Patent Publication Number: US-2013240267-A1

Title: Method for continuous formation core sampling

Description:
FIELD 
     The present invention regards a method for providing samples for testing of earth formation core knowing the location where the cuttings come from. 
     BACKGROUND 
     Generally there is a need for testing of the earth formation related to hydrocarbon exploration in order to determine the likelihood of finding such hydrocarbons. One solution for providing this is to provide core samples from the well, and then to perform tests on these core samples. There are several ways to obtain core samples; special drill bits which are used to take out core samples, for instance as described in US2009/0283326. There are developed different containers and methods for getting samples into the containers and removing the material from the containers, for instance as described in US2010084193 (Livingstone). There are also several methods for core taking while drilling, for instance as described in CN2690572 and U.S. Pat. No. 3,871,486, where the last of these describe a system for continuous drilling of a well bore and coring involving the use of a rotatable percussion drill having a central barrel, the drill being operated by air circulated through a dual concentric drill pipe and a swivel at the top of the drill pipe, the air, in major part, flowing upwardly through the bore annulus, outside the drill pipe to remove cuttings and other material from the annulus, and a minor part of the air flows upwardly in the central core tube of the drill pipe from the centre of the drill to carry uncontaminated core samples from the bottom of the bore hole. In U.S. Pat. No. 3,871,486 there is also described a system where one has used dual concentric drill pipes, with return fluid with cuttings through centre, which experienced problems. 
     SUMMARY 
     The aim with the present invention is to provide a system which is alleviated of at least some of the problems with known systems. 
     This is achieved with a system as described in the independent claims with additional favourable features given in the dependent claims or the following description. 
     There is provided a method for performing core sampling during drilling of hydrocarbon wells knowing the location where the cuttings comes from. According to the invention the method comprises the steps of providing a dual bore drill pipe with a drill bit at the end in the well bore, providing means to introduce a drill liquid into an annular space of the drill pipe and return the drill liquid together with drill cuttings to the surface through a centrally arranged return pipe in the dual bore drill pipe, removing larger cuttings at an outlet of return pipe, and perform core testing on these cuttings. 
     The dual bore drill pipe will be a concentric dual bore drill pipe but it is not necessary to have them concentrically arranged. It might also be possible to have the two pipes arranged side by side. The main issue is to provide a return pipe with a relatively large cross section, given the available cross section within the well bore. The drill bit may be a reverse circulation drill bit or it may be a conventional drill bit, but then with a cross-over arrangement between the drill bit and the dual bore drill string. Preferably the solution has a conventional drill bit. 
     According to an aspect the method may comprise adding drilling liquid at velocities of more than 1 meter per second measured at a flow control unit, typically located at the drill floor, by comparing volume pumped into the well per second and volume extracted out of the well per second (using Coreolismeter), measured as a main velocity at inlet of annulus at surface by calculations. Preferably the velocity of the drilling liquid through the drill string is around 1 to 3 meter per second. With such a velocity the possibility of having insufficient cuttings removal becomes negligible, and therefore it no longer represents a risk factor. Further, due to the speed for the flow the cuttings are transported to surface in minutes, with the extraction location known, providing a unique way of analyzing the formation properties while drilling providing valuable formation properties information. 
     According to an aspect the method may also comprise removing larger cuttings continuously or at intervals. By removing larger cutting continuously one will have a good picture of the properties of the earth formation. As one then also has a full representation of the larger cuttings one has a full picture representing all levels of the formations in the well. There is also the possibility of taking out the larger cuttings at intervals, where these intervals are recorded together with the samples taken at the intervals. Such a configuration of the sample takings will also give a good representation of the conditions in the well. With both systems one may also record wellbore ID in relation to the samples, to know wherefrom the samples are in the well bore. One also has the possibility of adding markers in the drilling liquid to establish the wellbore ID. 
     According to another aspect the method may comprise the step of removing cuttings of a given size range for testing. By limiting the cuttings to a given size range there is even less questions in relation to wherefrom in the well the cuttings are from, as these will all experience more or less the same lift from the drilling liquid out of the well bore. The cutting will travel with more or less the same velocity through the return pipe and thereby the determination of the position wherefrom the cuttings are from, is even more certain. There is also the possibility of separating out cuttings above a given size for testing. This may be done by for instance separating out all cutting which in one direction is larger than 1 cm, or alternatively separating out cuttings which are 1 cm times 1 cm, measured in perpendicular directions or cuttings which are from 0.5 cm 3  or 1 cm 3  and larger. Another possibility is to take out all cutting in the range 1 cm 3  to 2 cm 3 . It is also possible to set these limits at other values as for instance 1.5 cm or even 2 cm, 0.7 m 3 . Another possibility is to separate out all cuttings. A further possibility is to have a manual selection of cutting as they will land on a cuttings-chute in the mud handling system, for further testing of the properties of the formation. 
     According to another aspect the method may comprise providing a return pipe with an equal inner diameter between the two ends of the return pipe. By having the return pipe with an equal inner diameter, or a mainly equal inner diameter as there of course will be some variation, it limits the possibility of damaging the cuttings in their travel through the return pipe. By equal inner diameter, one should understand that there may be some variations in the diameter, the main issues is to not have elements extending in to the return pipe or obstructions in the return pipe or with other words to have a mainly smooth inner surface. 
     According to another aspect the method may comprise providing a light drill liquid that has a lower density than traditional drill mud and minimises the contamination of the cutting samples. This is to get a better reading of the conditions in the well, with the core samples taken from the cutting. 
     According to another aspect the method may comprise the step of establishing a flow of drilling liquid in the dual bore drill pipe before the drilling liquid is guided to the drill equipment as such. This may be achieved by having a three way valve down in the drill pipe close to the drilling equipment. The three way valve will initially allow a fluid flowing from the annulus and into the central bore above the drilling equipment and when drilling starts shuts this flow passage and opens the passages in the annulus down to the drilling equipment and from the drilling equipment and up to the central bore. This will give a better start of the drilling sequence and a better removal of the cutting at the start of the drilling sequence. This will also provide a better tracking of where the cuttings are taken from in the well. 
     The invention also regards a device for performing core sampling during drilling of oil wells. According to the invention the device comprising a dual bore drill pipe with a drill bit on one end, means for providing a drilling liquid to flow down the drill pipe in an annulus and return through a central pipe, a cuttings sample arrangement at outlet of return pipe, wherein the return pipe is formed with an equal inner diameter between the two ends. 
     According to an aspect the drill bit may be, and preferably is, a conventional drill bit where there is arranged a cross over element between the drill bit and the dual bore drill pipe. Alternatively there may be a reverse circulation drill bit attached to an end of the dual bore drill pipe. 
     According to another aspect the sample arrangement may be configured to remove cutting samples of a given size range continuously or at intervals. This may be done by having a filter arrangement at the outlet of the return pipe, or a separation device, which for instance may separate or filter out any cutting above a given size or within a given size range. The main issue is to provide samples of cuttings large enough for performing core testing on the samples. And by limiting them to a given size range one also have an increased certainty of wherefrom in the well they are from. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  shows a system for continuous formation core sampling. 
         FIG. 2  is a comparison of cuttings from a conventional drilling method and a method for continuous formation core sampling according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     The invention will now be explained with reference to an attached schematic drawing showing the principles of the invention  FIG. 1 , and a picture comparing cuttings provided with the present invention compared with conventional drilling,  FIG. 2 . 
     In  FIG. 1  there is schematically shown the principles of the invention in one possible embodiment. There is provided a dual bore drill string  1  with an outer pipe  2  arranged around an inner pipe  3 , thereby forming an annulus  4  for drilling fluid and a central bore  5  for the return of drilling fluid out of the well. The outer pipe  2  and inner pipe  3  are arranged concentrically and may be ordinary drill pipes or coiled pipes. At one end of the drill string  1  there is a drill bit  8 , drilling a well bore  9  into the ground. The well bore  9  has one part which is not cased and one part covered with a set casing  10 . The dual bore drill pipe  1  is connected to a conventional drill bit  8  through a cross over assembly  7 , guiding an annular flow in the drill pipe  1  to a central flow in the drill bit  8  and an annular flow around the drill bit  8  into the central flow passage or central bore  5  of the drill pipe. Alternatively there may be connected a reverse flow drilling bit to the drill pipe, then there is no need for a cross over assembly between the drill bit and the drill pipe. There is further in the drill pipe  1  also arranged a valve device  6 , which regulated the flow through and between the annulus  4  and the central bore  5 . The valve device  6  may be arranged to be a blow out preventer, BOP, for both the central bore  5  and annulus  4  and it may also provide for a controlled fluid passage between the annulus  4  and the central bore  5 . The drill pipe  1  is further bringing a casing element  11  to be set in the well when a new section of well is drilled. The top of the bore hole is closed by a rotating control device  12 , and there is provided a BOP at the top of the well bore as well. The opposite end of the dual bore pipe  1  compared with the drill bit  8 , is provided with a top drive adapter  13 , that is also guiding drilling fluid into and out of the drill pipe  1 . There is in relation to this provided a flow control unit  14  and a system  15  for treating the drilling fluid as it returns from the well to prepare it to be used in the well again. In relation to this there is according to the invention arranged a sample arrangement  16 , separating out cuttings for core testing. 
     The drill bit  8  in this embodiment is also assisted in its progression by the system with the piston  17  and fluid added in an annular space formed between the drill pipe  1  and the part of the well with the set casing  10 . This annular space is closed in by the drill pipe  1 , the set casing  10 , the piston  8  and a top assembly  10 . With adding pressure to this annular space with an upper annulus control unit  18  the drill bit  8  is pushed towards the bottom of the well  9 . The present invention will work also in a system without the progression system with piston  17  and control unit  18 . 
     With a system according to the invention there will at the outlet of the central bore  5  be cuttings which are much larger compared with conventional cutting, as typically shown in the picture of  FIG. 2 , where cuttings from conventional drilling is shown on the left of the picture and cuttings from drilling with a system as described above is shown in the right on the picture. 
     The invention has now been explained with reference to a non-limiting embodiment. A skilled person will understand that there may be made alterations and modifications to the explained embodiments which are within the scope of the invention as defined in the claims. The method and device may also be used for other kinds of well other than hydrocarbon well, where there is need for getting information about the formation into which one is drilling.