The use of porous particles to introduce oil field chemicals into a well is known for example from GB 2 284 223 A, and from GB 2 298 440 A. The particles might be used in the form of a pre-packed screen, or might be used in a gravel packing process, or in a formation fracturing process, or in a combination of such processes. Such a process has been proposed in particular for introducing scale inhibitor. US 3 199 591 suggests the use of a biocide impregnated into the pores of a porous proppant material and used in a fracturing process.

The introduction of bacteria into wells has also been suggested, as has the introduction of nutrients to encourage bacteria which are naturally present to grow.

There are several potential benefits that can derive from applying bacteria to hydrocarbon-producing wells, for example enhanced oil recovery, breaking of gels used during stimulation, and inhibition of corrosion, scale, wax, hydrate and asphaltene deposition. The benefits may derive from the bacteria's metabolic products, for example biosurfactants, organic acids, ketones and alcohols, or from direct metabolisation of crude oil or polymers used to increase the viscosity of fracture fluids. For example an article by R. S. Bryant and J.

Douglas (SPE 16284) presented at the SPE international symposium in San Antonio, Texas, Feb. 4-6,1987 describes experimental work studying the effects of injecting micro-organisms and nutrients into sandstone cores, while an article by F. G. Brown (SPE 23955) describes the use of micro-organisms for treating oil well systems and oil

reservoirs to control paraffin wax deposition and to increase production rates.

According to the present invention there is provided a method of treating an oil well wherein porous particles are supplied into the oil well, the porous particles containing micro-organisms.

The porous particles are preferably of a ceramic material, of generally spherical shape, and are preferably of porosity no more than about 30 percent, for example in the range ten percent to 20 percent. They are typically of size between about 0.3 mm to 5.0 mm, more preferably between 0.5 mm and 2.0 mm for example 1.0 mm.

They may be supplied into the well packed into a pre- packed screen in the form of a filter bed, or may be used as a gravel pack within the oil well and the perforations, or alternatively if the particles are sufficiently strong they may be used as fracture proppants in a fracture process and thereby be injected into cracks in the formation well away from the well bore. The porous particles may be used on their own, or may be mixed with other particles which may be non-porous or may be porous and contain oil field chemicals, nutrients for the micro-organisms, or other micro- organisms.

The porous particles preferably comply with the standards set out in the API recommended practices for testing gravel material, or those for proppant material (depending on how they are to be used). These specify criteria for particle shape, for acid resistance, and for crush resistance. The criteria for crushing strength depend on the particle size; for example particles of size 20-40 mesh (0.42-0.84 mm) for use as proppants must not lose more than 14 percent by mass at a closure

pressure of 4000 psi (28 MPa).

The micro-organisms are preferably bacteria, generally facultative anaerobic bacteria, because the down-hole environment usually lacks oxygen, and such bacteria can survive in the presence or absence of oxygen. The bacteria are typically of size about 0.1 to 0.3 micrometres across and 1 to 4 micrcometres long. Some types may be motile, so that they are capable of moving through a liquid. They may also attaclh themselves to surfaces within the particles. The miccro-organisms may initially be in a dormant state within the particles; and the particles may also include nutrientts for the micro- organisms. When contacted by an aqueou. zs phase after injection into the well, hydration of tthe micro- organisms, possibly combined with the effect of increased temperature, initiates their metabolic activity.

A benefit of this way of supplying bacteria down- hole is that the bacteria can grow and multiply in a substantially static environment withim the pores of the particles without the risk of being carried along with flowing fluids and so being carried outt of the well.

Nevertheless, bacteria may migrate from within such porous proppant particles into adjacentt parts of the formation and may be able to attach themselves to surfaces of the fractured formation. an any event metabolites produced and secreted by the bacteria will be released into the fluids, and well fluids will diffuse into contact with the bacteria within the pores.

The invention will now be further described by way of example only.

When it is desired to enhance the permeability of a formation comprising oil-bearing strata in the vicinity

of an oil well, it is known to inject a fluid into the well such that the pressure at the depth of those strata is sufficient to cause fracturing of the rocks of the strata. The fluid injected into the rocks may contain a dissolved polymer which may be cross-linked to form a gel (so it is of high viscosity), and may include particles of solid material such as sand grains or ceramic spheres which are carried into the fractures by the injected fluid. When the pressure is reduced the particles prevent the fractures closing. Such particles may be referred to as proppant particles. Typically the fractures may extend as much as 20 m or even 50 m or more out from the well bore, and the proppant particles will be distributed throughout the length of every fracture.

Where a producing section of an oil well extends through a poorly consolidated formation (or stratum) it is known to prevent inflow of sand particles from the formation by means of a gravel pack. This consists of a filter bed of small particles filling all the space between a tubular fluid-permeable screen within the well bore, and the wall of the well, and extending into the perforations. Such particles are usually referred to as gravel, although they may be substantially identical to those referred to as proppants; as a general rule particles for use as gravel do not have to be as strong as those for use as proppants.

In either of these situations it may be desired to reduce water inflow. This may be achieved by using, as the proppant or gravel particles, porous ceramic beads of diameter 0.7 mm and of porosity 15 percent containing nutrients and bacteria. The bacteria may be dormant, for example in the form of spores which will not grow until they are re-hydrated. Suitable bacteria include strains of Leuconostoc mesenteroides, which produce a long-chain

polymer of glucose, referred to as dextran; strains of Lactobacillus confusus, which produce a similar polymer from sucrose; strains of Zoogloea ramigera, which produce a polysaccharide polymer gel; and strains of Bacillus circulans, which produce a cyclodextrin. Such bacteria, if provided with suitable nutrients, such as a source of carbohydrate and organic phosphate, produce polymers which diffuse into the surrounding water phase and increase its viscosity, consequently reducing the flow rate of the water. It will be appreciated that the strains must be selected taking into account the temperature of the formation--it may be necessary to use mesophilic strains (where the expected temperature is in the range 20-45°C), or thermophilic strains (where the expected temperature is in the range about 45-65°C).

In the case of a fracture treatment, at least some of the proppant particles may comprise porous ceramic beads as described above containing bacteria and nutrients. The high viscosity fluid might for example contain a polymer such as guar gum and a cross-linking agent such as borate at a pH above 9.5; it is important to ensure this polymer subsequently breaks down.

Hydration of the proppant and the increase of temperature when the proppant is placed in the formation fractures initiates metabolic activity of the bacteria. In this example the bacteria may be of a strain which metabolises the polymer directly; alternatively the secreted products of the bacterial metabolism might reduce the pH to below the value at which the gel is stable. For example strains of Bacillus licheniformis can grow in anaerobic conditions, and produce acids when provided with sucrose as a nutrient.

In some oil wells, the crude oil contains a significant proportion of long chain hydrocarbon

molecules which may form wax deposits particularly in the well bore itself, as the pressure of the fluid decreases.

Such a well may be treated by providing porous ceramic particles (as described above), either as proppants in a fracture procedure or as gravel in a gravel pack or pre- pack, the porous particles containing bacteria which metabolise long chain molecules into shorter chain molecules. As a by-product the bacteria may also produce organic acids or alcohols which may act as surfactants or as solvents. For example the bacteria might crack the long chain n-alkane C1gH40, which is a soft wax, forming the shorter n-alkanes C8H18, C7H16 and C2H6 which are of considerably lower viscosity. It will be appreciated that the crude oil may contain waxy polymers of a wide range of molecular weights, and that it may be beneficial to provide a variety of strains of bacteria which preferentially metabolise polymers of different lengths.

In some wells the crude oil may contain significant quantities of the dangerous gas hydrogen sulphide (H2S), which may be produced by sulphate-reducing bacteria within the oil-bearing formation. It may be possible to suppress hydrogen sulphide generation by subjecting the formation to a fracture process in which at least some of the proppant particles are porous beads (as described earlier) which contain bacteria which consume sulphate ions, so that the sulphate ions are not available to any sulphate-reducing bacteria which may be present in the formation.

Another problem which occurs in some wells is scale formation, the scale consisting of a mixture of insoluble salts for example of carbonates and sulphates of calcium and barium. This can be suppressed by treating the well either by gravel packing or fracturing, using particles

at least some of which are porous beads (as described earlier) which contain bacteria. In this case the bacteria are strains whose metabolism produces products such as organic acids, polysaccharides, polyphosphates or glycolipids for example, which may act as chelating agents for the cations or which may act as antinucleation agents to prevent crystal growth.

From the above examples it will be seen that the present invention enables bacteria to be used to achieve a range of different results. In some cases the desired result is the metabolic breakdown of components in the oil; in other cases the desired result is the metabolic breakdown of chemicals added to the well; in other cases the desired result is the consumption or assimilation of chemicals which occur in the crude oil; and in other cases the desired result is brought about by the secretion of products of the metabolism of the bacteria.

In some cases the bacteria will require additional nutrients, which may be initially provided within the porous particles; under some circumstances it may be necessary periodically to replenish the nutrients by squeezing fluids containing such nutrients into the well.