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BACKGROUND 
       [0001]    The present invention generally relates to the stimulation of subterranean wells and, in a representatively illustrated embodiment thereof, more particularly relates to specially designed apparatus and methods for inhibiting a screen-out condition in a subterranean well fracturing operation. 
         [0002]    In zone fracturing of subterranean wells one previously proposed method employs a series of tubular sleeves longitudinally spaced apart along a tubular casing of an overall wellbore string. Each sleeve is slidable relative to the casing between a closed position in which the sleeve blocks associated casing side wall ports, and an open position in which the sleeve unblocks such ports to permit exit therethrough of a pressurized fracing slurry which is used to create and prop open subterranean formation fractures through which production fluid may be subsequently delivered through the wellbore string to the surface for recovery. Annular seats are secured to the sliding sleeves for movement therewith relative to the casing and are sized to sealingly receive valve actuating members, such as balls, which are successively dropped through the string. Via the use of packers or other types of seal-off structures interdigitated with the sliding sleeves, a series of fracturing zones are defined externally of the casing—each zone being associated with one of the sliding sleeves. 
         [0003]    In carrying out a typical zone fracturing operation, with the sleeves initially in their closed positions, a ball or other type of valve actuating member is dropped through the string and caused to sealingly engage the seat portion of the lowermost sleeve. Via downward fluid pressure exerted on the dropped ball, its associated sleeve is forced in a downstream direction to its open position in which its previously covered casing ports are opened to permit pressurized frac slurry to be discharged into the formation adjacent the now-opened set of casing ports. When the fracing of this first zone is complete, a second ball is dropped into sealing engagement with the seat of the closed sliding sleeve immediately uphole of the opened first sleeve. Downward fluid pressure is then exerted on the second ball to downwardly slide its sliding sleeve and thereby open a second series of casing ports to permit pressurized fracing fluid to flow outwardly therethrough to thereby frac a second formation zone above the first fraced formation zone while the second ball isolates the fracing fluid from the first dropped ball. This sequence is repeated for each of the upwardly successive closed sliding sleeves until the zone fracturing operation is completed. 
         [0004]    When fracing a well it is desirable to pack as much proppant into a formation as possible to keep the fractures open for production, especially close to the wellbore. A risk exists for plugging a well by packing too much proppant into a specific zone. This plugging is commonly known as a “screen-out” which may be defined as a condition arising when fracture fluids are no longer capable of carrying the proppant or the concentration of proppant becomes too great, causing the proppant to settle out in the piping and not be carried into the subterranean fractures. 
         [0005]    A screen-out condition may cause a severe disruption in well operations and significant cost overruns due to the well known difficulties encountered in eliminating the screen-out. Various techniques have been previously proposed to prevent a screen-out condition from occurring since unplugging a screen-out is quite time consuming and expensive. Each of these known techniques carries with it problems which makes it less than entirely desirable. As but one example, a common screen-out prevention method when initiating fractures upon opening a new zone is to send fluid with no proppant therein to the formation for a period of time, and later add maximum concentrations of proppant to the fluid to place the proppant into the subterranean fractures. Due to the cost of the fluid it is desirable to minimize its use in the fracing operation. This known technique, however, substantially increases the volume of fracing fluid required, thereby materially increasing the overall cost and time needed for the fracing operation. 
         [0006]    As can be seen from the foregoing, a need exists for improved apparatus and methods which eliminate or at least reduce the aforementioned problems created by the occurrence of screen-out conditions in well fracing operations as generally described above. It is to this need that the present invention is primarily directed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a cross-sectional view through a longitudinal portion of a deployed wellbore string with a sliding sleeve assembly therein opened to permit the fracturing of a subterranean formation zone adjacent the opened sliding sleeve assembly; 
           [0008]      FIG. 2  is a view similar to that in  FIG. 1 , but with an undesirable screen-out condition having been created within the wellbore string above the opened sliding sleeve assembly; 
           [0009]      FIGS. 3-5  are cross-sectional views through the deployed wellbore string portion and sequentially depict the representative use of improved apparatus and methods of the present invention in inhibiting in the wellbore string portion the creation of the screen-out condition shown in  FIG. 2 ; and 
           [0010]      FIG. 6  is an enlarged perspective view of a specially designed valve actuating member embodying principles of the present invention and used in the screen-out inhibiting technique shown in  FIGS. 3-5 . 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    Referring initially to  FIGS. 1 and 2 , a longitudinal portion of a downhole-deployed wellbore string  10  is shown which comprises a tubular casing  12  in which a longitudinally spaced apart series of sliding sleeve valve assemblies, including a representative uphole or upstream sleeve valve assembly  14  and a downhole or downstream sleeve valve assembly  16  below it. Packing elements  18 , or some other structures such as cement sections, prevent fluid flow between annular zones  20  disposed between the exterior of the wellbore string  10  and the borehole  21  through which the string  10  extends. 
         [0012]    Each sliding sleeve valve assembly  14 , 16  comprises a coaxial tube  22  that can be positioned over radial holes or ports  24  in an exterior tubing component  26  of the sliding sleeve valve assembly. Sealing structures such as O-rings  28  prevent fluid passage from the interior of the wellbore string  10  to the annular zones  20 . Each sliding sleeve valve assembly  14 , 16  may also have a structure, such as a seat  30  that can be engaged by a valve actuating member, representatively in the form of a ball  32 , to actuate the associated coaxial tube  22 . Most commonly, seats are designed to be engaged by balls of increasing size to selectively open zones with specific ball sizes. The present invention applies to, but is not limited to, systems with ascending ball sizes. Sliding sleeve systems utilizing expandable seats (as opposed to the representatively fixed diameter seats  30 ) can also benefit from principles of the present invention. 
         [0013]      FIG. 1  shows the wellbore string with fracing fluid passing therethrough in the downstream direction  34  to the open downstream sliding sleeve assembly  16 , the fracing fluid comprising a fluid laden with proppant such as sand. The fluid is directed through the opened radial ports  24  of the downstream sliding sleeve valve assembly  16  and into adjacent subterranean formation fractures  36  in the earth. It is desirable to lodge as much proppant as possible in these fractures to allow hydrocarbons to later be able to pass through the fractures  36  for delivery to the surface for recovery. It is also desirable to minimize the use of fluid when delivering the proppant due to the cost of the fluid. 
         [0014]      FIG. 2 , in which principles of the present invention are not utilized, illustrates the wellbore string  10  in a screen-out condition in which proppant  38  has become too dense and impacted within the string  10  to allow fluid to flow to the fractures  36 . The most common time for this screen-out condition to occur is at the point when fractures  36  can no longer accept any more proppant  38 . Another point at which a screen-out condition can occur is when the sliding sleeve assembly  16  is initially opened and fractures  36  have not yet been initiated. A current practice employed to prevent a screen-out condition is to send fluid without proppant therein to the formation for a period of time, and thereafter add proppant to the fluid once the fractures  36  have been created. As previously mentioned herein, this technique is often less than satisfactory due to increased cost and time delay considerations. In a screen-out condition such as that depicted in  FIG. 2 , it is not possible to flow a ball down the string  10  and open the upstream sliding sleeve valve assembly  14  since fluid cannot be pumped downwardly through the string  10  due to the proppant-blocked screen-out condition shown in  FIG. 2 . 
         [0015]      FIGS. 3-5  sequentially illustrate the representative use of improved apparatus and methods of the present invention in preventing in the depicted portion of the wellbore string  10  the screen-out condition shown in  FIG. 2 . With initial reference to  FIG. 3 , there is illustrated therein a specially designed valve actuating structure used to create a pressure differential across the seat of the upstream sliding sleeve valve assembly  14 . By way of non-limiting example, the valve actuating structure is a second ball  40  (see also  FIG. 6 ) with through-holes  42  (representatively three in number) suitably formed therein and extending along axes  44  so that the ball  40 , when seated on the upstream seat  30 , is not capable of completely plugging fluid flow therethrough to the downstream sliding sleeve valve assembly  16 . The valve actuating ball structure  40  could also function in this method without holes. However, the ability of the ball  40  to pass fluid is desired. In  FIG. 3 , ball  40  which is being downwardly forced through the string  10  by pressurized fracing fluid is shown at a point at which the ball  40  initially lands on the upstream seat  30 , but has not yet opened the upstream sliding sleeve assembly  14 . 
         [0016]    As will be appreciated by those of ordinary skill in this particular art, the dropping of the ball  40  takes place after the lower ball  32  has been dropped onto and blocks the downstream seat  30  which is then downwardly shifted by pressurized fracing fluid to open the downstream sliding sleeve valve assembly  16  and create the fractures  36  via pressurized fracing fluid outflow through the uncovered tubing string side wall ports  24  of the downstream sliding valve assembly  16 . 
         [0017]    The ball  40  is made of a suitable material hard enough to actuate the coaxial tube  22  of the upstream sliding sleeve assembly  14 . Upstream coaxial tube  22  (like the downstream coaxial tube  22 ) is representatively held in its closed position by means of shear pins or a shear ring (neither of which is illustrated herein). Upstream and downstream sliding sleeve valve assemblies  14 , 16  are designed to open at a pressure much lower than the pressure at which the formation is fraced. The ball  40  is strong enough to stay supported in the upstream seat  30  and open the upstream sliding sleeve valve assembly  14 , but is not strong enough to remain in the upstream seat  30  at the fracing pressure. 
         [0018]    Ball  40  can have a soft enough modulus to either extrude or shear through the upstream seat  30 . A suitable dissolving material may also be utilized in the construction of the ball  40  since a dissolving material used in downhole force-receiving structures are typically suitable for opening of the upstream sliding sleeve valve assembly  14 , but do not require future milling in the well. When in place upon the upstream seat  30 , the ball  40  only partially blocks the upstream seat  30 , thereby permitting a limited fluid flow downwardly through the upstream seat  30  and creating a downward pressure drop across the upstream seat  30  sufficient to downwardly open the upstream sliding sleeve valve assembly  14 . After the upstream sliding sleeve valve assembly  14  is opened, the ball  40  shears downwardly through the upstream seat  30  and arrives at its  FIG. 4  position. The ball  40  is initially dropped onto the upstream seat  30  during a final period of the fracing of the downstream subterranean formation zone associated with the downstream sliding sleeve assembly  16 . Representatively, the fracing fluid pressure is lowered somewhat during the dropping of the ball  40 , and then returned to its full fracing pressure after the ball  40  lands on the upstream seat  30 . 
         [0019]      FIG. 4  shows the well bore string  10  after the second ball  40  has downwardly moved the upstream tube  22  to its open position and then sheared downwardly through the upstream seat  30 . At this point the downstream sliding sleeve valve assembly  16  can still receive proppant concurrently with the upstream sliding sleeve assembly  14 . Since the proppant  38  is denser than its carrier fluid, most of the proppant will pass by the upstream sliding sleeve valve assembly  14  to the downstream sliding sleeve assembly  16  which is desirable at the end of a zone&#39;s fracture. This also works to the advantage of the upstream sliding sleeve assembly  14  since a low concentration of proppant will be present as initial fractures  46  are made adjacent the upstream sliding sleeve assembly  14 . 
         [0020]      FIG. 5  depicts the final step in the illustrated representative embodiment of the present invention. A ball  48  (of an imperforate construction like that of the downstream ball  32 ) is sent down the string  10  to plug fluid flow to the downstream sliding sleeve assembly  16  via the upstream seat  30  by landing on and sealingly blocking the upstream seat  30 . Even if the fractures  36  can no longer accept proppant  38 , the ball  48  can still land on the upstream seat  30  to concentrate the fracing fluid to the zone at the upstream sliding sleeve assembly  14 . The method can subsequently continue with a further ball (not shown) opening yet another zone upstream of the open upstream sliding sleeve valve assembly  14 . 
         [0021]    As can be readily seen from the foregoing, principles of the present invention may be utilized to reduce the risk of a screen-out condition during the initiation of a new zone and improves the amount of proppant close to the wellbore of a completed zone by dropping an intermediate plugging member (such as the illustrated perforated ball  40 ) to open the sliding sleeve valve assembly for a new zone while still allowing fracing fluid flow to the nearly completed zone. This uniquely allows two zones to be open for a period of time before dropping a ball to plug fluid from reaching the completed zone and diverting the flow through the most recently opened sliding sleeve assembly. During the period in which both zones are opened, a heavier amount of proppant-laden fluid can be pumped so that a high concentration of proppant surrounds the well bore when the ball serving as a plug closes the completed zone. Having a second zone opened at the time of initiating new fractures in the newly opened sliding sleeve valve assembly&#39;s zone also gives time for fractures to form and reduces the risk of a screen-out condition occurring during the initial fracturing stage. 
         [0022]    Principles of the present invention are suitable for use in all sliding sleeve valve applications that are actuated by drop systems, usually utilizing but not exclusive to ball-type plug members. Such principles of the present invention may also be utilized to advantage in both cemented and open-hole applications, with open-hole applications being defined herein as sleeves being partitioned by packing elements (as illustratively depicted in  FIGS. 3-5  in which principles of the present invention are utilized). The representatively described screen-out inhibiting process applies to both graduated size ball systems and to systems with seats capable of locking. In the case of locking seat-based systems, the ball sent to actuate the upper sliding sleeve valve assembly (for example, the ball  40  used to open the upstream sliding sleeve valve assembly  14 ) can be of a significantly smaller diameter than the lower ball (for example, the ball  32 ) while still being capable of actuating its associated seat and then passing therethrough at a pressure less than the full fracing pressure. 
         [0023]    The foregoing detailed description is to be clearly understood as being given by way of illustration and example only, the spirit and scope of the present invention being limited solely by the appended claims.

Summary:
A subterranean well fracturing system comprises a downhole well string having installed therein initially closed upstream and downstream sliding sleeve valve assemblies each openable to provide fracing fluid discharge outlets through well string side ports to an associated subterranean fracing zone. To inhibit an undesirable screen-out condition during formation fracturing, specially designed apparatus and methods are operative to sequentially (1) block the downstream valve seat, (2) open the blocked downstream valve seat using pressurized fracing fluid, (3) partially block the upstream valve seat, (4) open the partially blocked upstream valve seat using pressurized fracing fluid, a portion of which is flowed through the partially blocked upstream valve seat, and then (5) unblock the partially blocked upstream valve seat to permit a full flow of pressurized fracing fluid therethrough. In this manner, pressurized fracing fluid is simultaneously delivered to two fracing zones to inhibit a screen-out condition in the well string.