Patent Publication Number: US-2010126777-A1

Title: Drilling System with a Barrel Drilling Head Driven by a Downhole Tractor

Description:
TECHNICAL FIELD 
     The present invention relates to a drilling system for drilling out a sample of a subterranean formation, a well, or the like downhole, comprising a drilling head having an at least partly hollow cylindrical body with a circumferential rim, which has at least one cutting edge, a longitudinal axis going through a centre of the cylindrical body, and a driving unit for rotating the cylindrical body and the cutting edge for cutting a piece out of the formation. 
     BACKGROUND 
     Drilling systems are used when drilling a well in a subterranean formation or for taking out a sample of the formation before drilling any further. 
     Known drilling systems for taking out a sample are constructed from a long line of pipes, the system thus extending from above surface to a destination downhole where the sample is to be taken. The system is mounted by mounting one pipe onto another just before the drilling operation on the drilling rig. The extension of the pipes put together makes the mounting process complicated, and the process is moreover time consuming. 
     However, the long line of pipes creates a Weight On Bit (WOB) by the force of gravity which is needed for drilling downhole. Such a system creating weight on bit by the force of gravity is not very useful when drilling horizontally, e.g. for taking out a sample, since the force of gravity is substantially decreased when the pipes extend horizontally. 
     Another known type of drilling system uses coiled tubing, i.e. a long, continuous length of pipe wound on a spool. The pipe is straightened prior to pushing into a wellbore and rewound to coil the pipe back onto the transport and storage spool. Although substantially less time consuming than the system described above, the equipment for coiled tubing drilling still takes a long time to transport to the site. Moreover, coiled tubing drilling still uses the force of gravity to create WOB, and thus does not solve the problem of drilling horizontally. 
     Therefore, there is a need for a drilling system which is able to take out samples also when drilling horizontally. 
     Furthermore, in order to prevent a blowout when drilling downhole, the fluid in the well is mixed with a number of chemicals for making a differential pressure downhole. However, due to the differential pressure, the above-mentioned systems sometimes get differentially stuck. 
     Thus, there is also a need for a drilling system reducing or even eliminating the risk of downhole systems getting differentially stuck. 
     DESCRIPTION OF THE INVENTION 
     An aspect of the present invention is, at least partly, to overcome the disadvantages of the drilling systems mentioned above, and to provide an improved drilling system which is able to take out a sample of the formation without getting stuck. 
     This aspect and the advantages becoming evident from the description below are obtained by a drilling system for drilling out a sample of a formation, a well, or the like downhole, comprising: 
     a drilling head comprising:
         an at least partly hollow cylindrical body with a circumferential rim and a longitudinal axis going through a centre of the cylindrical body, the circumferential rim having at least one cutting edge,       

     a driving unit for rotating the cylindrical body and the cutting edge for cutting a piece out of the formation, and 
     a driving tool such as a downhole tractor for driving the drilling head forward in the well or formation. 
     By having a driving tool for driving the drilling head forward and provide a weight on bit, the drilling system can drill horizontally into the formation for drilling a branch well or taking out a sample for further investigation. 
     By combining a downhole tractor and a drilling head, the drilling operation can be performed substantially faster than when using coiled tubing or jointed pipes, since the equipment for using coiled tubing or jointed pipes takes longer time to transport to the operation site and takes longer time to prepare at the operation site. A downhole tractor may easily and quickly be transported to the operation site compared to the equipment necessary for coiled tubing operations, and the preparation time at the operation site is thus substantially decreased, if not eliminated. Moreover, the downhole tractor has proven to be efficient enough to use for drilling operations also, since it can provide a sufficient WOB. 
     In one embodiment, the downhole tractor may be connected to and powered through a wireline. 
     In addition, the drilling system may further comprise at least one tightening means for providing a closing of the well above the drilling head for preventing a blowout. 
     By having a tightening means, the well or hole in the formation can be divided into an upper part in relation to the bottom of the well or hole and a lower part comprising the drilling system or at least most of it. In this way, a blowout can be prevented without use of chemicals creating a differential pressure, and thus without risking that the system gets differentially stuck. 
     In one embodiment, the tightening means may be expandable. 
     Moreover, the cylindrical body may have a closed end opposite the cutting edge. 
     In another embodiment, the circumferential rim may have a plurality of cutting edges. 
     In addition, the circumferential wall may have at least one slot. 
     In yet another embodiment, the hollow body may have a length up to 10 metres. 
     Moreover, the drilling system may further comprise a gas cartridge or a pump for inflation of the tightening means with gas or fluid from the well. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in detail below with reference to the drawings, in which 
         FIG. 1  shows a drilling system according to the invention, and 
         FIG. 2  shows another embodiment of the drilling system according to the invention. 
     
    
    
     The drawings are merely schematic and shown for an illustrative purpose. 
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a drilling system  1  according to the present invention comprising a barrel drilling head  2 , a driving unit  6  for rotating the barrel drilling head  2 , and a driving tool  8  such as a downhole tractor. The drilling system  1  is submerged into the well or subterranean formation  10  by the driving tool  8  which is powered through a wireline  11  by a power means above surface. 
     The drilling head  2  is rotationally driven by the driving unit  6  at a certain speed, and the weight on bit (WOB) is controlled by the driving unit  6  so that the longitudinal displacement of the cutting edge  5  towards e.g. the formation  10  is held constant during the drilling process. 
     Due to the fact that the weight on bit (WOB) is created by the driving unit  6  and not just by the force of gravity, the drilling system  1  can also drill horizontally, e.g. for taking out samples  7  horizontally. 
     The drilling head  2  has a partly hollow cylindrical body  3  that is closed at one end  13  and has a circumferential rim  4  at the other end. The circumferential rim  4  has at least one cutting edge  5  which, when rotated by the driving unit  6 , cuts out a piece  7  or sample  7  of the formation  10 . In this way, the drilling system  1  can be used for both for drilling a well downhole and for taking out samples  7  for further investigation. 
     Due to the fact that the drilling head  2  has a hollow cylindrical body  3 , the drilling head  2  is able to receive the sample  7  that is being cut out and bring it up with the rest of the system  1 . In this embodiment, the inside of the hollow body  3  is 6 metres in depth, and the drilling system  1  is therefore able to cut out a sample  7  of almost 6 metres. In another embodiment, the hollow body  3  may be as long as 10 metres. 
     As shown in  FIG. 1 , the drilling system  1  further comprises a tightening means  9  positioned around the wireline  11  for closing a lower part of the well in order to prevent a blowout without using chemicals. 
     In this embodiment, the tightening means  9  is expandable. The expandable tightening means  9  is in the form of an expandable ring blown up by gas positioned in a gas chamber  12  or a gas cartridge  12  within the drilling system  1 . In another embodiment, the fluid surrounding the drilling system  1  is used to expand the tightening means  9  by use of a pump  14  and a filter for filtrating the fluid before it is pumped into the tightening means  9 . Preferably, the pump  14  and the filter are positioned in the part of the drilling system  1  between the connection to the wireline  11  and the downhole tractor  8 . 
     The tightening means  9  may also be positioned around the downhole tractor  8  as shown in  FIG. 2 . Hereby, the tightening means  9  need not be expanded as much as when positioned around the wireline, and the tightening means  9  can also be non-expandable, e.g. a rubber ring. Thus, the expansion of the tightening means  9  is quicker and easier since the volume of the tightening means  9  need not be as large as if positioned around the wireline  11 , and the gas chamber  12 , gas cartridge  12 , or pump  14  can be positioned in the side of the downhole tractor  8  providing a quick inflation of the tightening means  9 . 
     The driving unit  6  is an electrical motor which drives the drilling head  2 . The motor  6  has a shaft for driving the drilling head  2 . The shaft may be connected to the drilling head  2  through a gear connection. In this way, one drilling head  2  may be replaced by another drilling head  2 . 
     For providing a differential pressure below the expandable means  9 , the drilling system  1  comprises a pump  14  which may be any kind of suitable pump. In this embodiment, the pump  14  is a one-step centrifugal pump, but in another embodiment the pump  14  may be a multi-step centrifugal pump, a jet pump, or a piston pump.