A downhole well valve having a variable area orifice ( 26 ) is flow area adjusted by a sliding sleeve ( 20 ) that is axially shifted along a tubular housing ( 12 ) interior in a finite number of increments. A hydraulic actuator ( 60 ) displaces a predetermined volume of hydraulic fluid with each actuator stroke. An actuator displaced volume of fluid shifts the flow control sleeve by one increment of flow area differential. An indexing mechanism ( 40 ) associated with the sleeve provides a pressure value respective to each increment in the increment series.

BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of downhole well tools. More specifically, the invention relates to a downhole tool that provides a selectively variable fluid flow area between the well annulus and the interior flow bore of a well tube. 
     2. Description of Related Art 
     The economic climate of the petroleum industry drives producers to continually improve the efficiency of their recovery systems. Production sources are increasingly more difficult find and exploit. Among the many newly developed production technologies is directed drilling. Deviated wells are drilled to follow the layering plane of a production formation thereby providing extended production face within the production zone. In other cases, a wellbore may pass through several hydrocarbon bearing zones. 
     One manner of increasing the production of such wells is to perforate the well production casing or tubing in a number of different locations, either in the same hydrocarbon bearing zone or in different hydrocarbon bearing ones, and thereby increase the flow of hydrocarbons into the sell. However, this manner of production enhancement also raises reservoir management concerns and the need to control the production flow rate at each of the production zones. For example, in a well producing from a number of separate zones, or lateral branches in a multilateral well, in which one zone has a higher pressure than another zone, the higher pressure zone may produce into the lower pressure zone rather than to the surface. Similarly, in a horizontal well that extends through a single zone, perforations near the “heel” of the well (nearer the surface) may begin to produce water before those perforations near the “toe” of the well. The production of water near the heel reduces the overall production from the well. Likewise, gas coning may reduce the overall production from the well. 
     A manner of alleviating such problems may be to insert a production tubing into the well, isolate each of the perforations or lateral branches with packers and control the flow of fluids into or through the tubing. However, typical flow control systems provide for either on or off flow control with no provision for throttling of the flow. To fully control the reservoir and flow as needed to alleviate the above-described problems, the flow must be throttled. 
     A number of devices have been developed or suggested to provide this throttling although each has certain drawbacks. Note that throttling may also be desired in wells having a single perforated production zone. Specifically, such prior art devices are typically either wireline retrievable valves, such as those that are set within the side pocket of a mandrel or tubing retrievable valves that are affixed to the tubing. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is a downhole valve for well flow regulation that incorporates a sliding sleeve to alter the fluid flow area between the well annulus and well tube flow bore. The tubular valve housing is ported with fluid flow openings in cooperative alignment with fluid flow ports through the sliding sleeve. When the sleeve ports are aligned with the housing ports, fluid flow is accommodated between the well annulus and the tube flow bore. When the sleeve ports are axially offset from the housing ports, fluid flow between the well annulus and the tube flow bore is obstructed. Sleeve port alignment is in graduated increments between a fully open valve and a fully closed valve. 
     Each increment of sleeve displacement is driven by a predetermined volume of hydraulic fluid released from a novel stepping valve. In one directional sequence, a distinctive fluid pressure also is required to step the sleeve from the prior increment to the next. Accordingly, greater fluid pressure is required to increase the valve flow area from one area increment to the next. Moreover, the pressure required for each shift of the sleeve is distinctive to the flow area increment that the sleeve is advancing toward (or from). 
     At each incremental location of the sleeve, the sleeve position is secured by a respective detent channel that accommodates a resiliently expanding snap ring. Each ring detent is flanked by a channel wall set at a predetermined acute angle. Steepness of the channel wall dictates the pressure required to radially constrict the resiliently biased snap ring. Provision of a distinctive channel wall angle respective to each valve flow area setting of the sleeve translates to a distinctive hydraulic pressure from the stepping valve essential to shift the sleeve from a particular setting. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     For a thorough understanding of the present invention, reference is made to the following detailed description of the preferred embodiments, taken in conjunction with the accompanying drawings in which like reference characters designate like or similar elements throughout the several figures of the drawing. Briefly; 
     FIG. 1 is an axial length section of the invention presented in four longitudinal segments,  1 A,  1 B,  1 C and  1 D, respectively. 
     FIG. 2 is an axial section view of a first embodiment of the stepping valve actuator; and, 
     FIG. 3 is an axial section view of a second embodiment of the stepping valve actuator. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     In the following description, numerous details are set forth to provide an understanding of the present invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these details and numerous variations or modifications from the described embodiments may be possible. 
     As used herein, the terms “up” and “down”, “upper” and “lower”, “upwardly” and “downwardly” and other like terms indicating relative positions above or below a given point or element are used in this description to more clearly describe some embodiments of the invention. However, when applied to equipment and methods for use in wells that are deviated or horizontal, such terms may refer to a left to right or right to left relationship as appropriate. 
     Generally, preferred embodiments of the invention provide a variable flow area valve assembly that includes an axially sliding valve sleeve adapted to regulate the flow of fluid through one or more orifices in the valve housing. The sleeve is axially translated from one flow area position to the next by the pressure of a measured volume of hydraulic fluid bearing on a cross-sectional area of the sleeve. A valve actuator operably attached to the valve housing transmits, from a surface source, the measured volume of hydraulic fluid necessary to shift the valve sleeve position from one flow increment to the next in a sequence of several locations between a fully open position to a fully closed position. The change in fluid flow area as the sleeve is actuated through the incremental positions varies so that predetermined changes in flow condition can be provided. As used herein, flow condition may refer to pressure drop across the valve and/or flow rate through an orifice in the valve. 
     At each position increment of the sleeve translation range between fully open and fully closed, the sleeve is secured from uncontrolled displacement by a resilient snap ring set in a sleeve ring seat. At each designated flow area position, is a detent channel in the valve housing. The snap ring on the sleeve expands into a respective detent channel. Each detent channel is defined between parallel channel walls. At least one wall of each channel is formed at an acute angle to the housing axis with each angle being progressively steep. Consequently, a relationship may be established between the channel wall angle respective to a particular flow area setting and the hydraulic pressure from the valve actuator necessary to displace the sleeve from the particular flow area to another. 
     With respect to FIG. 1A, the “upper” end of the invention assembly includes an index housing  10  shown in cross-section to be a tubular element having a number of circumferential channels  40   a  through  40   g  turned about the internal bore perimeter  11 . The side walls of these channels are set at distinctive acute angles. The side walls of the channel  40   a  may be cut at 25°, for example. Representatively, the side wall cut for channel  40   b  may be cut at 30°, the sidewall angle of channel  40   c  may be 35°, the sidewall angle for channel  40   d  may be 45°, the sidewall angle for channel  40   c  may be 50° and the sidewall angle of channel  40   f  may be 60°. 
     As shown by FIG. 1B, the lower end of the index housing  10  threadably assembles with a tubular actuator housing  12 . The assembly joint between the index housing  10  and the actuator housing  12  compresses a chevron seal  30  that wipes the outer cylindrical surface of an axially shifted flow regulator sleeve  20 . 
     The lower end of the actuator housing  12  threadably assembles with a tubular sub  14  as shown by FIG.  1 D. The bottom end of the sub  14  threadably assembles with a tubular bottom housing  16 . The thread joint between the sub  14  and the bottom housing  16  compresses a chevron seal  34  against the outer cylindrical surface of the axially shifted sleeve  20 . 
     The tubular wall of the actuator housing  12  is perforated by a number of elongated orifices  28  as seen from FIG.  1 C. In open alignment with the actuator housing orifices  28  are the corresponding orifices  26  through a seal compression sleeve  24 . The compression sleeve  24  engages the intermediate chevron seal  36  and is secured by an outer clamp  18 . The chevron seal  36  wipes the regulator sleeve  20  surface. 
     Within the housing bore, a tubular sleeve  20  is disposed for a sliding seal fit with the chevron seals  30 ,  34  and  36 . Through the lower end of the sleeve  20  tube wall, a number of elongated orifices  22  may be provided to cooperate with the housing orifices  26  and  28 . The upper end of the regulator sleeve  20  carries a resilient snap ring  42  in a caging channel  44  shown by FIG.  1 A. The outer corners of the snap ring  42  are chamfered to facilitate radial constriction of the snap ring perimeter by an axial thrust on the sleeve  20 . The sleeve is designed for an operative stroke between the detent channels  40   a  and  40   g , inclusive. The snap ring  42  seats into each detent channel  40  for a respective fluid flow relationship through the orifices  22 ,  26  and  28 . When the snap ring  42  is seated in detent channel  40   a , the valve is fully closed. When the snap ring  42  is seated in detent channel  40   g , the valve is fully open. At each of the detent channel positions between  40   a  and  40   g , a progressively increasing flow area is provided by increased alignment between the sleeve orifices  22  and the housing orifices  26 ,  28 . 
     Along the outer surface of the sleeve  20  and aligned between the upper housing seal  30  and the intermediate seal  36  is a chevron seal  32  shown by FIG.  1 C. The seal  32  is secured to the sleeve  20  and moves with it as a load piston. The seal  32  wipes the internal bore wall of a housing cylinder  13  and divides it into two variable volume pressure chambers  46  and  48 . The upper pressure chamber  46  is served by a closing hydraulic conduit  50  from a surface source of hydraulic pressure supply as illustrated by FIG.  1 B. The lower pressure chamber  48  is served by a hydraulic conduit  52  from the control actuator  60  as shown by FIG.  1 C. The control actuator  60  is supplied with hydraulic fluid from the well surface through conduit  54  as shown by FIG. 1B for opening the valve. 
     One embodiment of the control actuator  60  is illustrated in detail by FIG.  2 . An actuation cylinder  61  contains a stepping piston  62  for control of hydraulic fluid flow through the cylinder  61  along a direction of orientation from the supply conduit  54  to the sleeve control conduit  52 . The stepping piston  62  has a sliding seal  65  with the wall of cylinder  61 . A return spring  66  exerts a resilient bias on the stepping piston toward the fluid in-flow end of the cylinder  61 . An orifice closure plug  63  projects axially from the out-flow end of the stepping piston to align with the entrance orifice of the sleeve control conduit  52 . Distinctively, the volume  64  of cylinder  61  that is displaced by translation of the stepping piston  62  from the in-flow end of the cylinder  61  as illustrated by FIG. 2 to closure of the conduit  52  by the plug  63  substantially corresponds to the displaced volume of the lower sleeve chamber  48  for advancement of a single opening increment e.g. to move the sleeve snap ring  42  from the detent channel  40   b  to the detent channel  40   c . A plurality of stepping piston  62  strokes may be required to move the sleeve  20  from an initial opening of the valve as illustrated by FIG.  1 A and the axial distance between detent channels  40   a  and  40   b.    
     The stepping piston  62  further comprises a fluid flow check valve  76  that is oriented to permit a reverse flow of fluid at a limited flow rate from the sleeve control conduit  52  toward the supply conduit  54  by lifting the valve closure off the valve conduit seat against the bias of closure spring  77 . 
     Also within the body of the stepping piston  62  is a stepping valve  70  that comprises an orifice closure pintle  74  acting against the valve seat  73  around the flow orifice  71 . A spring  75  exerts resilient bias on the pintle  74  to open the flow orifice  71 . However, a salient end  78  of the pintle  74  projects above the in-flow end-plane of the pintle  74  to close the orifice  71  when the stepping piston  62  is pressed against the in-flow end of the cylinder  61  by the bias of return spring  66 . 
     As illustrated by FIG. 1D, the regulator sleeve  20  is in the closed valve position. Opening of the valve to a minimum flow rate increment requires the sleeve  20  to be advanced upwardly to move the snap ring  42  from the detent position  40   a  illustrated to the adjacent detent position  40   b . Such linear displacement of the sleeve position relative to the housing requires a finite volumetric increase in the lower pressure chamber  48 . This finite volume of hydraulic fluid is displaced from the displacement chamber portion  64  of the actuation cylinder  61  by the stepping piston  62  as the piston is translated along the cylinder length. 
     Opening hydraulic pressure is directed from the surface along the opening hydraulic line  54  into the upper chamber  68  of the cylinder  61 . The initial pressure differential across the opposite faces of the piston  62  closes both piston valves  70  and  76  and overcomes the spring bias  66  to drive the piston  62  toward the control conduit  52  thereby displacing the fluid volume  64  from the cylinder  61 . 
     At the end of the piston  62  stroke, the plug  63  closes the entrance orifice of conduit  52  to terminate the fluid displacement from the actuation cylinder  61 . Closure of the conduit  52  is signaled to the surface by an abrupt increase in the pressure of opening line conduit  54 . The fluid displaced from actuation cylinder  61  is channeled into the lower sleeve chamber  48  to drive the sleeve snap ring  42  from detent channel  40   a  to  40   b . The resilient bias of the snap ring  42  into the channel  40   b  secures the sleeve position at that location. 
     Upon receipt of the abrupt pressure increase, pressure in the opening conduit  54  is released at the surface and the return spring  66  is allowed to drive the stepping piston  62  toward the in-flow end of the cylinder  61 . Without the high pressure differential across the stepping valve  70 , the spring  75  displaces the pintle  74  from the valve seat  73  to permit a bypass flow of fluid from the conduit  54  through the orifice  71  into the displacement chamber  64  of cylinder  61  until the pintle salient  78  abuts the end wall of the cylinder. 
     The foregoing procedure is repeated for each increment of sleeve opening except that the pressure supplied to the opening conduit  54  that is required to overcome the progressively increased angle of each detent channel wall  40   c  through  40   g  increases correspondingly. Hence, by the pressure value required to advance the sleeve an increment, the identity of the opening increment may be known. 
     From any position of relative opening, the valve may be closed by a surface directed pressure charge along closing conduit  50  into the upper sleeve chamber  46 . See FIGS. 1B and 1C. Correspondingly displaced fluid in the lower sleeve chamber  48  follows a reverse flow path along the actuator control conduit  52  into the cylinder  61  and past the stepping piston  62  through the check valve  76 . 
     An alternative embodiment of the invention control actuator  60  is illustrated by FIG.  3 . In this embodiment, the check valve  76  is omitted as separate apparatus. The bias force of stepping valve opening spring  75  is modified to keep the orifice  71  open against the closing bias of return spring  66  to permit a controlled bypass flow of fluid from the lower sleeve chamber when the valve is closed. 
     Use of sleeve retainer detent channels  40  having progressive side wall angles is one method of informational feedback for indicating the sleeve position. It should be understood by those of skill in the art that other devices may be used to accomplish the same end such as linear transducers. 
     Other applications for the actuator valve  60  described herein may include stepping control for under-reaming tools. It may also be used in a drill-stem testing tool to set an inflatable packer for pressure reversals without unsetting the tool. In another application, the actuator may be used to step set an inflatable packer to different inflation pressures. Similar to the present embodiments, the actuator may be used to step set a gas lift valve into different flow rate positions. 
     Although the invention has been described in terms of particular embodiments which are set forth in detail, it should be understood that this is by illustration only and that the invention is not necessarily limited thereto. Alternative embodiments and operating techniques will become apparent to those of ordinary skill in the art in view of the present disclosure. Accordingly, modifications of the invention are contemplated which may be made without departing from the spirit of the claimed invention.