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CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. § 119 to U.S. Provisional Patent Application Ser. No. 60/082,660, filed on Apr. 22, 1998. 
    
    
     BACKGROUND 
     The invention relates to controlling multiple downhole tools. 
     Referring to FIG. 1, for purposes of measuring characteristics (e.g., formation pressure) of a subterranean formation  31 , a tubular test string  10  is typically inserted into a wellbore which extends into the formation  31 . In order to test a particular region, or zone  33 , of the formation  31 , the test string  10  may have a perforating gun  30  that is used to penetrate a well casing  12  and form fractures  29  in the formation  31 . To seal off the zone  33  from the surface of the well, the test string  10  typically includes a packer  26  that forms a seal between the exterior of the test string  10  and the internal surface of the well casing  12 . Below the packer  26 , a recorder  11  of the test string  10  takes measurements of the test zone  33 . 
     The test string  10  typically has valves to control the flow of fluid into and out of a central passageway of the test string  10 . An in-line ball valve  22  is used to control the flow of well fluid from the test zone  33  up through the central passageway of the test string  10 . Above the packer  26 , a circulation valve  20  is used to control fluid communication between an annulus  16  surrounding the test string  10  and the central passageway of the test string  10 . 
     The ball valve  22  and the circulation valve  20  may be controlled by commands (e.g., “open valve” or “close valve”) sent downhole. Each command is encoded into a predetermined signature of pressure pulses  34  (see FIG. 2) that are transmitted downhole to the tool  11  via hydrostatic fluid present in the annulus  16 . A sensor  25  of the tool  11  receives the pressure pulses  34 , and subsequently, electronics and hydraulics of the test string  10  operate the valves  20  and  22  to execute the command. 
     For purposes of generating the pressure pulses  34 , a port  18  in the casing  12  extends to a manually operated mud pump (not shown). The mud pump is selectively turned on and off by an operator to encode the command into the pressure pulses  34 . A duration T 0  (e.g., 1 min.) of the pulse  34 , a pressure P 0  (e.g., 250 p.s.i.) of the pulse  34 , and the number of pulses  34  in succession form the signature that uniquely identifies the command. 
     SUMMARY 
     In one embodiment, the invention features a system for use in a subterranean well. The system has a command generator that is configured to furnish a first command stimulus downhole. The system includes a first assembly located downhole and has a first member. The first assembly is connected to move the first member in response to the first command stimulus, and the movement of the first member generates a second command stimulus. The system also includes a second assembly located downhole. The second assembly is connected to respond to the second command stimulus. 
     Advantages and other features of the invention will become apparent from the following description, from the drawing and from the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a schematic view of a test string in a well being tested. 
     FIG. 2 is a waveform illustrating a pressure pulse command for a tool of the test string of FIG.  1 . 
     FIGS. 3A, and  4 - 9  are schematic views of a string that includes multiple valves and packers. 
     FIGS. 3B and 3C are waveforms illustrating pressure pulses transmitted to tools of the test string. 
     FIG. 10 is a block diagram of a hydraulic system to control valves of the tools. 
     FIG. 11 is a block diagram of electronics to control valves of the tools. 
     FIG. 12 is a cut-away view of the test string illustrating operation of the ball valve. 
     FIG. 13 is a cut-away view of the test string illustrating operation of the circulation valve. 
     FIGS. 14 and 15 are flow diagrams illustrating the operation of electronics of tools of the test string. 
     FIG. 16 is a schematic diagram illustrating another test string in a well being tested. 
     FIGS. 17 and 18 are flow diagrams illustrating the operation of electronics of tools of the test string. 
     FIG. 19 is a cross-sectional view of a multi-lateral well. 
     FIGS. 20 and 21 are flow diagrams illustrating the operation of valve units of FIG.  19 . 
    
    
     DETAILED DESCRIPTION 
     As shown in FIGS.  3 A- 3 C, a tubular test string  40  having two in-line testing tools  50  and  70  is located inside a well. To send a command (e.g., “open valve” or “close valve”) downhole to the upper tool  50 , a mud pump  39  is used to encode the command into a series of pressure pulses  120  (i.e., a command stimulus) which are applied to hydrostatic fluid present in an upper annulus  43 . The upper tool  50  has a sensor  54  in contact with the hydrostatic fluid in the upper annulus  43 . The upper tool  50  uses the sensor  54  to identify the signature of the pressure pulses  120  and, thus, extract the encoded command. In response to the appropriate commands, the upper tool  50  is constructed to actuate an in-line ball valve  53  and/or a circulation valve  51 . 
     The upper annulus  43  is the annular space above a packer  56  which forms a seal between the exterior of the upper tool  50  and the interior of a well casing  44 . Because the lower tool  70  is located below the packer  56 , the fluid in the upper annulus  43  cannot be used as a medium to directly send pressure pulses (and thus commands) to the lower tool  70 . However, because a central passageway of the test string  40  extends through the packer  56 , this central passageway may be used as a conduit for passing commands to the lower tool  70 . As described below, commands are sent to the lower tool  70  by using the ball valve  53  of the upper tool  50  to form pressure pulses  122  in well fluid (e.g., oil, gas, water, or a mixture of these fluids) present in a lower annulus  42  below the packer  56 . The lower tool  70  has a sensor  74  in contact with fluid in the lower annulus  42 . The lower tool  70  uses the sensor  74  to receive the pulses  122  and, thus, extract the commands sent by the upper tool  50 . 
     Thus, commands are sent to the lower tool  70  by the upper tool  50 . More particularly, to send a command to the lower tool  70 , the mud pump  39  first creates pressure pulses  120  in the fluid in the upper annulus  43 . The pressure pulses may be either negative or positive changes in pressure, and the pressure pulses  120  form a signature that indicates a command for the lower tool  70 . In this manner, the upper tool  50  receives the pressure pulses  120 , decodes the command from the pulses  120 , and selectively opens and closes the ball valve  53  to send the command to the lower tool  70  via pressure pulses  122 . The pressure pulses  122  are applied to a column of well fluid existing in the central passageway of the string  40  where the string  40  extends through the packer  56 . Perforated tailpipes  90  of the string  40  establish fluid communication between the central passageway of the string  40 , the annulus  43 , an annulus  42  and an annulus  41 . For example, perforated tailpipes  90  may be located above and below a perforating gun  57  (of the string  40 ) that is located in the annulus  42 . In this manner, the tailpipes  90  establish fluid communication between the central passageway of the string  40  and the annulus  42 . Thus, due to this arrangement, the pressure pulses  122  that are formed by the upper tool  50  propagate to the lower annulus  42 . As a result, the lower tool  70  uses the sensor  74  to identify the unique signature of the pulses  122  and thus, extract the command. After extracting the command, the lower tool  70  executes the command. 
     The advantages of the above-described arrangement may include one or more of the following: tools below the packer may be controlled without extending wires or pressurized hydraulic lines through the packer; additional electronics may not be required; and additional hydraulics may not be required. 
     Besides the sensor  54  and the ball valve  53 , the upper tool  50  may include a circulation valve  51  and electronics that are configured to decode the signature of the pressure pulses  120  and to control the valves  53  and  51  accordingly. A recorder (not shown) may be located below the packer  56  for taking measuring characteristics of fluid in the lower annulus  42 . 
     In some embodiments, the string  40  may includes a perforated tailpipe  90  that is located above a ball valve  72  of the lower tool  70 . As controlled by the ball valve  72 , the tailpipe  71  allows fluid communication between the lower annulus  42  and a central passageway of the string  40  that extends through the packer  76 . The packer  76  forms a seal between the exterior of the lower tool  70  and the interior of the well casing  44 , thereby forming a test zone  45  and an annulus  41  below the packer  76 . 
     The lower tool  70  also has electronics to decode the pressure pulses  122  and to operate the ball valve  72  accordingly. Located below the packer  76  are a perforating gun  82  that may be between two perforated tailpipes  90  that establish fluid communication between the central passageway of the test string  40  (extending through the packer  76 ) and the annulus  41 , as controlled by the ball valve  72 . A recorder  80  may also be located below the packer  76  to take measurements in the test zone  45 . 
     As an example, the string  40  may be inserted into the well to perforate and measure characteristics of a formation  32  using a process, such as is described below. The circulation valve  51  remains closed except when fluid communication between the upper annulus  42  and the central passageway of the string  40  needs to be established. 
     To begin the process, as shown in FIG. 3A, the test string  40  is inserted into the well with both ball valves  53  and  72  opened. Next, as shown in FIG. 4, pressure is applied through the tubular test string  40  to detonate the perforating gun  82 . When detonated, shape charges in the gun  82  form lateral fractures  100  in the formation  32  and well casing  44  below the packer  76 . 
     As shown in FIG. 5, once the perforations  100  are formed, the mud pump  39  is used to send a command to the upper tool  50  to close the ball valve  53 . Tests are then conducted in the zone  45  to measure characteristics of the perforations  100 . After the tests are complete, a column of well fluid exists in the central passageway of the test string  40  below the ball valve  53 . 
     As shown in FIG. 6, once the testing of the zone  45  is complete, a process is performed to seal off the zone  45 . To accomplish this, the mud pump  39  instructs the upper tool  50  to open and close the ball valve  53  in a manner to generate pressure pulses in the column of well fluid below the ball valve  53 . These pressure pulses have a predetermined signature indicative of a command for the lower tool  70  to close the ball valve  72 . When the lower tool  70  recognizes this signature (via the sensor  74 ), the lower tool  70  closes the ball valve  72  and seals off the zone  45 . 
     As shown in FIG. 7, once the ball valve  72  has been closed, the perforating gun  59  is detonated to form another set of perforations  130  in another formation  33 . Because the ball valve  53  is open, the well fluid flows upwardly through the perforated tailpipe  57  and past the packer  56 . The formation  33  is then tested using the upper tool  50 . 
     As shown in FIG. 8, once the testing of the formation  33  is complete, the mud pump  39  then sends commands to the upper tool  50  to open and close the ball valve  53  in a manner to generate pressure pulses in the column of well fluid below the ball valve  53 . These pressure pulses have a predetermined signature indicative of a command for the lower tool  70  to open the ball valve  72 . When the lower tool  70  recognizes this signature, the lower tool  70  opens the ball valve  72 , and the formations  32  and  33  are tested together. 
     The testing procedure described above requires that a column of well fluid exists below the ball valve  53 . Sufficient pressure (typically exerted by the fluid in the formations  32  and  33 ) must also be exerted on the column so that the opening and closing of the valve  53  produces pressure variations (FIG. 3B) large enough for the sensor  74  to detect. If the formations  32  and  33  do not exert sufficient pressure, the circulation valve  51  may be opened and another fluid, such as a light gas (e.g., nitrogen), is injected into the central passageway of the string  40  above the ball valve  53 . The gas displaces the well fluid above the valve  53  to reduce the hydrostatic pressure above the ball valve  53  and create a pressure difference necessary for generating the pressure pulses  122 . Alternatively, a fluid, such as a formation “kill” fluid, may be injected into the central passageway of the string  40  and the lower annulus  42  so that the pump  39  may be used to send commands to the tool  70 . 
     Each of the tools  50  and  70  use hydraulics  249  (FIG. 10) and electronics  250  (FIG. 11) to operate the valves. As shown in FIG. 10, each valve uses a hydraulically operated tubular member  156  which through its longitudinal movement, opens and closes one of the valves. The member  156  is slidably mounted inside a tubular housing  151  of the test string  40 . The member  156  includes a tubular mandrel  154  having a central passageway  153  coaxial with a central passageway  150  of the housing  151 . The member  156  also has an annular piston  162  radially extending from the exterior of the mandrel  154 . The piston  162  resides inside a chamber  168  formed in the tubular housing  151 . 
     The member  156  is forced up and down by using a port  155  in the housing  151  to change the force applied to an upper face  164  of the piston  162 . Through the port  155 , the face  164  is subjected to either a hydrostatic pressure (a pressure greater than atmospheric pressure) or to atmospheric pressure. A compressed coiled spring  160  contacting a lower face  165  of the piston  162  exerts upward forces on the piston  162 . When the upper face  164  is subject to atmospheric pressure, the spring  160  forces the member  156  upward. When the upper face  164  is subject to hydrostatic pressure, the piston  162  is forced downward. 
     The pressures on the upper face  164  are established by connecting the port  155  to either a hydrostatic chamber  180  (furnishing hydrostatic pressure) or an atmospheric dump chamber  182  (furnishing atmospheric pressure). Four solenoid valves  172 - 178  and two pilot valves  204  and  220  are used to selectively establish fluid communication between the chambers  180  and  182  and the port  155 . 
     The pilot valve  204  controls fluid communication between the hydrostatic chamber  180  and the port  155 , and the pilot valve  220  controls fluid communication between the atmospheric dump chamber  182  and the port  155 . The pilot valves  204  and  220  are operated by the application of hydrostatic and atmospheric pressure to control ports  202  (pilot valve  204 ) and  224  (pilot valve  220 ). When hydrostatic pressure is applied to the control port the valve is closed, and when atmospheric pressure is applied to the control port, the valve is open. 
     The solenoid valve  176  controls fluid communication between the hydrostatic chamber  180  and the control port  202 . When the solenoid valve  176  is energized, fluid communication is established between the hydrostatic chamber  180  and the control port  202 , thereby closing the pilot valve  204 . The solenoid valve  172  controls fluid communication between the atmospheric dump chamber  182  and the control port  202 . When the solenoid valve  172  is energized, fluid communication is established between the atmospheric dump chamber  182  and the control port  202 , thereby opening the pilot valve  204 . 
     The solenoid valve  174  controls fluid communication between the hydrostatic chamber  180  and the control port  224 . When the solenoid valve  174  is energized, fluid communication is established between the hydrostatic chamber  180  and the control port  224 , thereby closing the pilot valve  220 . The solenoid valve  178  controls fluid communication between the atmospheric dump chamber  182  and the control port  224 . When the solenoid valve  178  is energized, fluid communication is established between the atmospheric dump chamber  182  and the control port  224 , thereby opening the pilot valve  220 . 
     Thus, to force the moving member  156  downward, (which opens the valve) the electronics  250  of the tool energize the solenoid valves  172  and  174 . To force the moving member  156  upward (which closes the valve), electronics  250  energize the solenoid valves  176  and  178 . The hydraulics of the tool are further described in U.S. Pat. No. 4,915,168, entitled “Multiple Well Tool Control Systems in a Multi-Valve Well Testing System,” which is hereby incorporated by reference. 
     As shown in FIG. 11, the electronics  250  for each of the tools  50  and  70  include a controller  254  which, through an input interface  266 , may monitor an annulus pressure sensor (e.g., the sensor  54  or  74 ). Based on the command pressure pulses received by these, the controller  254  uses solenoid drivers  252  to operate the solenoid valve set  172   a - 178   a  for the ball valve and a solenoid valve set  172   b - 178   b  for the circulation valve. 
     The controller  254  executes programs stored in a memory  260 . The memory  260  may either be a non-volatile memory, such as a read only memory (ROM), an electrically erasable programmable read only memory (EEPROM), or a programmable read only memory (PROM). The memory  260  may be a volatile memory, such as a random access memory (RAM). The battery  264  (regulated by a power regulator  262 ) furnishes power to the controller  254  and the other electronics of the tool. 
     As shown in FIG. 12, each of the ball valves  53  and  72  includes a spherical ball element  269  which has a through passage  274 . An arm  275  attached to the moving member  156  engages an eccentric lug  270  which is attached through radial slots  272  to the element  269 . By moving the member  156  up and down, the ball element  269  rotates on an axis perpendicular to the coaxial axis of the central passageway  150 , and the through passage  274  moves in and out of the central passageway  150  to open and close the ball valve, respectively. 
     As shown in FIG. 13, for the circulation valve  51 , the housing  151  has a radial port  304  extending from outside of the tool, through the housing  151 , and into the central passageway  150 . A seal  302  located in a recess  301  on the exterior of the member  156  is used to open and close the circulating port  304 . By moving the member  156  up and down, the circulation valve  51  is opened and closed, respectively. 
     As shown in FIG. 14, the controller  254  of the upper tool  50  executes a routine called AN_CNTRL to decode commands sent by the mud pump  39  and actuate the ball valve  53  accordingly. In the AN_CNTRL routine, the controller  254  monitors  350  the pressure via the sensor  54 . If the controller  254  determines  352  that a pressure pulse has not been detected, then the controller  254  returns to step  350 . However, if a pressure pulse has been detected, the controller  254  then decodes  354  the command. If the controller  254  does not recognize  356  the command, then the controller  254  returns to step  350 . Otherwise, the controller  254  determines  358  whether the command is for another downhole tool (i.e., the lower tool  70 ). If not, then the controller  254  actuates  360  the valves  51  and  53  to carry out the command and returns to step  350 . If the controller  254  determines  358  that the command was for the lower tool  70 , then the controller  258  actuates  362  the ball valve  53  to send the command down to the lower tool  70 . 
     As shown in FIG. 15, in a routine called TU_CNTRL, the controller  254  of the lower tool  70  performs a series of steps to decode commands sent by the upper tool  50 . In the TU_CNTRL routine, the controller  254  first monitors  364  the tubing pressure sensor  258 . If the controller  254  determines  366  that a pressure pulse was detected, then the controller  254  decodes  368  the command. If the controller  254  recognizes  370  the command, the controller  254  actuates  372  the circulation valve  71  and the ball valve  72  of the lower tool  70  to perform the desired function. The controller  254  then returns to step  364 . 
     In another embodiment, the ball valve  53  is located at the surface of the well. The ball valve  53  is controlled via electrical cables extending to the ball valve  53  (instead of through the pressure pulses  120  transmitted through the upper annulus  43 ). 
     Other embodiments include a test string with more than two downhole tools. For example, as shown in FIG. 16, in a test string  405 , one tool  400  generates commands for three tools  401   a-c  located downhole of the tool  400 . In order to select the correct tool  401   a-c,  the tool  400  generates the same command more than once. The number of times the tool  400  generates the command identifies the recipient of the command. For example, for the tool  400  to transmit a command to the tool  401   c,  only one command is sent by the tool  400 . For the tool  401   b,  the tool  400  sends two commands, and for the tool  401   a,  the tool  400  sends three commands. 
     As shown in FIG. 17, for the above-described sequencing method of addressing the tools  401   a-c,  the controller  254  in each of the tools  401   a-c  executes a routine called TU_CNTRL_MUL 1 . In the TU_CNTRL_MUL 1  routine, the controller  254  monitors the pressure tubing sensor  258 . If the controller  254  determines  452  that a pressure pulse was detected, then the controller  254  decodes  454  the command. If the controller  254  recognizes  456  the command, then the controller  254  increments  458  a parameter called TCOUNT (set equal to zero on reset of the electronics  250 ) which indicates the number of times the command has been detected. If the controller  254  determines  460  that the TCOUNT parameter indicates that the tool has been selected, then the controller  254  actuates  462  the valves to perform the command and returns to step  450 . If the commands are for a tool located further downhole, then the controller  254  determines  464  whether the ball valve of the tool is closed (i.e., thereby indicating the command did not reach the next tool downhole). If not, the controller  254  returns to step  450 . If, however, the ball valve was closed, then the controller  254   401  actuates the ball valve in a manner to send the command downhole. 
     As shown in FIG. 18, in another embodiment, the tool  400  uses pressure pulses in the central passageway of the test string  405  to send an address with the command. The address uniquely identifies one of the downhole tools  401   a-c.  In this embodiment, the controller  254  for each of the tools  401   a-c  executes a routine called TU_CNTRL_MUL 2 . The TU_CNTRL_MUL 2  routine is identical to the TU_CNTRL_MUL 1  routine with the exception that step  458  is replaced with a step  478  in which the controller  254  decodes  478  the address sent by the tool  400 . 
     As illustrated in FIG. 19, the control of downhole devices as discussed above may be extended beyond downhole testing strings. In FIG. 19, the principles are applied to an actual production environment. For example, a multi-lateral well  500  may have computer-controlled valve units  508 - 512  that control the flow of well fluid from lateral wellbores  502 - 506 , respectively, to a trunk  501  of the well  500 . Each of the valve units  508 - 512  has the same electronics  250  and hydraulics  249  discussed above along with a ball valve for controlling the flow of fluid through the central passageway of the valve unit. The flow of the well fluid through the trunk  501  is controlled by a valve unit  520 , of similar design to the valve units  508 - 512 . 
     As shown in FIG. 20, the controller  254  in each of the valve units  508 - 512  executes a routine called LAT_CNTRL 1 . In the LAT_CNTRL 1  routine, the controller  254  monitors  600  the pressure in the trunk  501 . If the controller  254  detects  602  a pressure pulse, then the controller  254  decodes  604  the command. If the controller  254  then recognizes  206  the command as being for the valve unit, the controller  254  actuates  608  the ball valve of the valve unit to execute the command. 
     As shown in FIG. 21, the controller  254  for the valve unit  520  executes a routine called TRUNK_CNTRL. In the TRUNK_CNTRL routine, the controller  254  monitors  620  the pressure in the trunk  501 . If the controller  254  determines  622  that the pressure has dropped below a predetermined minimum threshold, then the controller  254  performs  624 - 634  a series of operations to increase the pressure in the trunk  501 . The controller  254  first determines  624  whether the valve  508  is open, and if not, the controller  254  then actuates  626  the ball valve of the unit  520  to generate a command to open the valve unit  508 . The controller  254  then returns to step  620 . If the valve unit  508  is open, then the controller  254  determines  628  whether the valve unit  510  is open, and if not, the controller  254  actuates  630  the ball valve of the valve unit  520  to generate a command to open the valve unit  510  and returns to step  620 . If the valve unit  510  is open, then the controller  254  determines  632  whether the valve unit  512  is open, and if so, the controller  254  actuates  634  the ball valve of the unit  520  to generate a command to open the valve unit  512  and returns to step  620 . 
     If the controller  254  determines  636  that the pressure in the trunk  501  is greater than a predetermined maximum threshold, then the controller performs  638 - 648  steps to reduce the pressure in the trunk. The controller  254  first determines  638  whether the valve unit  508  is closed, and if not, the controller  254  actuates  640  the ball valve of the valve unit  520  to send a command to close the valve unit  508  and returns to step  620 . If the controller  254  determines  642  that the valve unit  510  is closed, then the controller  254  actuates  644  the ball valve of the unit  520  to send a command to close the valve unit  510  and returns to step  620 . If the controller  254  determines  646  that the valve unit  512  is closed, then the controller  254  actuates  648  the ball valve of the valve unit  520  to send a command to close the valve  512  and returns to step  620 . 
     In other embodiments, the valve unit  520  is located at the surface of the well. The valve unit  520  is controlled via electrical cables connected to the valve unit  520 . 
     Other embodiments are within the scope of the following claims. For example, instead of using the mud pump  39  to generate a single command to instruct the upper tool  50  to generate a command for the lower tool  70 , in an alternative embodiment, a series of commands is sent by the mud pump  39  to directly control the opening and closing of the ball valve  53  in the generation of the command for the lower tool  70 . 
     While the invention has been disclosed 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 the invention.

Summary:
A system used in a subterranean well has a command generator (e.g., a manually operated mud pump) that is configured to furnish a first command stimulus (e.g., pressure pulses having a predetermined signature) downhole. A first assembly (e.g., a testing tool) of the system is located downhole and has a first member (e.g., a valve) that is connected to move the first member in response to the first command stimulus. The movement of the first member generates a second command stimulus, and a second assembly of the system, also located downhole, is connected to respond to the second command stimulus.