Abstract:
A technique enables an improved filtering of sand, a desired distribution of produced or injected fluid, and a reduction in erosion of completion components positioned in a production or injection well. The technique employs a base pipe and a sand screen surrounding the base pipe. The base pipe comprises a plurality of flow restriction elements arranged in a selected pattern along the base pipe to provide a desired distribution of the fluid flowing into or out of the sand screen. The pattern of flow restriction elements also maintains a flow rate of the flowing fluid below an erosive flow rate across the entire sand screen.

Description:
BACKGROUND 
     In many gas wells, inflowing fluid passes through a sand screen which filters out particulates from the inflowing gas. Generally, the flow rate of the inflowing gas is very high such that any sand production can cause substantial erosion of components in a gas well completion. The sand production is controlled with sand screens employed either as stand-alone screens or in combination with a surrounding gravel pack. However, the velocity of the inflowing gas often can exceed an erosion velocity which causes erosion of the sand screen and ultimate failure of the sand screen. Once the sand screen fails, the risk of erosion arises with respect to other elements of the completion. Use of gravel packing may limit the velocity of particulates; however gravel packs are not necessarily uniform along the entire sand screen, resulting in high, erosive flow rates through poorly packed regions. 
     SUMMARY 
     In general, the present invention provides a technique for filtering sand; distributing a flow of fluid; e.g. distributing an inflow of gas or condensate; and limiting the potential for erosion of completion components in a wellbore. By way of example, the technique is useful in production applications, but the technique also can be used in fluid injection applications, e.g. gas injection applications. The technique employs a base pipe and a sand screen surrounding the base pipe. The base pipe comprises a plurality of flow restriction elements deployed in a selected pattern along the base pipe to provide a desired distribution of flowing fluid. The pattern of flow restriction elements also maintains a flow rate of the flowing fluid below an erosive flow rate across the entire sand screen. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Certain embodiments of the invention will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements, and: 
         FIG. 1  is a schematic illustration of one example of a sand screen assembly deployed in a well, according to an embodiment of the present invention; 
         FIG. 2  is a partial cross-sectional view of the sand screen assembly taken generally across an axis of the sand screen assembly, according to an embodiment of the present invention; 
         FIG. 3  is a partial cross-sectional view taken generally in an axial direction through a wall of the sand screen assembly, according to an embodiment of the present invention; 
         FIG. 4  is a partial cross-sectional view of an alternate example of the sand screen assembly taken generally across an axis of the sand screen assembly, according to another embodiment of the present invention; 
         FIG. 5  is a partial cross-sectional view taken generally in an axial direction through a wall of an alternate example of the sand screen assembly, according to another embodiment of the present invention; 
         FIG. 6  is a schematic illustration of one embodiment of the flow restriction elements, according to an embodiment of the present invention; 
         FIG. 7  is a partial cross-sectional view of an alternate example of the sand screen assembly taken generally across an axis of the sand screen assembly, according to another embodiment of the present invention; 
         FIG. 8  is a partial cross-sectional view taken generally in an axial direction through a wall of an alternate example of the sand screen assembly, according to another embodiment of the present invention; and 
         FIG. 9  illustrates one example of a flow profile along a sand screen when fluid inflow is controlled by flow restriction elements, according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous details are set forth to provide an understanding of the present invention. However, it will be understood by those of ordinary skill in the art that the present invention may be practiced without these details and that numerous variations or modifications from the described embodiments may be possible. 
     The present invention generally relates to a system and methodology for filtering sand from flowing fluid, such as from inflowing gas in a gas production well. As explained in greater detail below, the system and methodology also enable a desired distribution of the flowing fluid across the sand screen while keeping the flow rate of the flowing fluid below an erosion flow rate to protect the sand screen from degradation. 
     According to one embodiment, a well system is provided with one or more sand screen assemblies coupled into a completion and deployed downhole into a gas well. Each sand screen assembly comprises a base pipe surrounded by a sand screen which filters particulates from an inflowing stream of gas during gas production. The base pipe beneath the sand screen is equipped with a plurality of flow restriction elements through which the inflowing gas moves to an interior of the base pipe after passing through the sand screen. 
     The flow restriction elements are sized and distributed to provide a controlled pressure drop and to eliminate regions of high flow velocity along the sand screen. The flow velocity is restricted to a rate below an erosion rate of the sand screen to prevent degradation and failure of the sand screen during gas production. The flow restriction elements may be arranged in a variety of patterns to provide the controlled pressure drop and thus the controlled flow rate through the sand screen. For example, multiple flow restriction elements may be evenly distributed along the base pipe to provide an evenly distributed inflow of gas and a consistent pressure drop along the sand screen. However, other patterns of the flow restriction elements also may be selected to create a desired flow control, e.g. a desired variation in pressure drop and/or flow rate along the sand screen. 
     Referring generally to  FIG. 1 , one schematic example of a well system  20  for use in a well  22  is illustrated. Well  22  may comprise a production well for producing a desired fluid, e.g. gas or oil; or well  22  may comprise an injection well for injecting a desired fluid, e.g. gas or water. The well system  20  is designed to enable filtering of flowing fluid during production (or injection) of fluid from the well  22 . In this particular example, well system  20  may comprise a well completion  24 , e.g. a gas production well completion, deployed downhole into a wellbore of well  22 . The completion  24  may be deployed downhole via a conveyance  26 , such as coiled tubing, production tubing, or another suitable conveyance. Depending on the specific application, well  22  may comprise a wellbore  28  which is cased or lined with a casing  30  having perforations  32  to enable fluid communication between a surrounding reservoir/formation  34  and the wellbore  28 . However, completion  24  may be employed in open wellbores or in a variety of other wellbores, environments and wellbore configurations designed to maximize retrieval of the desired hydrocarbon based fluid, e.g. gas. The completion  24  also may be designed for fluid, e.g. gas, injection applications. 
     Well completion  24  potentially includes many types of devices, components and systems. For example, the well equipment may comprise a variety of artificial lift systems, sensor systems, monitoring systems, and other components designed to facilitate production operations, servicing operations, and/or other well related operations. In the example illustrated, well completion  24  further comprises a sand screen assembly  36 . 
     The sand screen assembly  36  has a sand screen  38  designed to filter sand from gas or other fluid flowing across the sand screen  38 . During gas production, for example, gas flows into wellbore  28  from formation  34  and passes through sand screen  38  which filters out sand while allowing the remaining gas to pass into completion  24 . The sand screen  38  may be used in cooperation with and/or be positioned between other components of the well completion  24 . Additionally, the sand screen assembly  36  may comprise a base pipe  40  positioned such that the sand screen  38  is mounted to surround the base pipe  40 . 
     Completion  24  also may comprise one or more isolation devices  42 , e.g. packers, positioned to enable selective isolation of a specific well zone associated with the sand screen assembly  36 . It should be noted that well completion  24  may further comprise additional sand control assemblies  36  and isolation devices  42  to isolate and control fluid flow, e.g. gas flow, from (or to) other well zones of the reservoir/formation  34 . 
     In  FIG. 1 , wellbore  28  is illustrated as a generally vertical wellbore extending downwardly from a surface location  44 . Additionally, completion  24  is illustrated as deployed downhole into the generally vertical wellbore  28  beneath surface equipment  46 , such as a wellhead. However, the design of wellbore  28 , surface equipment  46 , and other components of well system  20  can be adapted to a variety of environments. For example, wellbore  28  may comprise a deviated, e.g. horizontal, wellbore or a multilateral wellbore extending from surface or subsea locations. The well completion equipment  24  also may be designed for deployment into a variety of vertical and deviated wellbores drilled in a variety of environments. 
     Referring generally to  FIG. 2 , one embodiment of sand screen assembly  36  is illustrated. In this embodiment, base pipe  40  comprises a plurality of flow restriction elements  48 , and sand screen  38  is mounted around base pipe  40  and the plurality of flow restriction elements  48 . The flow restriction elements  48  are designed to allow gas flow through a sidewall  50  of base pipe  40  and into an interior  52  of the base pipe for production to a desired location. The plurality of flow restriction elements  40  are arranged in a desired, predetermined pattern to provide a controlled pressure drop across the base pipe  40 , and thereby to provide a controlled flow rate of inflowing gas through sand screen  38 . The flow restriction elements  48  also may be employed for use with other fluid, e.g. condensates, oil or water, flowing at a high flow rate into or out of the base pipe  40  during production or injection applications. 
     Various sizes, densities and patterns of flow restriction elements  48  may be located along the base pipe  40  which is positioned radially beneath the surrounding sand screen  38 . The sizes, densities and patterns of flow restriction elements  48  are selected according to the environment, downhole pressures, quality of the formation, presence of a surrounding gravel pack, and other environmental parameters. The size, density and arrangement of the flow restriction elements  48  establish the desired pressure drop along the base pipe  40  and also serve to sufficiently reduce the flow velocity of the gas or other fluid below an erosion flow rate. In specific applications, the arrangement of flow restriction elements  48  is selected to reduce the flow rate of inflowing gas (and particulates carried with the inflowing gas) to a rate which does not cause erosion along any region of the surrounding sand screen  38 . In many applications, the flow restriction elements  48  are evenly distributed along the base pipe  40  to provide a constant pressure drop along the base pipe  40  and an evenly distributed inflow of gas. However, the size, density and pattern of the restriction elements  48  also may be varied along the base pipe  40  in a predetermined manner to provide a controlled variation of pressure drop and/or flow rate of, for example, inflowing gas. 
     In  FIGS. 2 and 3 , cross-sectional views of portions of one specific embodiment of the sand screen assembly  36  are illustrated. In this embodiment, the flow restriction elements  48  comprise small holes or orifices  54  extending in a generally radial direction through sidewall  50  of base pipe  40 . The orifices  54  have a diameter selected according to the parameters of the downhole application, e.g. gas production application, so as to sufficiently reduce the rate of flowing fluid below an erosion rate of sand screen  38 . In many applications, the size of orifices  54  is in the range of one to five times the size of the slot openings/passages through the surrounding sand screen  38 . For example, if sand screen  38  is designed with screen openings, e.g. pore or slot openings, approximately 0.25 mm in diameter, the diameter of orifices  54  may be selected in the 0.3 mm to 1.0 mm range. However, formation parameters, e.g. particle size, and other downhole factors may encourage use of smaller or larger orifices  54 . The pattern of orifices  54  can be used to significantly reduce flow area through the base pipe  40  and to spread the flowing fluid over a desired perforation pattern. Consequently, the desired pressure drop occurs as fluid moves through sidewall  50  of base pipe  40 . The total inflow area created by the sum of flow restriction elements  48  is calculated to give the desired pressure drop and flow rate reduction along the base pipe. 
     The inflow area provided by flow restriction elements  48  is a function of perforation/orifice diameter and the number of orifices  54 . To achieve an even distribution of the flowing fluid, e.g. inflowing gas, as desired in some embodiments, many small holes may be created through sidewall  50  of base pipe  40  in a consistent or even pattern. This type of pattern through the base pipe  40  creates an even gas inflow pattern toward and through the sand screen  38 . 
     In the embodiment illustrated, sand screen  38  comprises a plurality of layers  56  designed to facilitate both filtering and flow through the sand screen  38 . Depending on the well environment and other downhole factors, the actual type and number of layers can vary substantially. However, several types of sand screens  38  comprise an internal drainage layer  58  surrounded by a filter media layer  60 . Alternate and/or additional layers also may be provided. 
     In  FIGS. 4 and 5 , another embodiment of sand screen assembly  36  is illustrated as having sand screen  38  positioned over base pipe  40 . In this embodiment, each flow restriction element  48  comprises a nozzle  62  in the form of an insert which is inserted into a corresponding perforation or opening  64  formed radially through sidewall  50 . The nozzle inserts  62  may be secured in their corresponding openings  64  by a variety of mechanisms. For example, the nozzle inserts  62  may be threaded into or press fit into corresponding openings  64 . The nozzle inserts  62  also may be tapered or conical to facilitate frictional engagement when press fit into corresponding opening  64 . It should be noted that in other embodiments, the nozzles  62  may be formed in sidewall  50  without creating separate inserts received in corresponding openings. 
     In the embodiment illustrated, each nozzle insert  62  comprises a passage  66  through which inflowing gas is routed through sidewall  50  and into the interior  52  of base pipe  40 . As described with respect to the previous embodiment, the size of each passage  66  as well as the number and pattern of inserts  62  may be calculated to achieve the desired pressure drop across the base pipe  40  and also the desired reduction in velocity of flowing fluid, e.g. inflowing or outflowing gas, to a flow rate below an erosion rate of the sand screen  38 . The nozzle inserts  62  also may be formed from an erosion resistant material, such as a hardened material, carbide material, or other suitable material. 
     Referring generally to  FIG. 6 , the nozzles  62  may be designed with flow passages  66  each having an expanded portion  68  downstream of a passage entry opening  70 . By way of example, the expanded portion  68  may be designed as a tapered region with a taper having an increasing diameter in the direction of flowing fluid. The expanded portions  68  help prevent plugging of passages  66  if particles pass through screen openings  72 , e.g. slots or pores, of sand screen  38 . In this design, the entry opening  70  provides the desired flow area, but this region only extends a short length to help prevent plugging. 
     By choosing nozzles  62  having passages equal to or slightly larger than screen openings  72  of the sand screen  38 , a self-healing effect is achieved. If the sand screen  38  undergoes any erosion, as illustrated by the widened screen opening  72  on the right side of  FIG. 6 , a particle  74  is able to pass through and plug the corresponding nozzle  62 . The plugged passage  66  reduces the fluid flow flux in this area and reduces or eliminates any further erosion. Consequently, the diameter/area of passages  66  may be selected based on formation particle size to make sure the particles are able to plug the passage  66  in the event of regional failure of sand screen  38 . In some applications, passages  66  may be smaller than screen opening  72  but then the nozzles are subject to unwanted plugging due to fines passing through the sand screen  38 . 
     To further improve this self-healing effect, the drainage layer  58  of the sand screen  38  may be separated into several compartments. The compartmentalization may be achieved by placing inserts or other types of flow blocking members in the axial flow channels of the drainage layer  58  to prevent movement of particles  74  in an axial direction along an exterior of the base pipe  40 . Preventing particles  74  from flowing axially or tangentially along an outer surface of the base pipe  40  ensures that a significant portion of the sand screen will not fill with sand even if a small part of the sand screen  38  is eroded. By way of example, the inserts or flow blocking members may comprise a ring in the drainage layer, a segment between structural members, e.g. between axial rods, of the sand screen, a shim placed between wrappings of the screen, or other suitable members designed to compartmentalize the screen and thus prevent any substantial transverse flow of fluid and particulates. 
     Referring generally to  FIGS. 7 and 8 , another embodiment of sand screen assembly  36  is illustrated as having sand screen  38  positioned around base pipe  40 . In this embodiment, each flow restriction element  48  comprises a small tube  76  disposed between an outer surface of the base pipe  40  and the surrounding sand screen  38 . In one example, multiple tubes  76  are oriented generally longitudinally between a drainage layer of the sand screen  38  and the outer surface of base pipe  40 , as best illustrated in  FIG. 8 . Additionally, each tube  76  is routed to and coupled with the corresponding hole  54  extending through sidewall  50 . 
     With respect to embodiments of the present erosion prevention system, such as those embodiments discussed above, the size of the passages/flow areas through the flow restriction elements is designed for optimal flow performance. However, various embodiments also may be constructed to provide the self-healing effects discussed above. Generally, each flow restriction element  48  provides a flow connection to the interior  52  of base pipe  40  and acts as a drain for inflowing fluid, e.g. gas, entering the sand screen  38 . As a result, the gas flow approaching sand screen assembly  36  tends to converge towards these drainage points. 
     The focusing effect of the flow may be controlled, at least somewhat, by the slot/opening density of the sand screen  38  and/or by the cross-sectional configuration of the drainage layer  58 , as illustrated schematically in  FIG. 9  which provides an example of a flow profile  78  across the sand screen  38 . With relatively small areas open to flow through the sidewall  50  of base pipe  40  versus a relatively large cross-sectional area of the drainage layer  58 /sand screen  38 , a more even flux is achieved with respect to fast flowing fluid, e.g. inflowing gas, approaching the sand screen assembly  36 . As the fluid enters the slot opening  72  of the sand screen  38 , a small pressure drop occurs. Additionally, a small pressure drop occurs as a fluid flows longitudinally/transversely within the sand screen  38  toward a flow restriction element  48  of base pipe  40 . To achieve a small flux variation, the sand screen assembly  36  may be designed so the pressure drop through the screen opening  72  is of a similar order of magnitude as the pressure drop along the drainage layer  58  over the distance between distant flow restriction elements  48 . 
     Desired patterns of flow restriction elements  48  may be selected and designed based on optimization of peak flow velocity versus average flow velocity. Knowledge of the peak flow velocity and the average flow velocity is used to design flow restriction element density and flow area to ensure the velocity approaching sand screen  38  stays below an erosion velocity, thereby reducing or preventing erosion of the sand screen  38 . 
     The overall well system  20  may be constructed to accommodate a variety of flow filtering applications in a variety of well environments while limiting or preventing erosion of the screen and other completion components. Accordingly, the number, type and configuration of components and systems within the overall system may be adjusted to accommodate different applications. For example, the size, number and configuration of the sand screen assemblies may vary from one application to another along the completion equipment. Additionally, many types of flow restriction elements and arrangements of those elements may be employed as dictated by the overall design of gas production equipment and by downhole environmental conditions. The base pipe configuration and the sand screen configuration also may be adjusted according to the specific application and environment. The sand screen assemblies and their erosion control elements may be combined into many types of well completions utilized in production and/or servicing operations. Also, the types and arrangements of other downhole equipment used in conjunction with the one or more sand screen assemblies may be selected according to the specific well related application in which the sand screen assemblies are employed. 
     Although only a few embodiments of the present invention have been described in detail above, those of ordinary skill in the art will readily appreciate that many modifications are possible without materially departing from the teachings of this invention. Accordingly, such modifications are intended to be included within the scope of this invention as defined in the claims.