GB2251446A - Control valve for well cementing operations - Google Patents

Abstract

A pressure differential valve 1 for preventing freefall of cement during cementing of a casing string comprises a low pressure chamber 10 filled with air at atmospheric pressure formed between O-rings 8b and 8d. Due to the difference in diameter of the O-rings 8b and 8d the hydrostatic pressure in the casing forces valve head 6 into a closed position. The valve 1 opens when a predetermined pressure difference is applied across the valve 1 by cement being pumped into the casing above the valve 1. <IMAGE>

Description

METHOD AND APPARATUS FOR CONTROLLING WELL CEMENTING OPERATION FIELD OF THE INVENTION The invention relates to a method and apparatus for controlling the flow of cementing fluid pumped into a well casing to effect the cementing of the casing in a wellbore by flow downwardly through the casing, out the bottom end of the casing, and upwardly around the casing.

BACKGROUND OF THE INVENTION Substantially every cementing operation faces the problem of cement "freefall". The densities of commonly used cementing fluid substantially exceed that of well fluids and drilling mud which are present within the casing and in the annulus between the exterior of the casing and the wellbore at the beginning of the cementing operation. As a result, when a cementing fluid is pumped into the well casing, the hydrostatic pressure on the well fluids at the bottom of the casing is significantly increased, causing a large rate of return flow of the well fluids upwardly around the casing, such rate of flow being substantially in excess of the flowrate of the cement being introduced into the casing.As a result, the imbalance between the densities of typical cements and muds leads to a period where the heavy column of cement in the casing "falls" away from the surface, and that creates one or more void spaces at the top of the casing. As a result, the surface lines are "on vacuum" and the pumps are pumping against zero gauge pressure. The return rate exceeds the pump rate part of the time and then is less than the pump rate in the later portion of the cementing operation.

The cement "freefall" condition can be aggravated if the pumping of the cement is interrupted for any reason when a portion of cement has been introduced in the wellbore annulus, due to the much higher weight of the cement contained in the casing bore compared to the total weight of mud, well fluids and little or no cement contained in the wellbore annulus.

The condition of freefall leads to the following potential problems: 1. Any air sucked into the casing by leakage during freefall periods leads to frothy cement; 2. High evaporation/low heat loss in cement under vacuum conditions can lead to dehydration of the cement; 3. Lower internal pressure while the vacuum pockets exist leads to higher collapse differential on the casing; 4. Water hammer results when pumping catches up with the cement column, and can cause damage to casing or equipment; and 5. Because the annulus flowrate can be higher or lower than the pump rate, this can result in nonturbulent flow during low flowrate periods while higher annulus flowrates lead to additional backpressure on production formations.

While the freefall problem has existed for many years, the industry has yet to develop a satisfactory solution for eliminating or significantly reducing this problem.

The present invention is directed to each of these problems.

One device that has been designed in an effort to stop the freefall of a cement slurry down a bore hole is described in UK Application No. GB2147641A. What is shown there is a spring-lQaded valve member intended to hold back the cement from freefalling. This is a pressure differential device rather than a pressure ratio device. To essentially hold up the column of cement, this device, if ever practically commer cialized, would require a spring so stiff that there would scarcely be room to insert it within the housing. This type of device does not make use of a low-pressure, variable-volume chamber which creates a pressure ratio device as in the present invention.It is the physical configuration of the apparatus of the present invention that makes it workable as opposed to that disclosed in the UK application which depends solely on the spring to hold up the entire column of cement in a no-flow condition.

Differential shoes and differential collars, such as that available under Models 108-05 and 109-11 from the Baker Line Company, are devices designed to automatically fill the casing as it is run to within 90 percent of the fluid level of the annulus. These devices result in greater buoyancy of the entire string, decreasing stress on threaded connections and on rig equipment. These devices also protect the formation from destructive high-pressure surges as the casing is run in.

Differential fill allows the operator to keep casing moving, thereby eliminating sticking associated with time-consuming surface fill operations. The operator can also circulate at any time while retaining the differential fill capacity. This device has a valve sleeve that is so constructed that its cross-sectional area at the bottom is 90 percent of the crosssectional area at the top. The difference in area between the top and bottom surfaces of the valve sleeve acts to limit the fill-up action so that the fluid level in the casing approximates 90 percent of the fluid level of the annulus. This device was made available through Baker Oil Tools, Inc. and described as Unit No. 7066 in a 1981 catalog.

SUMMARY OF THE INVENTION In accordance with one aspect of this invention, a differential pressure control regulator is installed at the bot tom end of the casing to be cemented, preferably just above a conventional guide shoe. Fluid passes through a constricted annulus defined between a downwardly facing annular sealing surface and a valve head which is axially movable relative to the annular sealing surface. The valve head is mounted on a stem portion which in turn is slidably and sealably mounted in a central bore. The sealing cooperation between the stem portion and the central bore is such as to provide a trap chamber containing air at atmospheric pressure. The result of the existence of such chamber is that the well hydrostatic pressure will force the valve head into engagement with the annular sealing surface when no pumping pressure is being applied at the top of the casing.

When pumping pressure is applied to the top of the casing, as by the introduction of a cementing fluid, the increased pressure will be translated down the column of fluid contained within the bore of the casing and will operate on the head portion of the valve regulator, thus moving the head portion downwardly when sufficient pressure differential exists across the valve. So long as the surface pump pressure plus density imbalance between the casing and the annulus is sufficiently large, the valve will stay open, thus allowing pumping of the cement. By proper sizing of the valve seat, if the cement pumping is interrupted for any reason, the valve head portion will move upwardly to its closed position and the "freefall" of the cement column is prevented.

Those skilled in the art will recognize that the area of the seat must be selected to conform to conditions encountered in the particular well being cemented because of the presence of so many variables. For example, the density of the cementing fluid may vary over a range from 1.1 to 1.7 times higher than the density of the well fluids which are to be displaced by the cementing fluid. The depth of the well, of course, results in higher hydrostatic pressures at the bottom of the casing, thus changing the amount of upward bias on the valve head of the regulating valve. Accordingly, it is necessary to provide from three to five different configurations of seat area, and annular valve heads to accommodate such variations.

A substantial reduction in the number of different sizes may be effected by utilizing two or more of the aforementioned regulating valves connected in series relationship. When so connected, the wells having the higher pressure ratios may be handled by two valves, each having dimensions that would ordinarily not accommodate such high pressure drops.

The reduction in the pressure drop across each of the series-connected regulating valves has the further advantage of reducing the fluid flow velocity through such valves and thus greatly reducing the erosion effects of such high fluid velocity.

In another aspect of the invention, one or more choke means are defined on the casing to provide a pressure drop within the cementing fluid when the cementing fluid is placed in the casing at a predetermined rate of flow. Such provision will abate the "freefall" effect.

Other advantages of the method and apparatus of this invention will be readily apparent to those skilled in the art from the following detailed description, taken in conjunction with the annexed sheets of drawings, on which is shown two preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a vertical sectional view of a flow regulating valve. embodying this invention, with the valve shown in its closed position.

Figure 2 is a view similar to Figure 1, with the valve shown in a first open position.

Figure 3 is a view similar to Figure 1, with the valve shown in a second open position.

Figure 4 is a vertical sectional view illustrating a modification of the regulating valve incorporated in this invention.

Figure 5 is a schematic view showing the location of two regulating valves in series for controlling the cementing of a casing wherein high pressure ratios are anticipated.

Figure 6 is a chart illustrating the relationship of upstream-downstream pressure ratios, cement-mud density ratios, and ratio of cement column to depth of well.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT An embodiment of the present Smrrtion will win be described in detail, by waxy of emnple, Referring to Figure 1, there is shown a differential pressure control regulator 1 embodying the present invention, which can be a choke means. Such an apparatus is serially connected at its upper end by threads la to the bottom end of a well casing (not shown). At a lower end, the threads lb permit connection of the apparatus to a conventional guide shoe (also not shown). Such guide shoe is, of course, provided with fluid passages leading to the exterior of the casing and hence communicating with the annulus between the casing and the well bore.

The differential pressure control regulator 1 has a tubular outer body portion or housing 1c which is provided with internal threads ld for mounting a valve housing 2 within the bore of housing ic. Threads ld are sealed by an O-ring le. Valve housing 2 defines a large internal bore 2a, having internal threads 2b at the upper end thereof. The lower portion of the valve housing 2 is traversed by a central bore 2c, which has a first counter bore and a second internally threaded counter bore 2e at its bottom end. A plurality of peripherally spaced fluid passages 2f are defined in the lower outer wall portions of valve housing 2 and are in communication with the top and bottom ends of the central bore.

A valve seat 4 is provided which is of cylindrical configuration and has external threads cooperating with internal threads 2b of the valve housing 2. The threads 2b are sealed by an O-ring 2g. Valve seat 4 defines a constricted central flow passage 4a in its upper end which is flared outwardly at its lower end as indicated at 4b and terminates in a downwardly facing, inclined annular seating surface 4c, which is disposed immediately above the upper end of the peripherally spaced, axial flow passages 2f.

Valve housing 2 defines a large bore chamber 2h which is disposed intermediate the annular seating surface 4c and communicates with each of the peripherally spaced flow passages 2f. A cylindrical valve head 6 is mounted in chamber 2h by having internal threads 6a secured to the upper end of a cylindrical stem portion 8, which extends downwardly through the central bore 2c on the counter bore 2d. Stem portion 8 is provided with an enlarged shoulder 8a mounting an O-ring 8b which sealably cooperates with central bore 2c. At the bottom end of stem portion 8, a further enlarged shoulder Sc is provided, which mounts an O-ring 8d which sealable cooperates with the counter bore 2d.Due to the difference in diameters in O-rings 8b and 8d, an enclosed chamber 10 is defined between the stem portion 8 and the lower portion of valve housing 2. Chamber 10 preferably contains air at atmospheric pressure which is trapped therein during the assembly of the tool at the well surface.

It will, therefore, be apparent that the valve head 6 is mounted for axial movements relative to the annular valve seat 4c and an annular, upwardly facing inclined seating surface 6b is formed on valve head 6 to seal ably cooperate with the annular seating surface 4c when the valve head 6 is in its uppermost position1 as illustrated in Figure 1. The valve head 6 is biased to this closed position relative to the annular seating surface 4c by the hydrostatic pressure existing at the bottom of the casing, which acts upon the differential pressure area provided by the chamber 10 containing air at atmospheric pressure, or another fluid at a known pressure or a vacuum.Thus, when no fluid pressure is applied to the casing fluid and the wellbore annulus other than the hydrostatic pressure, the valve head 6 will occupy the upward closed position relative to the annular seating surface 4c, and fluid flow through the apparatus 1 is thus eliminated.

Hence, the problem of the freefall of any height column of cementing fluid contained in the casing above the apparatus 1 when the pumping is eliminated is completely solved by the hydrostatic bias applied to maintain the valve head 6 in its closed position. When the apparatus 1 is defined as a choke, it is sized to provide a pressure drop at a predetermined flowrate to thereby abate the freefall of the cement fluid.

Stem 8 has a lower flange 30 which is disposed in bore 32 for relative translational movement. Seal 8d maintains contact with bore 32. Bore 34 is narrower than bore 32 and is separated from bore 32 by tapered surface 36. It can readily be seen that the downstream pressure in the annul us outside the casing is in fluid communication with bore 32 and flange 30. It can also be seen that flange 30 overlays chambet 10 which is filled with a fluid at essentially atmospheric pressure. The majority of the pressure acting on flange 30 represented by arrow 38 acts directly through cylindrical segment 40 of stem 8 and is opposed by the hydrostatic forces upstream in the casing bearing down on valve head 6.However, since the diameter of flange 30 exceeds the diameter 40 of stem 8, a portion of the downstream forces acting in the direction of arrow 38 are solely opposed by the atmospheric pressure in chamber 10. By adjusting the ratio of diameters of flange 30 to the diameter 40 of stem 8, the apparatus of the present invention can be configured to actuate from the closed to the open position with a variety of pressure ratios acting across valve head 6. The configuration as shown in the figures also promotes the ability of the well operator to drill through the internals of tubular body lc to compietely remove the internal valve apparatus after the completion of the cementing operation to permit further well operations.

To facilitate the drillability of the apparatus, it is constructed of materials that will readily lend themselves to being drilled while offering the necessary performance characteristics to make the apparatus function. Such components offer minimal resistance to drilling to facilitate their removal at the completion of the cementing operation.

When the cement pumps are energized at the surface to pump cementing fluid down the bore of the casing, additional downward pressure is applied to the valve head 6, causing the valve head 6 to move downwardly to a first open position illustrated in Figure 2, wherein the annular flow area defined between the stationary downwardly facing seating surface 4c and the upwardly facing inclined surface 6c formed on the top end of the valve head 6 is less than or substantially equal to the flow area through the constricted bore portion 4a.

Obviously, the rate of the fluid flow through the apparatus 1 is determined by the dimensions of the bore area 4a and the aforedescribed annular flow area.

Another unique feature of this invention is that the valve head 6 is capable of being moved downwardly to a second open position, shown in Figure 3, wherein the bottom face 6d of the valve head 6 abuts the bottom end of chamber 2h. In this position, a much larger annular flow area is provided around the valve head 6. Hence, if debris contained in the casing fluids impinges on the annular flow passage defined around the valve 6 in the first open position, the effect of such debris would be to increase the pressure on valve head 6 to move it downwardly toward the aforedescribed second open position wherein the annular flow passage is opened to a sufficient extent to permit reasonable sizes of debris to pass therethrough.

It was previously mentioned that a second internally threaded counterbore 2e is provided at the bottom end of the counterbore 2d. A small choke 12 may be threaded into threads 2e to provide a constricted communication passage between the bottom end of stem 8 and the fluids passing through the peripherally spaced flow passages 2f. Such choke eliminates the possibility of the very'rapid movements of the stem in response to changes in pressure of the fluid passing through the apparatus 1.

It will, therefore, be readily apparent to those skilled in the art that the apparatus 1 heretofore described provides substantially complete assurance that the flow of fluids out of the bottom end of the casing will always be sufficiently limited to prevent the occurrence of cement freefall. Additionally, if the pumping of the cement is interrupted for any reason, the apparatus 1 functions solely in response to hydrostatic pressure to shift the valve head 6 into a closed position, thus preventing any flow out of the casing and eliminating the possibility of.the freefall of the heavier density cement contained in the upper portions of the casing string.

In some installations, it may be desirable to maintain circulation prior to the cementing operation. This can be readily accomplished with the modification shown in Figure 4 wherein a central bore 14 is provided through the head portion 6, which communicates with chamber 2h through radial ports 15 so that fluid circulation may be maintained through the appa ratus 1, even though the head portion 6 is in its upper, closed position. The top end of central bore 14 is provided with an annular ball seat 14a and when it is desired to initiate the cementing operation, a ball B is dropped through the casing to seat on the ball seat 14a and thus close the circulation passage 14 and permit the apparatus 1 to operate in the manner heretofore described.

It will also be apparent to those skilled in the art that the number of variables inherent in the cementing of a plurality of wells would require substantial adjustments of the sizes of the valve head 6, the annular downwardly facing sealing surface 4c, and the diameter of the atmospheric pressure chamber 8. The relationship between these variables has been calculated and the results hereof are indicated graphically on Figure 6. The vertical coordinate represents ratios of the upstream pressure to the downstream pressure, meaning the fluid pressure exerted by the cement column contained in the casing to the fluid pressure existing below the apparatus 1. The horizontal coordinate represents the ratio of the maximum height of the cement column to the wall depth. Obviously, this would have substantial effect on the flow rate of the fluid through the apparatus 1.

The Figure 6 chart indicates that the full range of the ratios of upstream to downstream pressure may be accommodated by utilizing only five different sizes of valve seating surfaces 4c and valve head control surfaces 6c. These five sizes will readily accommodate pressure ratios between 1.13 and 1.33. For higher pressure ratios, the arrangement schematically indicated in Figure 5 may be utilized wherein two of the pressure regulating valves 1 are connected in series in the bottom portion of the casing C above a cement guide 20. With the two regulators in series, pressure ratios between 1.33 and 1.75 can be accommodated and will prevent freefall of the cement for the various ratios of the cement density to mud density ranging from 1.1 to 1.7. From this chart, the desired ratio of upstream to downstream pressure may be calculated for the conditions encountered in a particular cementing operation. Additionally, the need for utilizing two differential pressure control regulators in series for certain combinations of variables is clearly indicated.

Although the invention has been described in terms of specified embodiments which are set forth in detail, it should be understood that this is by illustration only and that the invention is not necessarily limited thereto, since alternative embodiments and operating techniques will become apparent to those skilled in the art in view of the disclosure.

Accordingly, modifications are contemplated which can be made without departing from the scope of the described invention.

Claims (18)

1. An apparatus for preventing freefall of cement during a casing cementing operation, comprising: a body; valve means in said body responsive to ratios of pressure acting on opposing sides of said body; said valve means remaining in a closed position in a nonflow condition; said valve means biased open upon the existence of a predetermined pressure ratio across said valve means.

2. The apparatus of claim 1, wherein said valve means further comprises: a valve head; a valve stem said from said valve head; a valve seat on said body, said valve head engaging said seat to close said valve; said valve stem forming a variable volume fluid chamber between itself and said body; whereupon a portion of any force applied to said valve stem in a direction to force engagement between said valve head and said valve seat to close said valve means is resisted by the pressure in said variable volume cavity to provide decreased resistance to applied forces tending to close said valve means to bias the valve means into a closed position.

3. The apparatus of claim 1, further comprising: low-resistance means operable on said valve means to reduce resistance to forces in a direction tending to close said valve means to keep said valve means closed, holding in the contents of a casing until the pressure ratio of the pres sure inside the casing to the pressure outside the casing exceeds a predetermined value.

4. The apparatus of claim 2, further comprising: a first sealing means operable between said stem and said body as said stem translates with respect to said body; a flange on the end of said stem having a larger cross-sectional area than said stem; a second sealing means operable between the outer periphery of said flange and said body, while said flange translates with respect tq said body; said variable volume cavity, extending between said first and second sealing means; said body extending away from said stem to accommodate said flange and second sealing means; whereupon forces on said flange tending to move said valve head toward said valve seat in part encounter resistance only by the fluid in said chamber, with the remainder of said flange meeting resistance from the casing contents bearing on said valve head.

5. The apparatus of claim 3, further comprising: bypass means in said valve head to selectively permit flow through said valve head, circumventing a seal formed by the contact between said valve head and said seat; plug means actuable remotely from said bypass means to interact with said bypass means to selectively defeat said bypass means.

6. The apparatus of claim 5, wherein: said bypass means is a flowpath having an entrance on a top surface of said valve head and exiting from said valve head at a point on an other side of the contact area between said valve head and said valve seat from said entrance to said valve head; said entrance to said passage formed to retain an object introduced through the casing for selective sealing off of said flowpath.

7. The apparatus of claim 1, further comprising: a plurality of valve head, stem, flange assemblies in at least one body mounted in series employed to selectively control the pressure ratio required to permit flow to commence through said assemblies, wherein said pressure ratio required to operate each individual valve head may be identical or different from the remaining valve heads to selectively obtain the desired overall pressure ratio to initiate movement of the assembly toward the open position.

8. The apparatus of claim 7, wherein each of said valve heads is configured to move toward the open position on a pressure ratio between about 1.03 to 1.35:1 such that when used in tandem, the overall pressure ratio required to move said assembly of valve heads used alone or in tandem can be within the ratio of about 1.03 to 1.8:1.

9. The apparatus of claim 3, wherein said valve means further comprises: a valve head; a valve stem extending from said valve head; an valve seat on said body, said valve head engaging said seat to close said valve; said valve stem forming a variable volume fluid chamber between itself and said body; whereupon a portion of any force applied to said valve stem in a direction to force engagement between said valve head and said valve seat to close said valve means is resisted by the pressure in said variable volume cavity to provide decreased resistance to applied forces tending to close said valve means to bias the valve means into a closed position.

10. The apparatus of claim 9, further comprising: a first sealing means operable between said stem and said body as said stem translates with respect to said body; a flange on the end of said stem having a larger cross-sectional area than said stem; a second sealing means operable between the outer periphery of said flange and said body, while said flange translates with respect to said body; said. variable volume cavity extending tset*#n mld firstarxlse rxl sealing means; said body extending away from said stem to accommodate said flange and second sealing means, whereupon forces on said flange tending to move said valve head toward said valve seat in part encounter resistance only by the fluid in said chamber, with the remainder of said flange meeting resistance from the casing contents bearing on said valve head.

11. The apparatus of claim 10, further comprising: bypass means in said valve head to selectively permit flow through said valve head, circumventing a seal formed by the contact between said valve head and said seat; plug means actuable remotely from said bypass means to interact with said bypass means to selectively defeat said bypass means.

12. The apparatus of claim 11, wherein: said bypass means is a flowpath having an entrance on a top surface of said valve head and exiting from said valve head at a point on an other side of the contact area between said valve head and said valve seat from said entrance to said valve head; said entrance to said passage formed to retain an object introduced through the casing for selective sealing off of said flowpath.

13. The apparatus of claim 10, further comprising: a plurality of valve head, stem, flange assemblies in at least one body mounted in series employed to selectively control the pressure ratio required to permit flow to commence through said assemblies, wherein said pressure ratio required to operate each individual valve head may be identical or different from the remaining valve heads to selectively obtain the desired overall pressure ratio to initiate movement of the assembly toward the open position.

14. The apparatus of claim 13, wherein each of said valve heads is configured to move toward the open position on a pressure ratio between about 1.03 to 1.35:1 such that when used in tandem, the overall pressure ratio required to move said assembly of valve heads used alone or in tandem can be within the ratio of about 1.03 to 1.8:1.

15. A method of preventing freefall of cement when cementing a casing string installed in a fluid-filled borehole, comprising: installing at the base of the casing string a pressure rati6-actuated valve to seal off the casing string in a no-flow situation and until a predetermined pressure ratio acts on opposing sides of said valve to allow flow through the casing and behind it into the wellbore.

16. The method of claim 15, further comprising the step of: mounting said valve to said casing in a manner where it can be readily drilled through at the conclusion of cementing.

17. The apparatus of claim 1, wherein: said valve means is mounted in said body in a manner to facilitate its removal by drilling through said valve means at the conclusion of the cementing operation.

18. The apparatus of claim 10, wherein: said valve means is mounted in said body in a manner to facilitate its removal by drilling through said valve means at the conclusion of the cementing operation.