FIELD OF THE INVENTION
- BACKGROUND OF THE INVENTION
The present invention generally relates to hydrocarbon production, and more particularly to well treatment compositions comprising one or more esters or polyesters of hydroxy acid or of glycerol for obtaining controlled acidization in a well.
The following paragraphs contain some discussion, which is illuminated by the innovations disclosed in this application, and any discussion of actual or proposed or possible approaches in this Background section does not imply that those approaches are prior art.
Natural resources such as gas, oil, and water residing in a subterranean formation can be recovered by drilling wells into the formation. Well drilling involves drilling a wellbore down to the formation while circulating a drilling fluid through the wellbore. Various types of drilling fluids, also known as drilling muds, have been used in well drilling such as mineral oil-based fluids and synthetic oil-based fluids. Such drilling fluids typically form a thin, slick filter cake on the formation face that provides for successful drilling of the well bore and that helps prevent loss of fluid to the subterranean formation. In the hydrocarbon bearing portions of a formation, drilling fluids that produce filter cakes of cellulose and starch derivatives and sized calcium carbonate are often employed.
Several stages may be used to produce oil found in subterranean formations. The first stage, which is known as the primary production stage, allows the oil to flow into a production well (or wells) under natural forces. At first, the natural forces may be sufficient to drive the oil to the surface where it is recovered. However, at some point, pumps may be required to displace the oil from the wellbore to the surface. The primary production stage usually yields only about 5% to 15% of the oil in the reservoir. A secondary recovery operation thus is typically performed to recover additional amounts of the oil from the reservoir. A common secondary recovery operation known as secondary flooding involves injecting a fluid such as water into a so-called injection well (or wells) to drive oil in the formation to the production well (or wells). Secondary flooding usually recovers up to an additional 50% of the original oil in the reservoir. Tertiary recovery operations such as tertiary flooding may also be used to drive the remaining oil from the formation to the production well. Unfortunately, the presence of the filter cake on the face of the subterranean formation may adversely affect the flow of fluid though the injection and production wells. The filter cake may occlude the pore structure of the formation. In the case of the injection wells, particularly in deepwater environments, the injected fluid usually is not flowed back to remove the filter cake left by the drilling fluid. However, the pump pressures (e.g., fracturing pressures) required to inject past the filter cake may be higher than desirable for achieving good sweep efficiency of the oil.
A procedure known as acidization has been used for filter cake removal for over a century. In particular, the cellulose of which the filter cake is primarily composed may be decomposed by applying acid to the filter cake. It is believed that the first acidization procedure involved directly injecting strong mineral acids such as hydrochloric acid (HCl) into the well. However, the high reactivity of such strong acids commonly result in the rapid consumption of the acid before it can reach the desired treatment region where the filter cake was located. Further, such acids are highly corrosive and thus attack the metal parts of the well structure, causing irreversible damage to the well.
New acidization treatment solutions have been developed to overcome the problems associated with the use of mineral acids alone. For example, one such treatment solution includes hydrochloric acid emulsified in crude oil such that the aqueous phase, i.e. the solution of acid in water, is surrounded by a continuous oil phase emulsifier that prevents the acid from adversely affecting the metal parts of the well structure. See U.S. Pat. No. 1,922,154 to de Groote. A variation on this treatment solution uses a higher concentration of emulsifier to prolong the stability of the emulsion. See U.S. Pat. No. 2,050,932 to de Groote. Another treatment method involves removing any water in contact with the metal parts of the well before introducing HCl gas absorbed in a mineral oil that is practically immiscible with or insoluble in water to insulate the metal of the well from being attacked by the acid. See U.S. Pat. No. 2,206,187 to Herbsman. Yet another method utilizes both an aqueous fluid and a non-aqueous fluid capable of forming or releasing an acid upon dilution with water. In particular, the well may be filled with oil to protect the metal from the acid, followed by pumping the aqueous fluid down to the formation. The non-aqueous fluid containing the acid-forming substance may then be introduced to the well. See U.S. Pat. No. 2,059,459 to Hund. A treatment solution that uses an ester, such as that derived from glycerol, as an emulsifying agent for an aqueous acid in oil is described in U.S. Pat. No. 2,681,889 to Menaul et al. The ester undergoes hydrolysis to break the emulsion and release the acid. A similar solution uses an acid anhydride such as acetic anhydride in a hydrocarbon carrier fluid to release acid upon reaction with water. See U.S. Pat. No. 2,863,832 to Pernine. A treatment solution comprising an anhydrous organic acid, such as formic acid, acetic acid, or propionic acid, in an anhydrous hydrocarbon has also been described in U.S. Pat. No. 2,910,436 to Alhambra et al. Unfortunately, such acids are as likely to be prematurely exhausted as mineral acids before reaching the desired treatment region. All of the above-mentioned patents are incorporated by reference herein.
- Well Treatment Compositions for use in Acidizing a Well
One modern acidization method involves the generation of acids in the wellbore via the action of enzymes and suitable acid precursors. However, this method is limited by the heat tolerance of the particular enzyme being used and the breakdown temperature of the acid precursor. Treatment at high temperatures results in fast acid exhaustion and enzyme deactivation which results in poor filter cake removal. The enzymes and acid precursors thus need to be stored and handled at the well site carefully to avoid being exposed to relatively high temperatures due to heat and sunlight. Another method relies on the triggered release of acid via the lowering of the pH of an aqueous solution comprising an ortho ester to below about 7. Unfortunately, at elevated temperatures this release may occur in a relatively short period of time. Yet another method growing in popularity relies on the time-dependent reaction of certain esters, such as diethyleneglycol diformate, in an aqueous solution to generate acid such as formic acid. The esters currently being used for this purpose hydrolyze at relatively slow rates at temperatures less than 60° C. However, at higher temperatures those esters hydrolyze too quickly to allow the aqueous solution to be adequately dispersed across the entire filter cake before the acid is consumed. The filter cake removal thus may be localized to a proportionately small area when using such methods, further resulting in the premature loss of the acid-generating fluid through pores that have been unclogged by this localized removal. It is therefore desirable to develop an acidization method in which the acid-releasing reaction occurs at a relatively slow rate over a wide temperature range, particularly at relatively high temperatures.
Some teachings and advantages found in the present application are summarized briefly below. However, note that the present application may disclose multiple embodiments, and not all of the statements in this section necessarily relate to all of those embodiments. Moreover, none of these statements limit the claims in any way.
BRIEF DESCRIPTION OF THE DRAWINGS
An improved acidization solution may comprise an aqueous medium and one or more esters or polyesters of a hydroxy acid or of a glycerol. In embodiments in which the solution comprises an ester or polyester of a glycerol, the solution is substantially absent of a hydrocarbon such as an oil. The hydrolysis of such esters or polyesters occurs at a slower reaction rate than that of other known acidization esters, such as diethyleneglycol diformate, at temperatures higher than 60° C. or even higher than 100° C. Thus, the improved acidization solution may be distributed substantially throughout a region in a well where acidization is required before hydrolysis is completed, despite being exposed to relatively high temperatures. The hydrolysis may result in the slow release of an acid that is capable of consuming undesirable substances in the well, e.g., a filter cake, without being concerned that this consumption might be localized. Moreover, the strength of this acid may be greater than that of the acid formed by diethyleneglycol diformate, i.e., formic acid. Accordingly, the use of esters or polyesters of hydroxy acid or of glycerol for downhole acidization procedures provides for the removal of all or most of the filter cake present in the well.
FIG. 1 depicts an ester hydrolysis apparatus used in the examples provided below.
FIG. 2 illustrates the hydrolysis of various esters described herein and of some control esters in pure water at 100° C.
FIG. 3 illustrates the hydrolysis of various acetins in neutral water at 100° C.
FIG. 4 illustrates the hydrolysis of butyl formate and butyl lactate in neutral water at 100° C.
FIG. 5 illustrates the hydrolysis of ethyl lactate in neutral water and in various brines at 100° C.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 6 illustrates the hydrolysis of methyl-, ethyl-, propyl-, and butyl- lactate esters in neutral water at 100° C.
Well treatment compositions, particularly compositions for performing downhole acidization, may include water combined with (a) one or more esters or polyesters of a hydroxy acid, e.g., lactic acid, and/or (b) one or more esters or polyesters of a glycerol in the absence of a hydrocarbon such as an oil. Such esters or polyesters exhibit a relatively high solubility in the water and are capable of undergoing hydrolysis to produce an acid when exposed to a temperature in a range of from about 60° C. to about 150° C. For example, lactate esters or polyesters release lactic acid when hydrolyzed. The relative amounts of the esters or polyesters and the water in the well treatment compositions may be effective to produce an effective amount of acid to remove most or all of a contaminant in the well such as a filter cake. In various embodiments, the amount of the esters or polyesters present in the aqueous treatment composition is in the range of from about 10% to about 23% by weight or volume of the composition.
Examples of suitable hydroxy acid esters or polyesters for use in the well treatment compositions include lactic acid derivatives, methyl lactate, ethyl lactate, propyl lactate, butyl lactate, trilactin (a trimester of lactic acid and glycerol), and combinations thereof. Ethyl lactate is considered a particularly good lactate ester due to its ability to hydrolyze over the wide temperature range of from about 60° C. to about 150° C., with its half-life being particularly useful at temperatures ranging from about 80° C. to about 140° C. Further, ethyl lactate is relatively inexpensive and is available worldwide, whereas other esters typically have limited availability. For example, diethyleneglycol diformate has an availability limited to Europe and possibly Africa. Examples of suitable esters or polyesters of glycerol for use in the well treatment compositions include tripropionin (a triester of propionic acid and glycerol), trilactin, and esters of acetic acid and glycerol such as monoacetrin, diacetin, and triacetin. Various combinations of the esters or polyesters of hydroxy acid and/or glycerol also may be employed to adjust the half-life of the hydrolysis reaction.
The water contained in the well treatment compositions may be pure (i.e., neutral) water or salt water. In various embodiments, the water may comprise one or more brines capable of forming well treatment compositions having pH values in the range of from about 6 to about 8. Examples of such brines include sodium bromide (NaBr) brine, calcium chloride (CaCl2) brine, sodium formate (NaCOOH) brine, potassium formate (KCOOH) brine, calcium bromide (CaBr2) brine, potassium chloride (KCl) brine, sodium chloride (NaCl) brine, zinc chloride (ZnCl) brine, zinc bromide (ZnBr) brine, and combinations thereof. The choice of brine is usually determined primarily by the weight of fluid desired.
In additional embodiments, the well treatment compositions may include one or more other types of esters for adjusting the half-life of the hydrolysis reaction. For example, diethyleneglycol diformate may be employed in a treatment composition along with an ester or polyester of a hydroxy acid or of a glycerol to reduce the half-life thereof. The diethyleneglycol diformate therefore could be used to shorten the completion time of the well. It forms formic acid as a result of hydrolysis and is commercially available from Halliburton Energy Services, Inc. under the trade name of BDF-325 diethyleneglycol diformate. Examples of other esters with which the esters or polyesters described herein may be combined include diethyleneglycolmonoformate, monoethyleneglycoldiformate, monoethyleneglycolmonoformate, and combinations thereof. Similarly, other esters could be added in conjunction with the lactate esters or polyesters described herein to give faster rates of reaction as necessary.
The foregoing well treatment compositions may be prepared by mixing the one or more esters or polyesters and any other components with water on-site near the well before the acidization operation is to be performed. Alternatively, the components of the treatment compositions could be mixed off-site and transported on-site for storage until its use; however, the extra space required to accommodate the water, both during transport and during storage, makes this option less desirable. In addition, mixing beforehand would require the components to be kept well below 60° C. to avoid hydrolysis of the ester or polyester prior to its addition to the wellbore.
Methods of acidizing a well may be performed by pumping a well treatment composition described herein down the well. The temperatures in the well may be sufficient to cause the hydrolysis reaction between the one or more ester or polyester and the water to occur. For example, the temperatures may be in the range of from about 60° C. to about 150° C. The half-life of the one or more lactate esters or polyesters may be effective to allow the treatment composition to be pumped substantially throughout a region in the well where the acidization is required before the hydrolysis is completed. In various embodiments, the one or more lactate esters or polyesters have half-lives in a range of from about 6 hours to about 16 hours, alternatively from about 8 hours to about 13 hours, or alternatively from about 10 hours to about 11 hours, when hydrolyzed with neutral water at 100° C. Particular examples of the half-lives of various esters of lactic acid and of glycerol are presented in Table 1 below. As used herein, “half-life” refers to the time it takes for half of the original amount of the ester or polyester to react.
| ||TABLE 1 |
| || |
| || |
| || ||Half-Life in Neutral ||Pseudo First Order |
| || ||Water at 100° C. ||Rate Constant |
| ||Ester ||(seconds) ||(sec.−1) |
| || |
| ||Triacetin ||9,840 ||7.04 × 10−5 |
| ||Diacetin ||14,600 ||4.75 × 10−5 |
| ||Monoacetin ||38,400 || 1.8 × 10−5 |
| ||Tripropionin ||32,344.83 ||1.05 × 10−5 |
| ||Methyl lactate ||9,746.19 || 2.1 × 10−5 |
| ||Ethyl Lactate ||31,363.63 || 2.2 × 10−5 |
| ||Propyl lactate ||93,033.7 || 7.4 × 10−6 |
| ||Butyl lactate ||76,704.55 ||9.03 × 10−6 |
| || |
Based on Table 1, the half-lives of such esters at relatively high temperatures are longer than that of other known acidization esters, e.g., diethyleneglycol diformate, which produces formic acid. For example, the half-life of ethyl lactate may be about 10-11 hours, whereas the half-life of diethyleneglycol diformate may be about only 18-20 minutes in neutral water at 100° C. Thus, the reaction of such esters with water proceeds at a slower rate, allowing the treatment composition to be diverted throughout the entire targeted treatment region in the well before the hydrolysis reaction is completed. The production of acid via the hydrolysis reaction occurs during the opportune time at which the acid is in contact with undesirable substances in the well. Moreover, the strength of the acid (e.g., the pH of acetic acid=4.76, the pH of propionic acid=4.86; and the pH of lactic acid=3.08 at 100° C.) is relatively high, and in the case of lactic acid, is even higher than that of formic acid (pH=3.75 at 100° C.). Therefore, the acid may consume all or most of the undesirable substances before all of the ester or polyester is consumed. The choice of which ester or polyester to use in the treatment composition may be based on the application temperature and desired half-life of the ester or polyester.
In various embodiments, the well treatment composition may be used during a well completion operation such as the removal of filter cake from the inner wall of the well. The relatively slow reaction rate of the one or more esters or polyesters may permit the treatment composition to be dispersed across the entire filter cake before the hydrolysis ends. The acid generated by the hydrolysis may decompose calcium carbonate present in the filter cake, which is a major component in the filter cake. The removal of the filter cake ensures that oil, gas, and/or water residing in a subterranean reservoir penetrated by the well can flow into and through the well during production. Otherwise, the filter cake might block migration pathways such as pores in the earth between the reservoir and the interior of the well, thereby preventing the oil, gas, and/or water from permeating through to the well.
The invention having been generally described, the following examples are given as particular embodiments of the invention and to demonstrate the practice and advantages thereof. It is understood that the examples are given by way of illustration and are not intended to limit the specification or the claims to follow in any manner.
In the following examples, various ester hydrolysis reactions were run in the presence of calcium carbonate (CaCO3) to simulate the consumption of filter cake material. The acid evolved in each reaction was measured per unit of time. The acid generated upon hydrolysis was rapidly consumed in a secondary reaction that produced carbon dioxide (CO2). The half-life was secured when half of the CaCO3 was consumed. That is, using PV=nRT, the amount of CaCO3 that would yield approximately 1 liter of CO2 was calculated to be about 4.47 grams. We doubled the amount of CaCO3 (8.94 grams) for the reactions and then calculated the half-life based on the yield of 1 liter of CO2. Unless otherwise specified, all half-life quotations are actually the time required for 4.47 grams of CaCO3 to be consumed.
- Example 1
The following general procedure was used in each of the examples. The closed system reactor shown in FIG. 1 was used to measure the amount of CO2 gas that evolved in each reaction. Water or brine was added to a boiling flask 10, which was heated to 100° C. The CaCO3 was insoluble in the water and thus was observed at the bottom of flask 10. Once the liquid was boiling, a weighed amount of ester was added to flask 10. The amount of ester employed was always in ratio to the water or to the make-up water used to formulate the brine except where commercially available brines were used. Following addition of the ester to the boiling liquid, a reflux condenser 12 was affixed to flask 10. The top of condenser 12 was fixed with an adapter 14 to an inlet tube 18 in a 2-hole rubber stopper 16. The inlet tube 18 was passed through stopper 16, providing a seal. An outlet tube 22 was run from the bottom of a flask 20 to a 1,000 mL graduated cylinder 24. The flask 20 was charged with XP-07 base oil, commercially available from Halliburton Energy Services, Inc. The outlet tube 22 served the purpose of discharging the oil into graduated cylinder 24. The hydrolysis reaction produced acid in flask 10 that reacted with the CaCO3 to produce CO2 gas. The gas displaced the oil in flask 20 into graduated cylinder 24. The rate of CO2 evolution thus could be monitored visually by reading of the amount of oil in graduated cylinder 24 per unit of time. A half-life of greater than 10 hours at 100° C. was desired.
- Example 2
Solutions containing various esters of lactic acid and of glycerol (i.e., monoacetin, ethyl lactate, triacetin, tripropionin), butyl formate (a control), and BDF-325 diethyleneglycol diformate (a control) in neutral water were prepared and combined with a molar ratio of CaCO3. Hydrolysis reactions were then carried out using the resulting solutions at 100° C. The hydrolysis reactions were monitored, and the half-lives of the esters were compared. The volume of oil displaced by the CO2 gas is plotted as a function of time in FIG. 2. The half-lives of the esters of lactic acid and of glycerol were longer than that of the BDF-325 diethyleneglycol diformate. The butyl formate to CaCO3 molar ratio was doubled for comparison purposes. The curve for the 1:1 molar ratio reaction was surprisingly similar to that for the 2:1 molar ratio reaction. Doubling the molar ratio of the butyl formate should have lead to an increase in the relative reaction rate; however, this did not happen. It is believed that this behavior was due to the relative insolubility of the butyl formate since the rate of hydrolysis was determined by the concentration of the butyl formate in solution and not by the amount added.
- Example 3
Solutions containing 10 volume % monoacetin, diacetin, and triacetin in deionized water were prepared and combined with a molar ratio of CaCO3. Hydrolysis reactions were then carried out using the resulting solutions at 100° C. The hydrolysis reactions were monitored, and the half-lives of the esters were compared. The volume of oil displaced by the CO2 gas is plotted as a function of time in FIG. 3. The hydrolysis rates of the triacetin and the diacetin were very similar. However, the monoacetin consumed 4-5 times more slowly relative to the other two acetins, and the monoacetin displayed a half-life within the targeted range. It is believed that the presence of the two hydroxyl groups in monoacetin slowed the rate of hydrolysis through hydrogen bonding with water. Another theory relies on the fact that monoacetin is a mixture of isomers. The acetate group may be on either a primary hydroxyl group or a secondary group. One isomer may be more reactive than the other. Despite the slight differences in rate between the triacetin and the diacetin, it is not believed that the differences in rate are the result of a kinetic effect since triacetin has three acetate groups and monoacetin has one. Thus, the differences in rate may be related to hydrogen bonding.
- Example 4
Solutions containing butyl lactate and butyl formate (a control) in deionized water were prepared and combined with a molar ratio of CaCO3. Similar solutions in which the butyl formate and butyl lactate to CaCO3 molar ratio were doubled were also prepared for comparison purposes. Hydrolysis reactions were then carried out using the resulting solutions at 100° C. The hydrolysis reactions were monitored, and the half-lives of the esters were compared. The volume of oil displaced by the CO2 gas is plotted as a function of time in FIG. 4. The reaction rate of the butyl lactate was faster than that of the butyl formate. This difference in reaction rates was probably due to the difference in solubilities of these two compounds in water. That is, butyl lactate is slightly soluble in water while butyl formate exhibits poor solubility in water. Doubling the concentration of butyl lactate doubled its reaction rate. The poor solubility of butyl formate governed its slow reaction rate, making it unsuitable for use in the acidization of a well. This behavior indicates that an ester needs to be soluble in the aqueous phase before any appreciable reaction can take place.
- Example 5
Solutions containing 10 volume % ethyl lactate in deionized water and various brines were prepared and combined with a molar ratio of CaCO3. Hydrolysis reactions were then carried out using the resulting solutions at 100° C. The hydrolysis reactions were monitored, and the half-lives of the esters were compared. The volume of oil displaced by the CO2 gas is plotted as a function of time in FIG. 5. The ethyl lactate performed consistently in the different aqueous solutions. These results indicate that ethyl lactate would be very suitable for use in the acidization of a well.
Solutions containing 10 volume % methyl-, ethyl-, propyl-, and butyl- lactate in deionized water were prepared and combined with a molar ratio of CaCO3. Hydrolysis reactions were then carried out using the resulting solutions at 100° C. The hydrolysis reactions were monitored, and the half-lives of the esters were compared. The volume of oil displaced by the CO2 gas is plotted as a function of time in FIG. 5. The reaction velocities of the lactate esters decreased as the relative reaction rates of the esters decreased based on the following order:
- MODIFICATIONS AND VARIATIONS
In various embodiments, methods of acidizing in a well comprise placing an ester or polyester of a hydroxyl acid and water in the well, thereby allowing the ester or polyester to undergo hydrolysis. In more embodiments, methods of acidizing in a well comprise placing an aqueous solution comprising an ester or polyester of glycerol in a well, thereby allowing the ester or polyester to undergo hydrolysis, wherein the aqueous solution is substantially absent of a hydrocarbon. In further embodiments, well treatment compositions for performing acidization in a well comprise an ester or polyester of a hydroxy acid and water. In more embodiments, well treatment compositions for performing acidization in a well comprise an ester or polyester of a glycerol and water.
In additional embodiments, the acidization procedure described herein also may be employed to repair damage to the subterranean formation surrounding the well. This damage may be in the form of hydrocarbon wax deposits and/or inorganic salt deposits, such as calcium carbonate deposits, in the pores of a lime sand, lime stone, calcareous, or magnesium formation. Such inorganic salt deposits may occur as a result of subterranean water becoming saturated with alkaline earth carbonates under pressure, followed by the precipitation of the carbonates when the pressure is released after the drill-in of the well. The lactic acid produced by the hydrolysis of the lactate ester or polyester may attack and dissolve the wax and salt deposits, thus increasing the porosity of the formation.
In alternative embodiments, the one or more lactate ester or polyester and water may be added to different types of carrier fluids commonly used in the well. Examples of suitable carrier fluids include but are not limited to a gravel packing fluid, a drilling fluid, a completion fluid, a displacement fluid, and a work-over fluid, all of which are known in the art.
While preferred embodiments of the invention have been shown and described, modifications thereof can be made by one skilled in the art without departing from the spirit and teachings of the invention. The embodiments described herein are exemplary only, and are not intended to be limiting. Many variations and modifications of the invention disclosed herein are possible and are within the scope of the invention.
Accordingly, the scope of protection is not limited by the description set out above but is only limited by the claims which follow, that scope including all equivalents of the subject matter of the claims. Each and every claim is incorporated into the specification as an embodiment of the present invention. Thus, the claims are a further description and are an addition to the preferred embodiments of the present invention. The discussion of a reference herein is not an admission that it is prior art to the present invention, especially any reference that may have a publication date after the priority date of this application. The disclosures of all patents, patent applications, and publications cited herein are hereby incorporated by reference, to the extent that they provide exemplary, procedural, or other details supplementary to those set forth herein.