GB2115040A - Method of limiting sand production in wells - Google Patents

Abstract

The co-production of sand in wells, for example in oil wells, gas wells or water wells, is limited by locating in the well, in the region of that part of the surrounding formation 3 from which the sand arises, a permeable metal tube 8 which has pores providing a tortuous path through the tube. <IMAGE>

Description

SPECIFICATION Method of limiting sand production in wells This invention is concerned with the problems resulting from the co-production of sand in wells and is a method of limiting said sand production.

Many oilfields around the world are based upon oil accumulations occuring in relatively poorly consolidated sand. These include both land-based and off-shore fields but shallow land fields are particularly liable to be of this type. In such a field, it is very difficult to prevent sand being carried into the well, and in turn to the well-head, with the oil extracted from the reservoir. Furthermore, many other oil wells which do not initially produce sand are found to enter a period, especially later in their oil-producing lives, in which sand production becomes a problem.

The consequences of sand being extracted with oil from a well are considerable and expensive an may even be irremediable. For example, erosion of well-head equipment may be substantial, sand blocking of equipment or even of a well itself may occur and the formation of cavities in the reservoir and consequent collapse of unsupported cap-rock or casing may lead to the irreversible loss of a well.

Therefore many attempts have been made to prevent or minimise sand production in oil wells.

One method attempted has been to inject a curable synthetic resin into the sand formation.

However this method inevitably reduces the flow rate of oil from the well and may even block off oil flow completely.

Another method attempted has been to prefabricate a sand filter from a combination of a coarse particulate material and a bonding synthetic material. For example, granular materials such as chopped walnut shells or chopped coconut shell have been bonded with a ureaformaldehyde resin in order to prefabricate a filter in this way. However, such materials have not proved viable in practice, as they have tended to break down under the influence of oil or salt water at well temperatures and pressures. In addition, they have given rise to a reduction in oil flow rate from the well.

Because of these difficulties, the most widely used method of sand control in oil wells is the "gravel-packing" method, which itself is difficult to practise successfully and extremely difficult to control. In this method, an approximately annular packing of gravel is formed in situ down the well by feeding a slurry of gravel in carrier fluid, e.g.

water, into the annular space between the oil reservoir and a slotted screen located centrally in the well. The difficulty of carrying out this operation remotely at the foot of a well, at a distance of up to three miles below the surface and perhaps several miles out to sea, will be immediately apparent. Particularly in the case of a highly-deviated well, that is one which is intentionally inclined at a substantial angle to the vertical, in which gravitational effects operate at an angle to the axis of the well, it is extremely difficult to produce a reasonably uniform, symmetrical gravel packing.Much research has been carried out in attempts to overcome this problem, for example as described by Peden and Oyeneyin in the article "Gravel packing in deviated wellbores - a research review" appearing in the journal "Offshire Services and Technology", November 1981 at pages 15-23. However, satisfactory controllable gravel packings continue to be an elusive target.

In water wells, that is wells in which for example fresh water is produced for drinking or irrigation purposes or salt water is produced for oilfield water injection, similar sand production problems can arise.

To some extent, sand blocking is less of a problem in gas wells than in oil or water wells in that the gas is more mobile than oil or water but nonetheless rock collapse and erosion or other damage to equipment is undesirable and may prove expensive. For this reason, efforts continue to be made to reduce sand formation in gas wells, for example in wells producing natural gas. Thus a recent report in the journal "European Offshore Petroleum" for March tilth, 1982, at page 6, describes the development of a chemical method for countering sand production in the large onshore natural gas field at Groningen in Holland.

Similarly, although we are unaware of any specific reported activity in the area, it is to be expected that sand production can also be a problem in wells producing gases other than naturally occurring hydrocarbon gases, for example in wells drilled specifically to produce carbon dioxide.

It is against this background of prior attempts'to reduce sand production, many of which have proved to be either difficult to apply or unsatisfactory in practice, that the present invention has been made. According to the present invention, a method of reducing sand production in a well comprises locating in the well, in the region of the reservoir sand, a permeable metal tube having pores which provide a tortuous path through the tube.

It is not intended that the word "tube" should imply a particular cross-sectional shape. For example, while it may well prove convenient in forming the tube that it should have a uniform circular crnssection - and indeed this is the preferred cross-sectional shape, it is also possible for the tube to be of a triangular, square, rectangular or other polygonal cross-section. The tube may be formed by extrusion but it is preferred that it be shaped from a flat sheet by rolling or folding. If desired, the tube may consist of a plurality of tubular lengths of permeable metal in end-to-end alignment.

The permeable metal of which the tube is made may be any material having the necessary tortuous porous structure. Thus it may be of knitted or woven cloth (which may have been strengthened by sintering), a crumpled mesh, a random or ordered mat or other mass of fibres, a sintered mass of metal particles or a reticulated foam. The tortuous porous structure may be enhanced in the case of metal cloths by using a superimposed plurality of layers. Among the above materials, the knitted metal cloth sold under the trade mark "Knitmesh" is one particularly suitable product and the woven metal cloth sold under the trade mark "Rigimesh" is another. However, much to be preferred is a metallic skeletal foam product, for example the product sold under the trade mark "Retimet".

Depending upon the length and structural strength of the selected permeable metal product of which the tube is formed, the tube may be selfsupporting or may be supported in an appropriate way. Thus a tube of "Retimet" metallic skeletal foam may be left self-supporting although in practice additional support will help to add life and strength to the tube. In a preferred arrangement, the permeable metal tube is disposed co-axial with a tubular, slotted liner and secured thereto before installing in the well. Thus the metal tube may be disposed within a metal liner, in which arrangement the tube is afforded by the liner some protection against impact damage in transit or in situ, or the tube may surround the liner, in which arrangement it is supported better in operation but is less well protected against impact damage.

Alternatively, the advantages of internal and external liners may usefully be combined by locating the tube between two liners, disposed generally co-axially with respect to the tube and to each other.

The overall dimensions of the tube are related to the dimensions of the well and thickness of the oil reservoir. Thus the length of the tube may conveniently be between 10 and 200 metres, especially between 20 and 100 metres. The outside diameter of the tube will be slightly less than the diameter of the wellbore and may therefore conveniently be between 10 and 25 centimetres, especially between 1 5 and 20 centimetres. Thus a typical tube may be about 30 metres long and about 1 5 cm. outside diameter.

In selecting the material of which the tube is formed, it is important to take average pore size into account. It is highly desirable that the pore size throughout the material be as uniform as possible but in any case the average pore size should be selected bearing in mind the sand grain sizes encountered in the well concerned. Most of the sands encountered in oil production are between 0.02 and 2.0 mm diameter grain size and those which are particularly undesirable as coproducts in oil production are those of grain diameter between 0.02 and 1.0 mm. The average pore size of the tube may range from a fraction of the size of the relevant sand grains to a multiple of the grain size. However, the preferred average pore size of the metal tube is of the order of 0.5 to 6, especially 2 to 5, times the diameter of the smallest sand grains which it is desired to retain.

In general, average pore diameters of about 0.01 to about 5 mm are usually preferred, especially average pore diameters of about 0.04 to about 1.0 mm.

In one form of our invention, two or more coaxially-disposed permeable metal tubes may be located in the well. Thus, where the sand to be excluded from the well consists of grains of widely varying sizes, coaxial tubes of different pore size may be used. For example, an outer tube of coarser pore size, say 1.0 mm, may surround an inner tube of finer pore size, say 0.2 mm. When two or more coaxial tubes are used in this way, the sintered woven product sold under the trade mark "Rigimesh" is of particular value for this purpose, as it is readily available in sheet form in a wide range of pore sizes.

It is one advantage of the method according to the present invention that it can be practised without requiring any new handling techniques.

This is an important consideration as it has heretobefore been necessary to develop speciai techniques for the remote emplacement of a gravel packing down a well. In contrast, well established methods are regularly used for directly placing generally tubular structures down wells.

Thus well casings and cylindrical screens and liners of various types are conventionally placed in well bottoms as a feature of normal operations; locating, according to the present invention, a permeable metal tube at an appropriate place within the well is an operation well within the skill of those experienced in the oil production art.

In the case of the previously proposed chemical treatments, in general they require more complicated procedures than that of the present invention, usually entailing careful control techniques. In addition, the method according to the present invention is far less likely to lead to irreversible reductions in flow rate than are chemical methods of reducing sand production.

During normal production operations, for example oil production, from a well having the feature according to the present invention, by careful choice of the average pore size of the metal tube, sand production may be considerably reduced without significant adverse effect on flow rates, since any sand displaced from the product bearing formation may simply redistribute itself, preferably upon the surface of but alternatively within the interstices of, the permeable material of the metal tube or tubes. If, however, any reduction in flow rate occurring during production appears to be attributable to possible blocking of the tube interstices, this is readily rectified by reversing the flow through the tube ("back-flushing") for a short period. If desired, the back-flushing may be carried out using a liquid introduced into the well for that specific purpose, e.g. using an aqueous acid or acid-former, diesel oil, brine or a sequence of washes with different liquids.

The invention will be further described with reference to the accompanying single figure of drawings, in which three wells incorporating three different embodiments of the invention are illustrated, largely diagrammatically.

Claims (11)

In the drawings, three undersea wells A, B and C extend from the sea-bed 1 through successive rock formations indicated generally by the nurneral 2 into an oil-bearing permeable layer 3 of CLAIMS

1. A method of limiting sand production in wells comprising locating in the well, in the region of the reservoir sand, a permeable metal tube having pores which provide a tortuous path through the tube.

2. A method as claimed in claim 1, in which the tube is of uniform circular cross-section.

3. A method as claimed in claim 2, in which the outside diameter of the tube is within the range 10 to 25 centimetres.

4. A method as claimed in any of the preceding claims, in which the length of the tube is within the range 10 to 200 metres.

5. A method as claimed in any of the preceding claims, in which the average pore size of the permeable metal tube is within the range from about 0.01 to about 5 millimetres.

6. A method as claimed in claim 5, in which said average pore size is within the range from about 0.04 to about 1.0 millimetres.

7. A method as claimed in any of the preceding claims, in which two or more co-axially-disposed permeable metal tubes are located in the well.

8. A method as claimed in claim 7, in which a radially outer tube of coarser pore size surrounds an inner tube of finer pore size.

9. A method as claimed in any of the preceding claims, in which the permeable metal tube comprises a knitted or woven cloth, a crumpled mesh, a random or ordered mat or other mass of fibres, a sintered mass of metal particles or a reticulated foam.

10. A method as claimed in claim 9, comprising locating in the well a permeable metal tube of metallic skeletal foam.

11. A method as claimed in any of the preceding claims, in which the permeable metal tube is disposed co-axial with at least one tubular, slotted liner to which it is secured before being installed in the well.