This invention relates to plugs for use in well- bores and, more particularly, to a top plug or bottom plug for use in the placing of cement between casing and the wall of a wellbore.

Top plugs and bottom plugs are used in wellbores during cementation. Typically, after a well has been drilled to a particular depth the drill is withdrawn and a string of casing having a float shoe at its lower end is lowered close to the bottom of the bore. The annulus between the casing and the bore is then filled with ce¬ ment.

The cement is introduced by first placing a bottom plug into the casing and then pumping a calculated quantity of cement down the casing. The cement forces the bottom plug down the casing until it comes into contact with the float shoe. The float shoe prevents further downwards movement of the bottom plug and as the pressure of the cement increases a bursting disk in the bottom plug ruptures to allow the cement to flow through the bottom plug and float shoe and into the annular space between the casing and the bore.

After the calculated quantity of cement has been pumped down the well a top plug is introduced into the casing and is pumped down the casing by drilling mud until it reaches the bottom plug. Pressure is retained in the well until the cement sets and the top plug, bottom plug, float shoe and any residual cement are then drilled out. It will be appreciated that the bottom plug serves to separate the cement from the fluid already in the casing and the top plug serves to separate the cement from the drilling mud. In addition, the bottom plug should wipe the side of the casing as it travels down the casing to ensure that the cement does not become

contaminated with debris. Similarly, the top plug should remove substantially all traces of cement from the casing since any residual cement could harden and ob¬ struct the passage of other well tools and/or provide a source of unwelcome debris when the well is in produc¬ tion.

At the present time top plugs and bottom plugs com¬ prise a plurality of wipers substantially uniformly spaced along the body of the plug and a rear sealing fin. The wipers are intended to maintain the plug centre in the casing and to clean the sides of the casing. The rear sealing fin is intended to act as a sealing member.

Typically, the wipers are relatively flexible and their leading faces are inclined at about 60° to the longitudinal axis of the plug.

In contrast, the rear sealing fin is relatively thick and its leading face is inclined at about 30° to the longitudinal axis of the plug.

Whilst conventional top plugs and bottom plugs have worked quite adequately in conventional wells they are not entirely reliable in deviated wells.

We have now developed a top plug and a bottom plug which are more particularly, but not exclusively, inten¬ ded for use in deviated wells. In its basic form the top plug and bottom plug are characterized in that in addition to the rear sealing fin they are provided with a second sealing fin.

Advantageously, the second sealing fin is adjacent the rear fin. Preferably, the leading face of the second sealing fin will be inclined to the longitudinal axis of the plug at an angle α of from 10° to 50°, more preferably from 20° to 40°, and most preferably 30°.

Advantageously, the trailing surface of the second sealing fin will be inclined to the longitudinal axis of

the plug at angle β from 30° to 60°, more advantageously from 35° to 55°, and most advantageously 45°.

Preferably, the leading end of the plug is provided with one or more wipers. Advantageously, the leading face of the wipers will be inclined to the longitudinal axis of the plug at an angle Ω of from 45° to 75°, more advantageously from 50° to 70°, and most advantageously 60°.

Preferably, the trailing face of the wipers will be inclined to the longitudinal axis of the plug at an angle δ of from 70° to 90°, more preferably from 75° to 85°, and most preferably 80°.

Advantageously, the plug is provided with at least two wipers and the trailing wiper is spaced from the second sealing fin by a distance greater than the mini¬ mum distance between adjacent wipers.

In a particularly preferred embodiment the leading end of the plug is provided with two wipers which are remote from the second sealing fin. Top plugs and bottom plugs should be distinguished from other types of plugs used in wellbore, for example opening plugs, closing plugs and wiper plugs which are not subject to the demanding physical requirements of top plugs and bottom plugs.

For a better understanding of the present invention reference will now be made, by way of example, to the accompanying drawings, in which: -

Figure 1 is a cross-section through one embodiment of a bottom plug in accordance with the invention; and

Figure 2 is a cross-section through one embodiment of a top plug in accordance with the invention.

Referring to Figure 1 of the drawings, there is shown a bottom plug which is generally identified by the reference numeral 1.

The bottom plug 1 comprises a body member 2 of rigid polyurethane and an outer casing 3 of relatively elastic polyurethane.

The leading end of the bottom plug 1 is provided with two wipers 4 and 5 which are intended to centralize the bottom plug 1 in a casing and to clean the inner surface thereof. The leading face 6 of each wiper 5,6 is inclined to the longitudinal axis X-X of the bottom plug 1 at an angle Ω of 60°. Similarly, the trailing face 7 of each wiper 5,6 is inclined to the longitudinal axis X-X of the bottom plug 1 at an angle δ of 80°.

Turning now to the trailing end of the bottom plug

1, the bottom plug 1 is provided with a rear sealing fin

8. The rear sealing fin 8 is intended to act as a seal and is thus constructed differently from the wipers 4 and 5.

In addition to the rear sealing fin 8 the bottom plug 1 is provided with a second sealing fin 15.

The leading face 16 of the second sealing fin 15 is inclined to the longitudinal axis of the bottom plug 1 at an angle α of 30°. Similarly, the trailing face 17 of the second sealing fin 15 is inclined to the longitudi¬ nal axis X-X of the bottom plug 1 at an angle β of 45°.

The leading face 9 and trailing face 10 of the rear sealing fin 8 are substantially parallel to the leading

face 16 and trailing face 17 of the second sealing fin 15 respectively. It will also be noted that the axial length of the tip 11 and root 12 of the rear sealing fin 8 are substantially the same as the axial length of the tip 18 and root 19 respectively of the second sealing fin 15.

It will also be noted that the axial length of the tip 15 and root 19 of the second sealing fin 15 is greater than at the tip 13 and root 14 respectively of the wipers 4 and 5.

It will be noted that there is a substantial dis¬ tance between the trailing wiper 5 and the second seal¬ ing fin 15 in contrast to conventional plugs where the wipers are disposed along the full length of the plug. The leading end of the bottom plug 1 is provided with a tapered male member 20 with a corrugated outer surface 21 which is more fully described in our co- pending application No. of even date and which is intended to engage in a corresponding socket in the top of a float shoe or float collar to inhibit the bottom plug 1 rotating in the casing when the bottom plug 1 is being drilled out. Similarly, the trailing end of the bottom plug 1 is provided with a recess 22 having a corrugated surface 23 which is intended to accommodate a tapered male member of a top plug as will be explained hereinafter.

A passageway 24 extends axially through the bottom plug 1 and has a portion 25 of enlarged diameter defin¬ ing a shoulder 26 on which a bursting disk 27 rests. Turning now to Figure 2, there is shown a top plug which is generally identified by reference numeral 101. The top plug 101 is very similar to the bottom plug 1 and parts in the top plug 101 having similar functions to parts in the bottom plug 1 have been identified by the same reference numerals as used in Figure 1 but in

the ' 100 ' series .

The only significant differences are that passage¬ way 124 does not extend through the top plug 101 and serves only to reduce the weight of the top plug 101 and facilitate drilling out. In addition the trailing por¬ tion of the top plug 101 is not provided with a recess similar to recess 22 although the leading portion of the top plug 101 is provided with a tapered male member 120 having corrugated outer surface 121 which can be snugly accommodated in the recess 22 of the bottom plug 1.

Various modifications to the plugs described are envisaged. For example, the plugs could be provided with additional sealing fin(s) and or wiper(s) if desired. However, the configuration shown is particularly recom- mended for use in deviated wells. If desired, the seal¬ ing fins could be disposed at opposite ends of the plug.