US20130146281A1

US20130146281A1 - Method and Acidizing Tool for Deep Acid Stimulation Using Ultrasound

Info

Publication number: US20130146281A1
Application number: US13/707,781
Authority: US
Inventors: Mohamed Nabil Noui-Mehidi; Mohammed H. Al-Khaldi
Original assignee: Saudi Arabian Oil Co
Current assignee: Saudi Arabian Oil Co
Priority date: 2011-12-08
Filing date: 2012-12-07
Publication date: 2013-06-13
Also published as: EP2788577A2; CN104081000B; NO2855231T3; CN104081000A; CA2858088C; EP2788577B1; WO2013086278A3; CA2858088A1; WO2013086278A2

Abstract

A method of deep acid stimulation for a zone to be treated in an underground formation using an acidizing tool, the method including the steps of introducing the acidizing tool into the well bore, introducing the acid formulation onto the well bore wall at the treatment zone and introducing ultrasound energy into the underground formation at the treatment zone. The subsequent acid penetration depth is deeper than the initial acid penetration depth. A method of stress fracturing a portion of an underground formation includes the steps of introducing the acidizing tool into a well bore and introducing the acid formulation and the ultrasound energy at the focused treatment point. The weakened acidized spots in combination with the stress on the underground formation causes oriented stress-induced fractures to form that are fluidly coupled with the well bore. An acidizing tool includes an acid delivery system and an ultrasonic transmitter.

Description

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application claims priority from U.S. Provisional Application No. 61/568,279, filed Dec. 8, 2011. For purposes of United States patent practice, this application incorporates the contents of the Provisional Application by reference in its entirety.

TECHNICAL FIELD OF THE INVENTION

The field of invention relates to a method and device for improving the effectiveness of a matrix acidizing technique by increasing the depth of penetration of the acid throughout a subterranean carbonate formation.

BACKGROUND OF THE INVENTION

It is a common practice to acidize subterranean formations in order to increase the permeability thereof. For example, in the petroleum industry, acidizing fluid can be injected into a well in order to increase the permeability of a surrounding hydrocarbon bearing formation, thereby facilitating the flow of hydrocarbonaceous fluids into the well from the formation. Such acidizing techniques may be carried out as “matrix acidizing” procedures or as “acid-fracturing” procedures.
In acid fracturing, the acidizing fluid is disposed within the well under sufficient pressure to cause fractures to form within the formation. An increase in permeability; therefore, is effected by the fractures formed, as well as by the chemical reaction of the acid within the formation.
In matrix acidizing, the acidizing fluid is passed into the formation from the well at a pressure below the fracturing pressure of the formation. In this case, the permeability increase is caused primarily by the chemical reaction of the acid within the formation with little or no permeability increase being due to mechanical disruptions within the formation as in fracturing.
In most cases, acidizing procedures are carried out in calcareous formations such as dolomites, limestones, dolomitic sandstones, and the like. However, a common difficulty encountered in acidizing these types of formations is presented by the rapid reaction rate of the acidizing fluid with those portions of the formation with which it first comes into contact. This is particularly serious in matrix acidizing procedures. As the acidizing fluid is forced from the well into the formation, the acid reacts rapidly with the calcareous material immediately adjacent to the well. Thus, the acid becomes spent before it can penetrate a significant distance into the formation. For example, in matrix acidizing of a limestone formation, it is common to achieve maximum penetration with a live acid to a depth of only a few inches to a foot from the face of the wellbore. This, of course, severely limits the increase in productivity or injectivity of the well.
Various methods have been attempted to reduce the reaction rate of the acid with the rock formation. For example, others have tried adding reaction inhibitors to the acid formulation. Additionally, other work has focused on ways to reduce the local temperature in order to slow down the reaction rate. However, all of these types of solutions suffer serious drawbacks by increasing the cost and complexity of the matrix acidizing operation. Therefore, it would be advantageous to have a method and a device that provided for an improved deep acid stimulation over those known heretofore.

SUMMARY OF INVENTION

The methods and device provides for matrix acidizing aimed at reaching deeper stimulation zones in the underground formation. The method uses ultrasound energy to push the stimulating acid deeper into the underground formation.
A method for performing a deep acid stimulation of a zone to be treated in an underground formation utilizes an acidizing tool. The method includes the step of introducing the acidizing tool into a well bore. The well bore is operable to permit access to the underground formation. The well bore is also defined by a well bore wall. The acidizing tool is operable to introduce an acid formulation onto the well bore wall. The acidizing tool is also operable to introduce ultrasound energy into the underground formation. The method includes the step of introducing the acid formulation onto the well bore wall at the treatment zone. The acid formulation includes an acid. The introduction of the acid formulation is such that the acid diffuses into the underground formation at the treatment zone to an initial acid penetration depth. The method includes the step of introducing ultrasound energy into the underground formation at the treatment zone. The acid diffuses into the underground formation at the treatment zone to a subsequent acid penetration depth. The subsequent acid penetration depth is deeper into the underground formation than the initial acid penetration depth.
A method of stress fracturing a portion of an underground formation includes the step of introducing the acidizing tool into a well bore such that it is positioned proximate to a focused treatment point. The focused treatment point is associated with a portion of the underground formation under stress. The acidizing tool is operable to direct the acid formulation and the ultrasound energy at the focused treatment point. The method includes the step of introducing at the same time the acid formulation and the ultrasound energy at the focused treatment point. The simultaneous introduction diffuses acid from the acid formulation into the portion of the underground formation under stress. The acid formulation is introduced at a pressure less than the fracture gradient pressure stressed underground formation. The diffused acid creates weakened acidized spots in the underground formation under stress. The weakened acidized spots in combination with the stress on the underground formation causes oriented stress-induced fractures to form that are fluidly coupled with the well bore.
An acidizing tool for use in a well bore traversing through an underground formation includes an acid delivery system operable to introduce an acid formulation onto a well bore wall of the well bore. The acidizing tool also includes an ultrasonic transmitter operable to introduce ultrasound energy into the underground formation.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the present invention are better understood with regard to the following Detailed Description of the Preferred Embodiments, appended Claims, and accompanying Figures, where:

FIGS. 1A-C show an embodiment of the method of using an embodiment of the acidizing tool in a cross-sectional view of a pre-formed well bore;

FIG. 2 show an embodiment of the method of using an embodiment of the acidizing tool in a cross-sectional view of a pre-formed well bore;

FIG. 3 shows a cross-sectional view of an embodiment of the acidizing tool;

FIG. 4 shows a cross-sectional view of an embodiment of the acidizing tool;

FIG. 5 shows the histogram depth analysis for both before and after acid formulation exposure on a first core plug; and

FIG. 6 shows the histogram depth analysis for both before and after acid formulation and ultrasound energy exposure on a second core plug.

In the accompanying Figures, similar components or features, or both, may have the same or a similar reference label.

DETAILED DESCRIPTION

The Specification, which includes the Summary of Invention, Brief Description of the Drawings and the Detailed Description of the Preferred Embodiments, and the appended Claims refer to particular features (including process or method steps) of the invention. Those of skill in the art understand that the invention includes all possible combinations and uses of particular features described in the Specification. Those of skill in the art understand that the invention is not limited to or by the description of embodiments given in the Specification. The inventive subject matter is not restricted except only in the spirit of the Specification and appended Claims.
Those of skill in the art also understand that the terminology used for describing particular embodiments does not limit the scope or breadth of the invention. In interpreting the Specification and appended Claims, all terms should be interpreted in the broadest possible manner consistent with the context of each term. All technical and scientific terms used in the Specification and appended Claims have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs unless defined otherwise.
As used in the Specification and appended Claims, the singular forms “a”, “an”, and “the” include plural references unless the context clearly indicates otherwise. The verb “comprises” and its conjugated forms should be interpreted as referring to elements, components or steps in a non-exclusive manner. The referenced elements, components or steps may be present, utilized or combined with other elements, components or steps not expressly referenced. The verb “couple” and its conjugated forms means to complete any type of required junction, including electrical, mechanical or fluid, to form a singular object from two or more previously non-joined objects. If a first device couples to a second device, the connection can occur either directly or through a common connector. “Optionally” and its various forms means that the subsequently described event or circumstance may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur. “Operable” and its various forms means fit for its proper functioning and able to be used for its intended use.
Spatial terms describe the relative position of an object or a group of objects relative to another object or group of objects. The spatial relationships apply along vertical and horizontal axes. Orientation and relational words including “uphole” and “downhole”; “above” and “below”; “up” and “down” and other like terms are for descriptive convenience and are not limiting unless otherwise indicated.
Where the Specification or the appended Claims provide a range of values, it is understood that the interval encompasses each intervening value between the upper limit and the lower limit as well as the upper limit and the lower limit. The invention encompasses and bounds smaller ranges of the interval subject to any specific exclusion provided.
Where the Specification and appended Claims reference a method comprising two or more defined steps, the defined steps can be carried out in any order or simultaneously except where the context excludes that possibility.

FIGS. 1A-C

FIGS. 1A-C show an embodiment of the method of using an embodiment of the acidizing tool in a cross-sectional view of a pre-formed well bore traversing an underground formation.
FIG. 1A shows underground formation 10 containing treatment zone 15, which is a portion of underground formation 10 to be treated. Underground formation 10 and treatment zone 15 are accessible through well bore 20. Well bore 20 extends from the surface downward to treatment zone 15 and is defined by well bore wall 22. Treatment zone 15 interfaces with well bore 20 at well bore wall 22 and extends radially from well bore 20. Treatment zone 15 has uphole bound 24, which is the uphole-most portion of treatment zone 15 accessible through well bore 20, and downhole bound 26, which is the downhole-most portion of treatment zone 15 accessible through well bore 20.
In FIG. 1A, acidizing tool 30 is introduced (arrow 32) into well bore 20 such that it is positioned proximate to uphole bound 24 of treatment zone 15. Acidizing tool 10 is introduced coupled to coiled tubing 34. Coiled tubing is operable to supply acid formulation and power from the surface to acidizing tool 30. Acid formulation is introduced to treatment zone 15 through acid delivery system 36, which includes acid flow channels 38, which are operable to direct the acid formulation onto well bore wall 22 in treatment zone 15.
An embodiment of the method includes where the treatment zone of the underground formation is made of carbonate rock.
FIG. 1B shows acidizing tool 30 introducing acid formulation (jets 40) to treatment zone 15 through acid flow channels 38. Acidizing tool 30 distributes acid formulation 40 radially onto well bore wall 22 from uphole bound 24 to downhole bound 26 of treatment zone 15. Acid formulation 40 coats well bore wall 22 where distributed, which allows the acid from acid formulation 40 to diffuse and penetrate into treatment zone 15, forming acid treated portion 42 of treatment zone 15. The acid penetrates into treatment zone 15 to initial acid penetration depth 44, which is the depth into underground formation 10 as measured from well bore wall 22. FIG. 1B shows acid formulation 40 introduction while acidizing tool 30 is being further introduced into well bore 20.
The acid formulation includes an acid. Diluted hydrochloric and sulfuric acids are useful examples of acids solutions for the acid formulation. An embodiment of the method includes using a weak acid as the acid in the formulation. Weak acids are acids that do not fully disassociate in the presence of water. Acetic acid, formic acid, fluoroboric acid and ethylenediaminetetraacetic acid (EDTA) are examples of useful weak acids. Weak acids are considered useful in that their reaction is not instantaneous and total with the minerals present in the formation upon contact but rather measured through known reaction constants, permitting application of ultrasound energy. An embodiment of the method includes where the acid has a pH value in a range of from about 2 to about 5.
The acid formulation as part of an applied gel or foam can prolong contact with the well bore wall. The gel or foam can also reduce the amount of the acid formulation that directly contacts the well bore wall, which increases the amount of unreacted acid formulation available for driving into the treatment zone using ultrasound energy. The foam or gel can also improve the locating of the acid formulation as the foam or gel adheres to the well bore wall proximate to where it is distributed. An embodiment of the method includes where the acid formulation is part of a gel that is operable to physically adhere to the well bore wall. An embodiment of the method includes where the acid formulation is part of a foam that is operable to physically adhere to the well bore wall. Pressurized gases, including nitrogen, air and carbon dioxide, are useful for creating a foam to carry the acid formulation.
Acidizing tool 30 also includes ultrasonic transmitter 50 (shown internally). FIG. 1C shows acidizing tool 30 introducing ultrasound energy (arrows 52) to treatment zone using ultrasonic transmitter 50. Acidizing tool 30 transmits ultrasound energy 52 radially into treatment zone 15 from uphole bound 24 to downhole bound 26 of treatment zone 15. Ultrasound energy 52 radiates through acid treated portion 42 of treatment zone 15. Ultrasound energy 52 pushes the acid in acid treated portion 42 deeper into treatment zone 15, forming ultrasonic treated portion 54 of treated zone 15. The acid penetrates deeper into treatment zone 15 to subsequent acid penetration depth 56, which is the depth into underground formation 10 as measured from well bore wall 22. Subsequent acid penetration depth 56 is a greater value (that is, deeper into underground formation 10 from well bore wall 22) than initial acid penetration depth 44. FIG. 1C shows ultrasound energy 52 introduction while acidizing tool 30 is being extracted from well bore 20, which allows acidizing tool 30 to reach the position proximate to uphole bound 24.
The ultrasonic transmitter can introduce the ultrasonic energy into the underground formation at a range of frequencies and a range of intensities based upon the concentration, types, and amount of acid formulation used. An embodiment of the method includes introducing ultrasound energy at a frequency in a range of from about 10 kiloHertz (kHz) to about 1 megaHertz (MHz). An embodiment of the method includes introducing ultrasound energy at an intensity of sonication in a range of from about 1 Watt per square centimeter to about 10 (W/cm2).
The acid formulation and the ultrasound energy are directed by the acidizing tool in the same general direction to promote the dispersion of acid deep into the underground formation. An embodiment of the method includes where both the acid formulation and the ultrasound energy are introduced radially from the acidizing tool. This permits total coverage of the underground formation from the well bore. An embodiment of the method includes where both the acid formulation and the ultrasound energy are introduced to a focused treatment point.
The acid in the acid formulation reacts with the mineral constituents of the underground formation. A useful acid formulation is one where the acid has a reaction rate with the mineral constituents of the underground formation that is lower than the rate of diffusion thought the underground formation. Using a weak acid can prevent all the acid being consumed upon introduction to the well bore wall surface. Also, incorporating the acid formulation into a gel or a foam can also prevent a majority of the acid from being consumed upon initial application to the well bore wall. This permits maximizing the distance of diffusion through the underground formation, which improves the quality of formation stimulation per treatment, instead of simply acidizing the surface of the well bore wall with the entire amount of applied acid. An embodiment of the method includes where a significant portion of the acid does not react with the underground formation until the acid is diffused into the underground formation by the introduction of the ultrasonic energy. In an embodiment of the method, a “significant portion” means at least 50% of the acid introduced with the acid formulation. In an embodiment, a significant portion means at least 60% of the acid introduced. In an embodiment, a significant portion means at least 70% of the acid introduced. In an embodiment, a significant portion means at least 80% of the acid introduced. In an embodiment, a significant portion means at least 90% of the acid introduced. In an embodiment, a significant portion means at least 95% of the acid introduced.
The difference in depth between initial acid penetration depth and the subsequent acid penetration depth depends on several factors, including the intensity of sonication and frequency of the ultrasonic energy, time between application of the acid formulation and application of ultrasonic energy, time of exposure to ultrasonic energy, the acid composition, and the composition of the underground formation. An embodiment of the method includes where the difference in depth between the initial acid penetration depth and the subsequent acid penetration depth, as measured from the well bore wall, is at least 50% greater. An embodiment of the method includes where the difference in depth between the initial acid penetration depth and the subsequent acid penetration depth, as measured from the well bore wall, is in a range of from about 50% to about 90% greater.
The method of treatment does not require introduction of the acid formulation in excess of the fracture gradient pressure of the underground formation. Although potentially useful as a hydraulic fracturing or “fracking” fluid, the acid formulation useful for deep acid stimulation is operable to permit diffusion of the acid into the underground formation through the well bore wall using fluid transport and diffusion mechanics. An embodiment of the method includes introducing the acid formulation at a pressure less than the fracture gradient pressure value of the underground formation.
An embodiment of the method includes not introducing an externally supplied surfactant.

FIG. 2

FIG. 2 show an embodiment of the method of using an embodiment of the acidizing tool in a cross-sectional view of a pre-formed well bore traversing an underground formation similar to FIGS. 1A-C. Acidizing tool 130 introduces acid formulation (jets 140) to treatment zone 115 through acid flow channels 138. Acid flow channels 138 are located in a downhole position along acidizing tool 130. Acidizing tool 130 distributes acid formulation 140 from uphole bound 124 to downhole bound 126 of treatment zone 15. The acid from acid formulation 40 diffuses and penetrates into treatment zone 115, forming acid treated portion 142. The acid penetrates into treatment zone 115 to initial acid penetration depth 144.
Simultaneously, acidizing tool 130 introducing ultrasound energy (arrows 152) to treatment zone 115 using ultrasonic transmitter 150 (shown internal). Ultrasonic transmitter 150 is located uphole of acid flow channels 138. Acidizing tool 130 is introduced such that for a fixed position in treatment zone 115 well bore wall 122 is exposed to acid formulation 140 before introduced to ultrasound energy 152. Acidizing, tool 130 transmits ultrasound energy 152 from uphole bound 124 to downhole bound 126. Ultrasound energy 152 radiates through acid treated portion 142, pushing the acid in acid treated portion 142 deeper into treatment zone 115 to form ultrasonic treated portion 154. The acid penetrates deeper into treatment zone 115 to subsequent acid penetration depth 156. Subsequent acid penetration depth 156 is greater than initial acid penetration depth 144.

FIG. 3

FIG. 3 shows a cross-sectional view of an embodiment of the acidizing tool. Acidizing tool 230 has an acid delivery system 236 with a plurality of acid flow channels 238. Acid flow channels 238 are such that they are operable to introduce acid formulation (jets 240) onto focused treatment point 260. Acidizing tool 230 also has ultrasonic transmitter 250 positioned such that it is operable to introduce ultrasound energy (arrows 252) onto focused treatment point 260. The embodiment of the acidizing tool permits simultaneous introduction of acid formulation 240 and ultrasound energy 252 onto focused treatment point 260, driving acid deep into underground formation 210. Acidizing tool 230 is shown coupled to the surface with coiled tubing 234, which supplies acid formulation, and power conduit 262, which supplies electrical power.
An embodiment of the method of deep acid stimulation includes where the acidizing tool both introduces the acid formulation and the ultrasonic energy simultaneously by directing both towards a focused treatment point. The focused treatment point is a point on or a short length along the well bore wall.
Introducing the acid formulation and the ultrasonic energy simultaneously at a focused treatment point using such an embodiment of the acidizing tool is useful for creating oriented fracturing within a portion of the underground formation under stress. The acidizing tool is introduced into the well bore such that it is located proximate to the focused treatment point. The focused treatment point is associated with the portion of the underground formation under stress.
Simultaneous introduction of both the acid formulation and ultrasonic energy at the focused treatment point diffuses the acid deep into the underground formation at that location. The acid formulation introduction does not require exceeding the fracture gradient of the portion of the underground formation under stress. The acid inside the underground formation reacts with the formation and causes weakened acidized spots to form.
Although shown in FIGS. 1-3 as applying the acid formulation and ultrasonic energy such that the applied acid is driven into the formation in a direction perpendicular to the orientation of the well bore, the methods of deep acid penetration and inducing stress-induced fractures are not limited merely to angles perpendicular to the well bore. In instances where the orientation of underground formation does not lend itself to deep acid penetration at a 90 degree angle relative to the well bore, as often is the case in vertical or deviated directional wells applying acid formulation and ultrasonic energy into thin bands of productive formation, the ultrasonic transmitter is operable for positioning, either remotely or pre-positioned before introduction into the well bore, such that the ultrasonic energy directs the applied acid formulation into the underground formation in a non-perpendicular angle to the orientation of the well bore. For example, FIG. 3 could show acidizing tool 230 having a first ultrasonic transmitter 250 positioned such that its ultrasound energy 252 is directed at an obtuse angle relative to the orientation of the well bore and a second ultrasonic transmitter 250 oriented such that transmitted ultrasound energy 252 is directed at an acute angle relative to the orientation of the well bore.
The creation of weakened acidized spots within the underground formation in conjunction with the stress in the formation causes stress-induced fracturing of the portion of the underground formation under stress. The stress-induced fractures are oriented fluid flow channels that not only fluidly connect with the well bore but also run deep into the underground formation. In an embodiment of the method the stress-induced fractures fluidly connect with the weaken acidized spots. Such oriented stress-induced fractures are fluid cannels useful for additional operations.
Introducing hydraulic fracturing fluid into the oriented stress-induced fractures at pressures greater than the fracture gradient of the underground formation can widen the fractures and open up previously tight underground formations to exploitation, but in a predictable and controllable manner versus simply hydraulically fracturing the underground formation.

FIG. 4

FIG. 4 shows a cross-sectional view of an embodiment of the acidizing tool. Acidizing tool 330 includes first acid delivery system 370 and first ultrasonic transmitter 372 coupled in series with second acid delivery system 374 and second ultrasonic transmitter 376. Acid formulation is distributed from the surface through coiled tubing 334. Both first acid delivery system 370 and second acid delivery system 374 fluidly couple to coiled tubing 334 and to one another. Power conduit 362 transmits power from the surface to both first ultrasonic transmitter 372 and second ultrasonic transmitter 376, which electrically couple together in series. Acidizing tool 330 permits greater acid formulation and ultrasonic energy distribution in a single pass through well bore 320.

Supplemental Equipment

Embodiments include many additional standard components or equipment that enables and makes operable the described apparatus, process, method and system.
Operation, control and performance of portions of or entire steps of a process or method can occur through human interaction, pre-programmed computer control and response systems, or combinations thereof.

Experiment

Examples of specific embodiments facilitate a better understanding of deep acid stimulation method. In no way should the Examples limit or define the scope of the invention.
Two similar carbonate core plugs having similar physical and permeability properties are used in order to test the effect of ultrasound waves on acid penetration depth. Both carbonate core plugs are cylindrical in form with opposing flat faces and are 35 millimeters (mm) in length from face-to-face. The first core plug has an initial permeability value of 6 milliDarcy (mD). The second core plug has an initial permeability value of 8 mD. Each core plug is prepared by wrapping the side of the cylinder in TEFLON (E. I. du Pont de Nemours and Co.; Wilmington, Del.) but keeping the faces exposed.
The acid formulation for the experiment is a composition of a 5 wt % aqueous acetic acid solution. The acid formulation is maintained at 25° C. and is not stirred to maintain static conditions.
Both the first and second core plugs are partially immersed in a bath containing the acid formulation such that one face of the plug is in fluid contact with the acid formulation. The first plug is maintained in its position for two hours without any additional changes to its environment. The second plug followed the same procedure except that the bath containing the acid formulation and the second plug is exposed to ultrasound energy from an ultrasound source for the two-hour acid formulation exposure period. The ultrasound source directs ultrasound energy (at 300 kHz) at the face of the second cylinder immersed in the acid formulation.
After the two hour acid formulation immersion period, the acid penetration distance in both the first and second plugs is determined using computerized tomography (CT) analysis. A CT scanner performs a scan on the two carbonate plugs at 5 mm intervals starting from the fluid-exposed face of the core plug to the non-exposed face. The CT scanner scans both core plugs before treatment to establish a baseline for comparison. For each core plug, 7 CT “slices” along the length of the first and second core plugs both before and after testing help to create histograms that are useful in determining the effects of ultrasound energy introduction on acid penetration depth.
FIG. 5 shows the histogram depth analysis for both before and after acid formulation exposure on the first core plug. FIG. 6 shows the histogram depth analysis for both before and after acid formulation and ultrasound energy exposure on the second core plug.
Histogram analysis shows that both the first and second core plugs reacted with the acetic acid in the acid formulation. A downward shift in the CT distribution values produced by the CT analysis reflects a change in overall density of the core plug at that distance from the face exposed to the acid. The downward shift reflects that the acid dissolved mineral content from within the core plug and lowered its overall density. At distances where no downward shift in CT distribution occurred indicates that the acid did not penetrate to that depth and dissolve minerals from the core plug.
The histogram analysis of the first core plug indicates that the acid penetrated the core plug to a depth no greater than 23 mm from the exposed face. Beyond this distance, there no difference in the CT distribution values before or after treatment of the first core plug, indicating that acid did not penetrate any further into the first core plug.
The histogram analysis of the second core plug indicates that the acid penetrated the core plug to a depth of almost 35 mm from the exposed face. Compared to the first core plug, the effect of introducing ultrasound energy into the core plug during acid formulation treatment increased the acid penetration distance by at least 50%. The experiment shows that the use of ultrasound improves acid penetration depth.

Claims

We claim:

1. A method of deep acid stimulation for a zone to be treated in an underground formation using an acidizing tool, the method comprising the steps of:

introducing the acidizing tool into a well bore, the well bore operable to access the underground formation and defined by a well bore wall, and the acidizing tool operable both to introduce an acid formulation onto the well bore wall and to introduce ultrasound energy into the underground formation;

introducing an acid formulation comprising an acid onto the well bore wall at the treatment zone such that the acid diffuses into the underground formation at the treatment zone to an initial acid penetration depth; and

introducing ultrasound energy into the underground formation at the treatment zone such that the acid diffuses into the underground formation at the treatment zone to a subsequent acid penetration depth, the subsequent acid penetration depth deeper in the underground formation than the initial acid penetration depth.

2. The method of deep acid stimulation of claim 1 where the treatment zone of the underground formation comprises carbonate rock.

3. The method of deep acid stimulation of claim 1 where the acid formulation is a gel operable to physically adhere to the well bore wall.

4. The method of deep acid stimulation of claim 1 where the acid formulation is a foam operable to physically adhere to the well bore wall.

5. The method of deep acid stimulation of claim 1 where the acid is a weak acid.

6. The method of deep acid stimulation of claim 1 where the acid has a pH value in a range of from about 2 to about 5.

7. The method of deep acid stimulation of claim 1 where the ultrasound energy is introduced at a frequency in a range of from about 10 KHz to about 1 MHz.

8. The method of deep acid stimulation of claim 1 where the ultrasound energy is introduced with an intensity of sonication in a range of from about 1 and about 10 W/cm².

9. The method of deep acid stimulation of claim 1 where the subsequent acid penetration depth is at least 50% deeper than the initial acid penetration depth.

10. The method of deep acid stimulation of claim 1 where both the acid formulation and the ultrasound energy are introduced radially from the acidizing tool.

11. The method of deep acid stimulation of claim 1 where a significant portion of the acid does not react with the underground formation until the acid is diffused into the underground formation by the introduction of ultrasonic energy.

12. The method of deep acid stimulation of claim 1 where the underground formation at the treatment zone has a fracture gradient pressure value and the acid formulation is introduced at a pressure less than the fracture gradient pressure value.

13. The method of deep acid stimulation of claim 1 where both the introduction of acid formulation and the introduction of ultrasonic energy occur simultaneously and directed both towards a focused treatment point.

14. A method of stress fracturing a portion of an underground formation, the method comprising the steps of:

introducing the acidizing tool into a well bore such that it is positioned proximate to a focused treatment point, the focused treatment point associated with a portion of the underground formation under stress, the acidizing tool operable to direct an acid formulation and an ultrasound energy at the focused treatment point;

introducing simultaneously the acid formulation and the ultrasound energy at the focused treatment point to diffuse acid from the acid formulation into the portion of the underground formation under stress, the acid formulation introduced at a pressure less than the fracture gradient of the portion of the underground formation under stress,

where the acid diffused into the portion of underground formation under stress forms weakened acidized spots, the weakened acidized spots in combination with the stress in the portion of the underground formation causes oriented stress-induced fractures to form that fluidly couple to the well bore.

15. The method of stress fracturing of claim 14 further comprising the step of introducing a hydraulic fracturing fluid into the oriented stress-induced fractures at a pressure greater than the fracture gradient of the portion of the underground formation under stress.

16. An acidizing tool for use in a well bore traversing through an underground formation, the acidizing tool comprising:

an acid delivery system operable to introduce an acid formulation onto a well bore wall of the well bore, and

an ultrasonic transmitter operable to introduce ultrasound energy into the underground formation.

17. The acidizing tool of claim 16 where the acid delivery system is operable to introduce a gel acid formulation.

18. The acidizing tool of claim 16 where the acid delivery system is operable to introduce a foamed acid formulation.

19. The acidizing tool of claim 16 where the acid delivery system is operable to introduce acid formulation radially.

20. The acidizing tool of claim 16 where the ultrasonic transmitter is operable to introduce ultrasound energy at a frequency in a range of from about 10 KHz to about 1 MHz.

21. The acidizing tool of claim 16 where the ultrasonic transmitter is operable to introduce ultrasound energy with an intensity of sonication in a range of from about 1 and about 10 W/cm².

22. The acidizing tool of claim 16 where the acid delivery system is operable to introduce acid formulation and the ultrasonic transmitter is operable to introduce ultrasonic energy to a focused treatment point.