Underwater cementing method

Abstract

 A method of cementing a casing string in a wellbore that has been drilled below a body of water wherein the wellbore penetrates at least one permeable fluid-bearing formation, the method comprising:
(a) injecting a cement slurry through the casing string (3) and into the annulus (5) formed between the casing string and the wellbore wall thereby displacing any wellbore fluid from the wellbore through a discharge line that is in fluid communication with the annulus wherein the cement slurry is injected into the annulus at a sufficiently high pressure to prevent fluid ingress from the permeable fluid-bearing formation into the annulus;
(b) when the cement has reached a desired height in the annulus above the permeable fluid-bearing formation, isolating the discharge line from the annulus by closing a discharge valve (V3);
(c) opening an isolation valve (V1) that isolates a one-way valve (V2) from the annulus wherein the one-way valve is in fluid communication with a column of liquid that is above the valve and the one-way valve opens when the pressure below the one-way valve falls to below the head pressure of the column of liquid such that the head pressure of the column of liquid maintains the pressure in the annulus adjacent to the permeable fluid-bearing formation at above the pressure of the permeable formation thereby allowing the slurry to set and bond the casing string to the wellbore wall without ingress of fluids into the annulus from the permeable fluid-bearing formation.

Description

[0001] This invention relates to a method of cementing a casing string in an offshore wellbore, typically, from the bottom of the casing string to the seafloor.

[0002] In the drilling of a wellbore under a water body, such as in the drilling of an offshore wellbore under a sea, lake or estuary, it is usually desirable to cement the first casing string in the wellbore from the bottom of the casing string to the floor of the sea, lake or estuary (hereinafter "seafloor"). The first casing string usually extends from the seafloor into the wellbore for several hundred to several thousand feet. Often the upper portion of the first casing string is cemented to a conductor pipe that is positioned in the uppermost portion of the wellbore.

[0003] A problem arises in setting of a cement in the annulus between the first casing string and the wellbore wall in that there may be a loss of pressure in the annulus above the unset cement. Where there is a loss of pressure in the annulus above the unset cement, the pressure in the annulus adjacent to an exposed permeable formation that contains a fluid may be insufficient to prevent fluid ingress into the annulus from the exposed permeable formation. The ingress of fluid can interfere with the setting of the cement. Thus, an object of the present invention is to maintain the pressure above the unset cement thereby preventing ingress of fluids into the annulus.

[0004] Accordingly, this invention is directed to a method of cementing a casing string in an wellbore that has been drilled below a body of water wherein the wellbore penetrates at least one permeable fluid-bearing formation, the method comprising:

(a) injecting a cement slurry through the casing string and into the annulus formed between the casing string and the wellbore wall thereby displacing any wellbore fluid from the wellbore through a discharge line that is in fluid communication with the annulus wherein the cement slurry is injected into the annulus at a sufficiently high pressure to prevent fluid ingress from the permeable fluid-bearing formation into the annulus;

(b) when the cement has reached a desired height in the annulus above the permeable fluid-bearing formation, isolating the discharge line from the annulus by closing a discharge valve;

(c) opening an isolation valve that isolates a one-way valve from the annulus wherein the one-way valve is in fluid communication with a column of liquid that is above the valve and the one-way valve opens when the pressure below the one-way valve falls to below the head pressure of the column of liquid such that the head pressure of the column of liquid maintains the pressure in the annulus adjacent to the permeable fluid-bearing formation at above the pressure of the permeable formation thereby allowing the slurry to set and bond the casing string to the wellbore wall without ingress of fluids into the annulus from the permeable fluid-bearing formation.

[0005] This invention is applicable to any wellbore drilled beneath a body of water, such as a sea, lake, or estuary. For simplicity of description, the wellbore will be referred to as an "offshore wellbore", the body of water as a "sea", the bottom of the body of water as a "seafloor", and the bottom of the casing string as a "casing shoe". The permeable fluid-bearing formation that is penetrated by the wellbore is referred to as an "exposed formation".

[0006] The pressure in the annulus adjacent to the exposed formation initially has two components, the slurry "head" which may be considered the pressure expressed as a height of liquid cement slurry in the annulus above the exposed formation and the pressure applied to the injected slurry. If the applied pressure dissipates, this reduces the pressure in the annulus adjacent to the exposed formation such that the pressure may be insufficient to prevent fluid ingress from the exposed formation. However, the present invention increases the pressure within the annulus adjacent to the exposed formation owing to the additional head pressure provided by the column of liquid.

[0007] The offshore wellbore is an open hole wellbore that penetrates at least one permeable formation that contains a fluid such as water, brine or gas. Optionally, a conductor pipe may have been driven into the upper portion of the wellbore. Accordingly, by "wall of the wellbore" is meant the wall of the open hole wellbore and the inner wall of any conductor pipe that may have been driven into the upper portion of the wellbore. Accordingly, the casing string is either cemented to the open hole wellbore or is cemented to the inner wall of the conductor pipe.

[0008] The casing string that is cemented in the offshore wellbore using the process of the present invention is a surface casing string (first casing string). Typically, the casing string is cemented to the wall of the offshore wellbore from the bottom of the casing string (casing shoe) to the seafloor. Generally, the casing string is positioned in the wellbore and a cement slurry is pumped down the casing string, around the casing shoe and into the annulus formed about the casing string and the wellbore wall. The injection of the cement slurry is terminated when the cement slurry has filled the annulus to a position above the exposed formation. Generally, injection of the cement slurry terminates when the cement slurry has filled the annulus up to the seafloor. Where the wellhead is located at the seafloor, injection of the cement slurry is generally terminated when the cement slurry has filled the annulus up to the wellhead. Typically, injection of the cement slurry is terminated when cement slurry begins to be discharged from the annulus into the discharge line. The cement slurry is then maintained in the annulus and allowed to set and bond the casing string to the wellbore wall while applying a sufficiently high pressure to the cement slurry that there is no ingress of fluid from exposed formation into the annulus. Thus, the pressure in the annulus adjacent to the exposed formation is maintained at above the formation pressure i.e. the pressure within the pore space of the exposed formation. If the pressure starts to fall in the annulus, the one-way valve opens so that fluid flows through the one-way valve from the column of fluid and into the annulus such that the head pressure of the column of liquid is applied to the cement in the annulus. Accordingly, in order to avoid ingress of fluid into the annulus from the exposed formation, the combined head pressure of the column of fluid and of the cement slurry adjacent to the exposed formation should be greater than the formation pressure.

[0009] The column of liquid may be a column of water that is of sufficient height that the hydrostatic head pressure that is transmitted to the unset cement in the annulus prevents ingress of fluid into the annulus from the exposed formation. Typically, the one-way valve may be in fluid communication with the body of water. Typically, the body of water has a depth of at least 200 metres (corresponding to a hydrostatic head pressure of at least 20 bar absolute).

[0010] The column of liquid may also be a column of liquid within an umbilical wherein the umbilical extends from at or near the surface of a body of water to the one-way valve. Thus, the umbilical is connected either directly or indirectly to the one-way valve. Typically, the height of the liquid in the umbilical is selected such that the head pressure of the column of liquid is sufficiently high that when the one-way valve is open, the head pressure transmitted by the column of liquid to the unset cement in the annulus is sufficiently high that the pressure in the annulus adjacent to the exposed formation is sufficient to prevent ingress of fluid into the annulus from the exposed formation. Thus, the pressure adjacent to the permeable formation (i.e. the head pressure applied by the column of liquid and the column of unset cement above the exposed formation) is greater than the pressure of the exposed formation. The liquid that forms the column of liquid within the umbilical may be selected from seawater, lake water, estuarine water and wellbore fluids such as drilling muds or completion fluids.

[0011] The one-way valve that is in fluid communication with the column of liquid is a simple check-valve or non-return valve of the type well known to the person skilled in the art. Examples of suitable valves include a flapper valve and a poppet type valve in the form of a check valve closure element that is biased to a closed position against a valve seat by a coil spring. The person skilled in the art will also understand that the pressure below the open one-way valve will be the head pressure of the column of liquid minus any pressure required to maintain the one-way valve in an open position.

[0012] Once the cement in the annulus has gained compressive strength, the isolation valve may be closed such that the annulus is no longer capable of being in fluid communication with the column of liquid.

[0013] The person skilled in the art will understand that wellbore may be extended after the first casing string has been cementing into the wellbore and that the extended wellbore may be lined using a further casing string. However, the sealing of the annulus formed between the first casing string and the wall of the wellbore prevents loss of pressure within the annulus formed between the second casing string and the wellbore wall.

[0014] According to another aspect of the invention there is provided a system for maintaining the pressure in an annulus of an offshore wellbore during setting of a cement that has been injected into the annulus wherein the annulus is formed between a first casing string and the wall of the offshore wellbore, the system comprising:

a T-piece having a first pipe section that is in fluid communication with the annulus of the offshore wellbore, a second pipe section that is connected to a discharge line and a third pipe section that is either open to a body of water or is connected to an umbilical that contains a column of liquid wherein a discharge valve is provided in the second pipe section, and a one-way valve and an isolation valve that isolates the one-way valve from the annulus are provided in the third pipe section, and when the discharge valve is closed and the isolation valve is opened, the one-way valve opens, in use, when the pressure in the annulus is below the head pressure of the body of water or the column of liquid in the umbilical to allow fluid flow from the body of water or the column of liquid through the

third and first pipe sections of the T-piece and into the annulus.

[0015] The method and system of the present invention are better understood by reference to Figure 1 which is a drawing illustrating the one-way valve when used in an offshore wellbore environment. Figure 1 shows an offshore wellbore 1 having a conductor pipe 2 in the upper portion thereof. A casing string 3 is run into the wellbore 1 from the wellhead 4 to the bottom of the wellbore. A drill string (not shown) may be positioned within the casing string and extends to the lower portion thereof to facilitate the circulating of a cement slurry from a cement injection line (not shown) through the wellhead 4, down the drill string around the bottom of the casing, and into the annulus 5 formed between the casing string 3 and the wall of the wellbore 1 and between the casing string 3 and the inner wall of the conductor pipe 2. Wellbore fluids such as drilling mud displaced by the cement slurry may be discharged from the annulus 5 via valve V3 with valve V1 closed. Pumping of the cement is continued until the cement rises to the top of the annulus 5 (reaches the wellhead 4 at the seafloor) as evidenced by discharge of slurry from valve V3. The injection of the cement slurry is then terminated and the drill string is cleared of cement slurry. Valve V3 is then closed and valve V1 which is provided to isolate the one-way valve V2 is opened. The one-way valve V2 permits fluid flow from the body of water 6 (sea, lake or estuary) into the annulus but no flow of fluids from the annulus to the body of water 6. The one-way valve V2 opens if the pressure in the annulus above the cement falls to below the hydrostatic head pressure of the body of water 6. The person skilled in the art would be able to provide a suitable one-way valve that opens at the desired pre-set pressure. Accordingly, the hydrostatic head pressure of the body of water is transmitted to the annulus thereby re-pressurising the annulus. Re-pressurising of the annulus prevents ingress of fluid from an exposed formation into the annulus during setting of the cement. The one-way valve will close when the hydrostatic head pressure in the annulus above the cement equalises with the hydrostatic pressure of the body of water above the one-way valve. The cement slurry is therefore maintained in the annulus and allowed to set and bond the casing string 3 to the open hole wall of the wellbore or the conductor tubing from the bottom of the casing string (casing shoe) to the seafloor. In addition, any potential flow of fluids that ingress from the permeable formation to outside of the wellbore is blocked by valves V2 and V3.

Claims

1. A method of cementing a casing string in an wellbore that has been drilled below a body of water wherein the wellbore penetrates at least one permeable fluid-bearing formation, the method comprising:

(a) injecting a cement slurry through the casing string and into the annulus formed between the casing string and the wellbore wall thereby displacing any wellbore fluid from the wellbore through a discharge line that is in fluid communication with the annulus wherein the cement slurry is injected into the annulus at a sufficiently high pressure to prevent fluid ingress from the permeable fluid-bearing formation into the annulus;

(b) when the cement has reached a desired height in the annulus above the permeable fluid-bearing formation, isolating the discharge line from the annulus by closing a discharge valve;

(c) opening an isolation valve that isolates a one-way valve from the annulus wherein the one-way valve is in fluid communication with a column of liquid that is above the valve and the one-way valve opens when the pressure below the one-way valve falls to below the head pressure of the column of liquid such that the head pressure of the column of liquid maintains the pressure in the annulus adjacent to the permeable fluid-bearing formation at above the pressure of the permeable formation thereby allowing the slurry to set and bond the casing string to the wellbore wall without ingress of fluids into the annulus from the permeable fluid-bearing formation.

2. A method as claimed in Claim 1 wherein the casing string that is cemented in the offshore wellbore is a surface casing string and the cement slurry is pumped down the surface casing string, around the bottom of the casing string and into the annulus formed about the casing string and the wellbore wall and injection of the cement slurry is terminated when the cement slurry has filled the annulus to a position above the permeable formation.

3. A method as claimed in Claim 2 wherein injection of the cement slurry terminates when the cement slurry has filled the annulus up to the floor of the body of water.

4. A method as claimed in any one of the preceding claims wherein the column of liquid is a body of water having a depth of at least 200 metres.

5. A method as claimed in any one of claims 1 to 3 wherein the column of liquid is within an umbilical that is connected either directly or indirectly to the one-way valve and wherein the umbilical extends from the one-way valve to a position at or near the surface of a body of water

6. A method as claimed in Claim 5 wherein the liquid that forms the column of liquid within the umbilical is selected from seawater, lake water, estuarine water and wellbore fluids.

7. A method as claimed in any one of the preceding claims wherein the one-way valve that is in fluid communication with the column of liquid is selected from a flapper valve and a poppet type valve.

8. A method as claimed in any one of the preceding claims wherein the isolation valve is closed when the cement in the annulus has gained compressive strength.

9. A system for maintaining the pressure in an annulus of an offshore wellbore during setting of a cement that has been injected into the annulus wherein the annulus is formed between a first casing string and the wall of the offshore wellbore, the system comprising:

a T-piece having a first pipe section that is in fluid communication with the annulus of the offshore wellbore, a second pipe section that is connected to a discharge line and a third pipe section that is either open to a body of water or is connected to an umbilical that contains a column of liquid wherein a discharge valve is provided in the second pipe section, and a one-way valve and an isolation valve that isolates the one-way valve from the annulus are provided in the third pipe section, and when the discharge valve is closed and the isolation valve is opened, the one-way valve opens, in use, when the pressure in the annulus is below the head pressure of the body of water or the column of liquid in the umbilical to allow fluid flow from the body of water or the column of liquid through the third and first pipe sections of the T-piece and into the annulus.

Drawing