Microencapsulated Acid with Perforation Strategies to Improve the Delivery and Treatment of Formations in Hydraulic Fracturing Applications

Abstract

Acid plays an important role in the hydraulic fracturing process, such as removing damage from the cement and formation which can result from perforating operations, thus providing a better path for the fracturing operations that follow. The disclosure relates generally to microencapsulated acid or acid precursor for targeted delivery and dosing of acid at the site of perforation in hydraulic fracturing applications, device and method of use the same. The targeted delivery and dosing of acid at the site of perforation provides the benefit of, including but not limited to, eliminating over acidizing (limiting near wellbore formation damage) and optimizing the removal of perforation residue and formation materials for lowering break-down pressure.

Description

TECHNICAL FIELD

[0001] The exemplary embodiments disclosed herein relate generally to microencapsulated acid or acid generators for targeted delivery and dosing of acid at the site of perforation in hydraulic fracturing applications, device and method of using the same. The targeted delivery and dosing of acid at the site of perforation provides the benefit of, including but not limited to, eliminating over acidizing (limiting near wellbore formation damage) and optimizing the removal of perforation residue and formation materials for lowering break-down pressure.

BACKGROUND

[0002] Fracturing operations start with a well that has been drilled to a desired vertical and horizontal depth. Casing is cemented in place to isolate the well from the surrounding geology and groundwater zones. A perforating gun is lowered into the well to a designated location, and one or more charges are fired to perforate the casing, cement and formation. These perforations form the flowpath through which a subsequent stimulation treatment is applied.

[0003] Stimulation treatments involve creating or inducing fractures or enhancing natural fractures in the formation, and may be performed in multiple stages to achieve a desired network of fractures. A mixture of water, sand and chemicals is injected into the wellbore under high pressure to create and propagate the fissures or fractures in the formation. Other types of treatment fluids may also be used depending on the downhole operation, such as drilling operations, perforation operations, sand control treatments, water control treatments, wellbore clean-out treatments, organic scale deposits and inorganic scale treatments, and the like.

[0004] Acid may be used in a hydraulic fracturing process for many reasons including, for example, near wellbore clean out, remove perforation residue, to lower the formation breakdown pressure, and/or to "etch" channels in the rock that comprise the walls of the fracture. Without targeted delivery or dosing, the amount of acid normally required is in very large quantities, which requires shipment and storage of hazardous materials. Additional damage from excess acid can lead to corrosion, scale, and precipitate formation.

[0005] Therefore, there is a need for targeted delivering and dosing of acid at the site of perforation.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] For a more complete understanding of the exemplary disclosed embodiments, and for further advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings in which:

[0007] FIG. 1 illustrates exemplary microencapsulated particulates in which hydrochloride acid is encapsulated;

[0008] FIG. 2 illustrates an exemplary release mechanism of acid or acid precursors from microencapsulated particulates;

[0009] FIG. 3 illustrates an exemplary perforation operation;

[0010] FIG. 4 illustrates a perforation gun assembly useful in an embodiment of the invention;

[0011] FIG. 5 illustrates an exemplary perforation gun assembly containing microencapsulated acid chambers according to an embodiment of the invention;

[0012] FIGS. 6A-6B illustrate an exemplary perforation gun assembly useful in a process of releasing the acid during perforation;

[0013] FIGS. 7A-7B illustrate an exemplary perforation gun assembly containing microencapsulated acid chamber and a process of releasing the acid during perforation;

[0014] FIGS. 8A-8D illustrate an exemplary perforation gun assembly containing an overbalance chamber and a process of releasing the acid according to an embodiment of the invention; and

[0015] FIG. 9 is a flow chart showing the steps for acid treating a wellbore according to an embodiment of the invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0016] The following discussion is presented to enable a person ordinarily skilled in the art to synthesize and use the exemplary disclosed embodiments. Various modifications will be readily apparent to those skilled in the art, and the general principles described herein may be applied to embodiments and applications other than those detailed below without departing from the spirit and scope of the disclosed embodiments as defined herein. Accordingly, the disclosed embodiments are not intended to be limited to the particular embodiments shown, but are to be accorded the widest scope consistent with the principles and features disclosed herein.

[0017] As mentioned above, the embodiments disclosed herein relate to microencapsulated acid or acid generators for targeted delivery and dosing of acid at the site of perforation in hydraulic fracturing applications, method and device of use the same. Although the term "microencapsulated" is used herein, it should be understood the disclosed microencapsulated particulates may range from 0.001 micrometer (.mu.m) to about 5000 .mu.m size particulates.

[0018] As used herein, the term "microencapsulated acid," and grammatical variants thereof, refers to any substance that is able encapsulate or contain of all or a portion of one or more acids, or precursors thereof, to allow targeted placement and reaction of the acid at a perforation site.

[0019] As used herein, the term "acids," or "acid precursors," and grammatical variants thereof, refers to any acids (such as strong mineral acids like HCl, H.sub.2S0.sub.4, HF, H.sub.3P0.sub.4, and HNO.sub.3, or organic acids like acetic acid, tartaric acid, formic acid, or lactic acid) or precursors that has the capability of generating acid in-situ, such as TiCl.sub.3 (solid) or TiCl.sub.4 (liquid).

[0020] Polymer Microencapsulation

[0021] Materials: Polymer microencapsulation is used to create isolated solid, liquid, gas, or blends into individual particles. The polymer particles can be made from different blends of monomers in order to control the reactivity with the internal contents or external environment. Additionally, the polymer microparticles will have a range in internal and external dimensions which controls the weight percent of encapsulated materials, robustness of the barrier, and overall particle dimension and treating mass. Embodiments herein contemplate using encapsulated acids and acid precursors. Some examples of polymer materials that are inert enough to handle the reactive acids include 1-vinylimidazole with N,N-methylenebis(acrylamide), ACN/VDC (Poly(vinylidene chloride-co-acrylonitrile), polystyrene, n-butyl acrylate, acrylic acid and many others.

[0022] One or more embodiments contemplate an acid, such as HCl, HF, acetic acid, tartaric acid, formic acid, or lactic acid or acid precursor, e.g., TiCl.sub.4, to be loaded into the polymer microparticles. Loading the microparticles may be achieved using the following strategies in one or more embodiments.

[0023] Water in oil in water (W-O-W) double emulsion: Surfactant, acid/acid solution, monomer/polymer organic solutions are emulsified into an aqueous surfactant solution creating microcapsules or micro-sponges based on the polymerization process and solvent evaporation step. For example, in the first step of the encapsulation, aqueous acid (e.g., HCl) droplets is dispersed into a hydrophobic monomer phase to form a single emulsion. This is followed by dispersing the single emulsion phase into a second continues aqueous phase to generate a double emulsion. The resulting mixture is then subjected to polymerization of the monomers to form core-shell particles encapsulated with acids.

[0024] Expandable preformed hollow particles: Polymer microparticles will swell and become permeable with elevated temperatures or pressure. This allows acids to be added to the polymer microparticles after synthesis. For example, US 2018/0282609 discloses that fabricated polymeric nano- or microcapsules (e.g., from polystyrene) may be added to a volume of pure titanium chloride, or a corresponding solution of TiClx, and the resulting mixture may be subjected to one or more successive vacuum/fill cycles with an inert gas to diffuse the pure titanium chloride, or corresponding solution, through the shell to fill the cavities of the capsules in order to encapsulate the TiClx in the polymeric shell, which may subsequently produce about 53 wt. % of pure HCl.

[0025] Gas in oil in water (G/O/W) emulsion: surfactant, acid and organic based polymer solution are emulsified with an aqueous surfactant solution creating large swollen micro-sponges when the solvent is evaporated.

[0026] Cleaning excess acid/acid generators can be achieved through additional washes or with centrifugation and drying steps.

[0027] Post encapsulation modification of the polymer shell can also be performed through annealing (heat treatment) or coating will further isolate the acid and render the materials more inert. If the polymer barrier is more robust, then more drastic triggers would be used to release the acid payload.

[0028] FIG. 1 illustrates exemplary microencapsulates in which 12 M hydrochloride acid is encapsulated in a Poly(vinylidene chloride-co-acrylonitrile polymer shell.

[0029] In one aspect, acid may be target-delivered at the perforation site, and used in a hydraulic fracturing process to remove perforation residue. Preliminary calculations indicate that at most .about.1 L of pure HCl is required, per perforation, to effectively clean the cement and perforation residue. This can be generated from approximately 250 g of TiCl.sub.4 (a concentrated acid precursor), assuming enough water will ultimately be available for the reaction:

TiCl.sub.4+2H.sub.2O.fwdarw.4HCl+TiO.sub.2

[0030] This equates to 1.5 kg of TiCl4 to treat a 6-perforation cluster. If the TiCl4 is encapsulated as described above, this quantity could be contained in a cylindrical chamber approx. 5-ft. long (assuming 21/2 in ID).

[0031] Release Mechanism

[0032] One of the applications of acids in hydraulic fracturing is to clean up the cement and perforation residue and lower the formation breakdown pressure, meaning that the acid is only needed within the perforation process. Additional damage from excess acid will lead to corrosion, scale, and precipitate formation. Therefore, there is a need for targeted delivering and dosing of acid at the site of perforation.

[0033] In one aspect of the present disclosure, release of acid can be triggered with time, temperature, pressure, or explosion (extreme heat/pressure). Controlled release of the acid can be achieved with tuning of the polymer compositions, thickness and acid loading amounts.

[0034] In one embodiment of the present disclosure, encapsulated acid is released by perforation guns. FIG. 2 illustrates an exemplary release of acid from microencapsulates 202 via a perforation gun 200. Extreme heat and/or pressure from the explosion created by the perforation gun 200 rupture the polymeric encapsulated shell 202 and releases the acid in the core.

[0035] In another aspect of the present disclosure, the acid is target-delivered into the perforation volume and damage zone, as illustrated in FIG. 3, to clean up the cement and perforation residue and lower the formation breakdown pressure. The microencapsulated acid is introduced from the wellbore 300, or from a perforating gun assembly (not expressly shown) in the wellbore 300, through the casing 301 and cement lining 302 into the perforation volume 303 and perforation damage zone 304. This perforation promotes oil flow from the reservoir 305. The perforation tunnel length 306 and perforation diameter 307 may be selected as a matter of design preference.

[0036] Targeted Delivery of Acid to the Perforation Site

[0037] Delivering the microencapsulated acid to the path of the perforation gun can be achieved through various embodiments of the invention. In one embodiment, the microencapsulated acid is loaded in pumping fluid that is used to drive the perforation gun into place. This embodiment is suitable for a conventional perforating gun such as shown with respect to FIG. 4. FIG. 4 shows a perforating gun system 400 adjacent plug 401. The perforating gun system 400 includes a setting tool 402 and any desired number of perforating guns 403a, 403b, 403c through 403n. Perforating guns 402a-403n may be conventional shaped charged perforating assemblies that are well known in the art. When perforating guns 403a-403n are fired, creating perforations in the casing and cement, the pressure of the pumping fluid in the wellbore forces the microencapsulated acid through the perforations and into the perforation volumes and perforation damage zone in the formation.

[0038] In another embodiment, microencapsulated acid can be loaded into a perforation gun assembly, as shown in FIG. 5. FIG. 5 shows a perforating gun assembly 500 according to an embodiment of the invention having a plug 501, a setting tool 502, and a perforating gun 503 with an adjacent acid payload chamber 504. The perforating gun 503 and the acid payload chamber 504 form a perforating gun sub-assembly, additional examples of which are indicated at SOS. If desired, any number of similar perforating gun sub-assemblies SOS could also be provided on the perforating gun assembly 500 as a matter of design choice. In this exemplary embodiment, the microencapsulated acid or acid precursor is placed within a dedicated chamber 504 ("acid payload chamber") of the perforation gun assembly 500 adjacent to a perforating gun payload chamber 503, as illustrated in FIG. 5. Once the perforation gun assembly arrives at the targeted perforation site, the acid payload chamber 504 is activated to release the acid/precursor payload into the wellbore just prior in time to activating the perforating gun 503. The acid payload chamber release mechanism may be any suitable release mechanism, including an opening valve, sliding sleeve, or explosively-opened port(s), or similar mechanisms.

[0039] The release of the acid from microencapsulates may be accomplished in still further embodiments. For example, with reference to FIGS. 6A and 6B, a perforating gun assembly 600 according to an embodiment of the invention could be lowered into the wellbore 601 to create perforations 602. The perforating gun assembly 600 could then be removed and an acid pad loaded with microencapsulated acid could be lowered into the wellbore 601 and introduced into the perforations 602.

[0040] FIGS. 7A and 7B show yet a further embodiment in which the microencapsulated acid can be released in a more precise manner using a perforation gun assembly 700, which may be similar to the perforation gun assembly discussed with regard to FIG. 5. Referring to FIG. 7A, perforating gun assembly 700 includes an acid payload chamber 701 adjacent to perforating gun chamber 702. In operation, the acid payload is first released from the acid payload chamber 701 into the wellbore prior to activation of the perforation gun. As shown in FIG. 7B, the position of the perforation gun assembly 700 is precisely adjusted in the wellbore 703 so that the perforation gun chamber 702 is aligned with the position of the acid payload in the wellbore 703. The perforation gun chamber 702 is then activated via a perforating gun firing command from the surface and/or with a pre-programmed time delay between the acid payload chamber 701 and the gun chamber 702. The encapsulated acid is released during the perforation and is target-delivered into the perforation volume or damage zones.

[0041] Another embodiment of the invention is with regard to FIGS. 8A-8D. In this embodiment, a perforation gun assembly 800 includes an acid payload chamber 801, a perforation gun chamber 802, and optionally an overbalance chamber 803 containing propellant or similar material. The perforation gun assembly 800 may also include setting tool 804, which is positioned adjacent to plug 805, and any desired number of additional perforating assemblies 806, arranged similarly to the assembly comprising acid chamber 801, perforation gun 802 and propellant chamber 803.

[0042] If sufficient overbalance does not exist to push the acid into the perforation volume or damage zone, the overbalance chambers 803, which contain propellant or similar material, can be activated to create a transient overbalance condition, thus pushing the recently-released acid or acid precursor into the perforation volume or damage zone. In one embodiment, the release of the overbalance chamber can be triggered by the same firing command from the surface, with another time delay if desired. FIGS. 8B-8D illustrate an exemplary timing sequence of the above 3-chamber operation. As shown in FIG. 8B, the perforation gun assembly 800 is first positioned in the wellbore 807 at a desired location. The acid chamber 801 is then activated to release microencapsulated acid contained inside the chamber into the wellbore 807. Next, as depicted in FIG. 8C, the perforation gun assembly 800 is repositioned in the wellbore 807 so that the perforation gun 802 is located at approximately the same location in the wellbore 807 where the microencapsulated acid was released. The perforation gun 802 is then activated by a signal from the surface or, in other embodiments, by a time delay following the release of the microencapsulated acid. Finally, as shown in FIG. 8D, the perforating gun assembly 800 is again repositioned in the wellbore 807 such that the overbalance chamber 803 is located at approximate adjacent to the perforations created in by the activation of the perforating gun 802. Activation of the overbalance chamber 803 creates a pressure overbalance in the wellbore 807 that pushes a strong acid into the perforations in the wellbore and into the perforation volume and perforation damage zone.

[0043] Various methods of delivering microencapsulated acid or precursor to treat formations in hydraulic fracturing applications are also provided according to embodiments of the invention. In one embodiment, by slowly pulling the perforation gun assembly uphole, the acid payload chamber, the perforation gun chamber, and the overbalance chamber can be activated sequentially and at substantially in the same location in the wellbore. This allows the microencapsulated acid to be target-released into the perforations.

[0044] Embodiments of the inventive method are described more fully with regard to FIG. 9. A perforating gun assembly according to embodiments of the invention is first lowered to a desired starting location in the wellbore. The operator, at step 901, then sends a firing command to the assembly from the surface. At approximately the same time, the perforating gun assembly is slowly pulled uphole. The firing command, at step 902, activates the acid chamber, which releases a payload of microencapsulated acid into the wellbore. A first time delay following the activation of the acid chamber is provided to allow the perforating gun assembly, which is continuously being pulled uphole, to position the perforating gun to approximately the same location in the wellbore at which the microencapsulated acid or precursor was released. At step 903, the perforating gun is activated, thereby perforating the casing and creating perforations into the formation. In still a further embodiment, the method may include step 904 if there is insufficient overbalance to ensure the acid is driven into the perforations. In step 904, after a second time delay to allow repositioning of the acid chamber to approximately the same location in the wellbore as the perforations, an overbalance chamber is activated to create an overbalance condition sufficient to put the acid into the perforations.

[0045] In yet another embodiment, the microencapsulated acid/precursor could be packaged within the perforating gun chamber itself, rather than separate dedicated acid chambers. Further, the acid payload chamber, and propellant/overbalance chamber (if required), could be combined into a single physical chamber serving both functions.

[0046] Thus, the targeted delivery and dosing of acid at the site of perforation in hydraulic fracturing applications disclosed herein provides a number of benefits over existing acid delivery mechanism. These benefits include, for example, eliminating over acidizing, limiting near wellbore formation damage, optimizing the removal of perforation residue and formation materials for lowering break-down pressure, and eliminating the need of transportation and storage of corrosive acids, among others.

[0047] Accordingly, as set forth above, the embodiments disclosed herein may be implemented in a number of ways. For example, in general, in one aspect, the disclosed embodiments relate to a perforation gun assembly, comprising a microencapsulated acid payload chamber and a perforating gun payload chamber, or a chamber containing both a microencapsulated acid payload and a perforating gun payload, and optionally an overbalance payload chamber. In another aspect, the disclosed embodiments relate to a method for fracturing operation using the perforation gun assembly disclosed in accordance with any one or more of the foregoing embodiments.

[0048] In accordance with any one or more of the foregoing embodiments, the acid payload chamber or the chamber containing both an acid payload and a perforating gun payload contains a microencapsulated acid, an overbalance payload chamber contains propellant or similar material and can be activated to create a transient overbalance condition, and/or the acid payload comprises microencapsulated acid particulates with a polymeric shell and an acid or acid precursor core.

[0049] In accordance with any one or more of the foregoing embodiments, an acid payload chamber and a perforating gun payload chamber are present, wherein the acid payload comprises microencapsulated acid particulates with a polymeric shell and an acid or acid precursor core, and an overbalance payload chamber is present, wherein the overbalance payload chamber contains propellant or similar material, and can be activated to create a transient overbalance condition.

[0050] In accordance with any one or more of the foregoing embodiments, acid release occurs simultaneously with creating a perforation, and/or an acid precursor is released by perforating microencapsulated acid particulates and simultaneously creating a perforation.

[0051] In accordance with any one or more of the foregoing embodiments, a perforation gun assembly is used, and the perforating operation comprises releasing an acid payload from the acid payload chamber, activating the perforating gun payload chamber, and simultaneously, releasing acid or acid precursor by creating a perforation.

[0052] In accordance with any one or more of the foregoing embodiments, performing a perforating operation comprises releasing microencapsulated acid payload from the acid payload chamber, and activating the perforating gun payload chamber, and simultaneously, releasing acid or acid precursor by perforating microencapsulated acid particulates and creating a perforation.

[0053] In accordance with any one or more of the foregoing embodiments, the acid payload chamber and the perforation gun chamber can be released/activated sequentially and substantially in the same location, and the acid and/or acids precursor can be target-released into the perforation.

[0054] In accordance with any one or more of the foregoing embodiments, performing a perforating operation comprises using the perforation gun assembly, releasing an acid payload from the acid payload chamber, activating the perforating gun payload chamber, and simultaneously, releasing acid or acid precursor by creating a perforation, and activating the overbalance payload chamber, pushing the released acid or acid precursor into the perforation.

[0055] In accordance with any one or more of the foregoing embodiments, performing a perforating operation comprises releasing a microencapsulated acid payload from the acid payload chamber, activating the perforating gun payload chamber, and simultaneously, releasing acid or acid precursor by perforating microencapsulated acid particulates and creating a perforation, and activating the overbalance payload chamber, pushing the released acid or acid precursor into the perforation. The acid payload chamber, the perforation gun chamber, and the overbalance chamber can be released/activated sequentially and substantially in the same location, and the acid and/or acid precursor can be target-released into the perforation.

[0056] In accordance with any one or more of the foregoing embodiments, the perforation gun assembly comprises an acid payload chamber and a perforating gun payload chamber, wherein the acid payload comprises microencapsulated acid particulates with a polymeric shell and an acid or acid precursor core, the microencapsulated acid particles range from 0.001 micrometers to 5000 micrometers, the microencapsulated acid particles comprise one of HCl, H2S04, HF, H3P04, or HNO3, the microencapsulated acid particles comprise an organic acid, the organic acid comprising one of acetic acid, tartaric acid, formic acid or lactic acid, and/or the microencapsulated acid particles are encapsulated in an inert polymer material.

[0057] While the invention has been described with reference to one or more particular embodiments, those skilled in the art will recognize that many changes may be made thereto without departing from the spirit and scope of the description. Each of these embodiments and obvious variations thereof is contemplated as falling within the spirit and scope of the claimed invention, which is set forth in the following claims.

Claims

1. A perforation gun assembly, comprising: an acid payload chamber and a perforating gun payload chamber, or a chamber containing both an acid payload and a perforating gun payload.

2. The perforation gun assembly of claim 1, wherein the acid payload chamber or the chamber containing both an acid payload and a perforating gun payload contains a microencapsulated acid.

3. The perforation gun assembly of claim 1, further comprising an overbalance payload chamber, wherein the overbalance payload chamber contains propellant or similar material, and can be activated to create a transient overbalance condition.

4. The perforation gun assembly of claim 1, wherein the acid payload comprises microencapsulated acid particulates with a polymeric shell and an acid or acid precursor core.

5. The perforation gun assembly of claim 1, further comprising an acid payload chamber and a perforating gun payload chamber, wherein the acid payload comprises microencapsulated acid particulates with a polymeric shell and an acid or acid precursor core.

6. The perforation gun assembly of claim 5, further comprising an overbalance payload chamber, wherein the overbalance payload chamber contains propellant and can be activated to create a transient overbalance condition.

7. A method for performing a perforating operation in a subterranean formation, comprising: using a perforation gun assembly having an acid payload chamber and a perforating gun payload chamber, or a chamber containing both an acid payload and a perforating gun payload; and releasing acid from the acid payload chamber while simultaneously creating a perforation in the subterranean formation.

8. The method of claim 7, further comprising releasing an acid precursor by perforating microencapsulated acid particulates and simultaneously creating a perforation in the subterranean formation.

9. The method of claim 7, wherein the acid payload comprises microencapsulated acid particulates with a polymeric shell and an acid or acid precursor core.

10. The method of claim 9, further comprising: releasing the microencapsulated acid payload from the acid payload chamber, and activating the perforating gun payload chamber while simultaneously releasing acid or acid precursor by perforating microencapsulated acid particulates and creating a perforation in the subterranean formation.

11. The method of claim 10, wherein the acid payload chamber and the perforation gun chamber can be released/activated sequentially and substantially in the same location, and the acid and/or acids precursor can be target-released into the perforation.

12. A method for performing a perforating operation in a subterranean formation, comprising: using a perforation gun assembly having an acid payload chamber and a perforating gun payload chamber, or a chamber containing both an acid payload and a perforating gun payload, and an overbalance payload chamber, wherein the overbalance payload chamber contains propellant and can be activated to create a transient overbalance condition; and releasing an acid payload from the acid payload chamber while simultaneously creating a perforation in the subterranean formation.

13. The method of claim 12, further comprising: activating the perforating gun payload chamber, and simultaneously, releasing acid or acid precursor by creating a perforation; and activating the overbalance payload chamber and pushing the released acid or acid precursor into the perforation.

14. The method of claim 13, further comprising releasing a microencapsulated acid payload from the acid payload chamber; activating the perforating gun payload chamber, and simultaneously, releasing acid or acid precursor by perforating microencapsulated acid particulates and creating a perforation; and activating the overbalance payload chamber, pushing the released acid or acid precursor into the perforation.

15. The method of claim 14, wherein the acid payload chamber, the perforation gun chamber, and the overbalance chamber can be released/activated sequentially and substantially in the same location, and the acid and/or acid precursor can be target-released into the perforation.

16. The method of claim 15, wherein the microencapsulated acid particles range from 0.001 micrometers to 5000 micrometers.

17. The method of claim 15, wherein the microencapsulated acid particles comprise one of HCl, H.sub.2S0.sub.4, HF, H.sub.3P0.sub.4, or HNO.sub.3.

18. The method of claim 15, wherein the microencapsulated acid particles comprise an organic acid.

19. The method of claim 18, wherein the organic acid comprises one of acetic acid, tartaric acid, formic acid or lactic acid.

20. The method of claim 15, wherein the microencapsulated acid particles are encapsulated in an inert polymer material.