Patent Application
20150068762
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.
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.
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.
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.
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.
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.
Referring initially to
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.
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.
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.
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
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's zone also gives time for fractures to form and reduces the risk of a screen-out condition occurring during the initial fracturing stage.
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
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.
