Hydraulically driven tractor

Abstract

A large scale hydraulically driven downhole reciprocating tractor to display substantially continuous advancement. The tractor may be configured to pull coiled tubing downhole in a continuous manner so as to substantially avoid static frictional resistance to the pulling. As a result, the tractor and coiled tubing may achieve up to about twice the depth into the well otherwise attainable. Additionally, the tractor may include a fail safe mechanism whereby a plurality of pilot operated valves are employed to ensure that immobilization of at least one anchor of the tractor is present at all times during downhole advancement to thereby avoid the undesired effects of coiled tubing spring-back.

1 . A hydraulically driven tractor for advancement within a well, the tractor comprising:
 a piston;   a first housing about a first head of said piston, the first head for moving responsively to an influx of hydraulic pressure into said first housing; and   a second housing about a second head of said piston, the second housing to display movable responsiveness to the moving relative to the piston.   
   
   
       2 . The hydraulically driven tractor of  claim 1  wherein said piston is coupled to a downhole conveyance line extending from a surface at an origin of the well, the moving to pull the downhole conveyance line downhole. 
   
   
       3 . The hydraulically driven tractor of  claim 2  wherein the downhole conveyance line is coiled tubing. 
   
   
       4 . The hydraulically driven tractor of  claim 1  further comprising a hydraulic series assembly through which said first housing is fluidly coupled to said second housing. 
   
   
       5 . The hydraulically driven tractor of  claim 4  wherein said hydraulic series assembly comprises:
 a first transfer line coupled to said first housing for receiving an output of hydraulic pressure from the moving;   a second transfer line coupled to said second housing to deliver the output of hydraulic pressure thereto for the responsiveness; and   a switching mechanism coupled to said first transfer line and said second transfer line for transferring the output of hydraulic pressure from said first transfer line to said second transfer line.   
   
   
       6 . The hydraulically driven tractor of  claim 5  further comprising:
 a first pressurization line to provide the influx of hydraulic pressure into said first housing for a first period; and   a second pressurization line to provide an influx of hydraulic pressure into said second housing for a second period exclusive of said first period, said switching mechanism to regulate the periods.   
   
   
       7 . The hydraulically driven tractor of  claim 1  further comprising:
 a first anchor coupled to said first housing for immobilization thereof during the moving; and   a second anchor coupled to said second housing to allow lateral mobility thereof for the responsiveness.   
   
   
       8 . The hydraulically driven tractor of  claim 7  wherein the lateral mobility of said second housing includes centralization of said second housing. 
   
   
       9 . The hydraulically driven tractor of  claim 7  wherein said second anchor is coupled to said second housing for periodic immobilization thereof. 
   
   
       10 . The hydraulically driven tractor of  claim 9  further comprising a primary pilot valve and a secondary pilot valve coupled to said second anchor, said second anchor to allow the lateral mobility for the responsiveness exclusively upon opening of both said primary pilot valve and said secondary pilot valve. 
   
   
       11 . A hydraulically driven tractor comprising:
 a first assembly with a first anchor for immobilization thereof,   a second assembly with a second anchor for immobilization thereof, and   a mechanism coupled to each of the anchors for selectively activating immobilization of the first anchor prior to deactivating immobilization of the second anchor to provide lateral mobility thereto.   
   
   
       12 . The hydraulically driven tractor of  claim 11  wherein said mechanism comprises a plurality of pilot valves. 
   
   
       13 . The hydraulically driven tractor of  claim 12  wherein said plurality of pilot valves comprises primary and secondary pilot valves coupled to the second anchor, the deactivating upon opening of both said primary and secondary pilot valves. 
   
   
       14 . The hydraulically driven tractor of  claim 12  wherein the activating is achieved upon closure of one pilot valve of the plurality of point valves. 
   
   
       15 . The hydraulically driven tractor of  claim 11  wherein the deactivating of the second anchor is for a first period exclusive of a second period of deactivating of the first anchor. 
   
   
       16 . The hydraulically driven tractor of  claim 11  further comprising:
 a piston;   a first housing of the first assembly to accommodate a first head of said piston, the first head for moving responsively to an influx of hydraulic pressure into said first housing during immobilization thereof, and   a second housing of the second assembly to accommodate a second head of said piston, said second housing to display movable responsiveness to the moving relative to the piston during deactivating of the second anchor.   
   
   
       17 . A method of pulling coiled tubing downhole in a well, the method comprising:
 securing the coiled tubing to a downhole tractor;   immobilizing a first housing of the tractor within the well;   pressurizing a piston head in the first housing for driving the piston downhole;   providing lateral mobility to a second housing of the tractor; and   moving the second housing downhole relative to the piston in response to the pressurizing.   
   
   
       18 . The method of  claim 17  wherein said providing further comprises centralizing the second housing. 
   
   
       19 . The method of  claim 17  wherein the piston head is a first piston head, the method further comprising:
 immobilizing the second housing within the well; and   switching said pressurizing to a second piston head within the second housing to continue the driving.   
   
   
       20 . The method of  claim 19  further comprising:
 deactivating said immobilizing of the first housing; and   moving the first housing downhole in response to said switching.   
   
   
       21 . The method of  claim 20  wherein said deactivating further comprises centralizing the first housing. 
   
   
       22 . The method of  claim 20  further comprising employing a plurality of pilot valves to ensure said immobilizing of the second housing occurs prior to said deactivating. 
   
   
       23 . A hydraulically driven tractor for advancement of a downhole conveyance line within a well, the tractor comprising:
 a piston coupled to the downhole conveyance line and having a first head and a second head;   a first housing about the first head of the piston, wherein the first head divides the first housing into a power chamber and a return chamber, and wherein the first head moves in response to an influx of hydraulic pressure into the power chamber of the housing; and   a second housing about a second head of said piston, wherein the second head divides the second housing into a power chamber and a return chamber, and wherein the second housing is fluidly coupled in series to the first housing, such that said influx of hydraulic pressure into the power chamber of the first housing causes fluid to be expelled from the return chamber of the first housing and into the return chamber of the second housing to move the second housing relative to the piston.   
   
   
       24 . The hydraulically driven tractor of  claim 23  wherein the downhole conveyance line comprises coiled tubing. 
   
   
       25 . The hydraulically driven tractor of  claim 23  further comprising:
 a first anchor coupled to the first housing for immobilizing the first housing as the first head moves; and   a second anchor coupled to the second housing, but allowing lateral mobility of the second housing as the second housing moves relative to the piston.