You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND OF INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates to valves for use in the wall of a string of tubulars, such as casing that is placed in wells. More particularly, a valve is provided that can be opened by a selected pressure inside the tubular and that may remain open. 
         [0003]    2. Description of Related Art 
         [0004]    To produce hydrocarbons from some reservoirs in the earth, long, horizontal holes are drilled through productive rock. To prevent collapse of the surrounding rock into the horizontal wellbore, casing must be placed in the holes before fluids can be withdrawn. The casing is normally pushed along the horizontal hole, but the frictional resistance between casing and the wall of the wellbore limits the distance that casing can be placed in horizontal wells using standard methods. One way that has been used to allow longer horizontal sections of casing in wells is to float the casing into the horizontal section using a low-density fluid inside the casing. In all wells, horizontal, deviated and vertical, openings in the wall of the casing must be provided for injection of fluids or production of fluids through the well. 
         [0005]    After casing is placed in a well, in some wells cement is pumped down the casing and up the annulus between the casing and the wall of the wellbore. Openings for fluid flow through the wall of the casing are commonly made by perforating guns, which shoot a hole through the casing wall, the cement layer and a short distance into the surrounding rock. In other wells, an “open hole” is left outside the casing (no cement) and packers, made of a rubber sleeve that can be inflated or swellable material are placed at selected distances along the wellbore to prevent flow along the annulus outside casing. In this case, valves in the casing wall that can be opened by a mechanical device or by pumping balls down the casing are normally provided between the packers. 
         [0006]    European Patent EP 0 681 088 discloses an annulus pressure-responsive valve that can be locked in the open position and then closed. The valve uses a power piston in a housing with an actuating piston. European Patent Application 2 458 139 discloses a valve that can be inserted into the wall of casing in a well by using the valve body to drill through the casing wall. UK Pat. App 2464009 discloses a method of using inflow control devices, which are commonly used to control the rate of flow into casing when there are multiple points of entry. The inflow control devices do not stop flow. U.S. Pat. Nos. 5,957,197 and 6,820,697 disclose downhole valves. It is common to place valves such as sliding sleeves in casing, which can be operated by mechanical tools inside the casing. Such device is disclosed in WO 2012/080487. Valves in casing operated mechanically by running tools or coiled tubing inside casing are often used in connection with hydraulic fracturing of multiple zones in a well. Many patents disclose downhole safety valves, which serve primarily the purpose of effecting a reliable positive closure of the bore of a production tubing string in the event of an emergency. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    A valve is provided that can be opened by a selected increase of pressure inside casing or other tubular followed by a selected value of pressure reduction in the tubular. The valve may be used in operations to float a casing into a horizontal section of wellbore, to open a casing at selected locations for production or injection (including hydraulic fracturing operations), and for any other uses where a valve that is opened by internal pressure in the tubular and remains open is needed. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
         [0008]      FIG. 1  is an isometric view of the valve disclosed herein and a section of the tubular where it is to be installed. 
           [0009]      FIG. 2  is a cross-section view of the valve installed in a tubular. 
           [0010]      FIG. 3   a  is a view of the proximate end of the valve.  FIG. 3   b  is a cross-sectional view of the valve in the closed position. 
           [0011]      FIG. 4  is a cross-sectional view of the valve in the open position while mounted in a tubular. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]    Referring to  FIG. 1 , inflow control valve  10  is shown in position to be inserted into the wall of tubular sub  12 , having a wall thickness, t, which may be a coupling for a string of tubulars. Alternatively, the sub may have any selected length. The wall thickness t is effective to provide the mechanical strength of a tubular string to be placed in a well with valve  10  in place. Receptor hole  13  is sized to receive valve  10  and may include internal threads adapted to receive external threads on valve  10 . Alternatively, valve  10  may be held in place in receptor hole  13  by a clip ring and indexed so as to be held in a selected direction in hole  13  and to align ports to increase flow area when the valve is opened. The indexing may be provided by a protrusion or indentation on body  30  or marking on the proximate end of body  30  such that the valve can be aligned in receptor hole in a selected direction. O-ring  30   b  provides a seal outside body  30 . Inner port  14  is drilled to intersect receptor hole  13  at a selected position and extend through the inner wall of sub  12 . Plug  14   a  is placed in the outer portion of inner port  14 . Outer port  16  is drilled through the outer surface of sub  12  to intersect receptor hole  13  at a selected position. A screen may be placed over outer port  16  to prevent particles entering the port and valve and plugging a flow channel after the valve opens. Screens outside flow control valves in casing are well known in industry. 
         [0013]    Referring to  FIG. 2 , valve  10  is shown installed in receptor hole  13  of sub  12 . Threads  22  may be used to fasten valve  10  into hole  13  of sub  12 , or a clip ring may be used. Torque may be applied to valve  10  by use of hexagonal key hole  20  to align or make up the threads. When the valve is installed, distal hole  24  of the valve intersects outer port  16  and proximate hole  26  of the valve intersect inner port  14 . The holes in the tubular may be drilled or milled with a CNC machine or a conventional machine. 
         [0014]    Referring to  FIG. 3   a , a view of the proximate end of valve  10  is shown. Shuttle  33  is preferably centered around the axis of the valve. Referring to  FIG. 3   b , valve body  30  has distal hole  24  and proximate hole  26 , which are disposed in body  30  so as to intersect inner port  14  and outer port  16  of sub  12  when valve  10  is installed in sub  12 , as shown in  FIG. 2 . Outside threads  30   a  match inside threads in sub  12 . O-ring  31  seals around the proximal end of shuttle  33  with a sealing area A 1 . O-ring  32 , having a larger diameter than o-ring  31 , exerts a force in the distal direction when differential pressure exists from inside to outside the tubular. Split retainer ring  36  is compressed to fit inside body  30  and sized to spring out when it passes shoulder  30   d  so as to hold shuttle  33  in the valve-open position. Shear pin  34 , selected to shear at a selected axial force on shuttle  33 , is inserted in body base  30   c  and shuttle  33  before base  30   c  is inserted into valve body  30 . Body base  30   c  may be fastened in place by threads or other fastening method. Pressure inside sub  12  is applied between o-ring  31  and o-ring  32 . This pressure exerts an axial force on shuttle toward the distal end of the shuttle. Shear pin  34  is selected such that it is sheared at a force corresponding to a selected differential pressure between the inside and outside of sub  12 . After shear pin  34  has failed, shuttle  33  moves to place the distal end of shuttle  33  in contact with the end of the inside opening in base  30   c.  The valve is still closed when pressure inside sub  12  is at or above the pressure to shear pin  34 . 
         [0015]    As pressure inside sub  12  is decreased, a value is reached such that spring  35  moves shuttle  33  toward the proximal end of the valve. The spring constant of spring  35  may be selected to control the pressure inside sub  12  at which shuttle  33  moves.  FIG. 4  shows the position of shuttle  33  after shear pin  34  has been sheared and pressure has decreased inside sub  12  to allow shuttle  33  to move toward the proximal end of the valve. In  FIG. 4 , shuttle  33  has moved past the position where o-ring  32  and retaining ring  36  pass shoulder  30   d.  This allows equalization of pressure across o-ring  32 . Distal hole  24  intersects outside port  16  and proximal hole  26  intersects inside port  14 . [The valve is open when o-ring  32  passes  26 . In this position, split ring  36  insures that the valve is permanently open, independent of pressures inside and outside sub  12 . Turkey button  40 , which may be a bright color, may be placed on the proximal end of shuttle  33  to allow easy inspection to determine if the valve is open. Alternatively, turkey button  40  may be replaced with a sensor to indicate if the valve is open. A signal from the sensor may be communicated to surface using known communication technology or the sensor may be read by a device run on slick line, coiled tubing or logging cable. 
         [0016]    The force required to shear a pin can be selected over a wide range. For example, valves may be placed in a tubular string and adapted to shear the pin in different valves over a range of differential pressures across the valve. All valves in a tubular string remain closed until the shear pin in one valve is caused to fail, so selected valves can be opened by increasing pressure in the tubular only to the value required to open a selected valve or valves. When some valves are open and other valves requiring pressure are to be opened, open valves may be temporarily closed by dropping one or more ball sealers to close the entrance to inside port  14  of open valves or degradable solid particles may be pumped into the well to plug open valves, using technology known in industry. 
         [0017]    Multiple valves can be placed around the circumference of a tubular at about the same axial distance along the tubular. This may be required to achieve a desired flow rate into or out of the tubular at the same depth or distance along a horizontal section of a well. 
         [0018]    Valve body  30  may be constructed from standard coupling stock. The valves in a tubular string, such as a casing string, may be inserted at the surface in drilled ports that are threaded to receive the valves or that are adapted to a clip ring. Beans may be inserted with the valves or inside the valves to control flow rate. The beans or valves may be constructed of erosion-resistant materials. Flow from the valves enters the casing tangentially to the inside wall, minimizing erosion of a tubular inside the casing. Grooves in the outside surface of sub  12  (not shown) may be used to increase flow area if an outside port is in contact with a wellbore. 
         [0019]    One application of the valve disclosed herein is to allow a casing to be floated during installation in a horizontal section of a wellbore. A horizontal casing string may have valves placed at selected locations along the casing. Packers may be placed between the valves to isolate flow in the annulus to different valves. The packers may be swell packers or mechanically expanded packers, using technology known in industry. Light fluid, usually nitrogen gas, is placed inside the casing, using technology known in industry. After the casing has been “landed” at the selected location, the gas may be partially or completely displaced with liquid. Pressure inside casing is increased to shear the pins in selected valves along the casing, Pressure in the casing is then reduced to allow the valves to open. All valves may be opened at substantially the same time. Alternatively, valves having different pressures to shear the shear pins in the valves may be used, such that valves may be selectively opened at different distances along a casing string, Open valves may be temporarily plugged to allow application of pressure across closed valves for their opening. 
         [0020]    The valves disclosed here may be used in a well that is to be acidized or hydraulically fractured. Valves may be opened selectively at different locations, if desired. The method and apparatus described here may be used in vertical or horizontal wells. When wells are hydraulically fractured, fluid is injected at a high rate. Then fluid containing solid particles, called a “proppant” is injected. It may be desirable to increase the area of the flow channel through the valves for production or injection of fluids. For such applications, it may be desirable for the flow path through a valve to increase over that shown in  FIG. 4  at the beginning of a fracture treatment. To facilitate increase in flow area, shuttle  33  and spring  35 .may be constructed of materials that erode quickly. Even valve body  30  may be made partially or completely of material that is easily eroded, such as a soft metal like brass or bronze. After fluid has been pumped through an open valve it may erode completely, increasing the area for flow of fracturing fluid or produced fluids. In wells to be produced through the valves, it may be desirable to use materials that are quickly eroded by fluid flow. 
         [0021]    In another application, multiple valves may be placed along a horizontal wellbore or at different depths in a vertical well and some valves in the casing opened to allow production for a period of time while other valves remain closed. When it is desirable to open new zones or intervals for production, open valves may be temporarily plugged by injecting balls or degradable particles into the well before increasing pressure to a higher value to open additional zones. Alternatively, open valves may be permanently plugged by a resin that solidifies in the valves or by other methods before new zones are opened by increasing pressure in the casing sufficient to shear the pins in other valves. 
         [0022]    Locations of ends, openings and holes are identified by the terms “proximate” and “distal.” In the figures, proximate is leftward and distal is rightward. It should be understood that these terms are used for identification, and the directions can be reversed in defining the terms to achieve the same results. 
         [0023]    Although the present invention has been described with respect to specific details, it is not intended that such details should be regarded as limitations on the scope of the invention, except to the extent that they are included in the accompanying claims.

Summary:
A valve and valve assembly are provided to allow opening of flow channels through the wall of a tubular by increasing and then decreasing pressure in the tubular. Slidable seals having different areas are used to apply an axial force to a shuttle in the valve in response to differential pressure across the valve when in the closed position. The axial force shears a pin, releasing the shuttle. A decrease of pressure in the tubular can allow the valve to open in response to a spring force. Multiple valves may be placed along a tubular string and the valves may be opened all at substantially the same time or at time intervals determined by pressures applied inside the tubular.