GB2293839A

GB2293839A - Tool for generating down hole axial force

Info

Publication number: GB2293839A
Application number: GB9420261A
Authority: GB
Inventors: Peter Douglas Paterson
Original assignee: Individual
Current assignee: Individual
Priority date: 1994-10-07
Filing date: 1994-10-07
Publication date: 1996-04-10
Also published as: GB9420261D0

Abstract

A down hole drilling tool which uses the harmonic oscillation of transient pressure waves in the mud within the drillstring, to generate axial force. The invention is particularly suited to extended reach drilling applications where drillstring drag in horizontal sections presents problems when trying to maintain weight on bit. With reference to Fig 1, the tool, being a pipe system, is bounded by an exciter valve 2 at the downstream end and a check valve 5 at the upstream end. The exciter valve 2 is oscillated at one of the critical frequencies of the pipe system in order to establish a resonant condition (water hammer) in the tool. An oscillating standing wave of considerable magnitude is generated, the peaks of which occur at the closed valves 2 and 5. The pressure acting at the closed valves 2 and 5 is transmitted as an axial force to the attached drillstring 1 and 6. <IMAGE>

Description

TOOL FOR GENERATING DOWN HOLE AXIAL FORCE This invention relates to a tool for overcoming problems of "drag" in extended horizontal sections of boreholes drilled in the earth's crust.

Extended reach drilling is now commonly used to intercept target zones which are laterally displaced from the drill site. The major advantage of the technique is that a wide area can be targeted from a single drill site. Lateral displacements of up to 6000 metres have been drilled.

The length of horizontal displacements that can be drilled is restricted due to the friction between the drillstring and the borehole. This is manifested as torque and drag acting on the horizontal section of the drillstring, and is considerably more than that experienced in vertical sections. The cause is primarily due to the effects of gravity which holds the drillstring against the bottom of the hole. Maintaining "weight on bit" in horizontal sections is accomplished by drillstring weight in the vertical and build sections of the hole. The amount of weight that can be applied is restricted by the buckling criteria of the drillstring. The drag forces acting on the drillstring in the horizontal section, eventually equal the weight force that can be applied leaving no useful force at the bit.

The invention is a tool which uses the mud flowing through the drillstring to generates axial force. One or more of these tools will be run in the horizontal section of the stringstring thereby providing additional axial force.

The invention is based upon the harmonic oscillation of transient pressure waves (water hammer) in a pipe system i.e. resonance. In order for resonance to develop in a system, there is required to be a net positive influx of energy, and an exciter with a period which is matched to one of the critical periods of the pipe system. This oscillating condition will lead to pressure amplification at some point or points in the system.

In order for there to be a system, there is required to be some form of boundary condition both at the upstream and the downstream ends of the effective length of pipe. A boundary condition can be defined as any form of change in the flow profile which results in a change of the flow velocity. A restriction in the flow profile, termed a "hard end", results in a reflected pressure wave of similar sign to the incident wave whereas an expansion of the flow profile, termed as a "soft end", results in the inverse.

In the case of the invention, the energy source will be the mud pumped through the drill pipe from the rig. The downstream boundary condition will be a valve which will also serve the function of the exciter. The upstream boundary condition may be in the form of an expansion of the flow profile resulting in a soft end, or a check valve which will act as a hard end. The exciter valve will be cycled at or near one of the critical frequencies of the system to establish resonance in the tool. As a result, pressure amplification will occur at the downstream end in the case of a soft end for the upstream boundary condition, or at both ends if a hard end is used upstream. This amplification will manifest itself as a cyclic force which acts at one or both ends of the tool depending on the boundary conditions.

A specific embodiment of the invention will now be described by way of example with specific reference to the accompanying drawings in which: Figure 1 shows in perspective, the transient pressure wave tool; Figure 2 shows the exciter valve assembly; Figure 3 shows the extendable joint assembly; Figure 4 shows the upstream boundary condition as a check valve.

Figure 5 shows the upstream boundary condition as a change in flow profile.

The exciter valve 2 shown in detail in Fig 2, will have the capability of restricting the mud flow and will be cycled at or near one of the critical periods of the system. It follows that control of the valve cycle frequency is critical to the function of the tool. The only means of achieving this down hole is by using the relative motion between the drillstring and the borehole. The proposed valve will comprise of a rota 7, that being the drillpipe, and a stator 8 which will be similar in appearance to a drillstring stablizer. The stator will be bearing mounted on the drillstring thereby enabling the drillstring to freely rotate within it. The mud flow will pass via drillings 9 in the drillpipe wall to ported channels in the stator 10 and back into the drilling string via matched drillings 9 further downstream.Oscillation of the flow is achieved by blades between the ported channels 11 which periodically cover the drillings as the drillstring rotates. The external blades 12 on the stator will be in contact with the formation 13 thereby resisting the reaction torque that is generated. By monitoring the pressure wave pulse that reaches the surface, the rotary speed of the drillstring can be varied to generate the maximum magnitude of resonance in the tool.

An extendable joint 15 shown in detail in Fig 3, may be included in the tool design in which case it will be located immediately upstream of the exciter valve. The joint will be sealed 16 to prevent the mud leakage to the annulus and will be appropriately splined 17 to carry the drillstring torque. The function of the extendable joint is to enable the downstream section of the drillstring 1, to be propelled forward without any resistance from the pipe wall of the tool body. In this way, all of the force generated at the valve will be transmitted to the downstream section 1 of the drillstring.

The body of the tool 4 shown in Fig 1, will be comprised of thick welled drill pipe in order to resist the high cyclic pressures. The through bore size will be such as to achieve highest fluid velocity at an acceptable pressure drop across the tool. The length of the tool body will be such that pipe period approximates the required cyclic frequency of the tool when taking into account the sonic velocity of the mud and the boundary conditions used.

The upstream boundary condition will comprise of one of the following: 1/ A spring loaded check valve 18 which closes to reverse flow i.e. flow from the bit, as shown in Fig 4; 2/ An expansion in the flow profile cross-sectional area of at least 100% 19, as shown in Fig 5.

The cycle description is as follows. The exciter valve 2 restricts flow instantaneously thereby generating a pressure wave which travels through the mud in the tool body 4 at a sonic velocity (a). The pressure wave is reflected from the upstream boundary condition 5 which is a distance (L) from the exciter valve 2, and returns to the exciter valve 2 after a period of (t = 2L/a). In the case of hard end resonance 18 the pipe period is equal to (2L/a). Soft end resonance 19 however requires a further cycle to return the sign of the pressure wave to that of the original, in which case the pipe period is equal to (4L/a). In either case, the sequential exciter valve 2 closure is timed to be the same as the pipe period, thereby amplifying the pressure wave. The magnitude of the pressure wave will continue to grow each cycle until the energy losses in the system match the energy input.

Claims

1/ A down hole tool using the harmonic oscillation of transient pressure waves in the mud to generate an axial force.

2/ A transient pressure wave tool as claimed in Claim 1 comprising of an exciter valve, a sealed and splined extendable joint, a body of thick walled pipe and an upstream boundary condition.

3/ A transient pressure wave tool as claimed in Claim 2 wherein the upstream boundary condition is a check valve thereby constituting a hard end.

4/ A transient pressure wave tool as claimed in Claim 2 wherein the upstream boundary condition is an expansion in the flow profile thereby constituting a soft end.

5/ A transient pressure wave tool as claimed in Claim 2 wherein the thick walled drill pipe is used for the tool body.

6/ A transient pressure wave tool as claimed in any preceding claim wherein there is no sealed and splined extendable joint.

7/ A transient pressure wave tool as claimed in any preceding claim wherein the exciter valve is a rotary valve.

8/ A transient pressure wave tool as claimed in Claim 7 wherein the exciter valve only partially restricts the mud flow in order to generate the pressure wave.