BACKGROUND 
     The invention generally relates to controlling flows in a well. 
     In the downhole environment, there are many applications which involve controlling flows. For example, a typical downhole completion may include an oil/water separator, which receives a produced well fluid mixture and separates the mixture into corresponding water and oil flows. The water flow may be reintroduced into the well, and for this purpose, the downhole system may be designed for purposes of generally establishing the rate at which water is introduced back into the well. 
     The conventional way of controlling a flow in the downhole environment involves the use of a lossy device, such as an orifice or other restriction. The size of the flow path through the device may be determined, for example, using simple hydraulic calculations, which are based on the assumption that the downhole hydraulic parameters are relatively constant over time. However, when the pressure and/or flow characteristic of one part of the hydraulic system changes, the whole flow balance may be disturbed, as the calculated size is no longer correct. 
     Thus, there is a continuing need for better ways to control flows in a well. 
     SUMMARY 
     In an embodiment of the invention, a technique that is usable with a well includes providing downhole equipment and regulating a ratio of flows that are provided to the equipment. 
     In another embodiment of the invention, a system that is usable with a well includes communication paths, which are located in the well to receive flows. A controller of the system regulates a ratio of the flows. 
     Advantages and other features of the invention will become apparent from the following drawing, description and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is a flow diagram depicting a technique to control flows in a well according to an embodiment of the invention. 
         FIG. 2  is a schematic diagram of a system to regulate flows in a well produced by a single input flow according to an embodiment of the invention. 
         FIG. 3  is a schematic diagram of a system to regulate flows in a well produced by multiple input flows according to an embodiment of the invention. 
         FIG. 4  is a schematic diagram illustrating a venturi-based flow split controller according to an embodiment of the invention. 
         FIG. 5  is a schematic diagram illustrating a mechanical feedback-based flow split controller according to an embodiment of the invention. 
         FIG. 6  is a schematic diagram of a well according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     In accordance with embodiments of the invention described herein, flows in the downhole environment are controlled by regulating a ratio of the flows. Thus, this approach overcomes challenges of conventional downhole hydraulic systems in which orifice sizes and other hydraulic parameters were designed based on the assumption that no changes would occur to downhole flow rates, pressures, etc. More specifically, referring to  FIG. 1 , a technique  10  in accordance with some embodiments of the invention includes providing (block  14 ) a hydraulic system in a well, which contains communication paths to communicate flows. A ratio of the flows is regulated (block  16 ) such that the ratio is relatively constant and is not sensitive to pressure and/or flow changes in the hydraulic system. 
     As a more specific example,  FIG. 2  depicts a system  30  to regulate flows in a well according to some embodiments of the invention. The system  30  includes two cross-coupled hydraulic flow control subsystems, which regulate outlet flows  60  and  70  that are produced in response to an inlet flow  40 . More specifically, the inlet flow  40  (communicated through a conduit  34 ) is split into two intermediate flows  42  and  46 , which are communicated through conduits  44  and  48 , respectively, to flow controllers  50  (a flow controller  50   a  for the intermediate flow  46  and a flow controller  50   b  for the intermediate flow  42 ). The control of the intermediate flow  42  by the flow controller  50   b  produces the outlet flow  60 ; and the control of the intermediate flow  46  by the flow controller  50   a  produces the outlet flow  70 . 
     Flow sensors  54   a  and  54   b  are coupled to sense the flows  46  and  42 , respectively, and provide positive feedback to the flow controller  50  in the other flow path. In this manner, the flow controller  50   a  controls the outlet flow  70  based on the outlet flow  60 , which is sensed by the flow sensor  54   b . Similarly, the flow controller  50   b  regulates the outlet flow  60  based on the outlet flow  70  that is sensed by the flow sensor  54   a . Due to the positive feedback provided by this control scheme, the flow controller  50   a  increases the outlet flow  70  in response to sensing an increase in the outlet flow  60 . Likewise, the flow controller  50   b  increases the outlet flow  60  in response to the sensing of an increase in the outlet flow  70 . 
     Although  FIG. 2  depicts a control scheme for use with a single inlet flow, a similar control scheme may be used to control the ratios of flows that are produced by parallel inlet flows, in accordance with other embodiments of the invention. More specifically,  FIG. 3  depicts an embodiment of such a system  76  in accordance with some embodiments of the invention. As depicted in  FIG. 3 , the system  76  receives parallel inlet flows  78 . The system  76  may contain, for example, a passive device  74  that regulates resultant outlet flows  80 , which are produced in response to the parallel inlet flows  78 , such that a ratio of the outlet flows  80  is relatively constant. Thus, for two outlet flows Q 1  and Q 2 , the system  76  generally maintains the following relationship:
 
 Q   1   /Q   2   =k,   Eq. 1
 
where “k” represents a constant.
 
     As a more specific example, the passive device  74  (see  FIG. 3 ) may be a venturi or orifice plate mechanism, in accordance with some embodiments of the invention. As an example,  FIG. 4  depicts a passive, venturi-based flow split controller  100  in accordance with some embodiments of the invention. Referring to  FIG. 4 , the flow split controller  100  receives a single inlet flow  104  (for this example) at an inlet  105 . The inlet flow  104  flows through a main flow path of a venturi  110  to produce a corresponding outlet flow  108  at an outlet  107 . The venturi  110  includes a suction inlet  115 , which exerts a suction force against a piston  120  in response to the flow through the main flow path of the venturi  110 . The suction caused by the flow through the main flow path of the venturi  110  causes the piston  120  to counter an opposing force, which is exerted by a spring  140  and move to open flow through a flow path  117 . The flow path  117 , in turn, is in communication with the inlet  105 . Thus, for a given flow through the venturi  110 , fluid communication is opened through the path  117  to create a corresponding outlet flow at another outlet  131  of the flow divider  100 . When the outlet flow  108  increases, this causes a corresponding increase in the suction at the suction line  115  to further open the path  117  to further increase the outlet flow  130 . Thus, the flow split controller  100  provides positive feedback for purposes of regulating the ratio of the outlet flows  108  and  130  to be relatively constant. 
     It is noted that the flow split controller  100  is depicted in  FIG. 4  and described herein merely for purposes of describing a passive flow divider, or flow split controller, that may be used in the downhole environment in accordance with some embodiments of the invention. Other passive or non-passive flow split controllers may be used in accordance with other embodiments of the invention. 
     Referring to  FIG. 5 , as another example, in accordance with some embodiments of the invention, a system  150  uses two positive displacement devices  160  for purposes of regulating the ratios of two outlet flows  180 . In general, the positive displacement devices  160  each includes fins, or turbines, which turn in response to a received inlet flow  152 . Due to a mechanical coupling  170  between the positive displacement devices  160 , the rotation of the displacement devices is controlled in part through the positive feedback from the other device  160 . Thus, an increased flow through one of the positive displacement devices  160  causes a corresponding increase in flow in the other positive displacement device  160 . 
     The flow control systems, which are disclosed herein may have many downhole applications. As a specific example, in accordance with some embodiments of the invention, the flow control systems may be used for purposes of downhole oil and water separation. The basic principle is to take produced fluid (an oil/water mixture, typically with eighty plus percent of water) and pump the produced fluid through a device that separates a proportion of the water from the mixture and reinjects the water into a downhole disposal zone. As a more specific example,  FIG. 6  depicts a well  200 , which includes a flow split controller  244  in accordance with some embodiments of the invention. 
     As depicted in  FIG. 6 , the well  200  includes a producing zone  220 , which is located below a lower packer  240  and a water disposal zone  260 , which is located between the lower packer  240  and an upper packer  241 . A pump  222  of the well  200  receives a produced well fluid mixture  221 , which contains oil and water. The pump  222  produces an output flow  230 , which passes into an oil/water separator  234 , which may be a hydrocyclone, in accordance with some embodiments of the invention. The hydrocyclone  234  produces two flows a water flow and an oil flow. 
     Without proper regulation of the ratio of the oil and water flows, several problems may be encountered. For example, if the amount of water production increases more than expected, the rate at which the water is reinjected into the disposal zone  260  must be increased, in order to avoid producing the water to the surface of the well  200 . If the water production is significantly less than expected, oil may be injected into this disposal zone  260 . Therefore, by controlling the ratio of the oil and water flows, the efficiency of the water removal and oil production processes is maximized. 
     As depicted in  FIG. 6 , the flow split controller  244  produces a water flow  270 , which is communicated through a conduit  250  into the disposal zone  260 ; and the flow split controller  244  also produces an oil flow  217  to the surface via a conduit, or production string  215 . 
     To summarize, the overall goal of the flow split controller is to maintain a flow split ratio at some constant ratio in the downhole environment. The flow split controller senses the changes in flow or pressure and responds to maintain the flow split ratio. This arrangement is to be contrasted to designing a hydraulic system based on an assumed (but possibly inaccurate) model of the flow split; using lossy orifices to force some sort of flow split; or placing a device in the system that maximizes water removal. The latter approach may be significantly more complicated than the use of the flow split controller, as this approach may require sensors for the water and feedback to a flow rate controlling valve. 
     Several practical issues arise when using flow split controllers in the downhole environment, both general and application specific. The devices are passive (i.e., no external energy required). Therefore, in order to affect the flow split, work must be done and this arises from the losses in the flow measurement device (can be small if a venturi is used) and more so in the flow controller which has to throttle the flow (dominant as typically a partially closed valve). The more control the device has to achieve the greater the losses will be. Thus, significant flow splits against adverse pressure gradients will create the highest pressure drops through the device. 
     The flow split controllers may have moving parts in order to restrict the flow, and therefore, the presence of solids in the downhole environment may present challenges and possibly preclude positive displacement-type flow controllers. Solids may also be an issue for hydraulic type flow controllers as the flow velocity through the flow sensor and flow controller is high. Usually a flow velocity of several meters per second (m/s) is used in order to achieve sufficient hydraulic forces in the hydraulic feedback. The upper boundary on the flow velocity may be limited by such factors as erosion and the potential for a high flow jamming moving parts. 
     The devices may have a finite dynamic range depending on the CD versus flow rate characteristic of the flow controllers, but a single device may be able to cover flow split ranging by 10:1 and changes in downstream pressure of one of the flows. 
     Other challenges may arise in the use of a flow split controller downstream of an oil/water separator, be it a gravity type, hydrocyclone or rotating cyclone. First, the pressures on the two separated flows may not necessarily the same, and secondly, the densities of the two flows may be different. The different inlet pressures may be compensated for in the design of the flow controller in one or both of the lines, either as an offset in the flow controller if the differences are small or as a lossy device (e.g., fixed orifice) in the pressure line. 
     Using a hydraulic controller involves a flow sensor that has a performance proportional to the square root of density. Thus, differences and changes in the density of one or both of the lines affect the control, but provided there is some knowledge of the initial fluid properties, the initial set point may be made to allow for initial conditions and the square root reduces the sensitivity to this effect. In this configuration the flow sensor for the oil rich line acts on the flow controller for the water rich line and vice versa, so there is a compounded effect of the density contrast between the two lines. 
     While the present invention has been described with respect to a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.
A technique includes providing equipment downhole in a well to receive flows. The technique includes regulating a ratio of the flows in the well. The regulation includes regulating the ratio of the flows such that the ratio is substantially independent of pressures of the flows downstream of a point at which the regulation occurs.