P/00/01il Regulation 3.2 AUSTRAILIA Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT Invention Title: Seat apparatus and method The following statement is a full description of this invention, including the best method of performing it known to us: 1001241790 SEAT APPARATUS AND METHOD BACKGROUND [0001] Seats such as, for example ball seats are well known in downhole industries and especially so in the drilling and completion industry. Commonly, ball seats are used to regulate the flow of fluids and actuate downhole devices. Although ball seat configurations are many and are ubiquitous in their use within the art, the number of stacked ball seats that can be employed with traditional systems is limited. Improving the number of ball seats that may be stacked in a borehole will be welcomed by the art. SUMMARY [0002] In a first aspect the invention provides a downhole fracturing system including a seat assembly having a plurality of like-sized openings therein, each of the like-sized openings having a cross sectional area complementary to a corresponding plurality of objects such that each of the objects is matable therewith to substantially inhibit fluid flow through each like-sized opening, the objects having dimensions insufficient to mate with an opening in a next adjacent upstream seat assembly, wherein a first opening of the plurality of like-sized openings is disposed on a first plane and a second opening of the plurality of like-sized openings is disposed on a second plane off-set from the first plane along an axis extending substantially longitudinally through the seat assembly. [0003] In a second aspect the invention provides a hydraulic fracturing system, comprising: a first seat assembly having a plurality of first openings, each of the first openings having a first cross-sectional area; a second seat assembly positioned upstream of the first seat assembly and including at least one second opening therethrough having at least one second cross-sectional area complementary to at least one second object such that a respective second object is matable therewith to substantially inhibit fluid flow through the respective second opening and the first seat assembly operatively arranged to receive a plurality of first objects each having a first size smaller than the second size after the first objects have passed through the at least one opening in the second seat assembly, each one of the plurality of first openings corresponding to a corresponding one of the first objects for obstructing fluid flow through the first openings when the first objects are landed at the first seat assembly, wherein a first opening of the plurality of 2 1001241790 first openings is disposed on a first plane and a second opening of the plurality of first openings is disposed on a second plane off-set of the first plane along an axis extending substantially through the first seat assembly. [0004] In a third aspect the invention provides method of hydraulic fracturing including dropping a plurality of first objects each having a first size; landing the plurality of first objects at a first seat assembly, each of the plurality of first objects associated with one of a plurality of first openings in the first seat assembly, wherein a first one of the plurality of first openings is disposed on a first plane and a second one of the plurality of first openings is disposed on a second plane off-set of the first plane along an axis extending substantially longitudinally through the first seat assembly; obstructing fluid flow through the first seat assembly with the plurality of first objects; and performing a fracturing operation with fluid upstream of the first seat assembly. [0004A] Reference to any prior art in the specification is not an acknowledgment or suggestion that this prior art forms part of the common general knowledge in any jurisdiction or that this prior art could reasonably be expected to be understood, regarded as relevant, and/or combined with other pieces of prior art by a person skilled in the art. [0004BJ As used herein, except where the context requires otherwise the term 'comprise' and variations of the term, such as 'comprising', 'comprises' and 'comprised', are not intended to exclude other additives, components, integers or steps. BRIEF DESCRIPTION OF THE DRAWINGS [0005] Referring now to the drawings wherein like elements are numbered alike in the several figures: [0006] Figure 1 is a perspective view of an embodiment of a ball seat; [0007] Figure 2 is a cut-away side view of the ball seat of Figure 1; [0008] Figure 2a is a cut-away side view of the ball seat of Figure 1 illustrating the parabolic embodiment; [0009] Figure 3 is a side cut-away view of a portion of an alternate embodiment of a ball seat; [00101 Figure 4 is a perspective view of another alternate embodiment of a ball seat; and [0011] Figure 5 is a side cut-away view of the ball seat of Figure 4. 3 1000465087_2.docx DETAILED DESCRIPTION [0012] Referring to Figure 1, an exemplary embodiment of a ball seat 10 is illustrated. The ball seat 10 includes a housing 12 that includes tubular orifices 14. The tubular orifices 14 define flow paths indicated by the arrows 11. Seats 16 are disposed at the apertures of the orifices 14. Seats 16 take the form of a plurality of like-sized openings having a cross-sectional area complementary to a corresponding plurality of objects 20, as described further below. The illustrated embodiment includes concave portions 18 defined by the housing 12. The concave portions may be, for example, conical (see Figure 2), parabolic (see Figure 2a), or cylindrical in shape. [0013] Figure 2 illustrates a cut-away view of the ball seat 10. In operation, the ball seat 10 may be placed downhole in a borehole. A first object 20 such as, for example, a spherical object may be introduced into the borehole and driven towards the ball seat 10 by, for example, hydraulic pressure or gravity. One of the concave portions 18 directs the first object 20 into engagement with a seat 16; blocking an orifice 14 and obstructing a flow path 11. A second object 22 may similarly be driven towards the ball seat 10 and directed by the concave portions 18 into the empty seat 16; blocking the second orifice 14 and obstructing the second flow path 11. Each of the objects 20 is mateable with a respective ball seat 10 to substantially inhibit fluid flow through each ball seat 10. The objects 20 advantageously have dimensions insufficient to mate with an opening 10 in a next adjacent upstream seat assembly in a downhole fracturing system. The obstruction of the flow paths allows an operator to pressure up against the obstructed ball seat 10 to facilitate a downhole pressure based operation. This may be a fracturing job or actuation of a desired downhole device, or to otherwise effect desired downhole operations. [0014] Previous ball seat devices using a single orifice and seat arrangement may be less effective when the cross sectional areas of the orifice (and the associated diameter of the object) are less than a defined threshold cross sectional area. The threshold diameter may be a different diameter for different borehole systems and is associated with the likelihood of a pressure increase upstream of the orifice due to its restricted flow area. 4 1000465087_2.docx Orifice cross sectional areas less than the threshold cross sectional area may undesirably restrict the flow of fluid and cause the undesired and premature actuation of tools or other premature operations uphole relative to the ball seat device. The illustrated embodiments having more than one orifice allow the cross sectional areas of individual orifices (and the associated diameter of the objects) to be reduced while avoiding the restriction of the flow of fluid since the use of multiple orifices allows the net cross sectional area of the orifices to remain greater than the threshold cross sectional area. [0015] Figure 3 illustrates a side cut-away view of a portion of an alternate exemplary embodiment of a ball seat 30. The ball seat 30 is similar in operation to the ball seat 10 (of Figures 1 and 2) however; the seats 16 in the illustrated embodiment are disposed in different planes (A and B). In the illustrated example, the plane B is located downstream in the flow path direction (as indicated by the arrows 11) relative to the plane A. The disposition of the seats 16 in different planes may improve the performance of the ball seat 30. [0016] Figure 4 illustrates another alternate embodiment of a ball seat 40. The ball seat 40 operates in a similar manner to the embodiments described above, and includes a plurality of seats 16 and concave portions 18. In the illustrated embodiment, a seat 17 and associated flow path 15 have an axis coincident to the longitudinal axis 19 of the ball seat 40. The seats 16 and associated flow paths 14 are disposed radially about the axis 19 in the housing 12. Figure 5 illustrates a side partially cut-away view of a portion of the ball seat 40 (of Figure 4). [0016A] In a method of hydraulic fracturing a number of ball seat assemblies may be stacked in a borehole. In this embodiment of the invention a plurality of first objects having a first size are dropped and landed at a first seat assembly. Each object is associated with a respective ball seat thereby obstructing fluid flow through the first seat assembly, enabling the performance of a fracturing operation with fluid upstream of the first seat assembly. 5 1000465087_2.docx [0016B] A second ball seat assembly having at least one second opening may be located upstream of the first ball seat assembly. The cross-sectional area of the opening in the second seat assembly is larger than the first objects enabling the first objects to pass through the second openings. [0016C] Second objects having a size greater than the first objects may then be dropped and landed on the second ball seat assembly. [0017] The Figures described above illustrate exemplary embodiments of ball seats. Other embodiments may include any number of ball seats having multiple seat portions, flow paths, alignment planes, and shapes that are operative to direct objects to engage the seats. [0018] While one or more embodiments have been shown and described, modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustrations and not limitation. 6