BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The invention relates generally to systems and methods for creating lateral tunnels within and stimulating subterranean formations surrounding wellbores.

2. Description of the Related Art

[0002] Well stimulation is used to increase flow of hydrocarbon fluids from hydrocarbon- bearing strata and formations which surround a wellbore. Well stimulation can include techniques such as fracturing (or fracking) wherein fracturing is performed by a fracturing arrangement located within the main wellbore in order to increase flow from the formation into the main wellbore.

SUMMARY OF THE INVENTION

[0003] The invention provides systems and methods for stimulating a hydrocarbon- bearing formation which radially surrounds a wellbore. In a described embodiment, a combined acid tunneling and fracturing arrangement is run into a wellbore and includes a running string, such as a coiled tubing running string, and a bottom hole assembly having an acid placement tool which is carried by the running string. In preferred embodiments, the acid tunneling and fracturing arrangement also includes a packer element. Acid is pumped from the surface through a flow bore in the running tool.

[0004] In a described embodiment, the acid placement tool has a wand with one or more nozzles through which acid can exit the tool. Preferably, the acid placement tool has at least one articulated joints which allow the wand to be angularly flexed with respect to a tubular base portion. [0005] In preferred embodiments, the tool preferably include one or more sensors which can measure one or more downhole parameters, including deviation, azimuth, pressure, temperature and gamma ray. These sensors are installed within or upon the bottom hole assembly. In some embodiments, the bottom hole assembly incorporates a casing collar locator for measuring depth and/or a lateral camera.

[0006] Data communication conduit is provides to communicate data indicative of the downhole parameters sensed by the sensors to a controller at surface. Preferably, Telecoil ^® is used to transmit information obtained by the sensors of the bottom hole assembly to surface. Other telemetry means, such as optical fiber, could also be used. Information obtained by the sensors is preferably used to control the acid placement tool. The information, such as location, azimuth, tool inclination, pressure, and temperature, is used to control and map tunnels in real time. For instance, if detected lateral tunnel length is less than desired, additional acid can be pumped to lengthen the lateral tunnel. In another example, if detected tunnel trajectory is not as planned, the tunneling tool can be indexed, flexed or straightened to extend the tunnel in the desired direction.

[0007] In general, wellbore stimulation is performed by, first, acid drilling at least one lateral tunnel from the main wellbore into the surrounding formation with an acid placement tool. Acid exiting the acid placement tool during acid drilling can return to surface through the annulus which is formed between the acid tunneling and fracturing arrangement and the wellbore wall. Next, at least a portion of the acid placement tool is run into the lateral tunnel which is formed. A packer is set uphole from the acid placement tool, sealing off the annulus. Thereafter, acid is injected through the acid placement tool under a pressure which is sufficient to fracture portions of the surrounding formation. Fracturing is initiated within the lateral tunnel rather than within the main wellbore.

[0008] In preferred embodiments, the combined acid tunneling and fracturing arrangement is provided with an adjustable acid injection wand wherein end nozzles and lateral nozzles can be selectively closed off or opened to direct acid injection where desired. In described embodiments, a sliding sleeve member within the wand is moved between a first position, wherein the lateral nozzles are blocked against fluid flow, and a second position, wherein the sleeve member does not block the lateral nozzles. The sliding sleeve member may be moved by either landing a ball or plug member upon an associated ball seat or by a linear actuator. The end nozzles may be selectively closed off by a landed ball or plug member. In certain embodiments, the ball or plug member is dissolvable so that fluid flow through the end nozzles can be reestablished after a period of time.

[0009] Upon completion of the acid fracturing operation, the packer is unset and the acid tunneling and fracturing arrangement is withdrawn from the lateral tunnel. Thereafter, the acid tunneling and fracturing arrangement may be withdrawn from the wellbore or moved to a new location within the wellbore to repeat the tunneling and fracturing process.

[0010] If desired, the acid tunneling and acid fracturing operations may be conducted by separate tool strings which are run into the wellbore separately. First, an acid tunneling tool string is run into the wellbore and used to form a lateral tunnel. The acid tunneling tool string is removed from the wellbore. Second, an acid fracturing tool string is run into the wellbore. At least a portion of the bottom hole assembly of the acid fracturing tool string is disposed into the lateral tunnel. Preferably, the portion of the bottom hole assembly is isolated from the main portion of the wellbore by setting a packer within the lateral tunnel. Acid is then flowed through the bottom hole assembly to cause fracturing of the formation radially surrounding the lateral tunnel.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] For a thorough understanding of the present invention, reference is made to the following detailed description of the preferred embodiments, taken in conjunction with the accompanying drawings, wherein like reference numerals designate like or similar elements throughout the several figures of the drawings and wherein:

[0012] Figure 1 is a side, cross-sectional view of an exemplary wellbore containing an acid tunneling and fracturing arrangement in accordance with the present invention.

[0013] Figure 2 is a side, cross-sectional view of the wellbore and acid tunneling and fracturing arrangement of Figure 1 now with tunneling beginning.

[0014] Figure 3 is a side, cross-sectional view of the wellbore and acid tunneling and fracturing arrangement of Figures 1 -2 now with tunneling having been completed.

[0015] Figure 4 is a further side, cross-sectional view of the wellbore and acid tunneling and fracturing arrangement of Figures 1 -2 now with acid fracturing being conducted.

[0016] Figure 5 is a side, cross-sectional view of distal portions of an exemplary acid injection wand.

[0017] Figure 6 is a side, cross-sectional view of the acid injection wand portions of Figure 5, now with particular nozzles closed off.

[0018] Figure 7 is a side, cross-sectional view of an alternative acid injection wand which incorporates an electrically-actuated sliding sleeve member. [0019] Figure 8 is a side, cross-sectional view of the acid injection wand of Figure 7 now in an actuated position.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] Figure 1 illustrates an exemplary wellbore 10 that has been drilled through the earth 12 from the surface 14 down to a hydrocarbon-bearing formation 16. It is noted that, while wellbore 10 is illustrated as a substantially vertical wellbore, it might, in practice, have portions that are inclined or horizontally-oriented. A portion of the wellbore 10 could be lined with a metallic casing (not shown). However, the portions of the wellbore 10 which are to be stimulated are preferably not lined with metallic casing.

[0021] An acid tunneling and fracturing arrangement 18 is disposed within the wellbore 10. The acid tunneling and fracturing arrangement 18 includes a running string 20 which is preferably made up of coiled tubing. A flowbore 22 is defined along the length of the running string 20.

[0022] A bottom hole assembly 24 is located at the distal end of the running string 20. The bottom hole assembly 24 includes an acid placement tool 26 and a packer assembly 28. Preferably also, the bottom hole assembly 24 includes a sensor sub 30. An indexing tool 31 is also incorporated into the bottom hole assembly 24 which permits components below the indexing tool 31 to be rotated angularly within the wellbore 10 with respect to the running string 20. The indexing tool 31 may be any of a number of commercially available indexing tools such as the "flow activated hydraulic jetting indexing tool" from National Oilwell Varco of Houston, Texas.

[0023] The acid placement tool 26 includes a cylindrical base portion 32 and an acid injection wand 34. An intermediate arm 36 is located between the base portion 32 and the injection wand 34. A first articulable joint 38 connects the base portion 32 and the intermediate arm 36. A second articulable joint 40 connects the intermediate arm 36 and the injection wand 34. Each of the first and second articulable joints 38, 40 allows the connected members to be moved angularly with respect to one another. The first and second articulable joints 38, 40 may be constructed and operate in the same manner as those used in the StimTunnel™ acid placement tool which is available commercially from Baker Hughes, a GE company, LLC of Houston, Texas.

[0024] Preferably, the acid injection wand 34 is provided with end nozzles 42 and a plurality of lateral nozzles 44 which are disposed through the wand body 45 to permit fluid communication between a central fluid passage 46 and portions external to the wand body 45. In described embodiments, the injection wand 34 includes a sliding sleeve member which can be axially moved within the injection wand 34 to selectively block or unblock nozzles. Figures 5 and 6 illustrate an exemplary injection wand 34 having a central fluid passage 46 defined along its length. A sliding sleeve member 48 is disposed within the fluid passage 46 and is axially moveable therewithin between a first position, wherein the sleeve member 48 covers or blocks fluid flow through the lateral nozzles 44 (Fig. 5), and a second position, wherein the sleeve member 48 does not block fluid flow through the lateral nozzles 44 (Fig. 6). Preferably, a ball seat 50 is formed at the distal end of the sliding sleeve member 48. Initially, as shown in Figure 5, the sliding sleeve member 48 is positioned so that it blocks the lateral nozzles 44. In this position, fluid flow through the central fluid passage 46 can pass through the end nozzles 42 to areas external to the distal end of the injection wand 34. This configuration is preferred for acid tunneling because the acid flow through the end nozzles 42 will be directed generally in the direction of intended tunnel creation (i.e., the direction toward which the acid injection wand 34 is pointing.

[0025] it may be preferable to close the end nozzles 42 to fluid flow and open the lateral nozzles 44 to fluid flow when the acid tunneling and fracturing arrangement 18 is used for acid fracturing. To do this, a ball or plug member 52 is flowed into the running string 20 at surface 14. The ball or plug member 52 will land upon the ball seat 50 and allow fluid pressure to build up behind the ball or plug member 52. Pressure behind the ball/plug member 52 will shift the sliding sleeve member 48 to the second position illustrated in Figure 6. The lateral nozzles 44 will be unblocked allowing fluid to be communicated from the central fluid passage 46 to lateral areas external to the wand 34. Fluid flow in this manner is preferred during fracturing since the acid will be directed radially outwardly into the formation 16 under increased pressure, resulting in fracturing which will radiate outwardly from the lateral tunnel 72 within the formation 16. Fluid flow through the end nozzles 42 will be blocked by the ball/plug member 52. Preferably, the ball or plug member 52 is dissolvable within the acid over a period of time so that access to the end nozzles 42 will be reestablished after a period of time has passed.

[0026] In an alternative embodiment, the sliding sleeve member is electrically actuated to move between the first and second positions. Figures 7 and 8 illustrate an exemplary acid injection wand 34' which is constructed and operates in the same manner as the acid injection wand 34 described previously except where indicated otherwise. Sliding sleeve member 48' is moveably disposed within the fluid passage 46 between first and second positions which, respectively, block and unblock the lateral nozzles 44. The sliding sleeve member 48' is moveable between the first and second positions by electrical actuation. Electrical conduit 54 is disposed within the fluid passage 46 and is interconnected with linear actuator 56 and communicates commands from surface 14 to the actuator 56. Linear actuator 56 has arm 58 which is interconnected with the sleeve member 48'. The electrical conduit 54 may be tubewire. Upon command or energization from surface 14 via the conduit 54, the linear actuator 56 can move the sliding sleeve member 48' between the first and second positions. Preferably, a ball seat 50 is also formed within the fluid passage 46. When it is desired to close off flow to the end nozzles 42 and open the lateral nozzles 44, a ball or plug member 52 is flowed into the running string 20 and lands on ball seat 50 to block fluid flow through the end nozzles 42. The linear actuator 56 is actuated to move the sleeve member 48' to its second position, as illustrated in Figure 8, thereby opening the lateral nozzles 44 to fluid flow therethrough. The arrangement shown in Figures 7-8 may be preferable since the mechanisms for opening and closing off flow through the end nozzles 42 and lateral nozzles 44 are independent of one another. As a result, it would be possible to open the lateral nozzles 44 to fluid flow and not closing off flow through the end nozzles 42 by not landing ball 52. Alternatively, the end nozzles 42 could be used for fracturing as well by increasing the pumping pressure at surface 14 so the pressure will drop across the nozzles. In general, any style or direction of nozzles could be used for fracturing if their fluid pressure is above the rock fracturing pressure.

[0027] The sensor sub 30 includes at least one, and preferably more than one, sensor 60 which can measure one or more downhole wellbore parameters, including depth, deviation, azimuth, pressure, temperature and gamma ray, which are useful for identifying a location or attributes of the surrounding wellbore 10. The sensor sub 30 also preferably includes electronics storage or memory 62 to receive and store information received from the sensor(s) 60. The sensor sub 30 may include a deviation/azimuth measurement device or other location detector which will help identify the exact position and orientation of the bottom hole assembly 24 within the wellbore 10. A data communications conduit 64, such as tube-wire, is preferably used to transmit the received information to a surface- based controller and storage medium 66 from memory 62 of the bottom hole assembly 26. Telecoil ^® is coiled tubing which incorporates tube-wire that can transmit power and data. Tube-wire is available commercially from manufacturers such as Canada Tech Corporation of Calgary, Canada. Tube-wire 64 is shown within the flowbore 22 of the coiled tubing running string 20 and is operably interconnected with the controller/storage medium 66 at surface 14. The controller/storage medium 66 may be programmable, and preferably includes suitable programming to use mathematical modeling to determine the location and orientation of the bottom hole assembly 24 within the wellbore 10. Suitable programming for this application includes CIRCA™ RT modeling software for coiled tubing applications which is available commercially from Baker Hughes Incorporated.

[0028] Acid can be selectively flowed from an acid supply 68 at surface 14 by pump 70 through the flow bore 22 of the running string 20 to the bottom hole assembly 26. The pump 70 is preferably a variable speed or variable capacity pump.

[0029] Preferred methods of operation, are illustrated in Figures 1 -4. The acid tunneling and fracturing arrangement 18 is run into the wellbore 10 until the bottom hole assembly 24 is located proximate a desired location within the wellbore 10 within which it is desired to stimulate. This is illustrated in Figure 1 . Acid is then flowed by the pump 56 from the acid supply 54 to the bottom hole assembly 24 to exit the end nozzle 42, as illustrated in Figure 2. Also, the acid placement tool 26 will flex to angle the injection wand 34 so that the end nozzle 42 is directed toward the wall of the wellbore 10. Flexture of the first and second articulable joints 38, 40 is preferably hydraulically-actuated. When pumping pressure is increased above a certain limit, the joints 38, 40 bend and stay bent until the pumping pressure drops below the limit. As acid is pumped, a lateral tunnel 72 begins to form.

[0030] In order to form the lateral tunnel 72, at least some portion of the process of forming tunnel 72 within the formation 16 is preferably controlled based upon one or more wellbore parameters sensed by the sensors 46 of the sensor sub 30. Data sent to the controller 54 at surface 14 is used to rotate or otherwise control the bottom hole assembly 24. For example, if sensor(s) 60 include an azimuth sensor, information as to initial tool face position could be sensed and used to control orientation of the acid placement tool 26. Monitoring of down hole parameters and control of the bottom hole assembly can be done in real-time.

[0031] Figure 3 shows a subsequent time during tunneling wherein the length of the lateral tunnel 72 has advanced. The injection wand 34 and then a significant portion of the remainder of the bottom hole assembly 24 have entered the lateral tunnel 72. Sensors 60 detect inclination of the bottom hole assembly 24 and signals indicative of this are sent to the controller 66 at surface.

[0032] Figure 4 illustrates a subsequent time wherein acid tunneling has been completed. The packer assembly 28 is now set within the lateral tunnel 72 thereby isolating the acid placement tool 26 from uphole portions of the wellbore 10. At this point, the lateral nozzles 44 are preferably opened to flow and the end nozzles 42 may be closed against flow to better direct injection into the formation 16 for fracturing and to accommodate increased pumping pressures used for fracturing. Acid flow to the injection wand 34 is increased by the pump 70. Acid preferably exits the lateral nozzles 44 of the injection wand 34, and fractures 74 form in the formation 16.

[0033] In general, acid tunneling can be done with lower acid flow rates, and the inventors have found that acid tunneling with lower flow rates can often achieve longer tunnels than tunneling at higher flow rates. Acid fracturing can be done when the acid pressure is higher than the rock fracturing pressure and, in general, at higher fluid pressures than is used for acid tunneling.

[0034] In accordance with an general exemplary method for acid tunneling and fracturing, an acid tunneling tool having a bottom hole assembly 24 with an acid placement tool 26 is disposed within the wellbore 10 on a running string 20. Acid is flowed through the running string 20 to the bottom hole assembly 24 and form a lateral tunnel 72 within the formation 16. Next, bottom hole assembly 24 is disposed within the lateral tunnel 72 and isolated within the lateral tunnel by setting packer assembly 28. Acid is flowed to the bottom hole assembly 24 to fracture the formation 16 surrounding the lateral tunnel 72.

[0035] It is noted that steps in the method described can be performed using a single acid tunneling and fracturing arrangement 18. Alternatively, the acid tunneling steps can be performed using a first acid tunneling tool string which creates one or more tunnels, such as tunnel 72, and is then removed from the wellbore 10. Thereafter, a second tool string which is adapted to perform the acid fracturing steps is then run into the wellbore 10 to perform the acid fracturing steps. The bottom hole assembly of the second tool string would be run into the lateral tunnel 72 created earlier and secured within the tunnel 72 by setting a packer assembly 28. Acid is pumped to the bottom hole assembly to fracture the formation 16 surrounding the lateral tunnel 72.