EP0020416A1

EP0020416A1 - Improvements in and relating to electrical power transmission in fluid wells

Info

Publication number: EP0020416A1
Application number: EP79901156A
Authority: EP
Inventors: Stephen Alexander Horchler
Original assignee: UK Secretary of State for Energy
Current assignee: UK Secretary of State for Energy
Priority date: 1978-09-29
Filing date: 1980-04-22
Publication date: 1981-01-07
Also published as: WO1980000727A1; GB2055131A; GB2055131B

Abstract

Methode et appareil de transmission de l'energie electrique le long d'un alesage d'un puit de petrole ou de gaz en cours de perforation ou en production. L'invention s'applique particulierement au controle en profondeur dans le trou, un capteur electrique (10) pour le parametre particulier envoyant ses signaux de sortie a A' B' et via, respectivement, une section centrale isolee du cordon du tube (2) et de l'enveloppe (1) ces signaux etant retires en A, B pour un controle ou surveillance approprie. Un sous-isolant (11) pour l'isolation electrique de la section centrale du cordon est decrit.Method and apparatus for transmitting electrical energy down a bore of an oil or gas well being drilled or in production. The invention is particularly applicable to in-hole depth control, an electrical sensor (10) for the particular parameter sending its output signals to A' B' and via, respectively, an isolated central section of the pipe bead (2 ) and the envelope (1) these signals being removed at A, B for appropriate control or monitoring. A sub-insulator (11) for the electrical insulation of the central section of the cord is described.

Description

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Improvements in and relating to electrical power transmission in fluid wells.

TECHNICAL FIELD OF INVENTION

This invention relates to the passage of electrical power along the tubing string of a borehole which has particular reference to the downhole monitoring of some parameter (e.g. pressure, temperature or fluid velocity) relevant to the pro¬ duction of a borehold for a fluid well or the extraction of fluid (e.g. oil or gas) from such a borehole after its produc¬ tion. The invention also extends to a borehole per se adapted for such passage of electrical power. BACKGROUND ART

It has long been appreciated that considerable economies in the control of drilling of a borehole or the extraction of fluid from a borehole, result from accurate downhole monitor¬ ing of suitable parameters. Thus for example, the extraction of oil from an oil well can be optimised (with a substantial increase in potential yield from a well), if the changes in pressure occurring at the bottom of the well can be accurately charted. Heretofore there has been no wholly reliable method of monitoring the downhole pressure without interrupting the flow of oil from the well. Accurate pressure sensing devices are available which will operate satisfactorily in the condi¬ tions existing at the bottom of the borehole, but the problem has been the telemetering of the information generated by the sensing device several thousand feet down in the earth to the wellhead where the appropriate control action has to be taken. It has been proposed to transmit the sensed information up to the surface using an electrical cable clipped to the outside of the tubing string or incorporated in the walls of the pipes making up the string, but neither of these possibilities has met with much success. A separate electrical cable is sus¬ ceptible to damage and difficulties arise in correctly con¬ necting together the adjacent ends of cable which have been incorporated in the walls of the pipes interconnected to make the tubing string. Because of the difficulties experienced with the prior art arrangements it has been the practice, to ensure good well management, to periodically stop the oil or gas flow for a few days every few weeks, and to make the

I. W1P necessary measurements during the interruption in flow, a special sensor being lowered down the string for measurement purposes. Naturally, however, any interruption in flow from a production well represents a substantial loss of revenue

5 to the oil company and is highly undesirable.

Similar considerations apply to the downhole monitoring of critical parameters in the actual drilling of the borehole so that an improved method of power transmission along a borehole which would permit reliable downhole monitoring has lOuseful applications in both activities. THE INVENTION

In its broadest aspect the invention relates to a method of transmitting electrical power along a borehole comprising a electrically-conducting string located within an electrically-

15conducting casing which is characterised in that a section of tubing string between a wellhead hanger and a downhole packer electrically insulated from upper and lower tube ends by means electrically isolating subs and from the surrounding casing by electrically insulating centralisers and in that power is pass

20along the borehole using the casing as one conducting link and the said section as the other conducting link.

For downhole monitoring an electrical sensor producing an electrical output related to a parameter sensed down the hole has its output connected between the lower end of said section 5and the lower tube and the output from the sensor is monitored at the wellhead by connections between the upper end of said section and an upper part of the casing.

The sensor can be of a kind powered by a low voltage dc a this can be fed down the said section from the wellhead or gen 0ated in situ using a turbine generator (powered by mud, oil or gas).

The insulating centralisers are employed to prevent short circuiting between the said section and the surrounding casing and insulating tube centralisers are preferred which are 5 clamped at intervals around the tubing string and bear on the internal surface of the casing. Crude oil or diesel oil can be located in the annular volume between the tubing string and the casing.

The invention finds particular utility in a downhole pressure monitoring system (using a pressure-sensing oscil¬ lator unit whose output frequency is related to the subsis- ting pressure), but the method of the invention can be used for example for measuring temperature, electrical conductivity, radioactivity, flow velocity, the presence of specified chemi¬ cals and indeed any useful parameter for which a suitable sensor can be found. To ensure good electrical connection at the pipe joints in the string, he pipe lengths may be torqued together to such a degree that cold welding occurs at each joint. A con¬ ducting paste (e.g. incorporating a silver or graphite pow¬ der) can be used at the joints if required. The invention also embraces the special equipment needed to isolate a major section of the tubing string from its ends and from the casing and thus extends to the isolating subs, an insulating collar for the tubing string and an insulating coupling for the DHSV control line. Thus according to a further aspect of the invention a borehole comprising an electrically conducting casing de¬ fining the outer extremity of the borehole, and an electrically conducting tubing string extending substantially concentri¬ cally through the casing between upper and lower regions there- of,. is characterised in that the tubing string has a central section which is electrically isolated from the said upper and lower regions by a pair of subs and from the surrounding casing by a plurality of spaced-apart electrically insulating centralisers, each sub comprising first and second aligned tube lengths, each of which lengths terminates at its axially outer end in a screw-threaded section and at its axially inner end in an annular face which confronts the annular face of the other tube length, a first electrically insulating ring dis¬ posed between said annular faces to space the tube lengths apart and a locking collar having a shoulder confronting a shoulder on said first tube length and a screw-threaded por¬ tion engaged in threads on the second tube length, a second electrically insulating ring to space the shoulders apart and an insulating sleeve to space the locking collar from the first tube length, the locking collar being tightened on the second tube length to compress the two insulating rings and rigidly clamp the tube lengths together, a trans- 5mitter and a receiver of electrical power disposed at oppo¬ site ends of the said central section, each connected elec¬ trically between the casing and the locking collar of the ad¬ jacent sub, whereby the electrical circuit between said transmitter and receiver includes the casing and the said lOcentral section.

Suitably each sub is constructed to withstand a tension applied between the tapered threaded sections of the outer ends of the tube lengths of at least 180000 Kgs.

Suitably the locking collar includes a cable terminal 15to ensure low ohmic contact to the second tube length.

Conveniently the interior of the tube lengths are coated with electrically insulating material to increase the length of any shorting path available between opposite ends of the sub through the medium of the liquid flowing through the 20tube lengths in use of the sub.

Each insulating centraliser can be a ring of electricall insulating material (e.g. PTFE or fibre-reinforced resin) clamped around the outside of the tubing string by metallic ring -shackles which include narrow bearings for engaging the 5casing over localised regions spaced-apart around the periphe of the inner wall of the casing.

Where the DHSV is controlled by a hydraulic control line, the isolator need only provide an electrical break while main¬ taining a seal capable of withstanding the operating pressure 0of the valve and a variety of simple connections are available for this. BRIEF DESCRIPTION OF DRAWINGS

The invention will now be more fully described, by way of example with reference to the accompanying drawings, in 5which:-

Figure 1 is a schematic representation of a borehole in accordance with the invention and adapted for downhole moni¬ toring in accordance with the method of the invention,

O Figures 2 and 7 show parts of the subs used in the well of Figure 1 in greater detail,

Figure 3 is an end view of the sub shown in Figure 2, Figure is a sectional view of the- insulating coupling in the wellhead of Figure 1 in the control line of the DHSV,

Figure 5 is an end view of a tube centraliser used in the well of Figure 1, and

Figure 6 is a sectional view of the centraliser of Figure 5 taken on the line VI-VI. SPECIFIC DESCRIPTION

Figure 1 represents schematically a production oil well, the casing being indicated at 1, the tubing string at 2, the wellhead tubing hanger at 3 and the downhole packer at 4. In conventional manner a DHSV is located near the upper end of the string 2 and the control line for this is shown at 5. In a typical well the casing 1 would have a diameter of about 25 cms and the string 2 a diameter of around 11cms. The dotted region of Figure 1 could be several thousands of metres in length. In Figure 1, 10 represents a known type of pressure- sensitive oscillator (e.g. a Hewlett-Packard HP 2δll8) which generates an ac output (pulsed) when fed with a low voltage dc input, the frequency of the ac output being related to the pressure to which the oscillator is subjected. In a typical case, the oscillator could measure pressures between 70 to 850 bars and over that range the output would range between 500 Hertz and 15000 Hertz. The oscillator 10 is to be used for continually monitoring the pressure in the oil in the vicinity of the packer 4, so that there is a need to supply it with a low voltage dc current (e.g. 24 volts at 30 mA) and to receive from it the pulsed ac in such wise that the fre¬ quency of the ac output can be determined reliably at the well¬ head. In accordance with the invention this is achieved by electrically insulating the major portion of the string 2 by means of electrically isolating subs 11 from the wellhead hanger 3 and the packer 4 and, by means of insulating central¬ isers 12, from the casing 1. To prevent the control line 5 shorting out the insulated string section, a control line iso- lator 13 is employed.

With the major part of the string 2 isolated from the casing 1 and the hanger 3, a dc voltage applied between the points A and B in Figure 1 will appear across the oscillator 10 (to energise the same) and the output from the oscillator 10 will be fed to the points A' and B' at the bottom of the borehole and will appear as an ac signal at A and B. Thus the string 2 and the casing 1 serve as electrical transmissio lines for the powering of the downhole sensor and for the transmission of output signals from the sensor to the wellhea The isolating subs 11 can each have the form shown in Figures 2 and 3, the sub being oriented so that the arrow X points upwards if the sub is used at the wellhead and so it points downwards if the sub is used at the downhole end of the string 2. It is convenient to use a similar design of su at each end of the isolated section of the string, although the strength requirement of the lower sub is much less than that of the upper sub.

Referring to Figures 2 and 3, it will be seen that the sub comprises a first tube length 14 and a second tube length 15 which are locked together by a locking collar lβ. The outer ends of the lengths 14 and 15 are provided with conven¬ tional tapered threads 17 (only shown for the first tube length) to enable the sub to be joined into the string by normal oilwell techniques. The confronting annular faces 14a and 15a of the tube lengths are spaced apart by a first in¬ sulating ring 18 located in recesses formed in the faces 14a and 15a. Annular seals 18a (e.g. Wills rings) ensure a liqui tight joint in the vicinity of the ring 18. To provide the clamping force on the ring 18, the locking collar lβ is threaded to the length 15 at 19 and has a shoulder 20 which compresses a second insulating ring 21 against a shoulder 22 of the first length 14. An insulating sleeve 23 ensures that the locking collar l^β does not make any electrical contact wit the tube length 13- The locking collar is tightened in place by means of four radial grooves 24 (see Figure 3) and carries an annular bushing 25 of electrically insulating material to space it from the casing 1 (shown by chain lines in Figure 2). The electrical connection (A in Figure 1) to the isolated section of the string (of which length 15 is a part) is made via the cable 26 shown in Figure 2.

Grooves 27 in the bushing 25 allow the control line 5 to pass within the casing to the DHSV.

The lower sub 11 is inverted with respect to the upper sub and thus serves to electrically isolate the central part of the string 2 from the lowermost part which is in electrical contact with the packer 4, and, via the packer 4, with the casing 1. Figure 7 shows one way of mounting the sensor 10. A recess 30 can be formed in the wall of the tube length 14 and the cable 26 can connect directly to one input terminal and one output terminal of the sensor 10. The connection 31 shown in Figure 7 allows the cable 26 to.pass into the re- cess 30 in a pressure-tight manner without making electrical contact with the tube length 14. The casing of the sensor 10 is electrically connected to the tube length 14 (via the clamping rings 32) and this electrically connects the other input terminal and the other output terminal to the bottom end of the string 2, and via the packer 4, to the casing 1. To ensure the complete electrical isolation of the cen¬ tral part of the string 2 from the casing 1, the annular cen¬ tralisers shown in Figures 5 and 6 are employed. One pipe of the string 2 is shown at 40 in Figures 5 and 6 and this is spaced from the casing 1 by a metal ring shackle 4l clamped around an insulating sleeve 42. Both the sleeve 42^' and the shackle 41 are in two parts for easy application around a pipe as the string is being formed at the wellhead. As can be seen from the drawings, the shackle is provided with four equally spaced ears 4la, which bear on the casing 1 to centralise the string 2 and prevent electrical contact between the two. The annular volume 43 is filled with an electrically insulating liquid (e.g. diesel oil) during use to ensure that a high ohmic resistance is maintained between the string and the casing, during operation of the monitoring method of the invention. Since any pockets of water in the annular volume would drama¬ tically reduce the ohmic resistance (particularly if they were saline and bridged across between the string and the casing) it may be desirable to include an emulsifying agent in the o to ensure that any water present is held as a water-in-oil emulsion.

Although silicon nitride has been mentioned as a materi suitable for the construction of the rings 18 and 21 and the sleeve 23, it should be realised that other materials (in pa ticular other sintered ceramic materials) can be used pro¬ vided they have the necessary strength and insulating pro¬ perties. Figure 4 shows the insulating coupling 13 in the contro line - From the drawing it can be seen that the insulating rings 13a and 13b prevent electrical connection between the inlet pipe and the remainder of the coupling.

The^" ac pulses appearing between points "A" and "B" at the wellhead can be taken off by conventional connections and leads to suitable monitoring equipment either in the vic nity of the wellhead (e.g. on an offshore platform) or tele¬ metered to a remote location. The equipment used for the pr cessing of the information received from the sensor 10 can b conventional and apart from mentioning the possible need of an amplifying stage across the points "A", "B" (the output signals can be seriously attenuated in their passage to the wellhead) will not be further commented on here.

As previously mentioned, the provision of the insulated section in the tubing string enables electrical power to be fed to downhole equipment from the wellhead and this represen a further advantage in practice.

Claims

- _CLAIMS

1. A method of transmitting electrical power along a borehole comprising an electrically-conducting string" located within an electrically-conducting casing, characterised in that a section of the tubing string (2) between a wellhead hanger (3) and a downhole packer ( 4) is electrically insulated from upper and lower tube ends by means of electrically isolating subs (11) and from the surrounding casing by electrically insulating centralisers (12) and in that power is passed along the borehole using the casing as one conducting link and the said section as the other conducting link.

2. A method as claimed in claim 1, for downhole monitoring, characterised in that an electrical sensor (10) producing an electrical output related to a parameter sensed down the hole has its output connected between the lower end of said section and the lower tube end and the output from the sensor is monitored at the wellhead by connections between the upper end of said section and an upper part of the casing.

3. A method as claimed in claim 2, characterised in that the sensor output varies in frequency with change in parameter .. magnitude.

4. A method as claimed in claim 2, characterised in that the centralisers are sleeves clamped around the said section of the tubing string at spaced-apart intervals therealong.

5- A borehole in which the method of claim 1 can be employed, said borehole comprising an electrically conducting casing defining the outer extremity of the borehole and an electrically conducting tubing string extending substantially concentrically through the casing between upper and lower regions thereof, characterised in that the tubing string (2) has a central section which is electrically isolated from the said upper and lower regions by a pair of subs (11)and from the surrounding casing (1) by a plurality of spaced-apart electrically insulat¬ ing centralisers (12), each sub (11) comprising first (14) and second (15) aligned tube lengths, each of which lengths terminates at its axially outer end in a screw-threaded section and at its axially inner end in an annular face (l4a,15a) which confronts the annular face of the other tube length, a first electrically insulating ring (18) disposed between said annular faces to space the tube lengths apart and a locking collar (iβ) having a shoulder (20) confronting a shoulder (22) on the first tube length and a screw-threaded portion ( 19) engaged in threads on the second tube length, a second electrically insulating ring (21) to space the shoulders apart and an insulating sleeve (23) to space the locking collar from the first tube length, the locking collar being tightened on the second tube length to compress the two insulating rings and rigidly clamp the tube lengths together, a transmitter (10) and a receiver of electrical power disposed at opposite ends of said central section, each connected electrically between the casing and the locking collar of the adjacent sub, whereby the electrical circuit between said transmitter and receiver includ the casing and the said central section.

6. A borehole as claimed in claim 5, characterised in that each sub is constructed to withstand a tension applied between the tapered threaded sections of the outer ends of the tube lengths of at least 180000 Kgs.

7. A borehole as claimed in claim 5, in which the locking collar on each sub includes a cable terminal (26) to provide a low ohmic contact to the second tube length.

8. A borehole as claimed in claim 6, in which the interior of the tube lengths of each sub are coated with electrically insulating material.

9- A borehole as claimed in claim 6, in which each centralis is a ring of electrically insulating material clamped around th outside of the tubing string by shackles (41) which include narrow bearings ( 4la) engaging the casing over localised regions.

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