GB2147641A

GB2147641A - Borehole drilling device

Info

Publication number: GB2147641A
Application number: GB08424960A
Authority: GB
Inventors: Peter John Michael Hogarth
Original assignee: Individual
Current assignee: Individual
Priority date: 1983-10-08
Filing date: 1984-10-03
Publication date: 1985-05-15
Also published as: GB8424960D0; NO844004L; GB8326961D0

Abstract

In order to control the free fall of cement slurry down a borehole casing (10), the float shoe (12) at the leading end of the lowermost casing section, or an annular internal collar in one of the casing sections, preferably the lowest, is fitted with a check valve to hold back the inflowing cement until the pressure in the casing reaches a predetermined value. The check valve comprises a spring-biassed piston (30) and the cross-sectional area of the flow passage (38) past the piston when the valve is open is preferably of the order of four times the area of the inlet (20) to the check valve. <IMAGE>

Description

SPECIFICATION Borehole Drilling Device This invention relates to devices for improving the pumping of cement into boreholes.

In drilling a borehole, the borehole is first formed and a casing is inserted to line the hole. The casing is made up of a number of cylindrical sections which are passed down the hole in sequence and which are screwed together end-to-end. As one moves down to lower depths the diameter of the casing is reduced. It is therefore the practice to run a number of lengths of casing of constant diameter into the hole, then to pump cement down the casing, out of the end of the casing and thence into the gap between the borehole wall and the outer wall of the casing to seal the structure and hold it in place, and then when this cementing operation is complete to pass further cylindrical sections of casing of reduced diameter down through the first casing section and to screw them together so that they extend downwardly from the first section.These procedural steps are then repeated with reducing diameter casing sections. It is also the practice to provide a shoe at the bottom end of the leading casing section or an internal collar part-way down the length of the leading casing section.

One problem which arises when carrying out such cementation techniques is the problem of "free falling" of the cement which occurs during the initial pumping of the cement slurry down the casing.

Particularly with smaller size casings, from 4 inch (100 mm) diameter up to 133/8 inch (340 mm) diameter, the cement slurry falls freely out of control, and because of the pressures prevailing at the bottom end of the casing the cement tends to form channels as it leaves the bottom of the casing, and these channels, once established, tend to restrict the subsequent flow of cement. This can cause problems in pumping and also results in reduced effectiveness of the cement in holding the casing firmly in position within the borehole.

It is an object of the present invention to provide means whereby the free falling of cement slurry down the casing is controlled.

This is achieved in accordance with the present invention by the use of a check valve either within the float shoe at the bottom end of the leading casing section or within a collar part-way down the length of one of the casing sections. By the use of a check valve within the float shoe or collar the shoe or collar will then hold back the inflowing cement slurry until there has been a sufficient build-up of pressure within the casing to open the valve and thereby allow the cement slurry to flow out of the shoe or collar as a continuously expelled mass.

In one preferred arrangement in accordance with the invention the check valve is arranged to open at a pressure of the order of 500 Ibs per square inch (35 kg/cm2).

One presently preferred embodiment of float shoe and collar incorporating a check valve will now be described by way of example and with reference to the accompanying drawings.

In the drawings: Figure lisa longitudinal sectional view through the end of the casing section equipped with the float shoe and check valve; Figure 2 is an end view of the retainer plate of the checkvalveofthefloatshoe in Figure 1; and Figure 3 is a longitudinal sectional view through a casing section equipped with a collar incorporating a check valve.

Referring to the drawings there is shown in Figure 1 the leading end, i.e. the bottom end, of a section of casing 10 which is equipped internally with a float shoe 12 made of cement. The upper, i.e. left-hand, end of the casing 10 is internally screw-threaded to receive another section of casing and a cylindrical hole is provided through the upper end of the shoe 12. Within the float shoe there is provided a two-part housing 14a, 14b which defines a chamber 16 within which is positioned a ball 18. As shown, the ball 18 rests on fins equispaced around the housing with spaces therebetween for the passage of cement past the ball. A bore 20 extends into the ball chamber 16 for the inflow of cement to the shoe.A sealing ring 22 is fitted in the upper wall of the ball chamber 16 for sealing engagement with the ball 18 when it is displaced upwards in use, i.e. to the left as shown in the drawing.

Also mounted within the shoe is a retaining plate 24 which serves also as a flow control plate. The plate 24 is screw-threaded to an annular sleeve 25 which itself is screw-threaded to the housing portion 14b. As shown in the end view of this plate 24, it is provided with three arcuate flow ports 26 which define the flow area out of the shoe. The retaining plate 24 also comprises a hollow cylindrical spigot portion 28 which extends axially inwardly from the outlet end of the shoe and which defines an outer cylindrical surface on which a hollow piston 30 can slide. The piston 30 comprises a piston head 32, an annular sleeve portion 34 which extends in overlapping relationship with respect to the spigot 28, with O-ring sealing means therebetween, and a central stem 35 which is located at its end remote from the piston head in a central hole through the retaining plate 24.A spring 36 is provided as a biasing means for the piston 30.

The spring 36 is seated at one end against the piston head 32 and is seated at the other end against the internal face of the retaining plate 24. With this construction a flow chamber 38 is defined around the outside of the piston sleeve 34 and spigot portion 28. A breather passage through the piston stem 35 to the inner spring chamber is indicated at 40.

In one preferred embodiment, by way of example, the diameter of the input bore 20 is 2 inches (50 mm), the diameter of the ball chamber 16 is 4 inches (100 mm), the ball 18 has a diameter of 2.5 inches (62 mm), the piston head 32 has a diameter of 4 inches (100 mm) and the chamber 28 has a diameter of 6 inches (150 mm). With these dimensions the cross-sectional flow area through the chamber 38 is approximately 4 times that of the flow area through the bore 20. The output ports 26 in the plate 24 are dimensioned so that the cross-sectional flow area out of the shoe is substantially equal to the flow area through the chamber 38. With such an arrangement, all the pressure can be held within the casing and the displacement of the cement down the casing can be controlled by the rig pumps.

The essence of the device lies in the check valve which is composed of the piston 30 and spring 36.

The spring 36 is preferably a phosphor bronze heat-treated spring which is installed within the float shoe under slight compression. The piston 30 is displaceable against the force of the spring 36 and has a predetermined maximum travel distance between fully closed and fully open positions. The length of travel can be made adjustable. In the case of the preferred embodiment, the dimensions of which were given above, the shoe operates satisfactorily if the spring is such that a starting pressure of 500 Ibs per square inch (35 kg/cm2) is required initially to displace the piston 30 from its seat. The spring-loaded piston 30 can be arranged to have a maximum of 2 inches (50 mm) of travel, with a pressure of 1000 Ibs per square inch (70 kg/cm2) being required to open the piston 30 to its full extent. If the starting pressure is too low, then an aluminium shim ring (not shown) could be fitted under the spring 36 to allow for this.

Although the check valve has been described above incorporated into a float shoe at the bottom end of the casing, it can alternatively be fitted into a collar positioned within one of the casing sections, preferably the lowest casing section, thereby to fulfil the same function of holding back the falling cement until a sufficient pressure has built up in the casing.

Such an arrangement is shown in Figure 3.

Claims

1. A borehole casing section comprising a metal tube equipped with a float shoe at the leading end thereof, and a check valve incorporated in the shoe to preventthe passage of flowing material out of the leading end of the casing section until the pressure in the casing section has reached a predetermined value.

2. A borehole casing section comprising a metal tube equipped with an internal annular collar, and a check valve incorporated in the collar to prevent the passage of flowing material through the collar in the direction towards the outlet end of the casing section until the pressure in the casing section on the inlet side of the collar has reached a predetermined value.

3. A borehole casing section as claimed in claim 1 or 2, in which the check valve is positioned on the central axis of the casing section with a flow passage therethrough, the cross-sectional area of said flow passage through the check valve being of the order of four times the cross-sectional area of an inlet port for material flowing into the check valve from the casing section.

4. A borehole casing section as claimed in any preceding claim, in which the check valve comprises a generally cylindrical housing, a piston displaceable longitudinally within the housing, spring means biasing the piston towards the inlet end of the casing section, a seating surface against which the piston bears when the pressure is less than said predetermined pressure, and retainer means at the outlet end of the check valve against which said spring means bears.

5. A borehole casing section as claimed in claim 4, in which the piston comprises a cylindrical stem portion which has its outer end located in a hole in said retainer means which comprises an apertured plate.

6. A borehole casing section as claimed in claim 4 or 5, in which the housing defines a chamber at the inlet side of the piston and into which the flowing material initially flows, said chamber having a ball valve therein to prevent reverse flow of material towards the inlet end of the casing section.

7. A borehole casing section as hereinbefore described with reference to Figs. 1 and 2 or Figs. 2 and 3 of the accompanying drawings.