WO1999036660A1

WO1999036660A1 - Bronze alloy casing centraliser

Info

Publication number: WO1999036660A1
Application number: PCT/GB1999/000033
Authority: WO
Inventors: Alan Leslie Male; Ray Mikolajczyk
Original assignee: RAY OIL TOOL CO Ltd
Current assignee: RAY OIL TOOL CO Ltd
Priority date: 1998-01-16
Filing date: 1999-01-18
Publication date: 1999-07-22
Anticipated expiration: 2000-07-16
Also published as: GB2339584A; GB9800885D0; GB9921792D0

Abstract

A casing centraliser which comprises a tubular body (11) adapted to fit closely about a joint of casing and a plurality of blades (12) extending longitudinally along the outer diameter of the body, the centraliser being formed of a material comprising a bronze alloy.

Description

BRONZE ALLOY CASING CENTRALISER

This invention relates to casing centralisers for use in the completion of oil and gas wells wherein one or more strings of casing are "cemented" within the well bore where the strings have to be centralised and stabilized within the bore .

In the completion of such wells, a cement slurry is pumped downwardly within each casing string and upwardly within the annulus thereabout, and thus between the well bore and the outermost string or between an inner string and the next outer string. Desirably, the cement column is of substantially uniform thickness about the casing, and, in any event, the casing should not lay up against the side of the well bore or outer casing. It is important that the casing be cemented in the center of the well bore in order to make better producing wells, prevent interzonal and intrazonal communication, and provide uniform loading which may prevent collapsed casing in the wells and zones that have a history of collapsing casing before the end like of the well. Properly centered casing is important not only in and across the production zones, but also in other zones to reduce bad cement jobs around protective casing shoe joints, and thus reduce expensive cement squeeze jobs for production and for getting good casing shoe seats.

It is therefore common practice in the industry to dispose so called centralisers along the desired length of the casing. The centralisers are so placed on the casing that after the casing is run in the well bore, the centralisers will be in a position to support the casing off the side of the well bore in zones of interest.

A well bore is not a true vertical hole in the earth, but has areas of directional turns, bends, and vertical deviation changes. When casing is run into the well bore it has to contour with the well bore. The outline of the casing in the well bore may result in several or continuous contact with the well bore, with more severe contour changes resulting in more severe contacts. This realisation makes a supportive casing centraliser a necessary tool towards centering the casing in a well bore to attain the desirable cement coverage around the casing.

Known centralisers which are used in the industry include aluminium centralisers, such as formed of Tenzalloy, and zinc alloy centralisers such as formed from the ZA range of Zinc alloys available from Brock metals. These centralisers, whilst being satisfactory for the purpose for which they were intended are not ideal.

Off shore oil and gas wells are now being drilled faster and deeper than was previously envisaged. In view of this the conditions in which the centralisers are placed are somewhat different to those which were intended when these centralisers were designed. AS the well bore increases in depth, so the working temperature within the well bore increases. In many cases, centralisers are now required which will function reliably in temperatures exceeding 250 °C.

It is therefore an object of the present invention to provide a centraliser which, like those described above may be disposed about a casing joint, but which is of sufficiently rigid construction as to prevent collapse or bending, support the casing in deviated and crooked well bores and yet not unduly restrict the flow of drilling fluids and cement slurrys therepast, whilst operating reliably in increasingly high temperatures. According to one aspect of the present invention there is provided a casing centraliser which comprises a tubular body adapted to fit closely about a joint of casing and a plurality of blades extending longitudinally along the outer diameter of the body, the centraliser being formed of a material comprising a bronze alloy.

Preferably the blades of the centraliser are generally equally spaced apart around the outer diameter of the body.

Preferably also each blade has opposite sides and ends which are tapered outwardly toward one another, and a relatively wide outer surface for bearing against the well bore or an outer casing in which the casing is disposed. Thus, the centraliser is sufficiently strong to prevent collapse or bending out of shape, and instead will maintain the outer bearing surfaces in position to engage the well bore or an outer casing and enable the string to be raised or lowered through obstructions in the well bore. At the same time, the design of the blades provides sufficient cross sectional area between them for the passage of drilling fluids and the cement slurry.

One embodiment of the present invention is shown in the accompanying drawings in which:

Fig 1 is a vertical elevational view of a casing string within a well bore and showing centralisers disposed about adjacent joints of the casing string;

Fig 2 is an enlarged elevational view of one of the centralisers of Fig 1; and

Fig 3 is a cross-sectional view of the centraliser as seen along broken lines 3-3 of Fig 2. Turning now to the Figures and in particular Fig 1, the well bore WB is substantially vertical, although it will be understood that it may deviate a rather substantial amount with respect to the vertical. As can be seen from Fig 1, the diameter of the well bore is uneven throughout its length, with restricted areas at random points along its length. The casing string CS is made up of a plurality of casing joints CJ, each having a box at one end and a pin at the other end for connection to adjacent joints making up the string.

Each of the centralisers is indicated in its entirety by reference number 10 and is disposed about a casing joint for maintaining the joint substantially centered within the well bore so that a cement column of substantially even thickness may form about the casing. It will be understood that the number and spacing of the centralisers along the length of the casing string may vary at the will of the operator of the well.

In any event , such centralisers comprise a tubular body 11 which is constructed to fit closely about the casing joint, as best shown in Fig 3, and a plurality of blades 12 which extend longitudinally along the outer diameter of the body thereto in generally equal spaced apart relation. More particularly, in the illustrated embodiment the body is of circumferentially continuous construction and of sufficient thickness for adequate strength. The side edges of adjacent blades are disposed apart to allow ample fluid passage, and yet provide bearing surfaces of ample width at the wall of the well bore. In order to assemble the centraliser on the casing joint CJ, the body of the centraliser is slipped over the pin end of the joint prior to make up of the pin with a box end on an adjacent casing joint. In the illustrated embodiment of the invention, the body and sleeve are made of a one piece mould casting of a bronze alloy which offers good corrosion resistance to the centraliser, superior strength and hardness along with self lubrication properties and a very low coefficient of friction. Furthermore, such a bronze alloy centraliser is able to withstand temperatures, maintaining the integrity of the alloy at temperatures exceeding 250°C, which more than doubles the temperature rating of any of the alloys presently being used as mentioned above. Additionally bronze alloys are non-sparking which allows the centralisers according to the present invention to be used in potentially hazardous working conditions.

As previously described, the opposite sides 12A of the blades as well as the opposite ends 12B thereof are tapered outwardly toward one another and intersect the edges of a relatively large bearing surface 12C adapted to engage the well bore or outer casing to maintain the casing substantially centered therein. AS will be appreciated, the tapered ends 12B of the blades facilitate movement of the centraliser vertically through obstructions in the well bore. The tapered sides 12A of the blades, on the other hand, provide a wide open area between adjacent blades for the flow of cement slurry therethrough.

In the illustrated embodiment, the centraliser is secured to the casing joint CJ as a desired position along the length thereof by means of set screws 13. As best shown in Fig 3, each set screw extends threadedly through holes in both the blade and the body so as to bear tightly against the outer diameter of the casing joint CJ. Since the screws extend through the thickest portion of the blade, they have the largest possible threaded connection thereto, As shown, two set screws extend through each end of the blade. If, on the other hand, the centraliser is not secured t the casing joint so that it is free to rotate with respect thereto, its axial movement may be limited to stop collars at one or both ends .

AS also best shown in Fig 3, additional holes are drilled through one blade and the sleeve, and a permanent magnet 14 is mounted within the inner end of the hole in the body near the inner bore diameter. The outer ends of the holes are preferably filled with a body 14A of plastics material. As above described, this provides a magnetic field which may be detected by a conventional wire line logging tool within the casing joint.

There are a growing number of wells that now use wire wrapped screen liners for production purposes. In the process of using this method of production these wells can be spotted with acid to prevent the screens from plugging up and also to assist in opening up the pores of the well formation to allow better production rates, so it is extremely important to select the right centraliser material which will resist the acid and to give a guaranteed stand off and thus allowing the well to produce more effectively.

Independent tests have been carried out to evaluate the corrosive effect of an acid solution containing 12% HCL and 3% HF on a bronze alloy centraliser as opposed to a zinc alloy centraliser.

Corrosion Rate Calculation: corrosion rate, mpy = 0.001 inch per year metal loss

Corrosion Rate, mpy 534 W

D A T

Where W = weight loss, mg D = density of specimen, g/cm³

A = area of specimen, in² T = exposure time, hours Results :

1. Tin/Copper Bronze Alloy 115/836 84.287% LG2 Copper ASTM 836

Test #1 200°F

Initial Weight 82.5276 grams

Initial Dimensions 0.486 inches in diameter by 3.072 inches long

Weight Weight Corrosion Remarks

Loss, Grams Loss, % Rate, mpy 6 hours 0.0322 gm 0.04% 69 mpy Slight pitting

12 hours 0.0771 gm 0.09% 83 mpy Slight pitting

24 hours 0.1714 gm 0.21% 92 mpy Slight pitting discolorat ion of metal

Test #2 300°F

Initial Weight 82.0882 grams Initial Dimensions 0.486 inches in diameter by 3.043 inches long

Weight Weight Corrosion Remarks

Loss, Grams Loss, % Rate, mpy

6 hours 0.0719 grams 0.09% 155 mpy Slight pitting

12 hours 0.1123 grams 0.14% 121 mpy Slight pitting, discolorat ion of metal 24 hours 0.1464 grams 0.18% 79 mpy Slight pitting, discolorat ion of metal 2. Zinc Alloy ZA-12

Test #1 200°F

Initial Weight 58.0319 grams

Initial Dimensions 0.495 inches in diameter by 3.038 inches long

Weight Weight Corrosion Remarks

Loss, Grams Loss, % Rate, mpy

6 hours 4.2479 gm 7.32% 13,218 mpy Moderate to heavy pitting

12 hours 4.3897 gm 7.56% 6,829 mpy Moderate to heavy pitting 24 hours 6.2390 gm 10.75* 4 , 853 mpy Moderate to heavy pitting, discolorat ion of metal

Test #2 300°F

Initial Weight 57.8650 grams

Initial Dimensions 0.494 inches in diameter by 3.046 inches long

Weight Weight Corrosion Remarks

Loss, Grams Loss, % Rate, mpy

6 hours 7.5047 gm 12.97= 23,369 mpy Severe pitting

12 hours 8.5750 gm 14.82% 13,351 mpy Severe pitting

24 hours 10.3614 gm 17.91% 8,066 mpy Severe pitting, discolorat ion of metal

The alloy 115/836 was the best overall alloy for both temperatures. The calculated corrosion rates were low for both temperatures. The slight pitting and discoloration were minor compared to other alloys tested under the same conditions. The alloy ZA-12 lost significant weight and integrity especially at 300°F. The coupon turned purple and the metal surface began to flake off. The acid used in the alloy 115/836 test remained relatively clear to light yellow throughout the test. The pH of the acid mixture remained less than 1.0 for the duration of both test. The low pH indicates that the acid was not reacting with the coupons. A slight amount of white residue was noted at the end of Test #2. The acid used in the alloy ZA-12 test had moderate hydrogen evolution which caused the bottles to leak. The acid became cloudy and a black and grey residue was noted. A purple scale formed on the coupons and the pH of the acid mixture increased to 3.4 in both test. The pH increase was a result of the acid attack on the coupon.

A bronze alloy centraliser as described above is particularly suitable for the running of liners into a well bore, and with superior bearing characteristics and low friction values such centralisers will increase the probability of liner rotation within the bore. Furthermore, as described above, such centralisers can be run in wells in which the working temperatures within the well exceed 250°C.

Whilst the embodiment shown in the drawings shows a centraliser having a straight blade configuration it is to be appreciated that a centraliser according to the present invention may have any blade configuration such as, for example, straight blade or spiral blade.

Claims

1. A casing centraliser which comprises a tubular body adapted to fit closely about a joint of casing and a plurality of blades extending longitudinally along the outer diameter of the body, the centraliser being formed of a material comprising a bronze alloy.

2. A casing centraliser according to claim 1, wherein the blades of the centraliser are generally equally spaced apart around the outer diameter of the body.

3. A casing centraliser according to claim 1 or 2, wherein each blade has opposite sides and ends which are tapered outwardly toward one another.

4. A casing centraliser according to any one of claims 1 to 3, wherein each blade has a relatively wide outer surface for bearing against the well bore or an outer casing in which the casing is disposed.

5. A casing centraliser according to any one of claims 1 to 3, wherein the design of the blades provides sufficient cross sectional area between them for the passage of drilling fluids and cement slurry.