Patent Application
20100323933
1. Field
This invention relates to fluids for use in the oil field services industry. In particular, the invention relates to methods and compositions for dissolving filter cakes.
2. Description of the Related Art
One particular means of subterranean stimulation includes exposure of a formation coated in a filter-cake to a dissolution fluid; the injected fluid is often shut-in for hours or days to slowly dissolve the filter-cake, whose coating on the formation face limits production from that formation. The filter-cake, formed often from filtration of a drilling fluid into the formation through the exposed porosity, comprises a mixture of polymer (used in the drilling fluid as a viscosifying agent and fluid-loss additive but which precipitates as a solid through leakoff into the formation), solid weighting agents (most often comprising calcium carbonate or barium sulfate), and other fine particulates which join the drilling fluid during drilling from breakdown of the formation.
The formations may comprise either carbonate or sandstone. In sandstone, these formation-particulates can include quartz, clays, shale derivatives, or any of their byproducts upon formation-exposure to aqueous or oil-based drilling mud. The filter-cake (also known as mud-cake) dissolution fluids currently used for these purposes primarily comprise acidic fluids and/or chelating agents (at varying initial pH but most commonly in acidic fluids), whose intention is to render the inorganic portion of the filtercake soluble. These acids and chelating agents most often include aqueous solutions of one or more of the following: hydrochloric acid, hydrofluoric acid, acetic acid, formic acid, methanesulfonic acid (MSA), ethylenediaminetetracetic acid (EDTA), hydroxyethylethylenediaminetriacetic acid (HEDTA), diethylenetriaminepentaacetic acid (DTPA), any of their salts or partial salts (including sodium potassium, ammonium, and other salts) as well as a number of other mineral acids, inorganic acids, organic acids, and salts and partial salts. Other components of the fluid may include “breaker” chemicals, to breakdown the polymeric portion of the filtercake that may coat the inorganic particulates; common breakers including oxidizers (such as ammonium persulfate or sodium bromate), enzymes, and mixtures of these breakers or fully encapsulated versions of these breakers.
One challenge with the execution of a filtercake removal treatment is that it is often very difficult to control the reaction rates of filtercake dissolution when subjected to downhole conditions. Premature, localized breaking of the filtercake, known as pinholing, can cause losses of the remaining filter-cake dissolution fluid into the formation and as a result insufficient removal of the filtercake along the entire interval. Therefore, being able to predict or control the rate or time of dissolution of a filtercake is highly sought-after in stimulation.
A second feature desired in filter-cake dissolution fluid is the ability to carry out other functions while downhole, as the fluid pumped downhole is rarely squeezed into the formation and followed by subsequent fluids (for the added functions). Therefore, downhole applications with the filter-cake dissolution fluid are often limited by the small volume of fluid that can be used for either filter-cake dissolution or subsequent functions. An example of an added function relates to swellable packers. Swellable packers are often in place downhole unswollen prior to filtercake dissolution. It is desired in some cases to have a fluid that can swell the packers (possibly after a shut-in period) and subsequently to carry out controlled dissolution of the filtercake under downhole conditions. These swellable packers can be optionally swellable in either aqueous or hydrocarbon media, though hydrocarbon media (such as diesel) is most common.
Thus, the oil field services industry has a need for a means of controlled dissolution of downhole filtercakes. This controlled dissolution that is obtained by controlled release of reactive chemicals in addition to a means of placing a less-reactive form of an acid downhole (for extended periods of time) which is only activated upon acid-fractionation into an aqueous post-flush fluid. Additionally, more effective compositions for an initial fluid (for packer-swelling) and a filtercake-dissolution fluid are needed.
This invention relates to a composition and a method for dissolving a filtercake in a subterranean formation comprising forming a mixture comprising a dissolution fluid and a fluid comprising hydrocarbon; introducing the mixture into a subterranean formation containing a filtercake; introducing an aqueous fluid to the mixture; and dissolving the filtercake. This invention also relates to further exposing the mixture to swellable packer. This invention relates to a method for a composition, comprising a fluid comprising hydrocarbon; and a dissolution fluid, wherein the fluid comprising hydrocarbon and dissolution fluid are combined to form a miscible mixture that dissolves a filtercake in a subterranean formation.
At the outset, it should be noted that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developer's specific goals, such as compliance with system related and business related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time consuming but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. The description and examples are presented solely for the purpose of illustrating the preferred embodiments of the invention and should not be construed as a limitation to the scope and applicability of the invention. While the compositions of the present invention are described herein as comprising certain materials, it should be understood that the composition could optionally comprise two or more chemically different materials. In addition, the composition can also comprise some components other than the ones already cited.
In the summary of the invention and this description, each numerical value should be read once as modified by the term “about” (unless already expressly so modified), and then read again as not so modified unless otherwise indicated in context. Also, in the summary of the invention and this detailed description, it should be understood that a concentration range listed or described as being useful, suitable, or the like, is intended that any and every concentration within the range, including the end points, is to be considered as having been stated. For example, “a range of from 1 to 10” is to be read as indicating each and every possible number along the continuum between about 1 and about 10. Thus, even if specific data points within the range, or even no data points within the range, are explicitly identified or refer to only a few specific, it is to be understood that inventors appreciate and understand that any and all data points within the range are to be considered to have been specified, and that inventors have disclosed and enabled the entire range and all points within the range.
An embodiment of the invention involves a means of controlling activation of filter-cake dissolving fluid that has been placed into a wellbore. Specifically, the initial fluid pumped downhole involves a miscible solution of diesel and dissolution fluid (often acid) and optional mutual solvent. This fluid initially has very low reactivity toward the acid-soluble component of the filter cake (i.e. calcium carbonate) due to the low reaction rates of solutions of hydrocarbon-borne acid toward the solids. Minerals such as calcium carbonate can be exposed to a miscible acid dispersion under downhole temperature and pressure for extended periods of time (multiple days, depending on the bottomhole temperature) with minimal reaction.
A dispersion, with high concentration of hydrocarbon, can optionally be used to swell oil-swellable packers already in place downhole over this extended period of time without filtercake dissolution. Swellable elastomers useful in the swellable elastomeric compositions may be selected from natural rubber and any substance emulating natural rubber in that they stretch under tension, have a high tensile strength, retract rapidly, and substantially recover their original dimensions. The term includes natural and man-made elastomers, and the elastomer may be a thermoplastic elastomer or a non-thermoplastic elastomer. The term includes blends (physical mixtures) of elastomers, as well as copolymers, terpolymers, and multi-polymers. Examples include ethylene-propylene-diene polymer (EPDM), various nitrile rubbers which are copolymers of butadiene and acrylonitrile such as Buna-N (also known as standard nitrile and NBR). By varying the acrylonitrile content, elastomers with improved oil/fuel swell or with improved low-temperature performance can be achieved. Specialty versions of carboxylated high-acrylonitrile butadiene copolymers (XNBR) provide improved abrasion resistance, and hydrogenated versions of these copolymers (HNBR) provide improve chemical and ozone resistance elastomers. Carboxylated HNBR is also known. In certain exemplary embodiments the swellable elastomer may be the reaction product of a linear or branched polymer having residual ethylenic unsaturation with an ethylenically unsaturated organic monomer having at least one reactive moiety selected from acid, acid anhydride, and acid salt. The swelling time may also be controlled by the identity and concentration of the solvent component of the fluid or other additives, such as surfactant.
Subsequent injection of an aqueous solution which is often denser than the hydrocarbon such as brine, acid, or other fluids (including fluids for acid-fracturing), will pass through the diesel-borne acid, will capture a large proportion of the acid from the diesel solution, and will carry the acid downhole to the filtercake to dissolve the soluble portions of the filtercake more rapidly. This aqueous fluid can be injected using standard equipment from the surface, and will reduce in pH as it travels downhole through the diesel in place (because of the lower diesel hydrocarbon density). Another means of placing the aqueous acid directly along the pay-zone coated with filtercake is using coiled-tubing and injecting or jetting the aqueous solution through the diesel-acid solution in place downhole along the filtercake. An alternate means of exposure to aqueous phase (and subsequent triggering of the acid separation from hydrocarbon) may involve production of either the water-based drilling fluid filtrate or formation water through the filtercake after the prescribed shut-in period.
Controlled chemical release or chemical reactions downhole are highly sought after in stimulation of downhole reservoirs. Embodiments of the invention use a controlled dissolution of filter-cake using a solution of acid that has been initially placed into a wellbore in an “inactive” state that is subsequently “activated” by exposure to a second fluid. Specifically, the initial fluid pumped downhole involves a miscible solution of hydrocarbon and acid and optional mutual solvent. One key to the success of this fluid is the initial high miscibility of all combined components.
Examples of the hydrocarbon include diesel, kerosene, mineral spirits, naphtha, aliphatic hydrocarbons such as hexane, cyclohexane, heptanes octane, and unsaturated hydrocarbons (such as toluene), or other hydrocarbon solvents that can render a density of the final fluid that is lower than a subsequent aqueous fluid.
Examples of the acid include organic acids such as acetic acid, formic acid, lactic acid, glycolic acid, sulfamic acid, malic acid, tartaric acid, maleic acid, methanesulfonic acid, aminopolycarboxylic acids, 3-hydroxypropionic acid, polyaminopolycarboxilic acid, and other organic acids or mixtures of organic acids and their salts or partial salts that are fully miscible in the combined solution. Other examples of the form of acid may include organic or inorganic acids such as hydrochloric acid that are stabilized in acid-internal emlusions.
Finally, mutual solvents that can be used in these fluids include ethyleneglycol monobutyl ether (EGMBE), dipropyleneglycol monomethyl ether (DPME), methanol, ethanol, isopropanol, ethylene glycol, propylene glycol, and oligomers of ethylene glycol and propylene glycol, and others.
In some embodiments, the fluid may include a surfactant. The surfactant may include an amphilphile, wettability modifier, viscoelastic surfactant, or a combination thereof.
Initial qualification of a 100 mL solution of 80% diesel+10% glacial acetic acid+10% EGMBE found that at 83 deg C., this solution alone led to a very low-dissolution of a 5 gram sample of sized-calcium carbonate over a period of days. The table below shows that this solution has very low reactivity toward the calcium carbonate under exposure to heat for a period of 6 days. However, upon addition of 50 mL of 2% aqueous KCl (to a separate ˜2 day sample), the aqueous fluid settled to the bottom of the beaker and dissolved the calcium carbonate sample in under 1 hour.
Therefore, similar diesel-solvent-acid fluids could be prepared by changing the relative concentrations of diesel and acid or diesel, acid, and solvent. Additionally, the diesel-based mixture could contain other additives such as surfactants, demulsifiers, corrosion inhibitors, breakers, encapsulated breakers, viscosifiers, and a number of other additives. Conversely, the aqueous activator solution (final fluid) could also carry one or a number of these additives, specifically enzymes or breaker chemicals toward breaking down the polymeric portion of the filter cake as well. Therefore, the initial solution in place downhole initially has very low reactivity toward the filter cake due to the low reaction rates of hydrocarbon-borne acid toward the acid-soluble component of the cake. Other acid-soluble minerals that may be present in the filter cake may include calcium sulfate and could be dissolved on demand similarly. Conversely, other aqueous fluids have a =pH-reduction tailored to occur on demand once downhole could be injected subsequent to placement of the hydrocarbon-borne acid. These fluids could include acid-fracturing fluids, fluids intended for scale dissolution, matrix-acidizing fluids, and similar aqueous fluids.
From a practical standpoint, this initial low-reactivity of the initial diesel-solvent-acid solution could be in place for extended periods of time, assuming there is a low concentration of water in the filter cake. Injection of sufficient quantity of the diesel-based fluid (or an optional diesel preflush) could ensure that the filtercake is sufficiently water-free to minimize premature filtercake breakthrough. However, only upon exposure of the final aqueous postflush would the acid be passed into the aqueous fluid (through preferential fractionation from the hydrocarbon fluid into the aqueous fluid) and be effective to rapidly break down the filter cake.
Again, the high concentration of hydrocarbon in this two or three-component mixture can optionally be used to swell oil-swellable packers in place downhole over this extended period of time without filtercake dissolution. Diesel is a sufficient fluid to swell the elastomers of these packers. However, other hydrocarbon solvents may be equally effective. Further, the properties of the hydrocarbon phase (and choice of solvent) may impact the speed of packer-swelling. In the case of the diesel-acid or diesel-solvent-acid fluid being placed in the presence of swellable packers, subsequent injection of an aqueous solution such as brine, acid, or other fluids, will pass through the diesel-borne acid, will capture a large proportion of the acid from the diesel solution, and will carry the acid downhole to the filtercake to dissolve the soluble portions of the filtercake more rapidly. This aqueous fluid can be injected using standard equipment from the surface, to carry the acid downhole through the higher density of aqueous fluid compared to diesel. It is understood that this subsequent aqueous stage should not reverse the packer-swelling process. Additionally, in the case of a higher-density hydrocarbon-acid fluid being used, heavy brines could be used to pass through the acid-diesel fluid and through enhanced density travel downhole to the lowest portion of the borehole to attack the filter cake. Another means of placing the aqueous acid directly along the pay-zone coated with filtercake is by using coiled-tubing and injecting or jetting the aqueous solution through the diesel-acid solution in place downhole along the filtercake. This technique could be used specifically in the case of desiring to target dissolution of several targeted zones that are discontinuous along a long pay-zone.
An added feature of the fluid is the low corrosivity of the acid contained in hydrocarbon. Water based filtercake dissolution fluids with equivalent acid to the proposed solution would require corrosion inhibitor to protect the tubulars and casing from corrosion during the shut-in periods. The proposed fluid has been shown in previous tests to have extremely low corrosion rates (e.g. CIDB experiment 128:<0.049 kg/m2 (0.01 lb/ft2) at 204.4 deg C. (400 deg F.) for 6 hrs on N80 steel with no corrosion inhibitor).
The following examples are presented to illustrate the preparation and properties of fluid systems, and should not be construed to limit the scope of the invention, unless otherwise expressly indicated in the appended claims. All percentages, concentrations, ratios, parts, etc. are by weight unless otherwise noted or apparent from the context of their use.
The procedure to obtain these results follows.
The solubility of filtered solid residue is calculated as:
Percent Solubility: [(W1+W2−W3)×100]/W1
The procedure to obtain these results follows.
Calculate solubility and dispersing with the following.
Percent Solubility: [(W2−W3)×100]/[W2−W1]
Fluid=80% Diesel+10% Acetic Acid+10% EGMBE (ethylene glycol monobutyl ether)
The coupon is a thin rectangle of rubber around 2 mm thick whereas the button is 2.54 cm in diameter and 1.27 cm thick
Test conditions: 82.2 deg C. (180 deg F.), no top-pressure, varied hours exposure
Here: AcOH=acetic acid; EGMBE=ethylene glycol monobutyl ether; HFo=formic acid; Na3HEDTA=solution of trisodium HEDTA
One key to the success of the fluid in maintaining controlled filtercake dissolution is the lack of water in the solution (until it is added intentionally to initiate dissolution). Therefore, the preferred embodiments of this fluid must be fully miscible. These observations show that only certain formulations are fully miscible (HCl, formic acid, and Na3HEDTA have varying amounts of water in their formulations)
The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.
