This invention relates to earth well drilling systems.  In particular, it relates to apparatus and methods for gravel packing one or more production radial tubes extending into an earth formation from a well bore. 
A number of techniques are known for passing a drill string down a well bore through a whipstock into adjacent underground formation.  One particularly effective technique is disclosed in Dickinson et al.  U.S. Pat.  No.  4,527,639 wherein a piston-like system permits the turning of a rigid pipe drill string through a short radius 90.degree.  turn.  This is accomplished by directing hydraulic fluid against the rearward side of a drillhead at the forward end of the drill string to provide a pulling force at the drillhead to move the pipe into the formation without buckling of the pipe.  An improvement on this system is described in co-pending Dickinson et al.  application entitled Earth Drilling Method and Apparatus Using Multiple Hydraulic Forces, filed simultaneously herewith, wherein pushing forces at the rearward end of the drill string are used in addition to the pulling forces to move the rigid pipe through the whipstock and to control the rate of movement of the pipe. 
Erectable whipstocks are known and described in Dickinson et al.  U.S. Pat.  No.  4,527,639 and in EPA Publication 0 100 230.  There, a retractable whipstock consisting of connected assemblies are disclosed which extend from a retracted position within the structure to form an arcuate tube bending guideway by applying hydraulic forces from the surface to a hydraulic piston assembly.  After placement of the production radial tube, it is severed near the whipstock, and the remaining drill string and whipstock may be withdrawn as by pulling from the surface.  The procedure is repeated to place multiple radial tubes into other portions of the formation. 
In co-pending Dickinson et al.  application entitled Mechanically Actuated Whipstock Assembly, an improved retractable whipstock is disclosed which includes a structure with a number of collapsed, connecting guideway assemblies and a retractable anchor connected to the rear side of the anchor assembly.  Erection means is provided which is slidable within the assembly and pivotally connected to a forward one of the guideway assemblies and at its other end to an extension member extending to the surface.  When the system reaches the desired position adjacent the formation, the anchor is locked in the earth well and the erection means is pulled by an extension arm from the surface to cause a forward one of the guideway assemblies to be pivotally swung so that the guideway assemblies in composite form a curved pathway extending into the formation.  After erection, a drill string is passed through the whipstock into the formation and used as for steam injection.  The radial tube is cut near the whipstock exit for production and a portion of the tube and the whipstock is pulled back from the surface.  The system also includes a deerection system in which the extension arm is again lowered to cause the guideway assemblies to move back into their retracted position.  The anchor means is collapsed and the entire assembly may be moved to another position within the well or pulled to the surface.  In this manner, multiple radial tubes may be placed into the formation. 
The present invention relates to a system of gravel packing which is particularly effective for gravel packing radials in conjunction with the above type of systems using multiple production radial tubes.  Gravel packing is a technique whereby gravel is packed around a production well extending into an underground formation.  The well typically is lined with a slotted liner which includes slots of a size sufficient to pass oil from the surrounding formation into the liner for pumping to the surface but small enough to screen out the gravel pack particles. 
Various gravel packing techniques are disclosed in Zublin U.S. Pat.  No.  2,434,239, Sparkin U.S. Reissue Pat.  No.  28,372 and Medlin U.S. Pat.  No.  4,378,845.  Zublin discloses gravel packing of lateral pipes which are withdrawn during gravel packing.  Medlin discloses gravel packing from a well through a lateral screen.  Sparkin discloses gravel packing a well by pumping through casing perforations.