NO850982L - PROCEDURE FOR IMPROVED BURNTING. - Google Patents

Description

The present invention relates to the cementing of wells. More specifically, the present invention relates to a method for removing drilling mud and filter cake from the annulus of a well in order to achieve improved cementation.

During the drilling of oil and gas wells, drilling mud is circulated

down the interior of a hollow drill string, through nozzles in a drill bit located at the well of the drill string, and back

up to the surface through the space between the drill string and the wall of the borehole. The drilling mud is usually either water-based or oil-based and contains a selection of components. The primary functions of the drilling mud are to lubricate the drill bit, and to transport rock cuttings to the surface and to maintain a hydrostatic pressure in the borehole that is sufficient to prevent the ingress of formation fluids and thereby prevent blowouts.

Following drilling, casing is cemented into the wellbore to prevent the hole from collapsing and to segregate the penetrated formations. A casing is lowered into the wellbore and a cement slurry is pumped into the annulus between the casing and the wall of the borehole. This is usually done by pumping cement slurry down through the interior of the casing and up through the annulus. When the cement slurry has filled the annular piece to be cemented, pumping is stopped and the cement is allowed to harden.

It is important for the cement to form a strong, continuous casing that binds the casing to the wall of the borehole. The cement should completely surround the circumference of the casing and should run uniformly throughout the vertical length of the annular interval being cemented. If the cement is weak, or if pockets are left in it, several undesirable consequences can result. A poor cementing job will not effectively segregate the formations penetrated by the well drilling, and unwanted communication between the formation can occur, sometimes resulting in the production of unwanted fluids. Production fluid from a petroleum-bearing formation can also flow through the channels in the cement and into another formation where it is lost. It is particularly disadvantageous when the second formation contains a water reservoir. Contamination of the petroleum-bearing formation itself can also occur, such as when salt water forms channels through the cement and flows into the petroleum-bearing formation. Other harmful consequences of a poor cementing job can be the loss of treatment fluids that are pumped down the well to stimulate production.

One of the most common causes of ineffective cementing operations is the failure to displace all the drilling mud and filter cake from the annulus before introducing the cement mud. The filter cake is a layer of solid components collected from the drilling mud, which most commonly builds up on the wall of the wellbore resisting permeable formations. Even relatively small amounts of drilling mud and cake can contaminate the cement mud and cause weak points in the cement. Large amounts can impede the flow of cement slurry and thus prevent the cement slurry from completely surrounding the guide pipe, thereby resulting in channels through the cement.

Great efforts have been made to develop methods and devices to effectively remove drilling mud and filter cake from the annulus so that the cement mud will not be obstructed or contaminated. Numerous prefluids have been developed, some with thickeners, which are pumped through the annulus in front of the cement slurry in an attempt to separate drilling mud and filter cake from the annulus. Changes to the properties of the cement slurry itself have also been tested. A common practice is to use centering splines and scrapers on the casing to scrape filter cake from the wellbore wall as the casing string is lowered into place. Although these methods and apparatus have created some advantages, inefficient cementing operations caused by incomplete displacement of drilling mud and filter cake are still common. When this occurs, expensive auxiliary cementing is often necessary, which brings with it the further costs of loss of income while the well is shut down for auxiliary work.

There is still a strong need in the field for a method for cementing wells that will prevent drilling mud and filter cake from causing weak points and channel formation in the cement. The present invention is aimed at providing

such a procedure.

The present invention is a method for cementing wells, where a fluid is oscillated in the annulus before cementing. The direction of the fluid flow in the annulus is changed at least once. The oscillating flow of fluid removes gelled drilling mud and filter cake more effectively than conventional unidirectional flow. The cement slurry is then pumped into the annulus, displacing the fluid and the cement is allowed to harden.

Fig. 1 is a side view, partly in section, of a well drilling and equipment for practicing the method according to the present invention,

fig. 2 is a partial side view of a selective check valve which may be used in the practice of the method of the present invention, the selective check valve being shown in an open position, and

fig. 3 is a partial side view showing the selective check valve in a closed position, with cement slurry therein.

The present invention is an oscillatory flow method for cementing casing in boreholes. The terminology oscillatory flow refers to the change in the direction of flow of the fluid in the annulus prior to the introduction of a cement slurry to the annulus piece to be cemented. This is in contrast to conventional cementing methods, where the fluid flow in the annulus is unidirectional. By changing the fluid flow in the annulus from a forward direction to a reverse direction or vice versa, at least once, more drilling mud and filter cake can be displaced out of the annulus so that this does not contaminate or obstruct the cement mud that follows. Forward flow refers to flow down the interior of the casing and up the annulus. Flow in the reversed direction refers to flow down the annulus and up the casing.

Reference is made to fig. 1 where the apparatus for practicing the method according to the present invention can be seen. The rig platform 11 and the mast 12 are located on the surface above the well 13. The well has been drilled to a desired depth and the drill string (not shown) has been removed. The casing 14 has been lowered into the borehole and is held in place and is suspended above the bottom of the borehole by the V-belt 10 in the rig platform. The casing passes through the wellhead 15. The casing shown is production casing extending from the surface through the surface casing 16 to the petroleum-bearing formation 17. The surface casing has already been cemented in place by cement 18. Although cementing the production casing has been used to show the method according to the present invention, it should be understood that the method can also be used to cement casing other than production casing, such as surface casing 16 or intermediate casing (not shown).

In addition to the wellhead 15, other surface equipment is used, the cementing head 19 provides fluid communication between the interior of the casing and the lines 20 and 21. The line 20 leads to the pumping unit 22 and is regulated by valves 23 and 24. The line 21 leads to fluid collection tanks (not shown) . The valve 25 regulates flow through the line 21.

Three conduits provide fluid communication with annulus 26. Two of these conduits, conduit 27 and conduit 28, connect to the wellhead. Alternatively, the wires 27 and 28 can be connected to coils in the drill safety valve stack (not shown). Line 27 leads to pump unit 22 and is regulated by valves 29 and 30. Line 28 is regulated by valve 31 and leads to fluid collection tanks (not shown). The third line communicates with the annulus in the mud return line 32, through which drilling mud flows from the annulus to a screen for drilling cuttings (not shown) and the mud tank (not shown). The drilling mud is recycled from the mud tanks to the pump unit. The bore protection valve 33 for the annulus functions to seal off the annulus from the mud return line.

Sludge from the sludge tanks is accessible to the pump unit 22 via line 34, which is regulated by valve 35. There are three other lines leading to the pump unit. Line 3 6 carries pre-flush fluid and is regulated by valve 34, line 38 carries cement sludge and is regulated by valve 39, and line 40 carries displacement fluid and is regulated by valve 41. The tanks containing the pre-flush fluid, cement sludge and displacement fluid are not shown.

Fig. 1 shows the casing and annulus when it is full of drilling mud 42, pre-flush fluid 43, cement mud 44 and displacement fluid 45. At the illustrated stage, the oscillatory phase of the method is underway. Before describing the oscillatory phase, however, it is necessary to describe how drilling mud, pre-flush fluid, cement mud and displacement fluid are introduced to the casing and annulus. In the description that follows, all valves and annulus valves are in the closed position unless otherwise stated.

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When the casing string is assembled and lowered into the wellbore, the wellbore is full of drilling mud from the drilling operation. Drilling mud is left in the well bore after removal of the drill string to control the well. At the bottom of the casing 14 is a selective check valve 46 which allows the drilling mud to flow into the casing as the casing is lowered into the wellbore. The selective check valve allows all fluids to flow in the forward direction, but prevents cement slurry from flowing back into the casing after it has been introduced to the annulus. The selective check valve is described in more detail below. Since drilling mud flows into the casing through the selective check valve as the casing is lowered into the wellbore, the casing is initially full of drilling mud.

As is known to the person skilled in the art, it is the first step in the preparation

of the annulus for cementation called conditioning (conditioning-ing). Conditioning is necessary because drilling mud acquires a greater gel strength when it is allowed to sit for a period of time in a passive state. During construction and placement of the casing string in the wellbore, the drilling mud is not circulated and thus tends to gel. In the conditioning step, drilling mud is circulated in an attempt to break the gel and thereby set as much of the drilling mud and filter cake in motion as possible. However, the conditioning step will not be able to set all the drilling mud and filter cake in the annulus in motion, and thereby some gelled drilling mud and filter cake will remain stationary.

Conditioning is achieved by opening the valves 23, 24 and 35 and the annulus fuse 33 by switching on the pump unit 22. This causes drilling mud to be pumped through the line 20 and into the casing 14. The drilling mud flows downwards through the casing, through the selective check valve 46 and upwards through the annulus 26 and through the mud return line 32. After passing through a screen for drilling cuttings (not shown) and mud tank (not shown), the drilling mud is recycled to the pump unit through the line 34. The drilling mud is circulated in this way for a period of time sufficient to set most of the drilling mud in the annulus in motion. Rig mud pumps (not shown) can be used instead of the pump unit 22 to circulate the drilling mud for conditioning.

When the conditioning step is completed, a first bottom plug 47 is dropped into the casing through the cementing head 19. Some cementing heads are equipped to automatically drop plugs into the casing when a switch is actuated, but others require manual release, necessitating that the pumping unit can be disconnected. The bottom plug 47 is dropped into the casing to separate drilling mud 42 from the pre-flush fluid 43, which is being introduced into the casing. By closing valve 35 and opening valve 37, the pump unit will draw pre-flush fluid from line 36 and will pump it through line 20 and into the casing. The release of the first bottom plug is coordinated with the closing of the valve 35 and the opening of the valve 37 so that the first bottom plug separates the drilling mud from the pre-flush fluid.

The valve 37 remains open until a desired amount of pre-flush fluid has been introduced into the casing. As is well known to those skilled in the art, pre-flush fluid is used to separate drilling mud and filter cake from the annulus before cement mud is introduced. The amount of pre-flush fluid used depends on a number of factors, including the volume of the annular interval to be cemented and the geometry of the wellbore. In fig. 1, the annulus interval to be cemented runs from the bottom of the wellbore to the level marked by lines 52. A large variety of preflushing fluids are available where water is commonly used. As used herein, pre-flush fluid means any fluid other than drilling mud that is pumped into the annulus prior to the introduction of cement mud into the annular interval to be cemented. When the pre-flush fluid is pumped into the casing displace-

es drilling mud downwards through the casing 14 and upwards through the annulus 26 and out through the mud return line 32.

When the desired amount of pre-flush fluid has been introduced to the casing, the second bottom plug 48 is released to separate the pre-flush fluid from the cement slurry. The valve 37 and the annulus fuse 33 are then closed, and the valves 31 and 39 are opened. This switches the pump unit from the preflush fluid to the cement slurry 44, which is drawn through conduit 38. As used herein, cement slurry means any material adapted to bond the casing to the wall of the wellbore. Portland cement slurry is usually the most widely used. The cement slurry is pumped through line 20 and into the casing behind the pre-flush fluid. This continues until the desired amount of cement slurry has been pumped into the casing. Naturally, the amount of cement slurry used depends on the volume of the ring romina slurry to be cemented. As the cement slurry is pumped into the casing, the preflush fluid and the drilling mud are displaced in front of it. Drilling mud is displaced upwards through the annulus, out through conduit 28 and into fluid collection tanks (not shown). Due to the high density of the cement slurry, it may be necessary to partially close the valve 31 to slow the sinking of the cement slurry into the casing.

When the desired amount of cement slurry has been introduced to the casing, the pump unit is turned on and the lines 20 and 38 are washed to prevent cement from hardening therein. A top plug 49 is dropped into the casing to separate the cement slurry in the casing from the displacement fluid that follows. Valve 39 is closed and valve 41 is opened and the pump unit is switched on. This causes the pump unit to draw displacement fluid from line 40 and pump it through line 20 and into the casing. Water or drilling mud is often used as displacement fluid. When displacement fluid is pumped, the cement mud, pre-flush fluid and drilling mud are displaced in front of it.

Located just above the selective check valve 46 near the bottom of the casing is a landing ring 50. The landing ring provides a restriction in the interior of the casing through which bottom plugs cannot pass. When the leading edge of the preflush fluid has been displaced to the bottom of the casing, the first bottom plug 47 will seat against the landing ring. The continued application of pressure by the pumping unit or by the weight of the cement slurry causes a diaphragm (not shown) to rupture in the first bottom plug. This creates opening 51 in the first bottom plug through which the pre-flush fluid can flow. Bottom plugs are well known to those skilled in the art. Although the use of two bottom plugs is preferred, one or both may be omitted.

In fig. 1, the first bottom plug is shown when it has already become and the preflush fluid has been displaced out of the bottom of the casing and up through the annulus as it pushes drilling mud in front of it. The pre-flush fluid is displaced by pumping the cement slurry and displacement fluid as described above up to the aft end of the pre-flush fluid and the second bottom plug 48

is just above the first bottom plug and the landing ring. Thus, most of the preflush fluid is in the annulus, with a relatively small amount left in the casing. This is the time at which the oscillating phase of the method according to the present invention begins. This is also the time shown in fig. 1.

In the oscillatory phase, the direction of the flow of fluid in the casing and the annulus changes at least once. By changing the flow direction of the pre-flush fluid in the annulus, more gelled drilling mud and filter cake will be displaced from it. To change the flow from the forward direction to the reverse direction, valves 23, 24, 31 and 41 are closed and valves 25, 29, 30 and 35 are opened. This causes drilling mud 42 to be pumped through the line 27 and down the annulus. As a result, the preflush fluid 43 changes direction and flows down the annulus, through the selective check valve and up the casing, thereby displacing cement slurry 44 and displacement fluid 45 up the casing. The amount of drilling mud that should be pumped into the annulus primarily depends on the volume of the annulus interval to be cemented. The quantity should not be so large that it displaces the cement slurry all the way up to the top of the casing.

When the desired amount of drilling mud has been pumped into the annulus to provide the reversed flow, the flow direction is changed back to the forward direction. This is done by closing valves 25, 29, 30 and 35 and opening valves 23, 24, 31 and 41. By doing this, the pump unit stops pumping drilling mud down the annulus and starts pumping pretraining fluid down the casing through line 20. If necessary, the valve can 31 be partially closed to slow the flow of drilling mud 42 and pre-fluid 43 through the line 28. This may be necessary due to the high density of the cement mud. Pre-training fluid is pumped into the casing until the aft end of the pre-flushing fluid and the second bottom plug 48 is again just above the first bottom plug 47 and the landing ring 50.

This completes a complete cycle of the oscillatory flow. If necessary, the cycle is repeated a sufficient number of times to adequately clean the annulus of gel-containing drilling mud and filter cake. The cycle is repeated by closing valves 23, 24, 31 and 41 and opening valves 25, 29, 30 and 35 to achieve reverse flow and by closing valves 25, 29, 30 and 35 and opening valves 23, 24, 31 and 41 to achieve forward flow. With each change in flow direction, the pre-flush fluid will further clean the annulus.

When the desired number of oscillatory flow cycles

is complete, cement slurry 44 is introduced into the annulus to cement the casing. With valves 25, 29, 30 and 35 closed and valves 23, 24, 31 and 41 open for flow in the forward direction, displacement fluid is pumped into the casing through line 20. As before, valve 31 may be partially closed to slow the flow rate and thereby slow the degree of sinking of the cement slurry 44 in the casing. As the cement mud sinks, the pre-flush fluid 43 and the drilling mud 42 are displaced up the annulus and out the conduit 28 into fluid collection tanks (not shown). Alternatively, the valve 31 can be kept closed and the annulus fuse 33 instead opened so that the fluids shifted up the annulus will flow out the sludge return line 32.

When the second bottom plug 48 reaches the first bottom plug 4 7 it will sit on this. The first bottom plug is placed on the landing ring 50. As pressure builds up in the casing from pumping displacement fluid, the diaphragm (not shown) in the second bottom plug will rupture. As a result, the cement slurry will be pumped downward through the second bottom plug, through the first bottom plug, through the landing ring and through the selective check valve 46. This forces the cement slurry into the annulus and fills the annulus interval to be cemented. Pumping is continued until the top plug 49 seats on the other bottom plug. The top plug has no diaphragm and thus does not allow the displacement fluid to enter the annulus. When the top plug seats, it detects an increase in pressure at the surface and the pump unit is shut down. All circulation is thereby stopped and the cement slurry is allowed to harden. This completes the cementing work.

Due to the high density of the cement slurry, it will tend to flow back into the casing before hardening.

In conventional cementing methods, this is usually prevented by including a float ring at the bottom of the casing string. Float rings contain check valves which allow fluids to flow in the forward direction out of the casing and into the annulus, but which do not allow reverse flow, such float rings are not suitable for use in the oscillatory flow method of the present invention because reverse flow is desired. Selective check valve 46 is used instead. It allows forward and reverse flow for the drilling mud and preflush fluid, but allows cement mud to flow in the forward direction only. This keeps the cement slurry from flowing back into the casing after it has been introduced into the annulus.

Fig. 2 shows a sectional side view of the selective check valve 46. The selective check valve is a tubular member threaded into the bottom of the casing 14. A constriction 60 in the selective check valve serves as a seat for the float 61, which is spherical in shape. A float basket 62 limits the downward movement of the float. The float curve is conical in shape and has its tip 63 at the lower end. The tip of the float curve is solid, while the cone portion has recesses 64 which allow fluid to flow in and out of the selective check valve. The float 61 has a density less than the density of cement mud, but is greater than the density of drilling mud, pre-flush fluid and displacement fluid. The intermediate density of the float allows the selective check valve

to distinguish between cement slurry and other fluids and thereby prevent reversed flow of only cement slurry.

All fluids, including the cement slurry, can flow in a forward direction out of the casing through the selective check valve and into the annulus 26. During forward flow, the float rests at the tip of the float curve while fluid flows downward through the constriction 60 and recesses 64. During reverse flow, all fluids that are less dense than the float to flow upwards through the recesses and constriction. The massive tip of the float curve shields the float ear from upward forces of the flowing fluid, thereby preventing the float from being pushed up against the constriction.

This prevents the pre-flush fluid from flowing back up the casing during the reversed flow of the oscillatory phase as described above.

Fig. 3 shows the selective non-return valve full of cement sludge 44. As discussed above, cement sludge is pumped through the selective non-return valve and into the annulus after the oscillatory phase. Because the float 61 is of less density than the cement slurry, it floats upwards and seats against the constriction 60. This prevents the cement slurry from flowing out of the annulus and back into the casing. In the unfortunate event that displacement fluid bypasses the top plug and is pumped into the annulus, the check valve will allow the displacement fluid to be forced out of the annulus and back up the casing by the weight of the cement slurry. The float, which is less dense than the displacement fluid, will allow such reverse flow, but will stop reverse flow when the cement slurry is fed back to the selective check valve. Thus, the selective check valve allows reverse flow as desired, but prevents unwanted reverse flow of cement slurry back to the casing.

Although the described selective check valve is preferred, it is not the only flow control device suitable for use in practicing the method according to the present invention. Regardless of which flow regulation device is used, it must allow forward and reverse flow of the fluid oscillated in the annulus, but must only allow forward flow of the cement slurry. Another suitable arrangement (not shown) would be the use of a latch-down top plug between the cement slurry and the displacement fluid. This type of device is well known to the person skilled in the art. The snap lock type top plug snaps onto a locking plate near the bottom of the casing when cement slurry has been pumped into the annulus. This prevents cement slurry from flowing back into the casing. The locking face used to secure the snap lock type top plug should be arranged to allow the bottom plugs to pass through. Alternatively, the well plugs can be arranged so that the first bottom plug snaps onto the locking surface, the second bottom plug snaps onto the first bottom plug and the top plug snaps onto the second bottom plug. This will prevent reverse flow of cement slurry back into the casing.

Another device (not shown) that can be used instead of the selective check valve described above is a stage cementing coiler. These devices are well known to those skilled in the art and have a sliding sleeve with openings therethrough which can be created with openings in the outer part of the ring when the openings are aligned allowing both forward and reverse flow. The stage cementing annulus will initially be in the open position for oscillatory flow and for pumping cement slurry into the annulus. Bottom plugs manufactured for use with step cementing rings are arranged to pass through the step cementing ring and are called bottom bypass plugs. The step cementing ring is installed in the casing string above the landing ring. The bottom bypass plugs seat against the landing ring after passing through the step cementing ring. To prevent reverse flow of cement slurry back to the casing, the sliding sleeve of the step cementing ring is forced into the closed position by downward pressure on the top plug, which separates the cement slurry from the displacement fluid. Unlike bottom bypass plugs, the top plug does not pass through the step cementing ring.

Another device (not shown) which can be used to regulate the flow is a ball actuated valve such as "Circulating Flexiflow Fill-Up Coilar, Product no. 161-03 from Bakerline Division of Baker International Corporation". This ring is installed near the bottom of the casing and allows both forward and reverse flow until the recoil ball is introduced to the ring. With the check ball in place, only forward flow is allowed. The ball is introduced to the ring by placing it in the cement slurry. As the cement slurry is pumped through the ring and into the annulus, the recoil ball is forced through a hole in a rubber diaphragm in the ring. The rubber diaphragm will not allow the ball to be pushed back out of the ring. If the cement slurry starts to flow back into the casing, the check ball sits on the rubber diaphragm to shut off the flow. Another ball actuated valve that can be used is the "Fitrol Insert Valve, Product No. FY14 from B&W Incorporated".

Reference is again made to fig. 1, recalling that the oscillatory flow method described above specifies pumping of drilling mud 42 through conduit 27 and into annulus 26 to achieve reverse flow. If the annular interval to be cemented extends a long distance relative to the length of the wellbore, this can result in drilling mud being reintroduced into the interval. It must be noted that re-introduction of drilling mud into the annular interval to be cemented may to some extent again contaminate the interval with filter cake and gelled drilling mud. For this reason, if conditions permit, it would be more desirable to displace all the drilling mud from the annulus before the start of the oscillatory phase, and to achieve reverse flow by pumping pre-flush fluid 43 into the annulus through line 27, rather than by pump drilling mud. However, the conditions may not allow this due to the fact that a sufficient hydrostatic must be maintained in the annulus at any time before hardening of cement to prevent blowouts. Pre-flush fluids are usually of lower density than drilling mud and thus cannot provide sufficient hydrostatic pressure.

If the conditions are such that the pre-flush fluid can be provided with sufficient hydrostatic pressure, it may be desirable to separate the drilling mud all the way out of the annulus and to achieve reversed flow by pumping pre-flush fluid down the annulus. In such a case, only pre-flush fluid will be in the annulus during the oscillating phase. Two modifications to the steps described above are necessary to accomplish this. First, the amount of separation fluid 43 that is pumped into the casing and the annulus in front of the cement mud 44 must be sufficient to displace all drilling mud 42 out of the annulus through the mud return line 32. Second, instead of opening the valve 35 during reverse flow, the valve 35 is held closed and valve 37 is opened instead. As a result, preflush fluid will be pumped to the annulus instead of the drilling mud.

By only having pre-flush fluid in the annular interval to be cemented during the oscillatory phase, displacement of gelled drilling mud and filter cake can be maximized. However, this does not mean that the use of drilling mud to achieve oscillatory flow will be ineffective. Although reintroduction of drilling mud into the annular interval to be cemented may result in some recontamination of the interval with filter cake and gelled drilling mud, these contaminants should be quickly separated by the separation fluid during the forward flow that occurs when the cement mud is pumped into the annulus. The drilling mud should be separated immediately because very little will have had a chance to gel or form a filter cake. The drill lamb will not gel because it is not left in a quiescent state for a sufficiently long time during the oscillatory phase. The filter cake should be separated immediately because very little will accumulate on the wall of the wellbore during the relatively short period of time that the drilling mud is in the annular interval to be cemented during the oscillatory phase.

Although oscillation of the pre-flush fluid in the annular interval to be cemented is preferred, the oscillatory flow method according to the present invention can also be used by using drilling mud. It is that the drilling mud itself can oscillate in the annulus to loosen and separate filter cake and gelled drilling mud. In this case, the differential fluid will remain in the casing during the oscillatory phase. After the oscillatory phase, the preflush fluid is pumped in a forward direction through the annular interval ahead of the cement mud to flush the drilling mud therefrom. Although the use of pre-flushing fluid between the drilling mud and cement mud is preferred, it can be omitted in the case where drilling mud is oscillated to clean the annulus. Even without the use of a pre-flush fluid, oscillating the drilling mud should be able to provide better cleaning of the annulus than conventional unidirectional flow methods using pre-flush fluids. As will be discussed below, oscillatory flow has several distinct advantages over unidirectional flow, which explains this surprising result.

To practice the oscillatory flow method according to the present invention using drilling mud, only a few modifications to the procedure described above are needed. The steps for performing the oscillatory flow method using drilling mud are identical to the steps described above for the case where pre-flush fluid is used, with the exception that pre-flush fluid is not pumped into the annulus until after the oscillatory phase. Thus, after a sufficient amount of displacement fluid has been introduced into the casing to displace the preflush fluid 43 and first bottom plug 47 downward until just above the landing ring 50, the valves 23, 24,

31 and 41 and valves 25, 29, 30 and 35 are opened. This initiates reverse flow by pumping drilling mud 42 downward into the annulus through line 27. To change the flow from reverse to forward, valves 25, 29, 30 and 35 are closed and valves 23, 24, 31 and 41 are opened. This causes the displacement fluid to be pumped downwards into the casing, which causes the drilling mud to change direction and flow upwards in the annulus. After the desired number of oscillatory flow cycles have been performed in this manner, the forward flow is used to introduce preflush fluid 43 and cement slurry 44 to the annulus. When the first bottom plug 47 reaches the landing ring 50, it will sit on this. The continued build-up of pressure from the pump unit 22 ruptures a diaphragm (not shown) in the first bottom plug 47 and the preflush fluid is forced through the selective check valve 46 and into the annulus displacing drilling mud 42 ahead of it. Continued pumping causes the second bottom plug 48 to sit on the first bottom plug 47 and break, and cement slurry 44 is pumped through the selective check valve 46 and into the annulus until the top plug 49 sits on the landing ring. The pumping unit is then switched on and the cement slurry is allowed to harden.

In conventional cementing operations, the drilling mud flow and the pre-flush fluid flow are unidirectional. Generally, the flow is in the forward direction, that is, downward through the casing and upward through the annulus. By using conventional unidirectional flow, significant quantities of gelled drilling mud and filter cake are often recirculated and left in the annulus, thereby preventing or contaminating the cement mud when it is pumped into place. As discussed above, the end result can be an ineffective cementing job, requiring auxiliary cementing to correct the situation. Recirculation of the gelled drilling mud and filter cake in conventional cementing operations means that the force exerted on the immobile gelled drilling mud and filter cake by the unidirectional flow fluids was insufficient either to erode this stationary material or to overcome the forces that tended to hold the material in place .

The ability of the oscillatory flow method according to the present invention to effectively displace gelled drilling mud and filter cake from the annulus can be attributed to the many distinct advantages that oscillatory flow has over conventional unidirectional flow.

Consideration must be given to the effect of the flowing fluid, either drilling mud or pre-flush fluid, on the stationary gelled drilling mud and filter cake under conventional unidirectional flow. Since the flow is unidirectional, erosion of the stationary material by the flowing fluid will tend to streamline the material. Streamlining reduces the cross-sectional surface area of the stationary material on the side facing the flow, and thus reduces the force exerted on the material by the flowing fluid. As a consequence of streamlining, the stationary material acquires an asymmetry from top to bottom. The oscillatory flow method according to the present invention takes advantage of this asymmetry. When the flow direction of the fluid in the annulus changes, the fluid exerts pressure against the non-streamlined side of the stationary material. The non-streamlined side presents a larger cross-sectional surface area for the flowing fluid, and thus the flowing fluid exerts a greater force on the stationary material, thereby increasing erosion and displacement.

A second advantage of the oscillatory flow over conventional unidirectional flow is that periodic change of flow direction contributes to a fatigue-like failure of the stationary filter cake and gelled drilling mud. When materials are exposed to an oscillatory stress, they fail at a lower stress value than when exposed to a uniformly directed stress. Because of this phenomenon, the oscillatory flow method of the present invention allows the available pumping power to more rapidly cause the stationary material in the annulus to fail and thereby be displaced.

The oscillatory flow method has another advantage over conventional cementing operations that use unidirectional forward flow. Gelled drilling mud and filter cake often have densities greater than the densities of the drilling mud and the pre-flush fluid used to displace them. The net gravitational force acting on the stationary material in the annulus is the weight of the material minus its buoyancy. The buoyancy is determined by the relative densities of the stationary material and the flowing fluid. If the stationary material has a greater density than the flowing fluid, then the net gravitational force acting on the material will be downwards. Since the unidirectional forward flow in conventional cementing operations causes the fluid to flow upwards in the annulus, the upward force exerted on the stationary material by the flowing fluid is opposite to the downward acting net gravitational force. In contrast to the oscillatory flow method of the present invention, the downward force exerted by the fluid on the stationary material during reversed flow is greater than the downward acting net gravitational force. This increases the likelihood of overcoming the forces that tend to hold the stationary filter cake and gelled drilling mud in place.

The oscillatory flow method according to the present invention has a further advantage over conventional cementing methods. With conventional cementing methods, larger volumes of pre-flush fluids must be pumped through the annulus to increase the contact time between the pre-flush fluid and the gelled drilling mud and filter cake. With the oscillatory flow method, the contact time can be increased just by repeating the oscillatory cycle without the need to use larger amounts of pre-rinse fluids. This advantage is particularly important if the volume of preflush fluid used is limited by cost or hydrostatic pressure considerations.

Attempts have been made to compare the effectiveness of the oscillatory flow method of the present invention with the effectiveness of conventional methods using unidirectional flow. The effectiveness was measured expressed in the displacement efficiency of the methods. Displacement efficiency is defined as the percentage of volume of the annulus at the test apparatus that is filled with cement at the methods. The volume of the annulus that is not filled with cement is filled with undisplaced filter cake and gelled drilling mud.

The test apparatus included a 3.05 m long cylinder of permeable compacted sand with an inner diameter of 16.5 cm, reinforced internally by a perforated tube casing. The consolidated co-cylinder was prepared using an epoxy resin and sand, and was constructed to simulate a permeable formation. This cylinder of compacted sand with its perforated tube housing was placed inside a filtrate jacket containing water and allowed to become saturated with water. The filtrate jacket was enclosed in a heating jacket containing heating oil, which was used to raise the temperature of the apparatus and thereby simulate conditions downhole, a 4.57 m long section of a steel casing with a 1.52 m outer diameter was placed in the cylinder with compacted sand to form an annulus with a volume of about 26.5 L. The casing was centered in the cylinder of sand to provide a radially symmetrical annulus between the casing and the sand.

Drilling mud and cement mud were used in all the experiments. Several of the trials also used pre-rinse fluid. The compressed sludge was 1.917 kg/l sludge. Each drum of drilling mud contained the following: 110.90 L of fresh water, 185.52 kg of barite, 6.80 kg of bentonite, 1.81 kg of lignosulfonates, 0.11 kg of carboxyl methyl cellulose and 0.76 L of a 20% solution of sodium hydroxide. The cement slurry was 2.01 kg/l of slurry with each 28.60 1 of the slurry containing the following: 42.64 kg of API Class H Portland cement, 14.80 1 fresh water, 0.50% retarder and 0.50% dispersant . The pre-rinse fluid was fresh water.

Each run was initiated by circulating the drilling mud down the casing and up the annulus for a period of 1 hour at a flow rate of 3 barrels/minute and a temperature of 82.8°C. This part of the experiment was to simulate the circulation of drilling mud during drilling. Due to the permeability of the compacted sand, filter cake formed in the annulus under this part of the sample. After one hour, circulation was stopped and the temperature was raised to 93.3°C. The drilling mud was left in the annulus in this warm, calm state for a period of approx. 24 hours to simulate the conditions that exist in a wellbore before the start of a cementing operation. Due to the quiescent state, the drilling mud became gel-like. After the gelation period, circulation was resumed at a rate of 3 barrels/minute and at 82.2°C for a period of 1 hour to simulate the conditioning step of the cementing operations.

Five tests (numbered 1-5) were carried out to determine the displacement efficiency for conventional unidirectional flow of drilling mud in the annulus. In these tests, after circulating drilling mud for one hour to simulate conditioning, 1,589.7 liters to 3,179.4 liters of cement slurry was pumped down the casing and up the annulus at a rate of 4 barrels/minute. The temperature was then raised to 110°C and the cement slurry was allowed to cure for a period of at least 24 hours, thereby cementing the casing of the cylinder with consolidated sand. Together, the casing, the cement and the cylinder of consolidated sand with its perforated casing is called the test section. After curing, the sample section was removed from the test apparatus and was divided into seven sections to allow displacement efficiency measurements. The results for the individual samples are given in the table below. The average displacement efficiency for the five samples with unidirectional flow of drilling mud was 64.2%. This means that 64.2% of the annulus was filled with cement. The remaining 35.8% was filled with undisplaced filter cake and gelled drilling mud.

Two experiments (numbered 6 and 7) were carried out to test the displacement efficiency for oscillatory flow of drilling mud in the annulus. In these experiments, after circulating drilling mud i

the forward direction for an hour to stimulate conditioning,

the flow was changed to the reversed direction and ten barrels of drilling mud were pumped down the annulus and up the casing. The flow direction was then changed back to the forward direction and ten barrels of drilling mud were pumped down the casing and up the annulus. In Experiment 7, the cycle was repeated one more time by changing the flow direction to reverse and then forward, pumping ten barrels of drilling mud in each direction. Thus, drilling mud was oscillated in the annulus for one complete cycle of oscillating flow in test 6 and for two complete cycles in test 7. A flow rate of 4 barrels per minute was spent during the oscillatory phase. After this oscillatory phase, 1589.7 1 of cement slurry was pumped down the casing and up the annulus at a rate of four barrels per minute. The temperature was then raised to 110°C, whereupon the cement slurry was allowed to harden for at least 24 hours and the sample section was removed and cut into seven sections. Inspection of these sections yielded surprising displacement efficiencies of 83.8% for trial 6 and 94.4% for trial 7, for an average displacement efficiency of 89.1%. This compares very favorably with the average 64.2% in the unidirectional flow tests.

Tests were also conducted to compare unidirectional flow with oscillatory flows where a preflush fluid was used. Three trials (numbered 8-10) were performed using unidirectional flow of pre-rinse fluid. These tests were conducted in the same manner as the five experiments described above for unidirectional flow of drilling mud, except that 10 to 50 barrels of preflush fluid were pumped down the casing and up the annulus at a rate of 4 barrels/minute after the conditioning step and before introduction. of cement slurry to the annulus. These tests gave an average displacement efficiency of 80.8% for unidirectional flow of pre-rinse fluid. Due to the use of the preflush fluid, these results were much better than the 64.2% displacement efficiency that resulted from the five tests with unidirectional flow of drilling mud alone. However, despite the improvement brought about by the use of pre-flush fluid, the displacement efficiency for unidirectional flow still fell short of the 89.1% displacement efficiency found for oscillatory flow of the drilling mud eels.

Two experiments (numbered 11 and 12) were performed using oscillatory flow with a pre-flush fluid. These experiments were identical to the two experiments described above for oscillatory flow of drilling mud alone, except that ten drums of pre-flush fluid were pumped with each change in flow direction, instead of into drums of drilling mud. Thus, pre-flush fluid was oscillated in the annulus for one complete cycle of oscillatory flow in experiment 11 and for two complete cycles in experiment 12. In experiment 11, a displacement efficiency of 94.5% was measured. In experiment 11, the displacement efficiency was 90.2%, which thus gives an average efficiency of 92.3%. This exceeds the 80.8% efficiency found for unidirectional flow of pre-fluid fluid, and even exceeds the 89.1% efficiency found for oscillatory flow of drilling mud alone. The following is a table containing the results of all the experiments discussed above:

These experiments, which were carefully designed to simulate real well cementing conditions, demonstrate that the oscillatory flow method of the present invention is superior to the unidirectional flow used in conventional cementing operations. These experiments really show the surprising result that oscillatory flow of drilling mud without the use of a pre-flush fluid has a better pretraining efficiency than unidirectional flow with the use of a pre-flush fluid. By increasing the displacement efficiency with the method according to the invention, more filter cake and gelled drilling mud can be displaced out of the annulus. As a consequence, there is less obstruction and contamination of the cement slurry, resulting in a strong, uniform sleeve of cement that surrounds the casing and bonds it to the wall of the wellbore. Problems associated with ineffective cementing work such as communication between formations, production of unwanted formation fluids and loss of well treatment fluids are thus minimized by the method according to the invention and therefore fewer expensive auxiliary cementing works will be necessary.

While the present invention is subject to many variations, modifications and changes in detail,

it is intended that everything discussed above and shown in the attached seals is intended to be illustrative and not in a limiting way. E.g. the oscillatory phase can be performed before pumping cement slurry into the casing. Such variations, modifications and changes in details are included within the scope of the invention as defined by the following claims.

Claims (14)

1. Method for cementing a wellbore having a casing extending longitudinally therein which provides an annulus between the casing and the wall of the wellbore, characterized in that the method includes: (a) introducing a fluid into said annulus; (b) causing the fluid to flow in said annulus; (c) changing the flow direction of the fluid at least once; (d) introducing a cement slurry into at least one interval of said annulus, thereby displacing said fluid from the interval; and (e) allowing the cement slurry to harden. 2. Method according to claim 1, characterized in that the fluid is drilling mud. 3. Method according to claim 2, characterized in that the fluid is a pre-rinse fluid. 4. Method according to claim 3, characterized in that the annulus contains drilling mud before the introduction of the pre-flushing fluid into the annulus, where the drilling mud is at least partially displaced from the annulus by the pre-flushing fluid. 5. Method according to claim 1, characterized in that the flow direction of the fluid is changed at least four times. 6. Method for cementing a wellbore which has a casing which runs longitudinally therein and which provides an annulus between the casing and the wall of the wellbore, where the annulus contains drilling mud, characterized in that the method includes: (a) causing the drilling mud to flow in said annulus; (b) changing the flow direction of the drilling mud at least once; (c) introducing a cement slurry to at least an interval of said annulus to thereby displace drilling mud from the interval; and (d) allowing the cement slurry to harden. 7. Method according to claim 6, characterized in that the flow direction of the drilling mud is changed at least four times. 8. Method according to claim 6, characterized in that the pre-flush fluid is introduced into the interval after step (b) and before step (c), where the pre-flush fluid displaces the drilling mud from the interval and where the pre-flush fluid is displaced from the interval during step (c). 9. Method for cementing a wellbore at a well site, where the wellbore has a casing running longitudinally therein which provides an annulus between the casing and the wall of the wellbore, where the casing has an opening at its lower end which allows fluid communication between the annulus and the inner of the casing, where the well site has devices to circulate fluid downwards through the casing and upwards through said annulus and to circulate fluid downwards through the annulus and upwards through the casing, where the casing and annulus contain drilling mud, characterized in that the method includes: (a) pumping a cement slurry downward through the casing to thereby displace drilling mud downward through the casing and upward through the annulus; (b) pumping a displacement fluid downward through the casing to thereby displace the cement slurry downward towards the lower end of the casing, thereby displacing the drilling mud upwards through the annulus; (c) pumping drilling mud downwards through the annulus to thereby cause the drilling mud in said annulus to change direction and flow downwards through the annulus and upwards through the casing; (d) pumping displacement fluid downward through the casing; thereby causing drilling mud to change direction and flow downwards through the casing and upwards through the annulus; (e) repeating steps (c) and (d) a desired number of times in an alternating sequence; (f) pumping a sufficient amount of displacement fluid downward through the casing to cause cement mud to be displaced out of the casing and into at least one interval of the annulus, thereby displacing drilling mud from the interval; and (g) allowing the cement slurry to harden. 10. Method according to claim 9, characterized in that pre-flush fluid is pumped into the casing before step (a), where the pre-flush fluid is displaced out of the casing and through the interval during step (f). 11. Method according to claim 9, characterized in that drilling mud is circulated for conditioning before step (a). 12. Method for cementing a wellbore at a well site, where the wellbore has a casing pipe running longitudinally therein which provides an annulus between the casing pipe and the wall of the wellbore, where the casing pipe has an opening at its lower end which allows fluid communication between the annulus and the interior of the casing, where the well site has devices for circulating fluid downwards through the casing and upwards through the annulus and for circulating fluid downwards through the annulus and upwards through the casing, where the casing and annulus contain drilling mud, characterized in that the method includes: (a) pumping a pre-flush fluid downward through the casing, thereby displacing drilling mud downward through the casing and upward through the annulus; (b) pumping a cement slurry downward through the casing to thereby displace the preflush fluid downward through the casing and upward through the annulus; (c) pumping a displacement fluid downward through the casing to thereby displace the cement slurry downward towards the lower end of the casing, thereby displacing the preflush fluid upwards through the annulus; (d) pumping drilling mud downwardly through the annulus thereby causing the preflush fluid to change direction and flow downwardly through the annulus and upwardly through the casing; (e) pumping displacement fluid downward through the casing to thereby cause the preflush fluid to change direction and flow downward through the casing and upward through the annulus; (f) repeating steps (e) and (f) a desired number of times in an alternating sequence; (g) pumping a sufficient amount of displacement fluid downward through the casing to cause the cement slurry to be displaced out of the casing and into at least one interval of the annulus, thereby displacing preflush fluid from the interval; and (h) allowing the cement slurry to harden. 13. Method according to claim 12, characterized in that the drilling mud is circulated for conditioning before step (a). 14. Check valve arranged for use in a casing string and having a passage through which fluid can flow, characterized in that the check valve has a float which is placed below said passage and which is arranged to block the passage and prevent flow when in contact with this, where the float has a density greater than water and less than the cement slurry, where the non-return valve has a basket that limits the float's movement downwards, where the basket has a solid lower part and an upper part with at least one opening through which fluids can flow, where the massive lower part is arranged to shield the float from the fluid forces flowing upwards through said non-return valve.