian, where they are <1 they are pseudoplastic, and where they are >1 they are dilatent. The VES is strongly pseudoplastic, and beneficial in acidizing applications % EHS ( ) in 30 wt% CaCl2 375F 350F 300F 250F 200F 150F 100F 75F Tempreratures ( F) k' (10-3 pa.s n ) n' ( u ) y y = x R² = y = 1935x R² = y = x R² = y = 11536x R² = y = x R² = y = x R² = y = 21.28x R² = y = x R² = Shear Rate (s-1) Figure 4. Power law plots of the 6% VES at different fluid temperatures in 30wt% CaCl 2

7 Armovis EHS 7 Improved iron (III) tolerance of the novel VES One of the major issues that has hampered the widespread adoption of VES-diverted acids is the occasional failure of acidizing treatments which have led to damage of the reservoir rather than leading to the desired stimulation. The literature, particularly work done by Nasr-El- Din, has shown the culprit is in that FeCl 4 - salt of the viscoelastic surfactant which precipitates from solution, not only eliminating the benefit of the surfactants viscoelastic behavior, but creating solid surfactant-iron chloride salts that can damage the reservoir. The iron source is usually corrosion products either from mixing tanks or from production pipework. Figure 5 illustrates the physical effects of mixing 4% of the new and comparative VES contaminated with iron (III) chloride. With only a few thousand ppm Iron, phase separation with conventional VES is observed, become a solid precipitate with continued iron chloride doping. Concentrations of up to 10,000ppm have been noted in the field and such phase separation could be damaging if injected into low permeability perforations. The VES by comparison, shows excellent compatibility, shows only minor loss in viscoelasticity, and only limited colloidal separation at >9,000ppm Fe (III). Armovis EHS should address the formation damage issues previously observed with VES. New VES system New VES (4 vol %), HCI (20 wt %) and Fe (III) (different concentration) 1000 ppm 3000 ppm 8000 ppm 9000 ppm 10,000 ppm clear solution solution solution solution with precipitate little precipitate Alternate VES fluids Old VES (4 vol %), HCI (20 wt %) and Fe (III) (different concentration) 1000 ppm 3000 ppm 8000 ppm 9000 ppm 10,000 ppm clear solution phase separation phase separation emulsion with precipitation precipitation Figure 5. Iron (III) tolerance of the new VES compared with conventional VES in 20% HCl

8 Armovis EHS 8 Low dose requirements One of the concerns of using VES to divert acid is the relatively high dose of surfactant required to achieve diversion in the treatment acid. Conventional systems require between 6 and 10% of as-supplied product to create the type of viscosity required to divert fluids during acidizing operations, and even then, only to a maximum of approximately 250 F/120 C. Figure 6 illustrates that the new VES requires only about 2% of as-supplied product to achieve the target 100cP at 100s -1. Not only does this dose reduction save in the logistics of getting the product to the field, but it also reduces pumping requirements, possibly manpower requirement and lowers container disposal costs. Post acidizing produced fluids need to be collected and returned to shore for separate remediation if designated marine pollutants. The new VES is significantly more environmentally benign and may not be treated as a marine pollutant based on the ecotoxicity data generated to date. The new VES provides a significantly more efficient solution with much lower overall environmental burden than conventional VES systems. Viscosity (cp) Dose performance of new VES at 250 F - 30% CaCl % 1000 T(F) 5% 800 4% 600 3% 2% 400 1% Time (min) Figure 6. Viscosity/dose effect of as-supplied product at 250 F/120 C illustrating the low dose performance Temperature (F)

9 Armovis EHS 9 Breaking of novel VES systems A critical element of viscosification of depleted acids is that the viscosification be temporary. To benefit from the wormhole network created by the VES-containing acid, the fluid viscosity must be broken to allow the reservoir hydrocarbons to more readily flow into the well. Apparent Viscosity (cp) Breaking of new VES gel by Hexane 6% VES in 30% CaCl % hexane 0.1% hexane 0.5% hexane 1% hexane 5% hexane Breaking of fluids can be achieved in a variety of ways. In-situ hydrocarbons emulsify the surfactant and eliminating the viscosifying effect Dilution in saline brines elimination of the wormlike micelles through diffusion of the surfactant monomers into the bulk phase Internal breakers a variety of internal breakers have been developed for current VES systems. Breaking of the new VES using in-situ hydrocarbons has been demonstrated using hexane as a hydrocarbon analogue. Complete breaking of depleted fluids is achieved with 5% hexane, with stepwise viscosity degradation at lower hexane doses. Postflushing the well with mutual solvent (such as EGMBE) has been shown to break the VES-induced viscosity Temperature ( F) Figure 7. New VES depleted acid breaking using hexane

10 Armovis EHS 10 Coreflooding with the novel VES Coreflooding with the new VES in a 12 inch desert red limestone plug was performed with 20wt% HCl and 4% new VES to determine the suitability of the new product for acidizing. Coreflood work was performed at two temperatures 250 F/120 C and 325 F/165 C with fluid injection to breakthrough. CT scans of the injected core were taken. A typical rising saw-tooth differential pressure profile was observed during injection of the acid until breakthrough - typical of acid diversion as a result of viscosification. Minimal face dissolution was observed with the core in spite of the very high temperatures of the acid and relative corrosivity of the strong acid. In each case, a single predominating wormhole was observed to penetrate the core. This observation confirms the deep penetrating nature of the VES modified acid. Enhanced fluid production results from such networks. Figure 8. Post acidizing coreface and CT Scan of limestone core treated with 20% HCl with 4% new VES at 250 F/120 C

11 Armovis EHS 11 Other benefits observed in testing Tolerance of the new VES to viscosify depleted fluids in the presence of corrosion inhibitors a known issue in the field Suitable for use in dolomitic and limestone reservoirs Viscosification of fluids independent of final fluid ph Suitable for use in fracturing and completion operations, as IP allows Conclusions The novel VES offers significant advances in carbonate reservoir acidizing. In particular: Viscosification of depleted acid at bottom hole temperatures up to 350 F/175 C Elastic fluids properties that can deliver enhanced diversion at higher temperatures An improved tolerance to iron that can limit the potential for phase separation and damage during acidizing operations Significantly improved ecotoxicity over peer VES Lower dose requirements, and hence lower environmental burden, to reach required viscosities to provide diversion, particularly at lower temperatures Effective breaking of depleted acids with insitu hydrocarbons Deep penetrating wormholes in coreflood studies at intermediate and elevated bottom hole temperatures, surpassing anything previously available.

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