GB2411668A - Filter element with a bypass - Google Patents

Abstract

A filter element has a first flow path adapted to restrict the passage of particles there through and a second flow path that is constantly open. Preferably, the filter element has multiple small openings 36 and at least one large opening 38 so that, when none of the openings are plugged, the probability of solid particles carried by a flow of fluid becoming trapped in the region of the first openings is higher than that of the particles passing through the second openings. The filter, in tubular form 40, can be used in a drill string to remove particles from drilling mud that operates a downhole tool 18 such as a stabiliser. Should the small perforations become blocked, mud can still reach the stabiliser via the bypass openings. The filter works with flow either radially outwards or radially inwards through the filter element.

Description

241 1 668

FILTER

This invention relates to a filter, and in particular to a filter suitable for use in downhole applications, for example for use in the bottom hole assembly of a drilling system for use in the formation of wellbores.

It is becoming increasingly common to use drilling fluid or mud, supplied under pressure to the wellbore during the formation thereof, to operate components of a bottomhole assembly. By way of example, drilling fluid driven motors are known for use in supplying rotary drive to a drill bit.

Another type of fluid operated component used in a downhole tool is a set of turbines or impellers. Such impellers are commonly used in steerable drilling systems or measurement while drilling (MOOD) systems in order to generate electrical power downhole to operate various electrical components.

A further type of fluid operated component used in a bottomhole assembly is the bias unit of a rotary steerable system. One type of bias unit comprises a housing having a series of bias pads pivotally mounted thereon.

Each pad is pivotable between a retracted position and a radially extended position. Pistons are associated with the bias pads to push the pads from their retracted positions to their extended positions, the pistons being arranged to be supplied with clean drilling fluid through a suitable valve arrangement to cause such movement.

Where a bias unit, or other fluid driven component, is located downstream of the bottomhole assembly, or downstream of any other fluid driven component, there is a risk that wear of the upstream component(s) may result in the drilling fluid supplied to the downstream component carrying with it solid particles. Although the presence of relatively small particles in the supply of drilling fluid may not be problematic, the presence of larger particles could result in the downstream component becoming jammed or otherwise failing. Clearly this is undesirable. The particles introduced into the drilling fluid may comprise, for example, rubber or other elastomer chunks of relatively large dimensions, cement, pieces of surface screens and/or other metallic or non-metallic debris.

In order to prevent or reduce the risk of damage to a downstream located component, it is known to locate a filter or junk basket above the critical components to capture or trap the potentially damaging particles.

One type of filter comprises a strainer or screen located to trap particles of dimensions larger than a predetermine size, the strainer or screen being held in position by shear pins. In use, as the filter traps particles carried with the supply of drilling fluid, the strainer or screen will gradually become plugged resulting in an increase in the pressure across the strainer or screen. When a predetermined pressure drop corresponding to a predetermined level of plugging is reached, the shear pins will shear allowing the strainer or screen to move to a position in which it is inoperative, thereby allowing the supply of fluid to continue, the fluid no longer being filtered. There is a risk, with such an arrangement, of the shear pins failing to operate correctly resulting in, for example, premature movement of the strainer or screen thereby unnecessarily terminating filtration of the drilling fluid, or failure of the pins to shear despite the strainer or screen being severely plugged causing a significant disruption to the supply of drilling fluid to a downstream located component which could impair operation thereof.

Another type of junk basket collects small particles by using the venturi effect to "suck" particles into a junk basket. However, this type of junk basket has very limited application.

A further design is similar to the filter described hereinbefore but instead of arranging for the strainer or screen to move in the event of plugging, a by-pass passage is opened by the increased pressure across the strainer or screen causing a diaphragm or disk to rupture. On opening of the by-pass passage, supply of drilling fluid to the downstream component continues, but the fluid is not filtered. Again, there is the risk of premature rupture resulting in unfiltered fluid being supplied to the downstream components at a time when the fluid could have been filtered, or in the failure of the diaphragm or disk to rupture, with the risk of incorrect operation of the downstream components due to an inadequate supply of fluid.

According to the present invention there is provided a filter comprising a filter element having a region having a plurality of first, relatively small openings formed therein, and at least one second, relatively large opening formed in the filter element, the number, size and location of the first and second openings being chosen such that, in use, when none of the openings are plugged, the probability of solid particles carried by a flow of fluid becoming trapped in the region of the first openings is higher than that of the particles passing through the second openings. The said probability is determined by the proportion of fluid passing through the first openings and by the locations of the openings.

Such a filter is advantageous in that fluid passing through the filter will tend to pass through the first, relatively small openings. However, in the event of a number of the first openings becoming plugged, fluid may still pass through the filter, the fluid passing, at least in part, through the second, larger openings. As no moving parts or rupturing are required to allow fluid to avoid having to pass through the small openings, the filter will operate reliably and consistently.

Preferably, the filter element is of tubular form, fluid conveniently flowing, in use, in a generally radially inward direction from the exterior of the filter element to the interior thereof. Alternatively, fluid may flow in the opposite direction. The second, relatively large openings are conveniently located upstream of the first, relatively small openings. As such, the risk of relatively large particles which are removed from the flow of fluid by the first, relatively small openings subsequently rejoining the flow of fluid and passing through the second, larger openings is reduced.

The filter element preferably includes, downstream of the openings, a IS region which is free of perforations serving to define, in part, a chamber or junk basket in which particles separated from the supply of fluid will tend to collect.

According to another aspect of the invention there is provided a filter comprising a housing, a filter element being located within the housing, wherein the filter element defines a first flow path, the first flow path being adapted to restrict the passage of particles there through, and a second, constantly open flow path.

The invention will further be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a sectional view illustrating a filter in accordance with an embodiment of the invention; and Figure 2 is an end view of the filter of Figure 1.

Referring to Figure 1 there is shown a generally cylindrical housing having an inlet opening 12 at an upstream end thereof and an outlet opening 14 at a downstream end thereof. The housing 10 is located such that drilling fluid, having for example been passed through a fluid driven motor 16 is supplied to the inlet opening 12, fluid exiting the housing to through the outlet opening 14 being supplied to a downstream fluid driven component, for example a bias unit 18. Although references are made herein to the upstream component comprising a motor 16 and the downstream component comprising a bias unit 18, it will be appreciated that this need not be the case, and other components could be located upstream and/or downstream of the filter.

Although not shown, the housing 10 could form part of, for example, a stabilizer or reamer. For example, the housing 10 could incorporate a sleeve stabilizer element for use in stabilization of the bottomhole assembly.

Within the housing 10 is located a filter element 20. The filter element 20 is of generally cylindrical form, extending coaxially with the housing 10. Along most of its length, the filter element 20 is of diameter significantly smaller than the inner diameter of the housing 10 so as to define, with the housing 10, an annular chamber 22. At its first, upstream lO end 24, the filter element 20 is formed with an enlarged diameter region 26, the outer diameter of which is substantially the same as the inner diameter of the adjacent part of the housing 10 such that engagement between the housing 10 and the region 26 holds the first end 24 of the filter element 20 in position. The region 26 is formed with a pair of arcuate openings 28 (see Figure 2) arranged to allow drilling fluid supplied through the inlet opening 12 to enter the annular chamber 22.

A second, downstream end 30 of the filter element 20 is also provided with an enlarged diameter region 32 arranged to engage the inner surface of the housing 10 to hold the filter element 20 in position. The engagement of the region 32 with the inner surface of the housing 10 is such as to restrict or prevent fluid from flowing directly from the annular chamber 22 to the outlet opening 14.

An axially extending, blind bore 34 is provided in the filter element 20, the bore extending from the second end 30 thereof to a position close to the first end 24. A plurality of first, relatively small diameter openings 36 are provided in the filter element 20 to provide communication between the annular chamber 22 and the bore 34. The number and diameter of the first openings 36 will depend, to some extend, upon the specific application in to which the filter is to be used. In the specific embodiment illustrated, the number of first openings 36 provided is in excess of 350. However, arrangements having more or fewer first openings are possible.

In addition to the first openings 36, a pair of second, relatively large diameter openings 38 (only one of which is visible in Figure 1) are provided, also providing communication between the annular chamber 22 and the bore 34. The second openings 38 are located closer to the upstream end of the filter than the first openings 36.

A region 40 of the filter element 20 between the first openings 36 and the second end 30 is free of perforations, the adjacent part of the annular chamber 22 forming a junk basket or collection area 42.

In use, with the filter positioned as described hereinbefore, fluid supplied through the motor 16, together with any particles or debris carried thereby enters the housing 10 and passes through the arcuate openings 28 to enter the annular chamber 22. The dimensions of the first openings 36 are chosen so as to be smaller than the maximum size of a particle able to pass through the downstream component without causing damage thereto or impairing the performance thereof. The number and dimensions of the first, relatively small openings 36 are also chosen such that the flow resistance created by the first openings 36 is relatively low compared to that created by the second openings 38, the first openings 36 together forming less obstruction to fluid flow than the second openings 38. As a result, although a small proportion of fluid from the annular chamber 22 may pass through the second openings 38 to enter the bore 34, the primary route taken by particles carried by the fluid is to flow through the first openings 36 to enter the bore 34.

Although the fluid can pass through the first openings 36, particles or debris carried by the fluid of dimensions larger than the dimensions of the first openings 36 cannot pass through the openings 36 and instead are retained within the annular chamber 22. At least some of the particles and debris collected in this manner will tend to gather in the collection area 42.

Over time, it is likely that the collected particles and debris will cause at least some of the first openings 36 to become blocked or plugged. As more of the first openings 36 become plugged, the difference in the pressure caused by the two sets of openings will reduce, and as a result a greater proportion of the fluid will flow through the larger second openings 38, thereby allowing fluid to continue to pass through the filter at a substantially unobstructed rate so as to allow the continued operation of the bias unit 18 or other component located downstream of the filter. Although, under such conditions, fluid can pass through the filter, it will be appreciated that at least a significant proportion of the fluid, the fluid passing through the second openings 38, is not actually filtered to have particles and debris separated therefrom. The first openings 36 can be viewed as forming part of a first, filtered flow path, the second openings 38 forming part of a second, constantly open flow path.

Unlike the prior art filter arrangements, no moving or rupturable components need to be provided in order to allow the continued, unfiltered supply of fluid, thus the risk of incorrect filter operation as described hereinbefore is reduced.

At appropriate times or intervals, the filter may be retrieved for cleaning or emptying of the particles collected thereby, ready for redeployment.

Although a specific embodiment of the invention has been described and illustrated herein, it will be appreciated that a number of modifications and alterations may be made thereto without departing from the scope of the invention as defined by the appended claims.

Claims (11)

1. A filter comprising a filter element having a plurality of first, relatively small openings formed therein, and at least one second, relatively large opening formed therein, the number and size of the first and second openings being chosen such that, in use, when none of the openings are plugged, the probability of solid particles carried by a flow of fluid becoming trapped in the region of the first openings is higher than that of the particles passing through the second openings.

2. A filter according to Claim 1, wherein the filter element is of tubular form.

3. A filter according to Claim 2, wherein the filter element is arranged such that fluid flows, in use, in a generally radially inward direction from the exterior of the filter element to the interior thereof.

4. A filter according to Claim 2, wherein the filter element is arranged such that fluid flows, in use, in a generally radially outward direction from the interior of the filter element to the exterior thereof.

5. A filter according to any of Claims 2 to 4, wherein the second, relatively large openings are located upstream of the first, relatively small openings.

6. A filter according to Claim 5, wherein the filter element includes, downstream of the openings, a region which is free of perforations serving to define, in part, a chamber or junk basket in which particles separated from the supply of fluid will tend to collect.

7. A filter according to any one of the preceding claims, further comprising a housing within which the filter element is located.

8. A filter according to Claim 7, wherein the housing is generally cylindrical.

9. A filter according to Claim 7 or Claim 8, wherein the housing forms part of a stabilizer element

10. A filter substantially as hereinbefore described with reference to the accompanying drawings.

11. A filter comprising a housing, a filter element being located within the housing, wherein the filter element defines a first flow path, the first flow path being adapted to restrict the passage of particles there through, and a second, constantly open flow path. s