Patent Grant
7717179
The present invention relates to an apparatus and associated method for setting plugs, in boreholes of oil, gas, water or geothermal wells or the like.
Setting cement plugs in a borehole is a common oilfield operation. A cement plug involves a relatively small volume of cement slurry placed in a borehole for various purposes: (i) to sidetrack above a fish (a piece of equipment stuck in a borehole that cannot be removed) or to initiate directional drilling; (ii) to plug back a zone or plug back a well; (iii) to attempt to solve lost circulation problems during the drilling phase; and (iv) to provide an anchor for open hole tests.
One problem in setting cement plugs is that it can be difficult to retain the cement slurry in position in the borehole, especially when the plug is being set above the lowest point of the borehole (“off bottom”). Since cement slurries are often denser than borehole or drilling fluids, there is a natural tendency for the slurry to sink to the bottom of the well. U.S. Pat. No. 5,667,015 proposes a well barrier for maintaining a separation between an upper and a lower part of the well.
There have been previous proposals that attempt to address this problem by using a downhole sleeve to confine the cement to a specific zone of the well. Examples of this can be seen in U.S. Pat. Nos. 2,796,134, 3,032,115 and 3,460,625. In those cases, a delivery pipe is placed in the well with openings to allow cement to pass into the annulus. A sleeve is located around the openings and cement is pumped through the pipe into the sleeve to inflate it and seal against the formation. Suitable materials proposed for these uses are plastic or rubber. Once the cement has set, the delivery pipe is disconnected above the sleeve and the portion in the sleeve can be drilled out leaving the cement sheath in place. In these schemes, an impermeable sleeve is used to ensure that the cement is confined to the area of interest. In another example, U.S. Pat. No. 5,738,171 from Szarka describes an inflatable packer, without connector, and U.S. Pat. No. 6,578,638 from Guillory et al. describes an inflatable packer to be set above a lost circulation zone, then cement is injected into that zone before disconnecting it mechanically. None of these proposals demonstrate particularly effective cement properties in the region of particular interest: the borehole wall.
Some improvements have been realized in patent application WO 03/042495. An apparatus for setting a plug in a borehole zone is proposed using a flexible and expandable sleeve surrounding a setting tube connected to a delivery pipe, the connector comprising also a disconnecting mechanism. The plugging fluid is delivered through the delivery pipe and the setting tube to the sleeve, producing inflation of the sleeve. Nevertheless, the design of the apparatus presents several drawbacks. One of these is that the sleeve design did not incorporate any device or solution to anchor it into the borehole after disconnection. Consequently there is a risk that the weight of a cement column pumped above the tool could push the filled sleeve downward.
It is an object of the present invention to provide an apparatus which obviates or mitigates this drawback.
The invention provides an apparatus to be lowered in a borehole, comprising: (i) a delivery section for delivering a plugging fluid; (ii) a setting section comprising a longitudinal element and a flexible and permeable sleeve into which the plugging fluid is delivered; and (iii) a disconnect mechanism to allow the delivery section to be disconnected from the setting section, characterized in that the flexible sleeve is connected by at least one floating means to the longitudinal element.
The sleeve is permeable to allow the prior use of the apparatus in other drilling or well operations. Effectively, the permeable sleeve allows flow of mud or non fibrous fluid through the sleeve, but stops flow for compact fluid as cement or fibrous fluid. Preferably, the disconnect mechanism disconnects the delivery section from the setting section when the flexible sleeve is inflated inside the borehole at a given pressure. The given pressure is defined below the burst pressure of the flexible sleeve. Then, the setting section is left in the borehole and acts as a plug.
Preferably, the flexible sleeve is connected by two floating means to the longitudinal element, the longitudinal element being a tube. The floating means allow a free displacement by translation on the longitudinal element. The floating means can comprise a brake. The longitudinal element can further comprise shoulder(s) (90A and/or 90B) which will act as a limit stop against the floating means. This system of floating means ensures a perfect anchoring of the setting section inside the borehole and positioning on the borehole.
Preferably, the disconnect mechanism comprises a pin end or box end located on the setting section, an opposite, respectively box end or pin end on the delivery section and a sliding sleeve retaining the pin end and box end in connected position. Further, the disconnect mechanism functions only thanks to differential pressure existing between the inside of the flexible sleeve and the borehole.
Preferably, the apparatus further comprises a closing mechanism of the setting section which is in close position when the disconnect mechanism is in disconnected position. The closing mechanism can be a valve which operates and closes simultaneously when the delivery section disconnects. Further, the closing mechanism functions only thanks to differential pressure existing between the inside of the flexible sleeve and the borehole.
In another aspect, the invention provides a method of installing a plug in a borehole, comprising: positioning an apparatus as described above in the borehole at a position at which the plug is to be installed, pumping fluid into the flexible sleeve via the delivery section so as to inflate the flexible sleeve, disconnecting the setting section from the delivery section, and withdrawing the delivery section from the borehole leaving the setting section at the position, said setting section acting as the plug.
Preferably, the method further comprises the step of pumping an excess of the fluid into the borehole above the plug. The tightness of the plug is enhanced.
Preferably, the fluid is a cement slurry comprising solid and liquid components to cause a solids enriched layer to build up inside the flexible sleeve.
Preferably, the fluid is a cement slurry further comprising fibers of different types with at least one type being adapted for sealing the flexible sleeve.
Further embodiments of the present invention can be understood with the appended drawings:
The present invention involves the use of a flexible sleeve. The sleeve is also further permeable made from a woven material to permit placement of a cement plug in a borehole under naturally unstable conditions while forcing the cement slurry to remain in the borehole at the desired position. The unstable conditions in which the apparatus and method of the invention can be used include highly deviated and horizontal wellbores, and lost circulation conditions such as can occur in massively fractured formations, or off-bottom positioning of the plug with a layer of borehole or drilling fluid being present in the borehole below the plug.
An apparatus 20 for setting plug according to the invention is shown in
Some parts of the apparatus 20 are further shown in details in the
Preferably, each floating means is equipped with a brake 7 to control the friction over the stinger 1. For example, the brake can be an elastic collet or spring collet, pressed against the stinger by means of spring or screws 8. The brake friction is set so that the weight of the plugging fluid 13 filling the inflated bladder 3 cannot move the entire system {plugging fluid inside the bladder, bladder, floating means}. The spring or screws 8 can be pre-adjusted before operation or adjusted during operation with an added automatic system of regulation (not shown).
The upper extremity 9A of the stinger 1 further contains a shoulder 90A which ensures a stop for the floating means 14A. The delivery pipe further comprises an orifice 25 which ensures communication of the plugging fluid 13 from the delivery pipe to the internal cavity created by the sliding sleeve 24. The system {pin end, box end, sliding sleeve} corresponds to the connection/disconnection mechanism 17. The upper extremity of the setting tool further comprises an upper closing mechanism so that the bladder can be inflated but the plugging fluid can not flow back. The upper closing mechanism will be described in more details here below.
The floating means 14A and 14B brings two main advantages. First, the bladder will never be submitted to a tensile load higher than the brake friction. Useless stresses being eliminated, the optimum pressure rating is guaranteed. Secondly, as said before, the drawback of prior art system is that the bladder design did not incorporate any device or solution to anchor it into the borehole after disconnection. The applicants demonstrate that a setting tool comprising floating means can act as anchoring means. The setting tool with floating means plays cleverly on pressure applied on the bladder and thanks to those pressure differences remains in place.
Pext·A2=Pint·A3 (Equation 1)
A2=A3+A1 (Equation 2)
wherein, Pext is the external pressure in the borehole, Pint is the internal pressure inside the bladder, A1 is the area of the stinger in cross-section, A2 is the area of the borehole in cross-section and A3 is the area of the bladder inflated in cross-section.
According to the size of the stinger, the ratio between internal and external pressure can reach up to 1.2, i.e. the internal pressure always stays up to 20% above the external pressure. The internal pressure of the bladder is interesting, as it creates some friction against the borehole, which tends to lock the plugging element in place. As a result, the friction is proportional to the differential pressure applied on the bladder, and the plugging element stays in place with this “hydraulic lock”, whatever load is applied, as long as the internal bladder pressure stays below the burst bladder pressure. For example, a test has been realized with a plugging element set in a slick metal tube. The test was simulating a borehole with a very low friction, the worst case imagined, and enough pressure was applied on one extremity to generate a 42 tons load that tends to move the plugging element of the slick metal tube. Even in that extreme condition, the plugging element perfectly stayed in its initial position.
The disconnecting mechanism 17 allowing a disconnection of the setting tool or setting section 16 from the delivery section 15 presents also an advantage. In prior art systems, the connector is actuated by a physical means (dart or ball) pumped down after the volume of plugging fluid required for the sleeve inflation. This method requires a preliminary calculation of the open-hole volume, which cannot be accurate as the formation can be washed out during drilling. Consequently a safety margin for the volume of plugging fluid must be applied, and there must be one or several safety ports, initially plugged by a shear membrane or a pressure operated valve, which adds complexity to the design. The plugging fluid in excess will be vented through the ports to avoid bursting of the sleeve.
In the present invention, the connector 27 acts as a “hydraulic connector” located between the stinger 1 and the delivery pipe 19.
The upper closing mechanism from prior art presents some drawbacks. Effectively, in prior art systems, after disconnection, a non-return valve closes the sleeve to prevent plugging fluid flowing back and deflating the sleeve. The upper extremity of the stinger can be equipped with a non-return valve, so that the bladder can be inflated but the plugging fluid cannot flow back. However, the non-return valve induces several drawbacks. First, no reverse circulation is possible during running in hole, when the lower extremity of the stinger is still open. Reverse circulation offers the advantage of a higher return velocity, which is good to remove cuttings and solids out of the hole. Secondly, the actuation of such a valve is weak, as a spring is involved and the geometry does not allow a large size. Thus solids or fibers contained into the plugging fluid could potentially prevent a correct closure of the valve. In that case the sleeve would deflate and fall down into the borehole. Lastly, the flow path being very small because the spring tends to maintain the valve closed, there is a risk that solids or fibers contained into the plugging fluid can plug the valve and prevent any further inflation of the bladder.
Therefore in the present invention, another upper closing mechanism has been designed, with some major advantages. An automatic valve, taking advantage of the hydraulic connector, is used.
All the parts of the apparatus 20 can be machined with very common piece of equipment in the industry; enhancing the easy manufacturability.
The delivery section 15 is generally a drill pipe or coiled tubing or other types of tubes that can supply plugging fluid; it can also be a casing for special primary cementing, for example in total loss cases. Furthermore, the delivery section 15 can be another type of delivery system than a tube, for example an apparatus as described in patent application WO 04/072437 can be used downhole to move the setting section where desired and further supplying energy and plugging fluid. The setting section 16 is generally a stinger or longitudinal tube; it can also be a perforated casing or slotted liner.
The apparatus and method described above has the advantages of: prevention of fluid swapping—the cement slurry is not mixed with the fluid left underneath the tool; reduced loss of fluid to the formation; and strong mechanical properties of the cement, allowing for instance side-tracking (this is made possible by either the squeeze step, or the use of metallic fibers or both together).
The cement slurry used in this process typically includes fibers or mixtures of fibers. These fibers act in various ways, first by helping building a cake on the internal surface of the bladder, then by preventing loss of cement from the borehole above the bladder and finally by increasing the mechanical properties of the set cement to a point such that it will withstand subsequent drilling operations. When only a single type of fiber is used, flexible fibers are preferred: the use of such fibers has previously been proposed for use in lost circulation situations and they prevent the cement sheath from disintegrating after being drilled. When a mixture of fibers is used, a first type of fiber can provide the cement slurry with strong mechanical properties, which are beneficial for instance for kick-off cement plugs. These fibers are for instance the metallic fibers described in WO 99/58467. The second type of fiber can be similar to the flexible fibers described above. The fibers do not need to be added homogeneously to the whole slurry. For example, the flexible fibers can be used for the part of the slurry that inflates the bladder, while metallic fibers can be used in the second part (filling the borehole above the bladder), which needs strong mechanical properties.
When the delivery pipe is a drill pipe and once the cement has set, the drill pipe is reintroduced with a drill bit attached and drilling resumes, drilling through the stinger and cement inside the bladder to leave a remaining part of the bladder and a sheath of cement around the borehole in the zone. It acts as an impermeable barrier between the borehole and the formation that can sustain the hydrostatic pressure of the drilling fluid and so avoid the fluid loss problem. The presence of the cement cap on top of the sleeve and stinger assists in effective resumption of drilling and removal of the stinger. It can be further advantageous to put a whipstock above the apparatus 20 to guide the drill pipe and initiate deviation (kick-off).
In order for the cement slurry to build a cake inside the bladder, it is preferable that the slurry contains a large volume fraction of solids and does not possess too large fluid loss control properties. A composition that provides such properties can utilize an optimized particle size distribution for the solid components of the slurry such as is described in EP 0 621 247. Where a low density cement slurry is required, the approach proposed in WO 01/09056 is preferred. An example of such a base low density slurry is given in Table 1 below:
A suitable base higher density slurry is given in Table 2 below (same abbreviations as Table 1):
Fibre material is mixed with the base slurry to provide structure to the mass. Such fibres can be metallic (see, for example, WO 99/58467) or polymeric (see, for example, PCT/EP02/07899). Two suitable fibre materials and a proposed level of use in the cement slurries are given in Table 3 below:
The strength of the sleeve or bladder 3 material and the cement are important parameters in designing an operation in accordance with the invention. One of the most severe conditions lies in the case of the absence of support from the formation, for example when plugging caverns or highly unconsolidated formations. The strength requirement can be calculated using the following equations:
σt=KΔPa (Equation 4)
With ΔPa designating the differential pressure in the borehole, σt the tangential stress in a solid annulus at the wall of the borehole and K a stress intensity factor which, in the case of a solid unsupported annulus, is equal to
In which rb and ra represent respectively the outside diameter and the inside diameter of the solid annulus.
Using these equations, it is possible to estimate the strength requirements for the mesh and the cement.
If it is assumed that it is wished to consolidate a 10 foot section (304.8 cm) composed of broad cavities, the following procedure can be followed. The bottom of the area is situated at 3000 feet (914 m) with a pressure gradient of 0.3 psi/foot (total losses) (6.8×103 N/m3), that is to say the pressure at the bottom of the section is 3000×0.3=900 psi (6.2×106 N/m2). A cement slurry has, for example, a density of 0.8 psi/foot (1.8×104 N/m3). In order to ensure good coverage of the zone, a height of cement of 100 feet (30.48 m) might be appropriate. At 2900 feet (884 m), the hydrostatic pressure at the top of the column of cement is approximately 900−(100×0.44)=850 psi (5.9×106 N/m2). For simplicity of calculation, the borehole fluid is taken to be water and with a water level at approximately 950 feet (291.39 m), and a total loss situation is assumed. At the bottom of the section, the slurry will impose a pressure on the mesh of 850 psi+(0.8×100) psi=936 psi (6.4×106 N/m2). The differential pressure through the mesh is therefore 36 psi (0.25×106 N/m2). The cement, in the hardened state, must support in that part of the borehole a pressure of 1320 psi (9.1×106 N/m2) if the borehole fluid is water (0.44 psi/foot (9.9×103 N/m3) with a column height of 3000 feet (914 m)). The differential pressure for the cement is therefore 1320−900=420 psi (2.9×106 N/m2). If the borehole fluid is heavier than water, the differential pressure increases accordingly.
The strength of the mesh forming the sleeve is an important parameter. For example, assuming an 8×8 hard drawn, high carbon content steel cable mesh with a nominal yield strength of 300,000 psi (2068.4×106 N/m2), having a mesh diameter of 0.71 mm, an opening of 2.47 mm (a 5 mm steel fiber is not capable of passing through such an opening), the average tangential force over the volume occupied by the mesh is approximately 250 times the differential pressure, that is to say approximately 9000 psi (62×106 N/m2) (using equation 2 above) and an outside diameter of the mesh of 355.6 mm. The tensile stress on the cables is approximately 9000×(2.47+0.71)70.71=40,000 psi (275.8×106 N/m2).
In this calculation the average stress applied to the volume of the mesh is redistributed over the volume of the fibers. This simplified approach suggests that the mesh selected is capable of supporting and withstanding approximately 7 times the differential pressure of 36 psi (0.25×106 N/m2) before beginning to yield. The actual tensile strength of the mesh itself will depend in fact on many other parameters such as the orientation of the steel cables, the material used, etc. For example, a carbon fiber mesh has a tensile strength of approximately 640,000 psi (4414×106 N/m2). It appears at the present time that the mesh can provide appreciable support for the cement. It is also possible to envisage the use of a cement of lower density.
Similar calculations are made for cement. Assuming that the cement is not supported by the formation because of the presence of cavities, and the support afforded by the mesh is ignored, it is possible to apply the following reasoning: for a mass of cement having an outside diameter (equal to the diameter of the mesh) twice that of its inside diameter, the tangential tensile stress in the cement at the wall of the borehole is 5/3 times the differential pressure, according to equation 2, that is to say 700 psi (4.8×106 N/m2). This means that it is necessary to use a reinforced cement with a tensile strength of at least 700 psi (4.8×106 N/m2), typically comprising metallic fibers. Well cements with a tensile strength of 1000 psi (6.89×106 N/m2) are known.
In order to increase the reliability of the system, the mesh must be sufficiently strong to support the cement. The use of a cement with lower density or application to a shorter length of the stabilization zone will reduce the strength requirement of the mesh during the placing of the cement. The outside diameter can also be increased in order to reduce the tensile stress on the cement sheath.
The sleeve is preferably highly flexible in order to adapt to the dimensions and shape of the borehole whilst retaining good mechanical strength. Therefore carbon fibre, Kevlar or steel bands can be used. An appropriate material has a high tensile strength under downhole conditions and is not excessively degraded by fluids present in the well, at least until a permanent casing is installed. The structure of the mesh affords the required flexibility. However, it may also be necessary to be able to drill through the sleeve, the cement providing an impermeable layer, which makes it possible to drill the borehole without loss of circulation and increases the strength of the structure.
| Number | Date | Country | Kind |
|---|---|---|---|
| 05291785 | Aug 2005 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2006/006954 | 7/14/2006 | WO | 00 | 8/18/2008 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2007/022834 | 3/1/2007 | WO | A |
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| 5167710 | Leroux et al. | Dec 1992 | A |
| 6513601 | Gunnarsson et al. | Feb 2003 | B1 |
| 20040188954 | Naquin et al. | Sep 2004 | A1 |
| Number | Date | Country | |
|---|---|---|---|
| 20090183875 A1 | Jul 2009 | US |
