Preventing fluid loss

Sealing particles are used to stop or reduce undesired fluid loss. The sealing particles may be swellable or have effective cross-sectional areas greater than five square millimeters or are both swellable and have effective cross-sectional areas greater than five square millimeters. The sealing particles are disposed in one or more locations in which there is undesired fluid flow and, once lodged therein, stop or at least reduce the undesired fluid loss. A tubular having a bypass flow path may be used to deploy the sealing particles. The bypass flow path may use a biased or unbiased sleeve that is selectably movable to expose or block exit ports in the tubular. A retrievable sealing disk may be deployed to move the sleeve. The sealing particles may be made of a bi-stable material with extenders and may be actuated using swellable material. The sealing particles may extend in multiple dimensions.

BACKGROUND

When a formation is drilled under normal conditions, the well is almost always filled with drilling fluid that serves to carry rock cuttings to the surface, lubricate the drill bit, and provide an overpressure in the borehole to prevent the flow of formation fluids into the wellbore (i.e., a blow out). The overpressure provided by the drilling fluid also plays a key role in stabilizing the formation. As a result of the overpressure, the liquid part of the drilling fluid enters the formation (filtering) while the solid part accumulates at the formation surface (borehole wall). The accumulated solid contains materials (such as bentonite, for example) that act to form a hydraulic seal. The sealing layer is called “mudcake” and, once formed, prevents any further filtering of the drilling fluid into the formation. Thus, although a relatively small volume of the drilling fluid filters into the formation, the process is normally self-limiting.

Mudcake is able to form and sealing occurs because the pore size in the subsurface formation is smaller than the particle sizes in the drilling fluid. As a result, the bulk of those particles cannot pass through the pore entrance (though a small portion of very fine particles can pass and produce what is known as “fine invasion”). The bulk of the solid drilling fluid material is pressed against and sticks to the pore entrance and gradually builds the impermeable layer of mudcake. However, this process fails to occur when the size of the pore entrance is larger than the solid particles in the mud (drilling fluid). One common example is when fractures are encountered. Some natural fractures have apertures larger than the particles in the mud. This results in a fluid loss problem wherein a large volume of drilling fluid is lost into the formation, with its consequential economic and safety issues.

Fluid loss in fractures is manageable and remedial actions exist. One such remedy is to use solid materials in the drilling fluid that are proportionally larger. With this approach pore sizes of up to 2.5 millimeters have been sealed. More recently, the use of water swellable materials has been proposed. In this case, smaller, water swellable materials are used in the formulation of the mud. These materials enter the fracture, absorb water, and increase their volume, thereby forming a seal. Certain water swellable materials are capable of increasing their weight by over ten-fold in the course of a few hours. The rate and extent of swelling depends on the type of water available. The best results are obtained with fresh water.

A “super k layer”, also known as a “cavernous formation”, is a source of huge permeability and, when encountered during drilling, can take in large volumes of drilling fluid, even to the point there is not enough drilling fluid left in the borehole to reach the surface. This is referred to as “circulation loss”. Because super k layers have very large pores (on the order of tens of centimeter), there is no possibility of forming a mudcake at the borehole wall. As a result, the fluid loss can continue indefinitely so long as the fluid pressure in the borehole is higher than the fluid pressure in the formation.

SUMMARY

Sealing particles are used to stop or reduce undesired fluid loss. The sealing particles may be swellable or have effective cross-sectional areas greater than five square millimeters or are both swellable and have effective cross-sectional areas greater than five square millimeters. The sealing particles are disposed in one or more locations in which there is undesired fluid flow and, once lodged therein, stop or at least reduce the undesired fluid loss. A tubular having a bypass flow path may be used to deploy the sealing particles. The bypass flow path may use a biased or unbiased sleeve that is selectably movable to expose or block exit ports in the tubular. A retrievable sealing disk may be deployed to move the sleeve. The sealing particles may be made of a bi-stable material with extenders and may be actuated using swellable material. The sealing particles may extend in multiple dimensions.

DETAILED DESCRIPTION

Some embodiments will now be described with reference to the figures. Like elements in the various figures may be referenced with like numbers for consistency. In the following description, numerous details are set forth to provide an understanding of various embodiments and/or features. However, it will be understood by those skilled in the art that some embodiments may be practiced without many of these details and that numerous variations or modifications from the described embodiments are possible. As used here, the terms “above” and “below”, “up” and “down”, “upper” and “lower”, “upwardly” and “downwardly”, and other like terms indicating relative positions above or below a given point or element are used in this description to more clearly describe certain embodiments. However, when applied to equipment and methods for use in wells that are deviated or horizontal, such terms may refer to a left to right, right to left, or diagonal relationship, as appropriate. It will also be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another.

A system and method to prevent fluid loss from a pressurized region are described herein. A water swellable material may be used to seal a source of fluid loss such as a super k layer in a subsurface formation while drilling. The system and method may also apply to leaky tubing (i.e., tubulars) such as a pipe in which a leak has developed. As stated above, super k layers may have pore sizes on the order of tens of centimeters, which is very large. For such large pore sizes, the use of normal size water swellable materials becomes ineffective. However, larger particles (made from water swellable materials or not) may be constructed that are well-suited for sealing super k layers. Those larger particles can be delivered to the site of super k layers to provide a sealing surface.

FIG. 1shows a scenario in which a drilling operation has encountered a super k layer140. The drill bit120has penetrated formation150in which layer140has large enough pores to qualify as a super k layer. Because of the extremely high permeability of layer140, the drilling fluid filling well130will flow into layer140until the pressure in well130equals (or drops below) the pressure in layer140. Since the pores in layer140are too large to be blocked by the solids in the conventional mud, no mudcake is formed to counteract this extreme invasion process.

To stop the flow of mud into the super k layer140, one may introduce materials into the drilling fluid that are on the order of or bigger than the pore sizes in the super k layer. During the normal operation of drilling, the drilling fluid containing the (typically-sized) solid particles are pumped through the central passageway170of the drill pipe110. The drilling fluid travels to the drill bit120in which special orifices (jets)180are cut, allowing the mud to leave passageway170and enter the annular region between the inner diameter of the wellbore130and the outer diameter of drill pipe110. For a six inch drill pipe, for example, the passageway170is about four inches in diameter, while the orifices180in the drill bit are generally less than one centimeter. The jets180are intentionally made small to create a jetting action. As a result, although larger particles could be introduced in the formulation of the mud and carried through central passageway170in drill pipe110, the orifices180in the drill bit120would prevent them from entering the annulus and coming into contact with the formation wall130. Currently there is no apparatus available that can deliver such large particles (i.e., greater than approximately one cm) to the bottom of the well. Thus, using existing technology, the largest particle sizes that can be delivered to the super k layer are limited by orifices180in drill bit120rather than the large central passageway170in drill pipe110.

To circumvent this limitation, one may choose from at least two possible courses of action. One is to use larger particles, but avoid sending those particles through the jetting holes (orifices)180. Another is to send smaller particles that can grow and become large on site, after they pass through orifices180. A possible third course of action may involve some combination of the first two.

FIG. 2shows a bottom hole assembly (BHA) similar to that ofFIG. 1, but to which a bypass section or bypass opening230has been added. Bypass section230may employ many different mechanical designs that are conventionally used to stop or start a flow. Bypass section230has one or more holes235in the wall of drill pipe110that are large enough to allow desired large particles to pass into the annular region, thus bypassing jets180in drill bit120. In the embodiment ofFIG. 2, two holes235are shown that are diametrically opposed. The holes235open and close by rotating a cylindrical sleeve210that has matching holes250, but in which the remaining part of its cylindrical structure is solid. Under normal drilling operations, holes235are blocked by sleeve210, rotated to have its solid body facing holes235. When a super k layer or any layer with large pore or aperture size is encountered, the drilling process is stopped, the rotatable sleeve210is rotated (using, for example, a motor (not shown)) so that its holes250align with holes235in drill pipe110. This provides a new (temporary) flow path that offers much less resistance to flow, especially for large particles. The drilling fluid thus passes through the flow path formed by aligned holes235and250and enters the annulus. Under these conditions the drilling fluid may contain sealing particles that are only slightly smaller than the diameter of holes250or235, whichever is smaller. Those larger particles then serve to block the large pores in super k layer140and build a mudcake. Once a mudcake forms and the super k layer is sealed, the fluid loss is controlled and sleeve210may be rotated back to the closed position. That allows the mud to once again pass through the drill bit and, at this point, normal drilling operations can proceed.

FIGS. 3aand 3bshow an example pore in the super k layer140that has been invaded by a particle that is sufficiently large that it can not move past a certain length into the super k layer140. The large particles may be constituents of a special drilling fluid that contains particles with sizes ranging from the size of particles found in normal drilling fluid up to the maximum size that the downhole equipment (such as one having a bypass section230) can handle. A pore310has an aperture340. If the size of the large particle320is larger than aperture340, large particle320will at least partially block the aperture340, thereby reducing the effective aperture size, but not necessarily sealing the aperture, as smaller openings may still exist between aperture340and the blocking large particle320. That is, large particle320serves to at least reduce the flow into the super k layer dramatically, but may not stop it completely. However, with large particle320at least partially blocking aperture340, other, smaller particles in the mud can fill the resulting smaller effective aperture and particles can act in concert to form a seal and stop the undesired flow.

In some cases the size of aperture340near the wellbore may be larger than particle320, but a pore's size is generally not uniform and can reduce as one moves farther into the pore space, away from the borehole. A reduced pore size350, some distance into the formation, is conceptually illustrated inFIGS. 3aand 3b. Particle320may be small enough to initially pass through aperture340, but as the rush of the mud invasion into the super k layer140continues, particle320will be carried deeper into the super k layer140, where it eventually encounters a reduced aperture (pore throat)350. If the reduced aperture350is smaller than the size of particle320, particle320cannot pass beyond that point. While the entrapped particle320effectively further reduces the size of reduced aperture350, there may still be gaps360around particle320that remain unobstructed to fluid flow, similar to that described above. However, since the drilling fluid contains a distribution of particles having sizes ranging from small to large, the smaller particles can, as above, enter and seal off gaps360. Thus, when the large particle320is introduced to the super k layer140and gets stuck either at the pore face or in the pore throat at some point slightly removed from the borehole wall, it reduces the effective pore size and restricts the flow so that the remainder of the particles in the drilling fluid can seal the flow path, thereby stopping the invasion. In the example ofFIGS. 3aand 3b, one large particle320is shown, but in practice there will be more. The combined effect of all will be more effective in sealing the super k layer140than the single particle shown.

In the embodiment just discussed, one delivers the large particles320to the super k layer140via the bypass section230.FIGS. 4aand 4bshow an alternative embodiment of a bypass section230. Drill collar (or pipe)110inFIG. 4ais provided with exit ports410to allow large particles320to pass through the drill collar110and enter the annulus. InFIG. 4aone exit port410is shown, but, in general, more exit ports are possible and they can be located at various locations along the length of drill pipe110. The size and shape of exit ports410are selected to be larger than the largest particle320that is expected to be delivered to the formation.FIG. 4bshows a cross-section of a drill collar110having two exit ports410. During normal drilling operation, those exit ports410are closed so that mud can pass down to and through the drill bit (as shown inFIG. 1). A sleeve210having no holes is provided that, during normal operations, forms a barrier to prevent the drilling fluid from exiting through exit ports410. Sleeve210is supported by a spring420that, during normal drilling operations, is maintained in a compressed, neutral, or elongated state that keeps exit ports410closed. When a super k layer140is encountered, sleeve210may be forced in a direction that compresses or elongates spring420. As a result, sleeve210moves past exit ports410, allowing the pumped fluid to enter the annulus. Once the pumping-to-seal operation is completed, sleeve210is returned to its normal operational position by the spring420, as shown inFIG. 4b, whereupon normal drilling operations may resume.

One possible mechanism for displacing sleeve210is shown inFIG. 5. In this embodiment, a sealing disk510is sent down from the surface. Disk510is attached to a cable520, enabling its retrieval once the pumping-to-seal operation is terminated. In operation, once a super k layer140is encountered, a rapid fluid loss reduces the fluid level in the well or at least reduces the fluid pressure. Once those symptoms are observed, disk510and cable520may be deployed through central passageway170of drill pipe110. Disk510has a diameter that is slightly smaller than the inner diameter of drill pipe110or sleeve210, according to particular embodiments. As a result, it forms a loose piston, pushing the old drilling fluid located below (i.e., ahead of) it through the drill bit. Behind (i.e., above) disk510, the fluid containing the large particles320is pumped into the well. The pumping pressure acts to push disk510down until it reaches bypass section230. Restriction dogs430may be located somewhere along sleeve210to reduce the effective diameter of drill pipe110or sleeve210. Restriction dogs430provide a surface on which disk510can seat and make a seal. The seal causes the pumping pressure to bear on sleeve210, forcing it downward and compressing (in this embodiment) spring420. Once sleeve210passes by exit ports410, the drilling fluid takes the less restrictive path into the wellbore and, from there, enters the adjacent super k layer140.

While this operation is in progress, drilling operations are stopped and the pressure is monitored. As the super k layer becomes more and more sealed by the large particle mud, the pressure in the mud column climbs until it reaches an expected level. At this point, some volume of normal drilling fluid is pumped into the well to flush the heavy particles320that did not get deposited in the super k layer out of the well. Cable520is then used to pull disk510up, breaking the disk510/restriction dog430seal. To facilitate the movement of disk510in the uphole direction, drilling fluid may be pumped into the annulus from the surface and withdrawn from central passageway170(this is the opposite flow direction from normal pumping operations). That helps prevent any cavitation effect caused by drawing disk510upward through the drilling fluid. Note in this embodiment disk510forms an effective barrier between the different fluids being pumped, similar to a plug. That is, it separates the “large particle drilling fluid”, having a full distribution of particle sizes, from the normal drilling fluid being used before a super k layer was encountered. This allows a metered volume of the large particle drilling fluid to be pumped into the well.

In an alternative embodiment (shown inFIG. 6), the length of disk510is chosen to be large to facilitate the downward motion of sleeve210. In particular, the length of disk510can be as long as sleeve210. If disk510is made of a dense material such as metal and the spring constant is chosen appropriately, disk510will weigh enough to compress spring420and expose exit ports410. In this embodiment, pumping pressure is not needed or at least is not the only mechanism available to compress spring420. Also in this embodiment, the large particle drilling fluid filling the volume above disk510can be delivered to the super k layer at pressures that are not excessive since the fluid is not used to compress the spring. That helps reduce further fluid loss.

In another embodiment sleeve210is attached to a motor that can be activated to slide the sleeve up or down to open exit ports410. The motor can be activated using mud pressure coding, for example, as is commonly used in directional drilling. The motor can also be connected to a flow or pressure sensor that senses, for example, the rapid loss of mud or a pressure drop.

In yet another embodiment, smaller particles are pumped into the fluid loss layer, such as a super k layer, but the particles are able to absorb another material, such as water, for example, and expand to increase their size. This is a common practice in fluid loss layers that have fractures with moderate aperture sizes. In this case, in a fashion similar to that shown inFIGS. 3aand 3b, the particles enter the pore space and, upon expanding, form a seal. Water swellable materials have been successfully used for fractures having aperture sizes on the order of one or two millimeters. In super k layers, however, the aperture is on the order of centimeters and the existing practice of using smaller particle swelling material does not work well. The time required for the water swellable particles to enter the fluid loss layer is much shorter than the time it takes them to swell and form an effective seal. For this scenario, it is preferable that the aperture of the fluid loss layer decreases as one moves away from the borehole wall to the point that its size becomes comparable to the size of the particles before swelling. If this is not the case, then the common practice is to allow a certain amount of the particles to enter the fluid loss zone and then shut off the well for a few hours, allowing time for the swellable particles to swell. Sealing apertures of up to 2.5 millimeters in diameter (i.e., pores having effective cross-sectional areas less than five square millimeters) has been accomplished in this manner.

In another embodiment, large water swelling particles are delivered to the super k layer using a by-pass apparatus such as is described above. In practice, a known volume of drilling fluid containing a distribution of larger particles is placed slightly above disk510, forming a first band of fluid, and delivered to a depth of interest. When ports410open, this fluid flows out of the drill pipe and into the super k layer. A second band can be a buffer layer of normal drilling fluid, followed by an activating band, which in most cases will be fresh water. The water swelling particles are known to absorb the fresh water and swell rather quickly. Delivering the fresh water to the super k layer having large swellable particles already in the pore structure expedites the swelling and causes a pressure seal to develop. Note that during this operation, the fluid pressure in the inner diameter of the drill pipe has to be higher than in the annulus to prevent the drilling fluid in the annulus from entering the interior region of the drill pipe. The pressure can be regulated by a combination of drilling fluid density and pumping speed.

Using (water) swellable particles allows the pre-swollen particles to be smaller and pass more freely through small passages than particles that are not swellable and of comparable size to the swollen particles. Smaller water swellable particles (e.g., 1-3 centimeters) can be delivered to the super k layer as described above. Those particles subsequently swell when they come in contact with fresh water and grow four to ten times in length. Thus, the effective particle size is on the order of ten to thirty centimeters. These particles are also more flexible and can form a better seal than conventional, non-swellable particles. The swellable materials not only are able to increase their size, but can also grow to conform to the inner diameter and shape of the pore in which they are disposed.

In yet another embodiment, use is made of bi-stable materials to fill up the pore space and create a hydraulic seal. Bi-stable structures are mechanical objects that are stable in two different shapes or configurations. A common and illustrative example of a bi-stable structure is a “snap” bracelet. That is, a straight piece of bi-stable material is gently struck against a person's wrist and the material “snaps” into its second stable form—an open loop that wraps around the wrist. Bi-stable materials are stable in both configurations, but retain residual stresses that can be used to trigger transitions to their alternate forms. In an embodiment contemplated to seal off freely flowing structures, the bi-stable particles initially resemble closed umbrellas. Those closed-configuration particles are pumped into the high permeability (freely flowing) structure. The particles are then triggered to open up like umbrellas, causing a large restriction in the flow path.

FIG. 7ashows a short length of a bi-stable material710in its straight form. When the straight material ofFIG. 7ais pressed on its two ends, it is triggered and snaps to a curved shape720, shown inFIG. 7b. The transition is reversible and if the two ends of the curved shape720are pushed open, the material snaps back to the linear shape710. In the example ofFIGS. 7aand 7b, the length of the material is intentionally chosen to be short enough so that the object inFIG. 7bdoes not form a complete or closed loop. If two pieces of normal material730are attached to the two ends of the object inFIG. 7b, the object ofFIG. 7cis formed. The object ofFIG. 7chas rather large length but small width and behaves similar to the linear “closed umbrella” structure ofFIG. 7a. Note that the curvature of curved shape720has caused the distal ends of730pieces to come close to one another and may even be touching. If these two ends are pulled apart by some force, the curved shape720will snap to its straight form710and the structure740ofFIG. 7dis formed. This shape may have, for example, twice as much length as the object ofFIG. 7c, and despite its still small width may therefore behave similar to the “open umbrella” referred to above due to its increased length.

The triggering mechanism for the transition from bent (curved) to straight forms can be provided by (water) swellable materials.FIG. 8ashows a composite structure800similar to that ofFIG. 7c, with water swellable materials810added to the interior region of the structure. Once this object comes into contact with water, the swelling of material810causes the two ends of normal material730to separate, forming the configuration shown inFIG. 8b. The swelling continues until the two ends of curved shape720pass a transition zone and curved shape720snaps to the linear shape710ofFIG. 7aor7d.

Composite structures800can be made with small enough width to be pumped through inner passageway170of drill pipe110and pass through orifices180of drill bit120. Once these composite structures800are in the annulus, the rush caused by the invasion into the formation (rapid fluid loss) will convey them into the super k layer, where they will form random conglomerates by compaction. The band (volume) of mud containing composite structures800can be followed by a band of fresh water that will be absorbed by the water swellable particles810, causing them to expand and, in turn, causing curved shape720to snap to the increased length linear structure815(FIG. 8c). This structure has large length and, depending on its orientation relative to the flow direction, can lodge inside the pore and restrict or even stop the flow.

When the increased length linear structure815is aligned with the flow direction, it may be carried by the flow deep into the super k layer. The deeper those particles invade the super k layer, the more fluid is lost.FIGS. 9aand 9bshow another embodiment in which the embedding object is not linear and, once snapped open, will lodge in the pore. An example of a closed embedding object910is shown inFIG. 9ain which two curved shape structures720are attached together to form a cross (when snapped to their linear forms). Four legs made of normal material730(three legs are shown inFIG. 9a) are attached to the four ends of the curved shapes720(ends of the cross). When the water swellable material810expands, it causes closed embedding object910to snap into open embedding object920(FIG. 9b). The open embedding object920has four legs in a cross shape; thus, as soon as it snaps open and encounters a pore space of corresponding dimensions, it lodges in the pore, independent of its orientation relative to the fluid flow direction. This embodiment is not limited to four legs and structures with more than two cured shapes720attached together can be made that, when expanded, form a plurality of legs. Since the water swellable material expands in all directions, its unexpanded volume can be chosen such that when it swells, not only does it snap closed embedding object910open, but it also fills the entire cross section of open embedding object920and forms a very effective seal.

In the alternative embodiment shown inFIG. 11a, curved shapes (i.e., bi-stable materials)721and722share (i.e., each connect to opposite ends of) a common piece of normal material732. In this case, the other ends of the curved shapes721,722attach to ends of normal material731,732, respectively. In this embodiment, the snapping point is not in the middle of the structure as it is for the open embedding object920. Rather, there are two snapping points offset from the middle of shared piece732. The curved configuration ofFIG. 11ais roughly the same length as that ofFIG. 9a, but, once in its snapped configuration (FIG. 11b), it can be as much as one and a half times longer. Although an embodiment having two bi-stable material sections is shown inFIGS. 11a, 11b, other embodiments may have more bi-stable material sections joined to corresponding normal materials.

In the embodiments shown thus far, the straight (i.e., normal) materials are connected in line with the two ends of the curved material. These embodiments lead to straight structures when they are snapped open. In another set of embodiments, exemplified by that shown inFIGS. 12aand 12b, it is possible to connect the pieces720,735,736at different angles. In that case, the resulting snapped structure will not lie along a straight line.FIG. 12bshows a snapped form in its non-linear configuration.

It is easy to see if the embodiments ofFIGS. 12aand 11aare combined, it is possible to obtain three dimensional snapped forms that can be more effective in sealing large pores. That is, the embodiments shown inFIGS. 11a, 11b, 12a, and 12bmay be extended to other dimensions, similar to that shown inFIGS. 9aand 9b, by joining, for example, multiple bi-stable materials together. The added dimensions may be more effective in trapping the structure within the pore space of the rock and facilitate the build up of a plug that can serve to block the fluid loss into the formation. In addition, the joining can be done using a normal material in addition to or instead of a bi-stable material.

FIG. 10shows a flowchart or workflow to stop or reduce undesired fluid flow using sealing particles. Sealing particles that are swellable or have effective cross-sectional areas greater than five square millimeters or are both swellable and have effective cross-sectional areas greater than five square millimeters are provided (1002). The sealing particles are disposed in one or more locations in which there is undesired fluid loss (1004) and thereby stop or reduce the undesired fluid loss (1006).