Patent Application
20050274522
The present invention relates generally to an improved method and system for fracturing a subterranean formation to stimulate the production of desired fluids therefrom.
Hydraulic fracturing is often utilized to stimulate the production of hydrocarbons from subterranean formations penetrated by wellbores. Typically, in performing hydraulic fracturing treatments, the well casing, where present, such as in vertical sections of wells adjacent the formation to be treated, is perforated. Where only one portion of a formation is to be fractured as a separate stage, it is isolated from the other perforated portions of the formation using conventional packers or the like, and a fracturing fluid is pumped into the wellbore through the perforations in the well casing and into the isolated portion of the formation to be stimulated at a rate and pressure such that fractures are formed and extended in the formation. Propping agent may be suspended in the fracturing fluid which is deposited in the fractures. The propping agent functions to prevent the fractures from closing, thereby providing conductive channels in the formation through which produced fluids can readily flow to the wellbore. In certain formations, this process is repeated in order to thoroughly populate multiple formation zones or the entire formation with fractures.
Wellbores having horizontal or highly inclined portions present a unique set of problems for fracturing. For instance, in many horizontal or highly inclined wellbores sections the wellbore has no casing or the annulus between the pipe and formation may not be filled with cement. In such completions, it may be difficult or impossible to effectively isolate portions of the formation in order to effectively fracture the formation. In other cases where solid pipe has been used in the horizontal or highly inclined wellbore section, fluid may exit the solid pipe section to a non-cemented annulus. In such situations, control of fracture placement or the number of fractures may be difficult.
Even with cemented casings, these typical techniques are not without problems. Fracturing certain formations may require multiple repositioning and multiple placement of conventional packers and fracturing equipment to properly fracture the entire formation. Such activities often result in delay, and therefore additional expense, as downhole equipment is repositioned and the formation repeatedly fractured. In addition, each time packers are repositioned, there are risks that packers may unseat or leak, possibly resulting in unsuccessful fracture treatment, tool damage, and loss of well control. Further, it may be desirable to fracture the entire formation in a single operation, for instance to reduce costs. In addition, when horizontal sections of wells are fractured, there is usually a tendency for most of the created fractures to be concentrated at areas that are weaker or may have been mechanically damaged during the drilling process. Quite often, such concentrated fracturing occurs near the turn in the well from the vertical to the horizontal section. In some instances, concentrated fracturing may be located near naturally-occurring weak zones due to the non-homogeneous nature of many reservoir rocks. This may result in inadequate stimulation of the well due to failure to fracture along the entire formation and may greatly reduce overall well production compared to the potential production had the producing zones of the formation been more completely fracture-stimulated.
The present invention is directed to an apparatus and method for effectively fracturing multiple regions or zones in a formation in a controlled manner.
More specifically, one embodiment of the present invention is directed to a method of fracturing a subterranean formation penetrated by a wellbore by first positioning a liner fracturing tool within the wellbore to form an annulus between the liner fracturing tool and the wellbore. The liner fracturing tool has a liner outer wall, one or more jets, an upstream portion, a downstream portion and a fluid passageway. The jets of the liner fracturing tool in one embodiment are hollow and extend through the liner outer wall into the wellbore forming nozzles. The jets are capable of allowing fluid to flow from the fluid passageway to the subterranean formation. A fracturing fluid is introduced into the fluid passageway of the liner fracturing tool and fracturing fluid is jetted through at least some of the nozzles against the subterranean formation at a pressure sufficient to form cavities in the formation, which are in fluid communication with the wellbore. The fracturing fluid is maintained in the cavities at a sufficient static pressure while jetting to fracture the subterranean formation.
Another embodiment of the present invention is directed to a liner fracturing apparatus with a liner, wherein the liner has an outer wall, an interior fluid passageway, and at least one port in the outer wall. The liner fracturing apparatus also has one or more jets, wherein the jets are mounted within the ports and extend through the outer surface of the liner, forming nozzles.
Still another embodiment of the present invention is directed to a liner fracturing tool having a liner with an outer wall, an interior fluid passageway, and one or more ports in the outer wall, a jet holder that is mounted within the port or ports, and one or more jets that are mounted within at least one jet holder and extend beyond the outer surface of the liner, forming at least one nozzle.
The features and advantages of the present invention will be readily apparent to those skilled in the art upon a reading of the description of the exemplary embodiments, which follows.
A more complete understanding of the present disclosure and advantages thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings:
In wells penetrating certain formations, and particularly deviated wells, it is often desirable to create relatively small fractures referred to in the art as “microfractures” in the formations near the wellbores to facilitate creation of hydraulically induced enlarged fractures. In accordance with the present invention, such microfractures are formed in subterranean well formations utilizing a liner fracturing tool having at least one fluid jet.
The liner fracturing tool is positioned within a formation to be fractured, and fluid is then jetted through the fluid jet against the formation at a pressure sufficient to form a cavity therein and fracture the formation by stagnation pressure in the cavity. A high stagnation pressure is produced at the tip of a cavity in a formation to be fractured because of the jetted fluids being trapped in the cavity as a result of having to flow out of the cavity in a direction generally opposite to the direction of the incoming jetted fluid. The high pressure exerted on the formation at the tip of the cavity causes a microfracture to be formed and extended a short distance into the formation.
In order to extend a microfracture formed as described above further into the formation in accordance with this invention, additional fluid is pumped from the surface into the wellbore to raise the ambient fluid pressure exerted on the formation while the formation is being jetted by the fluid jet or jets produced by the hydrajetting tool. The fluid in the wellbore flows into the cavity produced by the fluid jet and flows into the fracture at a rate and high pressure sufficient to extend the fracture an additional distance from the wellbore into the formation.
The details of the present invention will now be described with reference to the accompanying drawings. Turning to
In one embodiment of the present invention, one or more ports 140 extend from liner inner wall 116 through liner outer wall 112 of mostly-horizontal liner section 130. Ports 140 are generally approximately circular openings, although other shapes may be used depending on the particular design parameters. Ports 140 are designed to allow the mounting of jets 150 within ports 140, and optionally, as further shown in
In an alternative embodiment, jet holders 160, shown in
Jet orientation and location are dependent upon the formation to be fractured, the process of which is described below. Jet orientation may coincide with the orientation of the plane of minimum principal stress, or the plane perpendicular to the minimum stress direction in the formation to be fractured relative to the axial orientation of wellbore 20 penetrating the formation. Jet location along liner 110 may be chosen to optimize formation fracture, i.e., typically to allow formation fracture throughout the portion of formation 40 to be fractured. In particular, one of ordinary skill in the art will recognize the importance of allowing adequate distance between jet 150 positions along the liner to reduce or eliminate intersecting or interfering fractures. Jet circumferential location about liner 110 should be chosen depending on the particular well, field or, formation to be fractured. For instance, in certain circumstances, it may be desirable to orient all jets 150 towards the surface for certain formations or 90° stations about the circumference of liner 110 for other formations. It is further possible to alter the internal diameter of jets 150 dependent upon the location of particular jet 150 along the wellbore, the formation, well, or field. One of ordinary skill in the art may vary these parameters to achieve the most effective treatment for the particular well.
The open end of liner 110 is typically plugged, such as with open-end plug 200 as shown in
In certain circumstances, it may be desirable to install thermally melting or dissolvable nozzle plugs 180 in nozzle 170 of jets 150 as shown in
In order to fracture a subterranean formation, liner fracturing tool 100 is lowered into wellbore 20 until jets 150 reach the desired formation to be fractured. When nozzle plugs 180 have been installed in nozzles 170, the liner may be washed in if necessary as described above. Following wash in, nozzle plugs 180 may be melted, for instance through the use of a fluid with a temperature above the melting temperature of nozzle plugs 180, or dissolved through the use of an acid wash or other chemical wash so designed as to dissolve the particular material. In some formations, the temperature of the formation may be such as to thermally degrade nozzle plugs 180 over time, thereby melting nozzle plugs 180 after completion of the wash-in procedure.
Fracturing fluid may then be forced through jets 150. The rate of pumping the fluid into liner 110 and through jets 150 is increased to a level whereby the pressure of the fluid which is jetted through jets 150 reaches the jetting pressure sufficient to cause the creation of the cavities 50 and microfractures 52 in the formation 40 as illustrated in
A variety of fluids can be utilized in accordance with the present invention for forming fractures, including aqueous fluids, viscosified fluids, oil based fluids, and even certain “non-damaging” drilling fluids known in the art. Various additives can also be included in the fluids utilized such as abrasives, fracture propping agent, e.g., sand or artificial proppants, acid to dissolve formation materials and other additives known to those skilled in the art.
As will be described further hereinbelow, the jet differential pressure (Pjd) at which the fluid must be jetted from jets 150 to result in the formation of the cavities 50 and microfractures 52 in the formation 40 is a pressure of approximately two times the pressure (Pi) required to initiate a fracture in the formation less the ambient pressure (Pa) in the wellbore adjacent to the formation i.e., Pjd≧2×(P1−Pa). The pressure required to initiate a fracture in a particular formation is dependent upon the particular type of rock and/or other materials forming the formation and other factors known to those skilled in the art. Generally, after a wellbore is drilled into a formation, the fracture initiation pressure can be determined based on information gained during drilling and other known information. Since wellbores are often filled with drilling fluid and since many drilling fluids are undesired, the fluid could be circulated out, and replaced with desirable fluids that are compatible with the formation. The ambient pressure in the wellbore adjacent to the formation being fractured is the hydrostatic pressure exerted on the formation by the fluid in the wellbore or a higher pressure caused by fluid injection.
When fluid is pumped into the wellbore or liner annulus to increase the pressure to a level above hydrostatic to extend the microfractures as will be described further hereinbelow, the ambient pressure is whatever pressure is exerted in the wellbore on the walls of the formation to be fractured as a result of the pumping.
At a stand-off clearance of about 1.5 inches between the face of the jets 150 and the walls of the wellbore and when the jets formed flare outwardly from their cores at an angle of about 20°, the jet differential pressure required to form the cavities 50 and the microfractures 52 is a pressure of about 2 times the pressure required to initiate a fracture in the formation less the ambient pressure in the wellbore adjacent to the formation. When the stand off clearance and degree of flare of the fluid jets are different from those given above, the following formulas can be utilized to calculate the jetting pressure.
Pi=Pf−Ph
ΔP/Pi=1.1 [d+(s+0.5)tan(flare)]2/d.2
As mentioned above, propping agent may be combined with the fluid being jetted so that it is carried into the cavities 50 into fractures 60 connected to the cavities. The propping agent functions to prop open fractures 60 when they attempt to close as a result of the termination of the fracturing process. In order to insure that propping agent remains in the fractures when they close, the jetting pressure is preferably slowly reduced to allow fractures 60 to close on propping agent which is held in the fractures by the fluid jetting during the closure process. In addition to propping the fractures open, the presence of the propping agent, e.g., sand, serves as an abrasive agent and in the fluid being jetted facilitates the cutting and erosion of the formation by the fluid jets. As indicated, additional abrasive material can be included in the fluid, as can one or more acids which react with and dissolve formation materials to enlarge the cavities and fractures as they are formed.
As further mentioned above, some or all of the microfractures produced in a subterranean formation can be extended into the formation by pumping a fluid into the wellbore to raise the ambient pressure therein. That is, in carrying out the methods of the present invention to form and extend a fracture in the present invention, liner fracturing tool 100 is positioned in wellbore 20 adjacent the formation 40 to be fractured and fluid is jetted through the jets 150 against the formation 40 at a jetting pressure sufficient to form the cavities 50 and the microfractures 52. Simultaneously with the hydrajetting of the formation, a fluid is pumped into wellbore 20 at a rate to raise the ambient pressure in the wellbore adjacent the formation to a level such that the cavities 50 and microfractures 52 are enlarged and extended whereby enlarged and extended fractures 60 are formed. As shown in
Liner fracturing tool 100 can be operated so as to fracture multiple sites of formation 40 approximately simultaneously, or portions of formation 40 can be fractured at different times. When liner fracturing tool 100 is operated to fracture multiple sites of formation 40 approximately simultaneously, fracturing fluid is pressurized throughout fluid passageway 132 of liner 110. In this way, fracturing fluid reaches all jets 150 approximately simultaneously and microfractures 52 are formed approximately simultaneously. Alternatively, when it is desirable to fracture different portions of formation 40 at different times, fracturing fluid is pressured through only some of jets 150 at any one time. This may be accomplished by installing a straddle packer type device immediately upstream and downstream of the portion of formation 40 to be fractured. Fracturing fluid is then pressured through jets 150 between the upstream and downstream portions of the straddle packer type device. The straddle packer type device may then be moved to a different set of jets 150 and the process repeated as desired. In this way, one portion at a time of formation 40 may be fractured.
Following the fracture of formation 40, the annulus or wellbore may be “packed,” i.e., a packing material may be introduced into the fractured zone to reduce the amount of fine particulants such as sand from being produced during the production of hydrocarbons. The process of “packing” is well known in the art and typically involves packing the well adjacent the unconsolidated or loosely consolidated production interval, called gravel packing. In a typical gravel pack completion, a sand control screen is lowered into the wellbore on a workstring to a position proximate the desired production interval. A fluid slurry including a liquid carrier and a relatively coarse particulate material, which is typically sized and graded and which is referred to herein as gravel, is then pumped down the workstring and into the well annulus formed between the sand control screen and the perforated well casing or open hole production zone.
The liquid carrier either flows into the formation or returns to the surface by flowing through a wash pipe or both. In either case, the gravel is deposited around the sand control screen to form the gravel pack, which is highly permeable to the flow of hydrocarbon fluids but blocks the flow of the fine particulate materials carried in the hydrocarbon fluids. As such, gravel packs can successfully prevent the problems associated with the production of these particulate materials from the formation.
In another embodiment of the present invention, the proppant material, such as sand, is consolidated to better hold it within the microfractures. Consolidation may be accomplished by any number of conventional means, including, but not limited to, introducing a resin coated proppant (RCP) into the microfractures.
In another embodiment of the present invention, following well fracture and any optional packing or consolidating steps, jet holders 160 may be dissolved using acids, such as when using jet holders 160 made of materials such as aluminum. When jet holders 160 are composed of PLA, they will automatically decompose into lactic acid after a designed period of time when exposed to water at desired temperatures. The time period will largely be controlled by the formulation of the PLA material and the ambient temperature around the tool. By dissolving or melting jet holders 160, ports 140 are opened to receive hydrocarbons from the reservoir. Thus, in at least one embodiment of the present invention, during production, hydrocarbons are allowed to flow through ports 140 into liner 110.
Therefore, the present invention is well-adapted to carry out the objects and attain the ends and advantages mentioned as well as those which are inherent therein. While the invention has been depicted, described, and is defined by reference to exemplary embodiments of the invention, such a reference does not imply a limitation on the invention, and no such limitation is to be inferred. The invention is capable of considerable modification, alteration, and equivalents in form and function, as will occur to those ordinarily skilled in the pertinent arts and having the benefit of this disclosure. The depicted and described embodiments of the invention are exemplary only, and are not exhaustive of the scope of the invention. Consequently, the invention is intended to be limited only by the spirit and scope of the appended claims, giving full cognizance to equivalents in all respects.
