MX2014009745A - PROCESS FOR HYDRAULIC FRACTURING WITH pH CONTROL. - Google Patents

Abstract

A process for hydraulic fracturing including the steps of (a) providing a fracturing fluid by combining water, proppant, an oxidizing biocide, and a friction reducer; (b) introducing the fracturing fluid into a well; and (c) controlling the pH of the fracturing fluid to a pH of at least about 4.5.

Description

PROCESS FOR HYDRAULIC FRACTURATION WITH PH CONTROL FIELD OF THE INVENTION The present invention relates to the stimulus for the production of underground hydrocarbon formations or stimulation of fluid injection in underground formations in a hydraulic fracturing process. More particularly, the present invention relates to an improved process for hydraulic fracturing in which the performance of additives, in particular, friction reducers, is improved.

BACKGROUND OF THE INVENTION The production of oil and natural gas from a deep well (underground formation) can be stimulated by a technique called hydraulic fracturing in which a fracturing fluid is introduced into an oil or gas well through a conduit, such as pipe or housing, to a flow regime and a pressure to create, reopen and / or extend a fracturing in the well, which allows access to oil or gas within the formation.

The fracturing fluid is usually a water-based solution and may comprise components such as suspended proppant agents (e.g., sand, bauxite); biocides to inhibit the growth of bacteria and other microorganisms; corrosion inhibitors and inhibitors Ref. 250121 incrustations that reduce the formation of rust in the duct; and friction reducers to promote the laminar flow of the hydraulic fracturing fluid in the formation and reduce the pumping pressure necessary to achieve the desired fracture fluid flow rate.

Problems have been encountered in the performance of the friction reducers of organic polymers such as anionic polyacrylamide polymers (either hydrolyzed to produce functionality or copolymerized with acrylic acid). There is a theory that the use of oxidizing biocides in combination with these polymers causes the polymers to degrade and therefore it is necessary to increase the pumping pressure during furation operations. The replacement of an oxidant biocide with a non-oxidizing biocide alleviates the problem of friction reducer performance. However, many non-oxidizing biocides are not acceptable due to environmental concerns and low efficacy. Non-oxidizing biocides include glutaraldehydes and combinations of quaternary amine-glutaraldehyde.

In addition, the friction reduction polymers can be sensitive to pH. It is known, for example, that the performance of a hydrolyzed (anionic) polyacrylamide friction reducer is reduced at or below pH 4.5. At low pH, the protonation of the friction reducers of Polyacrylamide can inhibit its ability to hydrate and release quickly, therefore reducing its ability to reduce friction.

In hydraulic fracturing processes, after the fracturing fluid is introduced into the well, a substantial portion of the fugitive fluid is recovered as production begins in the well. The initial recovery fluid, referred to as "return water" contains contaminants such as hydrocarbons, minerals, and salts that are extracted from the formation during the fracturing process in addition to the fractions of fracturing fluid components, including biocides, friction reducers , etc. that were introduced as part of the fracturing fluid. Since the production continues from the well, the water becomes "produced water", which is the water of natural origin in the formation. The return and produced water can not be simply discarded in a shallow local stream, river, or aquifer, but must be treated to remove contaminants. Due to the shortage of fresh water and the cost of treatment and / or the waste of produced water, it is desirable to reuse at least a part of the water produced in subsequent hydraulic fracturing treatments; however, the presence of contaminants, which includes oxidizable metal ions that produce acids, such as soluble Fe2 + and Mn2 +, it also negatively affects the performance of friction reducers. Once the Fe3 + and Mn4 + are formed, hydrolyze to produce Fe (OH) 3 and Mn02 and acid.

It is highly desirable to avoid the need to increase the pumping pressure to maintain the flow velocity during hydraulic fracturing and to avoid performance problems related to contamination of, or chemical reaction with, friction reducers. It is desirable to use more acceptable biocides, including oxidizing biocides, due to environmental concerns. It is convenient to reuse at least a part of the water produced in a fracturing fluid, which reduces the need for fresh water with each fracturing operation. The present invention satisfies these and other needs.

BRIEF DESCRIPTION OF THE INVENTION The present invention provides a process for hydraulic fracturing in which a fracturing fluid comprising a friction reducing polymer and an oxidizing biocide is introduced into a well. The process comprises providing a fracturing fluid by (a) the combination of water, proppant, an oxidant biocide and a friction reducer; (b) introducing the fracturing fluid into a well; and (c) controlling the pH of the fracturing fluid at a pH of at least about 4.5. These process steps (a) to (c) can be carried out in any suitable order.

PH control surprisingly stabilizes the performance of a friction reducer when an oxidizing biocide is used. In addition, the process of this invention has a greater tolerance to contaminants of the water produced.

In the process of this invention, the pH can be controlled by: (i) measuring the pH of (1) at least one of water, biocidal oxidant, or friction reducer, before step (a) or (2) the fracturing fluid, before or after the fracturing fluid is introduced into a well in step (b); (ii) comparing the measured pH with a set point of a desired pH; (iii) calculate the difference between the desired pH and the measured pH; and (iv) generating a signal corresponding to the difference calculated in (iii) that provides a feedback response to a controller for the addition of a base to at least one of the water, oxidant biocide or friction reducer or fluid of fracturing to control the pH of the fracturing fluid at a pH of at least about 4.5.

As an alternative to the use of a feedback response, any or all of steps (i) to (iv) can be carried out manually. For example, the pH of the desired component or fracture fluid can be measured either manually or by an automatic monitoring system. The comparison and calculation steps can be carried out manually or electronically. The step of generating a Signal can be a response to an automatic online computer or online control system. Alternatively, people can monitor the pH and manually operate a feed pump to add base to the water, biocidal oxidant or friction reducer, or fracture fluid. An operator may, for example, be located in a monitoring room to observe the pH and may contact a second operator (ie, generate and send a signal) with instructions (ie, a feedback response) to add the base, for example of a manual feed pump, to control the pH.

It has been found that in the process of this invention, when the pH of a fracturing fluid comprising a friction reducer and an oxidant biocide is controlled at a pH of at least about 4.5, the performance problems previously encountered are substantially reduced. with deterioration of performance of the friction reducer, such as, increase of the pressure that is required to maintain a desired flow velocity. It has also been found that pH control allows the use of mixed produced water in the hydraulic fracturing water without similar detrimental effects.

BRIEF DESCRIPTION OF THE FIGURE Figure 1 illustrates the provision of a fracturing fluid and the control of the pH thereof according to one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION The present invention is a process for hydraulic fracturing in which a fracturing fluid is introduced (injected) into an oil or gas well at a flow rate and pressure to create, reopen and / or extend a fracturing in the formation around the well, allowing access to oil or gas within an underground formation.

The fracturing fluid comprises water, propping agents, an oxidizing biocide and a friction reducer. The fracturing fluid may further comprise other components such as scale inhibitors and corrosion inhibitors. Surprisingly, it has been found that the performance of the friction reducers does not deteriorate, ie, there are no undesirable pressure increases, in the presence of oxidizing biocides when the pH of the fracturing fluid is controlled at a pH of at least about 4.5, preferably at least about 5.5. In general pH of the fracturing fluid is maintained at a pH less than about 7.

By "oxidizing biocide" is meant herein a compound having biocidal activity, which means that it reduces the amount of bacteria and other microorganisms that may be present in the fracturing fluid, as well as having the potential to oxidize other components in the fracture fluid. he fracturing fluid. Examples of oxidizing biocides include decolorizing agent (sodium hypochlorite, NaOCl), peroxides, such as hydrogen peroxide, peracids, such as peracetic acid, persulfates, ozone and chlorine dioxide. Preferred biocides include decolorizing agent, peracetic acid and chlorine dioxide.

The oxidizing biocide is generally added in an amount to provide a free residual agent in the fracturing fluid. The residual agent may be about 1-5 ppm of the oxidizing biocide. For example, approximately 50-90% of the applied biocide can be consumed. When the biocide is chlorine dioxide, for example, a dose as large as 150 ppm C102 it may be necessary to provide an objective of 1-5 ppm residual agent to achieve an adequate level of disinfection.

Chlorine dioxide is a preferred oxidant biocide. Chlorine dioxide is a gas and can be generated in situ at the location of the oil or gas well. Various methods for generating chlorine dioxide are known, including chemical and electrochemical processes such as those described for example in Ullmann's Encyclopedia of Industrial Chemistry, Wiley Online Library, http://onlinelibrary.wiley.com/doi/lO .1002 / 14356007, a06 483, p ub2 / pdf, accessed February 14, 2012. A particular method for generating chlorine dioxide involves the reaction of an alkali metal chlorite, such as sodium chlorite with an acid, such as hydrochloric acid or sulfuric acid as illustrated below. 5 NaCl02 + 4H + - 4 C102 (g) + 2 H20 + Cl "+ 5 Na + If an excess of acid is used to generate chlorine dioxide, the generated product may have an undesirably low pH which can reduce the pH of any fluid in The one that is injected By controlling the pH of the fracturing fluid as set forth herein, the problem of overfeeding acid from any source is solved without adversely affecting the performance of the friction reducer.

A friction reducer is added to a fracturing fluid to promote the laminar flow of the fracturing fluid, which is important to achieve the desired fracturing at lower pressures, while maintaining high flow velocities within the formation. The performance of the friction reducer is critical to achieve desirable flow rates at the desirable pressure. Poor performance of a friction reducer causes increased pressure or reduced flow velocity, any of which will have a negative impact on the fracturing process, increasing energy costs for increased pressure or increase in time and / or efficiency to achieve the desired fracturing at a lower pressure.

Friction reducers include organic polymers such as polymers and copolymers of acrylic acid and acrylamide. The friction reducers can be anionic, cationic and non-ionic. The anionic friction reducers have lower cost and are the most widely used. Anionic friction reducers are typically modified polyacrylamide polymers (either hydrolyzed to produce acid functionality or copolymerized with acrylic acid) and have carboxylate functionalities. Anionic friction reducers having carboxylate functionality may be more susceptible to performance problems, since these may be protonated at low pH, such as pH values of 4.5 or less.

The friction reducers may be an acrylic-acid-AMPS-polyacrylamide terpolymer, a PS-polyacrylamide salt dispersion copolymer, or a non-ionic polyacrylamide polymer. The friction reducers can be supplied in the form of aqueous dispersions or mixed aqueous / distillate dispersions of the petroleum of a polymer concentrate. A preferred friction reducer is a polyacrylamide.

Friction reducers are typically dosed in an amount of 50 to 1000 ppm (parts per million per volume of polymer dispersion), based on the volume of the fracturing fluid.

Large volumes of water are used in the hydraulic fracturing process. An advantage of the present invention is that when the pH is properly controlled, at least a portion of the water used for fracturing, which can average about 11,000 liters (3000 gallons) per minute, or more, can be return water and produced . The return water recovered from the fracturing operation and the water produced can comprise metal salts including ferrous and ferrous metal salts, hydrocarbons and residual biocide, friction reducer and other additives. In the absence of pH control as set forth herein, the presence of metal salts in the produced water interferes with the performance of the friction reducer. In certain regions, such as the Marcellus Shale region, the water produced is characterized as having a high iron content.

The proppant, which maintains an induced hydraulic fracturing open during or after a fracturing treatment, is most commonly sand, but can also be any other particulate material with adequate mechanical properties to withstand the stresses of closure including, for example, Ceramic, glass and bauxite.

The fracturing fluid may comprise other components, including, for example, polymers, agents of breakage, scale inhibitors, corrosion inhibitors, etc. These other components can be added to the biocide or water, or other options are still available for addition.

The process of this invention comprises providing a fracturing fluid by the combination of water, proppant, an oxidant biocide and a friction reducer. This combination step can be in a single step or multiple steps.

For example, water can be treated with the biocide and with other components such as a scale inhibitor and a corrosion inhibitor before the combination of the treated mixture with proppant and the friction reducer. The water treated with biocide can be stored, for example, for periods of about 30 minutes or less before combining it with a friction reducer. The water used may consist of all fresh water, usually from a stream, pond, or local drinking water supply, or a mixture of fresh water and the water produced. The fresh and produced water can be supplied to the treatment manifold in a single stream or in multiple streams.

"Fracking tanks" are often used as a source of water to supply a constant flow of water for the fracturing process. Water can be supplied to a combination / mixing device, such as a belt mixer in which the friction reducer, proppant and biocide agent or mixture of biocides are added to produce the fracture fluid.

Once the actuation fluid is produced, there is little time (perhaps a matter of seconds) before the fracturing fluid is introduced into an oil or gas well. The fracturing fluid is introduced, or injected, into the well at a pressure of 13.8 to 103 Pa (2000-15000 psi), typically 55-69 MPa (8000-10000 psi). The flow rate is typically several thousand gallons per minute, such as 15,000 liters per minute (4000 gallons per minute).

In the process of this invention, the pH of the fracturing fluid is controlled at a pH of at least about 4.5. Preferably, the pH is controlled at a pH of at least about 5.5. Generally, the pH is controlled at a pH of less than about 7, preferably less than about 6.5.

To control the pH of the fracturing fluid, a pH measurement is required. This measurement can be taken from one or more of the components of the fracturing fluid, in which the components of the fracturing fluid can be selected from the water, the biocide, or the friction reducer. Alternatively, the pH of the furation fluid can be measured. The pH of the fracturing fluid can measure before or after the introduction into the well. The pH measured is compared to a set point of a desirable pH for the component or fracturing fluid. The difference between the desirable pH and the pH measured is calculated. A signal corresponding to the calculated difference is generated, which provides a feedback response to a controller for the addition of a base to at least one of the water, biocide or friction reducer or fracturing fluid to control the pH of the fracturing fluid at a pH of at least about 4.5, preferably at least about 5.5.

The base may be an alkali metal salt or alkaline earth metal salt of hydroxide, oxide, bicarbonate, carbonate, or combinations of two or more thereof, as well as the base produced by the ammonia dissolved in water (ammonium hydroxide base). ). Preferably, the base is soluble in water, more preferably, the base is an aqueous solution.

In particular, when the produced water is used, it is preferred that the base does not cause precipitation of the metal ions present in the produced water. For example, if the water produced comprises barium, calcium and magnesium, the base is preferably an alkali metal hydroxide, more preferably an aqueous solution of ammonium hydroxide or an alkali metal hydroxide.

In general, the preferred alkali metal hydroxides bases are lithium, sodium and potassium hydroxides, more preferably as aqueous solutions. More preferred are aqueous solutions of ammonium hydroxide, or, sodium hydroxide, potassium hydroxide or a combination thereof.

Figure 1 illustrates the provision of a fracturing fluid and the pH control of the fracturing fluid according to one embodiment of the present process for hydraulic fracturing. Certain detailed characteristics of the present process, such as pumps, flow controllers, feed tanks and other auxiliary process equipment are not shown for reasons of simplicity and in order to demonstrate the main characteristics of the process. Such auxiliary features can be easily designed and used by one skilled in the art without undue difficulty or experimentation.

With reference to Figure 1, the water in the feed line 12 is treated by contact with biocide of the chlorine dioxide from the feed line 15 and supplied to the tank 21 commonly referred to as a "fracturing tank". The pH of the treated water 18 is measured and a suitable amount of the base is injected via the line 16 to control the pH to a value at least greater than 4.5. Water with chlorine dioxide treated with pH higher than 4.5 is extracted from the fracturing tank through the line 32 and contacted the friction reducer of line 34 and proppant agent of line 35 in mixing vessel 37 to form the finished fracturing fluid, which is then introduced through line 38 to water well.

EXAMPLES The identity of the commercial polymeric friction reducers used in these examples, all of which were obtained from Kemira (Kennesaw, GA, USA), is as follows: KemFlow ™ A4251 is an anionic hydrolyzed polyacrylamide polymer; KemFlow ™ A4358 is an acrylic acid copolymer of anionic polyacrylamide; and KemFlow ™ C4107 is a cationic polyacrylamide polymer.

Example 1 This example provides the results of the friction cycle tests demonstrating the effect of pH on the friction reduction of various friction reduction polymers.

Friction cycle tests were carried out at Stim-Lab, Inc. located in Duncan, OK, by the use of a standard apparatus known to those skilled in the art. For each test, approximately 34.1 1 (9 gallons) of test fluid was distributed at 37.9 l / min (10 gallons / minute) (Reynolds number approx. 75000). The reduction in friction of the decrease in pressure through an accurate length of the test cycle.

The water used to prepare the test fluids was a mixture of 40% water produced from the Marcelo shale formation and 60% surface water obtained from a location in Pennsylvania. The unadjusted pH of the water was 5.8. For tests run at a lower pH, the water is acidified with sulfuric acid. In some cases acidified water containing about 10 mg / 1 residual C102 was also treated. The test temperature was about 24 ° C (75 ° F).

For each test, a differential pressure of the baseline and flow velocity was established for the fluid without friction reducer. The friction reducer was then injected into the test fluid at a rate of 500 parts per million (ppm) on a volume basis, and the friction reduction percent was determined as compared to the base value. The results were recorded after 5, 10 and 14 minutes of continuous operation and are shown in Table 1. The tests in each of the friction reducers were performed under varying conditions of solution pH and residual C102 concentration, as indicated in the 1A-H tests in the following table.

Table 1 (a) - A4251, an anionic hydrolyzed polyacrylamide polymer. (b) - A4358, an acrylic acid copolymer of anionic polyacrylamide. (c) -C4107, a cationic polyacrylamide polymer.

It can be seen that the lower pH has a substantial negative effect on the friction reduction capacity of the anionic polyacrylamides A4251 and A4358. Therefore, the friction reduction of A4251 at pH 3.4 (IB) is much less than the reduction of friction of the same material at pH 5.8 (1A). Likewise, the friction reduction of A4358 at pH 3.5 (1E) is much lower than the friction reduction of the same material at pH 5.8 (ID). The friction reduction capacity of cationic polyacrylamides such as C4107 (1F and 1G) is substantially affected by pH changes in the range of 3.5 to 5.8. Test fluids 1C and 1H illustrate that the adjustment of the ascending pH with an alkali source, such as sodium bicarbonate (NaHCO3), after the C102 treatment, resulted in the conservation of an acceptable yield of friction reduction.

Example 2 This example demonstrates the effect of pH decrease of oxidant biocide in combination with divalent iron.

The deionized water without regulated pH was adjusted to a pH of 6.0-6.2 with 1 N NaOH or HCl as necessary. Samples 2B and 2C were prepared from this water and 99.5% iron heptahydrate (II) in such a way that the aggregate Fe2 + content was 25 and 50 mg / 1, respectively. Sample 2A was a control sample and did not contain aggregate iron. To each of the samples 2A-2C, 30 mg / 1 of C102 / were applied after which the pH and residual C102 were measured approximately 30 minutes later. The following table summarizes the results that were collected at laboratory ambient temperatures of approximately Sample Fe2 + pH pH C102 mg / 1 C102 mg / 1 mg / 1 initial initial applied residual 2A 0 6.18 6.24 30 28.5 2B 25 6.10 3.61 30 18.1 2C 50 6.10 3.3 30 14.5 The control of sample 2A, without Fe2 +, does not show substantial changes in pH with the addition of C102 and substantially no C102 is consumed (the amount applied and residual is substantially the same). In contrast, C102 in addition to Fe2 + -containing samples 2B and 2C causes substantial decrease in pH to levels below 4.5. According to the oxidation of the iron, the level of residual C102 in 2B and 2C is lower than in the control sample 2A.

From these results, it can be seen that water at relatively neutral pH taken from the environment, which comprises common minerals such as iron, can be reduced to pH below 4.5 when treated with oxidizing biocides such as chlorine dioxide.

It is noted that in relation to this date, the best method known by the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Claims (10)

CLAIMS Having described the invention as above, the content of the following claims is claimed as property.

1. A process for hydraulic fracturing characterized in that it comprises (a) provide a fracturing fluid by the combination of water, proppant, an oxidizing biocide, and a friction reducer; (b) introducing the fracturing fluid into a well; Y (c) controlling the pH of the fracturing fluid at a pH of at least about 4.5.

2. The process in accordance with the claim 1, characterized in that the pH is controlled by (i) measuring the pH of (1) at least one of water, oxidant biocide, or friction reducer, before combining step (a) or (2) the fracturing fluid , before or after the fracturing fluid is introduced into a well in step (b); (ii) comparing the measured pH with a set point of a desired pH; (iii) calculate the difference between the desired pH and the measured pH; and (iv) generating a signal corresponding to the difference calculated in (iii) that provides a feedback response to a controller for the addition of a base to at least one of the water, biocide or friction reducer or fracturing fluid to control the pH of the fracturing fluid at a pH of at least about 4.5.

3. The process in accordance with the claim 3, characterized in that some or all of the steps (i) to (iv) are carried out manually.

4. The process according to claim 3, characterized in that the base is ammonium hydroxide, an alkali metal salt or alkaline earth metal salt of hydroxide, oxide, bicarbonate, carbonate or combinations of two or more thereof.

5. The process according to claim 1, characterized in that the oxidizing biocide is selected from the group consisting of bleaching agent, peroxides, peracids, persulfates, ozone, chlorine dioxide, and combinations thereof.

6. The process according to claim 1, characterized in that the friction reducer is an anionic polyacrylamide or polyacrylamide copolymer.

7. The process according to claim 1, characterized in that the pH is at least about 5.5.

8. The process according to claim 1, characterized in that the pH is less than about pH 7.

9. The process in accordance with the claim 2, characterized in that the pH of the fracturing fluid is measured and measured before introducing the fracturing fluid into the well.

10. The process in accordance with the claim 2, characterized in that at least a portion of the water is water produced.