WO2016209084A1

WO2016209084A1 - Enhanced lateral chipping in deep hole drilling

Info

Publication number: WO2016209084A1
Application number: PCT/NO2016/050135
Authority: WO
Inventors: Svein Hestevik
Original assignee: Resonator As
Priority date: 2015-06-22
Filing date: 2016-06-22
Publication date: 2016-12-29
Also published as: NO344328B1; NO20150816A1

Abstract

Percussive drilling hammer assembly (20) arranged for temporary reduction of hydrostatic pressure locally at the points of material removal by periodically interrupting a flow of liquid towards the drill bit through at least one liquid channel (26). The drilling hammer assembly is provided with a liquid flow interrupting device which is arranged to abruptly and periodically interrupt the flow of liquid in a manner synchronized with the percussion frequency of the drill hammer impact piston (23), to thereby induce negative pressure waves from the point of interruption only in a direction towards the drill bit (21).

Description

Enhanced lateral Chipping in Deep Hole Drilling

The present invention concerns a percussive drilling hammer assembly as defined by the preamble of claim 1. According to another aspect the invention concerns a method for temporary reduction of hydrostatic pressure as defined by the preamble of claim 11. Background

Hydrostatic pressure (Bore Hole Pressure) encountered in deep hole drilling will substantially lower the drilling rate. This is because the high hydrostatic pressure will prevent a) efficient lateral chipping of the rock under the drill bit inserts and b) the disintegration of chip from the hole bottom (Chip Hold Down Effect). This is illustrated by Figures la-c, detailing the result of a single impact at three different pressures. Figure la shows a situation where a drill bit insert 101 hits the rock surface 102 under low pressure and creates a crack 103 in a mainly vertical direction. Fractions of a second thereafter, secondary cracks 104 are formed in directions mainly horizontal from the point of impact. These cracks eventually hits the rock surface and a lateral "chip" is thus released from the rock and is free to be removed by circulating drilling fluids. In general, the larger the lateral chips (being) formed, the more efficient drilling, typically measured as penetration rate (e.g. meters per hour). As shown by Fig. la, the typical lateral dimension of the chip formed is represented by L_L. the index L representing low pressure.

Figure lb shows the same situation as Fig. la but at a moderate pressure, i.e. a pressure somewhat higher than the pressure illustrated in Figure la. The typical lateral chip formed as a result of an impact is illustrated as L_M, the index M representing moderate pressure. The length L_M is typically shorter than the length L_L in Figure la, indicating that the released chip volumes formed per unit of time under moderate pressure, are smaller than the released chip volumes formed under low pressure. This is caused by the "hold down" effect which increases with increasing pressure thus resulting in gradually smaller lateral chips with gradually increasing pressure.

Figure lc shows same general situation as Figures la and lb, this time at even higher pressure. Thus it is no surprise that the length of a typical lateral crack 104 in this case is even shorter than the ones formed at moderate pressure and significantly shorter than the ones formed at low pressure. The figures la-c provide a good illustration on how and why the penetration rate drops off at increasing depth and resulting increased pressure. Prior art

The closest prior art may be the a percussive, liquid driven downhole drilling machine made by Wassara AB, published as WO 2004/085791. The Wassara hammer comprises a hydraulic drill string device in the form of a percussive in-hole drilling machine having a piston hammer with an axial through bore into which a tube extends. The tube forms a channel for flushing fluid from a spool valve and the tube contains channels with ports cooperating with the piston hammer for controlling the valve.

In one embodiment the piston is arranged to intermittingly establish fluid communication between the pressure discharge channel outlet and the discharge space that in operation is maintainable at a lower pressure than the pressure chamber. Such discharge space may have the form of a flushing channel. In this embodiment, the pressure chamber is intermittingly pressure relieved in sync with the reciprocating movement of the piston means.

Objectives

It is thus an object of the present invention to improve systems and/ or methods for improving lateral chipping with percussive impact drilling systems used at significant depths.

It is an object derived from the mentioned object above to provide means for eliminating the negative consequences of increased "hold-down" effect and reduced penetration rate experienced when drilling at significant depths.

The present invention The above object is achieved by the percussive drilling hammer assembly according to the present invention as defined by claim 1.

According to another aspect the present invention also concerns a method as defined by claim 11. Preferred embodiments are defined by the dependent claims.

Common for all embodiments is locally decreasing hydrostatic pressure at the vicinity of the drill bit inserts to enhance the lateral chipping (and drilling rate) in percussive deep hole drilling.

By "drilling fluid" as used herein is understood any fluid circulated through the drill bit during drilling.

A significant advantage with the present invention over the prior art technology is that a pressure wave, or negative pressure pulse, is generally created in one direction, namely in a direction towards the drill bit. The amplitude of this (negative) pressure wave is thus not reduced as a consequence of a similar pressure wave travelling upwards from the point of flow interruption. Thus a significant advantage is obtained with regard diminishing or eliminating negative consequences resulting from increased static pressure in deep hole drilling. For consistency we have in the following used the term "pressure wave" while "pressure pulse" may be an equally precise term, especially when taking into account the fact the abruptness of the change. To obtain a pressure wave only in downwards direction, the liquid conduit, path or channel in the reverse - or upwards - direction is automatically closed at - or before - each pressure wave generating occurrence generating pressure waves in the desired, downwards direction. The most reliable way of doing this is by letting the movement of the impact piston close the upwards passage at a certain position. The timing of the pressure wave generating occurrences compared to the impacts from the drill bit on the formation is also a critical parameter, as elaborated below.

The term "downwards" as used herein should be interpreted as the direction towards the drill bit, irrespective of the orientation of the well, vertical, deviated or even horizontal.

With the present invention the means for powering the percussive impact piston as such is not the core of the invention, and is therefore not discussed in any detail below. There are a number of mechanisms suggested for operating percussive drill hammers in the literature, including pneumatic, electric, and hydraulic powering mechanism, and any such mechanisms may be used with the present invention, including a combination of electric and hydraulic powering mechanisms. As will be seen below, preferred embodiments of the present invention offers also other advantages.

Detailed description of the invention with reference to enclosed drawings.

Figures la -lc show the principles of lateral chipping in drilling at various pressures.

Figure 2a shows a side sectional view of a percussive drilling hammer assembly according to a first embodiment of the present invention.

Figure 2b shows a bottom view of the drill bit according to Figure 2a.

Figure 3a shows a side sectional view of a percussive drilling hammer assembly according to a second embodiment of the present invention.

Figure 3b shows a bottom view of the drill bit according to Figure 3a.

Figure 4 shows a side sectional view of a percussive drilling hammer assembly according to a third embodiment of the present invention. Figure 5 shows a side sectional view of a percussive drilling hammer assembly according to a fourth embodiment of the present invention.

Figure 6 shows in 7 steps (a-g) the operating stages of the percussive drilling hammer assembly according to the first embodiment of the present invention.

Figures la-c, describing the concept of lateral chipping during percussive impact drilling, has been discussed in the background section above.

Figure 2a shows a side sectional view of a first preferred embodiment of the percussive drilling hammer assembly according to the present invention. A percussive drilling hammer assembly according to this embodiment comprises a drill bit 21, a drill bit shank 22 and an impact piston 23. The drill bit is provided with drill bit inserts 24 at its face (face inserts) and along its periphery 25 (gauge inserts) in a fairly conventional manner. Fluid for flushing at the drill bit face during drilling and for removal of the released cuttings or chips, are supplied through an inlet conduit 26 external of the impact piston 23 and into a bore 27 through the drill bit shank. In the drill bit the bore 27 is split into a number of small channels 28 having their respective orifice close t the drill bit gauge inserts 25. Such channels may be also be provided to one or more of the face inserts 24, but as discussed below, it is more important to have the orifices of these channels along the periphery of the drill bit and close to the gauge inserts 25.

During drilling, fluid is supplied from the surface through the inlet conduit 26 via the bore 27 and the channels 28 to the surface at which drilling occurs and circulated back to the surface from there in a region or annular space surrounding the assembly. However, for each and every impact of the impact piston 23 to the top face of the drill bit shank 22, the passage from conduit 26 into the bore 27 is momentarily blocked for fluid passage, causing an even series of negative pressure waves travelling down from the impact point between the piston and the shank all the way to the orifice of the channels 28, with a frequency corresponding to the frequency of the percussion of the impact piston. The time of arrival of the pressure wave is dependent upon the distance L from the rock surface and point of closure of the fluid line as shown in Figure 1 and of the speed of sound in the fluid at the actual pressure.

The negative pressure waves reaching the orifice of the channels 28 close to the gauge inserts of the drill bit, causes a momentarily and very short-lasting reduction in pressure locally around the orifice and thus locally near the gauge inserts. The speed of sound in the fluid is always less than the speed of the shock wave travelling through the drill bit shank, therefore the negative pressure waves reach the channel orifices at a point in time some fraction of a second after that the drill bit inserts have hit the rock. With a conveniently short drill bit shank this may be timed perfectly in relation to the mechanism described in relation to Figures la-c, bearing in mind that the first event at each blow is the formation of a vertical crack and only thereafter the mainly lateral cracks, resulting in chips of rock being released, are formed. One issue of importance is that the desired effect of reduced pressure at the vicinities around the drill bit inserts is greatest when each negative pressure wave reaches the rock surface at the point in time known as the "unloading phase" of the drill bit; i.e. when the force resulting from an impact of the drill bit inserts is no longer at its maximum. The synchronization is done by a) defining a proper timing for the valve closure with respect to the time of the impact and b) defining proper channel length between the valve section and the bit face.

This is described in further detail below. As also indicated by Fig. 2a, the fluid line may conveniently include a pressure wave dampener 29. Another issue of importance is the arrangement of the mechanism interrupting the flow being designed so as not to allow any negative pressure waves to "backfire", i.e. to escape upwards in the fluid line which in case would have reduced the amplitudes of the negative pressure waves travelling downwards through the drill bit

Figure 2b is a bottom view of the drill bit 21 provided with face inserts 24, gauge inserts 25 and channel orifices 28a close to each of the gauge inserts. A person skilled in the art will readily understand that there may be more or fewer channels 28 and orifices 28a than the ones shown in Figure 2b and that there may also be one or more channel(s) leading to face inserts 24 of the drill bit. It has proven to be even more effective to reduce the pressure at the gauge inserts of the drill bit compared to reducing the pressure at the face inserts.

While not expressly shown in any one of the drawings, a person skilled in the art understands that the drilling/ flushing fluid circulated down is also circulated back up, typically in an annular void surrounding the percussive drill hammer assembly.

Figure 3a shows a second embodiment of the percussive drilling hammer assembly which is somewhat different from the one of Figure 2a. Compared to the embodiment of Figure 2a, the arrangement of the fluid line and the design of the impact piston are modified to accommodate this function, while the design of the drill bit may be identical for these embodiments. In this embodiment the fluid flow is kept internally and is supplied to a central bore in the impact piston 33 by means of a conduit 36 being arrange coaxially and fluid tight therewith. From the central bore of the impact piston the fluid flows annularly around an upper part of the drill bit shank 32 and into a channel 37 therethrough, said channel being split into a number of small channels 38 in the drill bit 31. The passage from the central bore of the impact piston to the annular void around the upper part of the drill bit stem is open at all times except when the impact piston hits the drill bit stem, thereby effectively and momentarily shutting off the fluid flow in a manner quite similar to the one described in relation to fig. 2a above. Thus, also in this case the impact of the impact piston triggers the combined effect of forcing the drill bit into the surface of the formation and at the same time generating a negative pressure wave travelling downwards in the fluid line, all the way to the vicinities around the drill bit inserts and in particular the gauge inserts.

The fluid flow as such is, like for the first embodiment, generated by means of a pump or the like (not shown) being arranged upstream, such as at the surface of the well being drilled. The pump may be any suitable fluid pump conventionally used for circulating drilling fluid during drilling.

The fluid tight connection between the conduit 36 and the impact piston 33 allows at least a part of the impact piston to be surrounded by a lubricant without leakage of lubricant into the drilling fluid.

Figure 3b shows the bottom side of the dill bit 31 which is principally no different from that of first embodiment, shown in Figure 2b.

Another embodiment is shown in Figure 4. Here, like the embodiment of Figure 3a, the fluid flow of circulating is passed through an inlet conduit coaxially arranged with the impact piston and further on through a central bore in the impact piston. The only principle difference is that the stroke of the piston is arranged in a manner allowing the impact piston to work also as the piston of a pump; i.e. fluid is advanced past the impact piston when it retracts and is forced in front of the impact piston when it advances. To make this possible a kind of non return valve 431 is arranged in the impact piston, allowing flow of liquid therethrough during upwards backstroke of the piston, but closing for passage of fluid flow therethrough during downwards impact stroke. a) A mathematical approach to the second embodiment can be expressed as follows: During the impact stroke the piston nose is pushing out water with the flow rate of q=A_Nose*V_imp. This flow rate corresponds to liquid velocity of v_Liquid = q/A_F|_USh = A_Nose/A_F|_USh*v_imp. Water is guided through the side flushing to the bit face; where q is the volumetric flow rate, A_Nose is the cross-sectional area of (the front end of) the impact piston, V_imp is the linear velocity of the impact piston, V_Liquid is the linear velocity of the fluid in the flush channel, and A_F|_USh is the cross-sectional area of the flush channel b) During the impact the piston will stop and 'Water Hammer effect' will take place. The impact will also close the connection between the flushing hole and the inner bore of the piston. Therefore only one pressure wave is created which will have the full amplitude according to p = c_Liquid*p_Liquid*v_Liquid. where C_Liquid is the speed of sound in the liquid, p_Liquid is the density of the liquid and c) The negative pressure wave is guided to the vicinity of the inserts at the bit face to create a local pressure drop on the location where it is the most beneficial.

The speed of sound in the liquid can be found from the bulk modulus B of the liquid and dividing that by the liquid density and finally taking the square root thereof, c_Liquid = sqrt(S/p_Liquid).

Now with reference to figures 5 a fourth embodiment of the present invention is described. Again the difference from the first and second embodiments lies in the manner with which the fluid flow is interrupted to provide the negative pressure wave. While the mechanical parts of the drill bit assembly is quite similar to that of the second embodiment, shown in Figure 3a, here a quick acting valve 59 is used to interrupt the fluid flow from the fluid line 56 into the bore 57 in the drill bit shank. The activation of the valve 59 is determined by the position of the impact piston 53. Only when the impact piston has moved so far down that fluid connection is established via a conduit 591 between the valve 59 and a top volume above the impact piston, surrounding inlet conduit 56, the valve is activated and the flow of drilling fluid immediately interrupted with the same result as described above with regard to formation and travel of a pressure wave.

As in the two previous embodiments the point of interruption of the fluid flow is about at the top end of the drill bit shank, the length of which therefore determines the travel path and hence the "delay" of the pressure wave in relation to the hit of the drill bit against the rock.

The lower end of the drill bit may be identical for all three embodiments since there is no principal difference in their function between these embodiments. As mentioned the number of channels may vary and is preferably related to the number of drill bit inserts or at least the number of gauge inserts, since the lateral chipping predominantly is formed from the periphery of the drill bit. Typical steps encountered in drilling process where the percussive drilling hammer assembly is one according to this embodiment may be summarized as follows: a) A separate flow interruption valve is used to control the flushing flow. b) When the piston comes to a pre-defined position, the working pressure acting on the top volume is connected to the flow interruption valve and the valve switches its position from Open' to 'closed'. This will interrupt the flow and the negative pressure wave is created. c) After the impact the control valve of the impact piston (not shown here) connects the top volume to low pressure and the impact piston starts the reverse stroke. This pressure drop at the top volume causes also the flow interruption valve to switch back from 'closed' to 'open'.

Now with reference to Fig. 6, seven stages of the process utilizing the percussive drilling hammer assembly according to the first embodiment are explained in further detail. a) This stage is represented by steady flushing flow through the drill bit shank to the bit face. b) In stage b the piston hammer is shown impacting the bit shank and interrupting the flow. Positive pressure wave upstream and negative wave downstream are created. Pressure wave amplitude is p = c_Liquid*p_Liquid*v_Liquid. In addition, impact induced mechanical stress wave created in the drill bit shank, travelling towards the drill bit with a speed of about 5200 m/s. c) At this stage the mechanical stress wave reaches the bit face and starts to push the drill bit into a rock (loading phase starts). The negative pressure wave has fallen behind since the wave in the fluid travels with considerably lower speed. d) In stage d the stress wave has completely reached the drill bit face and the drill bit has reached its maximum penetration (loading phase ends). The negative pressure wave is about to enter the bit face. e) Stage e may be seen as the critical phase of the process. The drill bit starts to reverse due to elastic forces by the rock (unloading phase). Side cracking and lateral chips start to form around the inserts and in particular the gauge inserts. The negative pressure wave has entered the bit face causing a drastic drop in hydrostatic pressure at the vicinity of the inserts. High pressure drop enhances the lateral chipping process, resulting in considerably increased rate of penetration.

At the same time the impact piston starts the reverse stroke and opens the flow path. Opening the flow path creates counteracting pressure waves which eventually restores original pressure conditions. f) The counteracting pressure wave enters the bit face and the cancels out the pressure drop at the bit face. g) The drill bit is indexed to a new position and new cycle is about to start.

While the details of Figure 5 is explained in relation to the first embodiment provided above, similar development, step by step, takes place also for the second and third embodiment of the present invention. Calculation example

If the actual speed of sound in the liquid is 1200 m/s, and the point of interruption is 1.2 meters from the points of impact between drill hammer and formation, the interruption of the liquid flow should be arranged to take place at a Δΐ =1.2m/1200m/s = 1/1000 second or 0.001 seconds before each point in time at which impact occurs, for the pressure wave to reach the formation surface at the time of impact. In practice, however, it is sufficient that the pressure waves reach the formation surface at a point in time fractions of a second later, during the "unloading" phase of the drill bit against the formation. The optimal point in time may be found by trial and error for different types of rock, and adjustments may be performed by use of drill bits having different lengths of their shanks.

Preferred embodiments

At least one part of the liquid channel or channel system for drilling fluid is arranged through a central bore in the drill bit shank. However, other parts of the liquid channel may be arranged externally in relation to the piston and through a passage into the central bore in the drill bit shank. <such passage may be radial in relation to the axis of the piston and the drill bit shank.

The liquid flow interrupting device may be a quick-acting valve, said valve e.g. being triggered to close the channel for drilling fluid at an interruption point by the position of the impact piston.

The liquid flow interrupting device may alternatively be comprised by a piston, more accurately the impact piston, acting as a closing valve or acting a liquid pump, abruptly stopping the liquid flow when the piston hits the drill bit shank or more generally when its movement temporarily stops.

Especially preferred is the feature that the liquid flow interrupting device is formed by an impact piston arranged to transmit percussive impacts to the shank of the drill bit, the piston and shank being designed to close the liquid channel during each operational cycle of the piston. In some embodiments the impact surface of the piston is designed to close a radial fluid passage at the time of impact.

Particularly preferred is the feature that the central bore through the drill bit shank is divided into a plurality of channels in the drill bit and that each of these channels has an orifice in the vicinity of a gauge insert in the drill bit, thus maximizing the effect of the pressure reduction in the areas where it is most useful. Using drill bit shanks (22, 32, 42) of different lengths allow synchronization of the flow

interruptions with the percussive impacts.

The arrangement of the fluid flow interruption device and the bit face is preferably made in a manner fulfilling the requirement that

L valve^— k [2Lpi_Ston/((C_to0|/C|_iqUid)-l))] where L_piston is the length of the impact piston conveying the mechanical energy which in turn is transmitted as stress waves trough the steel down to the drill bit, L_va|_Ve is the distance between the contact point between drill bit and formation and the point of interruption of the fluid flow, k is a real number in the range 0.5 to 1.5, C _t00| is the speed of sound in the tool (drill bit shank and drill bit), and C_Liquid is the speed of sound in the drilling liquid. The factor k is set to a number between 0.5 and 1.5 to allow the pressure wave in the fluid to reach the drill bit face at the time of unloading of the drill bit from the formation and may be optimized within this range in accordance with the properties of the drilling fluid and specifically the speed of sound in the drilling fluid.

Claims

1. Percussive drilling hammer assembly (20, 30, 40, 50) arranged for temporary reduction of hydrostatic pressure locally at the points of material removal by periodically interrupting a flow of liquid towards the drill bit through at least one liquid inlet conduit (26, 36, 46, 56),

characterized in that the drilling hammer assembly is provided with a liquid flow interrupting device which is arranged to abruptly and periodically interrupt the flow of liquid in a manner synchronized with the percussion frequency of the drill hammer impact piston (23, 33, 43, 53), to thereby induce negative pressure waves in the liquid from the point of interruption only in a direction towards the drill bit (21, 31, 41, 51).

2. Percussive drilling hammer assembly as claimed in claim 1, wherein one part of the liquid channel is arranged through a central bore (27, 37, 47, 57) in the drill bit shank.

3. Percussive drilling hammer assembly as claimed in claim 1 or 2, wherein the liquid flow interrupting device is formed by an impact piston (23, 33, 43) arranged to transmit percussive impacts to a drill bit shank (22, 32, 42), the impact piston and drill bit shank being designed to close the liquid channel during each operational cycle of the impact piston.

4. Percussive drilling hammer assembly as claimed in claim 2 or 3, wherein the liquid inlet conduit (26) is arranged externally in relation to the impact piston (23) and through a passage into the central bore (27) in the drill bit shank (22), said passage being radial in relation to the axis of the impact piston (23) and the drill bit shank (22).

5. Percussive drilling hammer assembly as claimed in claim 4, wherein the impact surface of the impact piston is designed to close the radial passage at the time of impact with the drill bit shank (22).

6. Percussive drilling hammer assembly as claimed in claim 1 or 2, wherein the liquid flow interrupting device is comprised by an impact piston (43) acting as a liquid pump, abruptly stopping the liquid flow when the impact piston hits the drill bit shank (42).

7. Percussive drilling hammer assembly as claimed in claim 1 or 2, wherein the liquid flow interrupting device is a quick-acting valve (59).

8. Percussive drilling hammer assembly as claimed in any one of claims 2 - 7, wherein the central bore (27, 37, 47, 57) through the drill bit shank is divided into plural channels (28, 38, 48, 58) in the drill bit (21, 31, 41, 51).

9. Percussive drilling hammer assembly as claimed in any one of claims 1 - 8, wherein drill bit shanks (22, 32, 42) of different lengths allow synchronization of the flow interruptions with the percussive impacts.

10. Percussive drilling hammer assembly as claimed in any one of claims 1 - 8, wherein

^Lv_alv_e ^— k [2L_piston/((C_to0|/C|_iquid)-l))] where L_piston is the length of the impact piston conveying the mechanical energy which in turn is transmitted as stress waves trough the steel down to the drill bit, L_va|_Ve is the distance between the contact point between drill bit and formation and the point of interruption of the fluid flow, k is a real number in the range 0.5 to 1.5, C_t00| is the speed of sound in the tool (drill bit shank and drill bit), and C_Liquid is the speed of sound in the drilling liquid..

11. Method for temporary reduction of hydrostatic pressure locally at the points of impact under a percussive drilling hammer (to improve drilling efficiency) by providing a liquid flow towards the drill bit in a liquid channel, characterized in abruptly and periodically interrupting the liquid flow in a manner synchronized with the percussion frequency of the hammer, thereby inducing negative pressure waves travelling from the point of interruption only in a direction towards the drill bit.

12. Method as claimed in claim 11, wherein synchronization of the flow interruptions with the percussion impacts is achieved by using drill bit shanks of different lengths.