EP1356185B1

EP1356185B1 - A method and drill bit for reducing the cuttings size to a predetermined maximum size

Info

Publication number: EP1356185B1
Application number: EP01985461A
Authority: EP
Inventors: Per Thomas Moe
Original assignee: Individual
Current assignee: Individual
Priority date: 2000-12-21
Filing date: 2001-12-21
Publication date: 2006-09-06
Anticipated expiration: 2021-12-21
Also published as: WO2002052122A1; NO316856B1; ATE338877T1; NO20006591L; DE60122933D1; EP1356185A1; DE60122933T2; NO20006591D0

Abstract

In a method for percussive drilling of the holes in the ground, e.g. geothermal wells, use is made of water as drilling fluid and of a percussive drill bit (1) which is arranged to crush the cuttings to such small particles that the drilling water can carry them along up to the surface, notwithstanding its low viscosity and density as compared to usual drilling mud. For this purpose the return flow channels (9) of the drill bit are provided with such a small depth that only particles small enough may pass. The drill bit is provided with several rows of hard metal buttons (5-8), whereof the outermost (5) are relatively closely spaced and, in addition, are provided with a diamond coating. The buttons (6 and 7) on the inside do not have such a coating, so that they will be worn faster than the outer buttons (5) and give warning that the drill bit should be replaced before the outer and dimension determining buttons (5) have been subjected to noticeable wear. An outlet (10) for drilling fluid is placed centrally enough in the drill bit to overlap with the central axis (11) thereof.

Description

The present invention relates to a method for drilling deep holes in the ground by means of a drill string, a percussion drilling hammer at the lower end of the drill string, and a percussive drill bit having a diameter of at least 75 mm and being connected to the percussion drilling hammer, where drilling fluid is supplied to the percussion drilling hammer and drill bit through the drill string and is returned at a given velocity through an annular space formed between the drill string and the hole.
In drilling wells by means of a percussion drilling hammer it is customary to use a drill bit which on its front face is provided with inserts or buttons of hard metal. When the drill bit impacts against the bottom of the borehole, the buttons knock pieces of varying size loose from the ground material. Some of these may have a size above 10 mm, somewhat dependent on the type of rock to be drilled. Due to the density and viscosity of the drilling fluid, these large particles of the cuttings will nevertheless be carried along by the drilling fluid up to the surface through the annular space between the well wall and drill string, where the flow velocity typically may lie in the range 0.4-2 m/sec. The cuttings pass up along the head of the bit through channels in the side surface of the head. The drilling fluid is usually supplied through two or more openings in the front face of the bit, which at least partially direct the flow radially outward towards the periphery of the bit in order to flush the cuttings out to said channels.
The drilling fluid usually has a number of additives which makes it both expensive and harmful to the environment. Consequently, it is necessary to recover the drilling fluid, requiring a cleaning process, which in turn implies space consuming and expensive equipment for sifting and centrifuging the drilling fluid before it can be used over again.
The purpose of the present invention is therefore to avoid, or at least reduce, the drawbacks and casts connected to the use of common drilling fluids.
According to the invention, this is obtained by a method of the type mentioned by way of introduction where the characteristic features are that water is used as drilling fluid, and in that cuttings knocked loose by the drill bit are crushed to a predetermined maximum dimension before they are allowed to pass up along the bit to the annular space between the wall of the borehole and the drill string, said maximum dimension giving the cuttings a lower sinking velocity in water than the water velocity in the annular space.
By using water for the drilling fluid one does not need any recovering equipment because the water is usually available in unlimited supply and at a very low cost. Since the water which flows up through the annular space, generally speaking, will contain nothing but crushed rock which will not have any polluting effect on the environment, it may in the majority of cases be discharged without any treatment.
However, since water has a substantially lower viscosity and density than common drilling fluids, the cuttings will have a much higher sinking velocity in water than in drilling fluids. This entails that the heavier particles of the cutting will not be carried along by the water in the annular space unless the flow velocity is increased correspondingly. Particularly when drilling at larger depths, e.g. by means of coiled tubing, the flow resistance in the drill string and other pressure losses will make it practically impossible to provide a sufficient flow rate to carry the largest particles up to the surface. As a result, the cuttings will pack in the annular space and make further drilling impossible.
The invention has solved this problem by ensuring that the cuttings, before being allowed to pass by the bit and up into the annular space, are crushed to such an extent that there do not exist particles of such a size that the water flow in the annular space is not able to carry these to the surface. This maximum particle size will vary according to the type of rock to be drilled. In shale rock the large particles will generally take the form of flakes, which should be crushed to a length of maximum 6 mm. If the rock is gneiss or granite, the particles usually have a more rounded form and should in such cases be crushed to a largest dimension of about 4 mm.
Several methods may be envisioned in order to obtain the necessary crushing of the cuttings. A preferred solution according to the invention is, however, very simple, inexpensive and efficient. It consists of limiting the flow cross-section, particularly the depth, of the return channels in the side surface of the bit. As a result, the cuttings cannot pass by the bit until they have been crushed to a sufficient extent. This maximum depth will of course also be dependant upon the form of the cutting particles, but for bits above a certain size it will usually lie in the range of 1.5-4 mm.
In percussive drilling not only the ground material is knocked loose in pieces. It also happens that pieces of the hard metal buttons or inserts of the bit are knocked loose. The hard metal pieces are very heavy and therefore have a tendency to collect at the bottom of the borehole. Since the flushing holes on existing bits usually are placed at a radial distance from the central axis of the bit and, in addition, are directed radially outward, the flushing action centrally in the borehole becomes poor and, as a result, the hard metal pieces have a tendency to collect at this point and destroy the central portion of the bit so that the closest buttons in time will loosen and the bit cease to drill.
The present invention has a solution also to this problem. This consists of arranging a flushing channel opening, preferably as the only one, close to or even partly overlapping with the central axis of the bit. This provides effective flushing also of the central portion of the bottom of the borehole, resulting in that any hard metal pieces are flushed out to the side and follow the drill fluid up through the side channels of the bit. The hard metal pieces will often be too heavy to continue further up the annular space, but since they will be relatively few in number, they may be permitted to remain on the top side of the widest portion of the bit without doing noticeable harm.
For almost all drilling the drilling costs are strongly dependant upon the service life of the bit. Frequent changing of the bit is very cost consuming both due to the high cost of the bits and the time spent in running the drill string out of the well and in again at each replacement. Particularly when drilling geothermal wells, e.g. such wells described as in WO98/22760, the economy of the plant is totally dependant upon the ability to drill deep wells in hard rock in an effective manner. Here it is therefore desirable to be able to drill a relatively long distance before the bit has to be replaced and, in addition, it is very desirable that the diameter of the borehole is maintained to a highest possible degree even in the later phase of the life time of the bit.
For previously known percussive bits it has often been the case that the hard metal buttons along the periphery of the bit have been the first to be worn, resulting in that the diameter of the borehole has diminished gradually before the bit is so worn that this may be detected as reduced penetration rate. The resulting slightly conical portion in the lower part of the well will easily result in that the new bit jams and possibly is destroyed before it reaches the bottom of the well.
In order to avoid this problem, and concurrently also obtain substantially increased service life of the bit, it is according to the invention suggested to increase the number of buttons in the outermost row along the periphery of the bit. Furthermore, it is suggested to provide these outermost buttons with a diamond coating which increases their service life, while in at least the majority of the buttons on the inside are not provided with such a coating. This insures that buttons on the inside are worn out and stop the penetration of the bit before the buttons along the periphery are worn to such an extent that the diameter of the hole has been reduced to a noticeable degree.
Further advantageous features of the invention will appear from the patent claims and the following description of the exemplifying embodiment of a percussive bit according to the invention which is schematically shown in the appended drawing, where

fig. 1 is a view of the bit seen from the front, and
fig. 2. is a partial section along the line II-II in fig. 1.

The percussive bit 1 according to the invention shown in the drawing comprises a head 2 and a peripheral side surface 4, where hard metal inserts or buttons 5, 6, 7, 8 are inserted in the front face 3 thereof. The buttons 5 are arranged in an outer row adjacent the side surface 4 of the head 2 of the bit. The number of buttons 5 is so large that the average distance between the buttons is less than the diameter of the buttons. The buttons 5 are also coated with a diamond material, this being indicated in the drawing by a cross-hatching consisting of crossing lines. The buttons 6 in the row on the inside have a much larger spacing and are not provided with a diamond coating. Inside the buttons 6 is a further row consisting of two diametrally opposite buttons 7, and innermost there is a single button 8, which may have a diamond coating.
In the side surface 4 of the head there are axially extending recesses forming channels 9 in which drilling fluid, in this case water, carrying along cuttings may flow through up along the bit to the annular space between the wall of the hole made by the bit and the drill string (not shown). The channels 9 can have somewhat varying shape and depth, but the depth, measured with respect to an imaginary continuation of the side surface 4, must not supersede a given value determined by i.a. the type of rock to be drilled, the cross-sectional area of the annular space and the flow rate of the drilling water. A practical range for this maximum depth is 1.5-4 mm, the upper limit preferably being less than 5 mm, preferably maximum 3.5 mm in order to keep the flow rate of drilling water on a reasonable level.
It will also appear from the drawing that the front face 3 of the bit has an outlet opening 10 for drilling water which is so centrally located in the bit that it overlaps with the central axis 11 of the bit. This ascertains effective flushing of the entire bottom area of the borehole.
Even though the invention has been described in the above with references to a preferred exemplifying embodiment, it will be understood that the invention may be varied and modified in a number of ways within the frame of the appended claims. For instance, it is not necessary to have a channel 9 between each pair of the outer buttons 5. Furthermore, the bottom shape of the channels 9 may depart from the elliptically curved shape shown in the drawing, and the width in the tangential direction may of course also vary. The skilled person will also understand that the percussive bit according to the invention usually will be equipped with a shaft for attachment for a percussion hammer, which in turn may be driven in rotation by a drilling motor or the like.

Claims

A method for drilling deep holes in the ground by means of a drill string, preferably of the coiled tubing type, a percussion drilling hammer at the lower end of the drill string, and a percussive drill bit (1) having a diameter of at least 75 mm and being connected to the percussion drilling hammer, where drilling fluid is supplied to the percussion drilling hammer and drill bit (1) through the drill string, and is returned at a given velocity through an annular space formed between the drill string and the hole,
characterised in that water is used as drilling fluid, and in that cuttings knocked loose by the drill bit are crushed to a predetermined maximum dimension below 6 mm before they are allowed to pass up along the drill bit to the annular space between the wall of the borehole and the drill string, said crushing to said maximum dimension being ascertained by limiting the depth of return flow channels (9) for drilling fluid in the drill bit (1) to less than 5 mm as measured with respect to an imaginary continuation of the side surface (4) of the drill bit (1).
A method according to claim 1,
characterised in that said maximum dimension is kept below 4 mm.
A method according to claim 1,
characterised in that said depth is limited to a value in the area of 1.5-4 mm.
A drill bit for use in a method according to one of the preceding claims, comprising a head which has a diameter of at least 75 mm and which comprises a front face (3) and a peripheral side surface (4), where the front face (3) of the head (2) is provided with hard metal inserts (5-8) arranged in several rows at different radial distances from the central axis (11) of the drill bit (1), and with at least one outlet (10) for drilling fluid, and where the side surface (4) of the head (2) is provided with the substantially axially extending recesses forming channels (9) for the passage of the drilling fluid which through the use of the drill bit is supplied through said outlet (10),
characterised in that the largest depth of the channels (9) is less than 5 mm, preferably less than 3.5 mm, as measured with respect to an imaginary continuation of the side surface (4) of the drill bit (1), and that the average distance between the hard metal inserts (5) in the radially outermost row is less than the diameter of the inserts at least within groups of the inserts (5).
A percussive drill bit according to claim 4,
characterised in that the number of channels (9) is approximately as high as the number of hard metal inserts (5) in the radially outermost row on the front face (3) of the head (2).
A percussive drill bit according to claim 4 or 5,
characterised in that the hard metal inserts (5) in the radially outermost row has a coating of diamond material, while at least some of the buttons (6-8) on the inside are without such material.
A percussive drill bit according to one of the claims 4-6,
characterised in that the outlet (10) for drilling fluid lies adjacent to or partly overlapping with said central axis (11).