This invention relates to a method for preparing a plugging liquid on emulsion basis and on polysaccharide basis for plugging a zone of a subterranean formation penetrated by a bore hole.

A large number of gel forming or thickening compositions are known for use as well control liquids in the drilling of wells for recovery of oil and gas, for instance in cases where a well liquid is lost to the formation, or liquid or gas is flowing into the well from the formation. It is previously known to use polysaccharides for thickening purposes in such known plugging liquids.

Thus, for plugging a zone of a subterranean formation around a bore hole, Norwegian Patent Application No. 931954 discloses a plugging liquid consisting of an emulsion comprising: a) 5 - 50% by volume of a continuous phase containing: 800 - 998.5 ml/l of a hydrophobic liquid, 0.5 - 100 ml/l of an emulsifier, and 1 - 100 g/l of a crosslinking agent for a polysaccharide, b) 50 - 95% by volume of a discontinuous phase containing: 950 - 997 ml/l of water, and 3 - 50 g/l of a polysaccharide.

When this plugging liquid is subjected to high shear forces, e.g. by being forced through the nozzles of a drill bit, wherein the pressure drop will be in the range of about 50 to 120 bars, it thickens to a gelatinous solid. The mechanism of the thickening of the plugging liquid is a crosslinking of the polysaccharide with the crosslinking agent. It is therefore decisive for the performance of the plugging liquid that the polysaccharide and the crosslinking agent be kept separated from one another until the thickening of the drilling liquid is desired. According to said NO 931954, such separation is obtained by said crosslinking agent being contained in the

hydrophobic liquid (e.g. an oil), whereas the polysaccharide is contained in the discontinuous phase of the emulsion, which is an aqueous phase.

As long as the emulsion is at rest or is only subjected to moderate shear forces, the polysaccharide and the crosslinking agent remain separated from one another in their respective phases of the emulsion and no crosslinking reaction of any significance takes place. In this condition (Condition 1), the plugging liquid will have physical characteristics allowing it to be easily pumped, e.g. through a mud system. However, when the emulsion is subjected to high shear forces, e.g. by being forced through a drill bit in a bore hole, the polysaccharide and the crosslinking agent will come into contact with one another, whereby crosslinking of the polysaccharide takes place and the plugging liquid thickens.

For the plugging liquid to fulfil its function it is important that it (1) has the lowest possible viscosity in Condition 1, before it is subjected to high shear forces, (2) thickens to a high thickness in Condition 2 after having been subjected to high shear forces, (3) maintains its acquired thickness for a desired period of time, and (4) to the least possible extent undergoes a thickening in Condition 1 as a result of the emul- sion gradually deteriorating during storage or before having passed through the nozzles of the drill bit in the bore hole.

The plugging liquid according to NO 931954 satisfies these requirements to a surprisingly high extent.

However, the plugging liquid disclosed in NO 931954 is encum- bered with certain disadvantages regarding its preparation per se, because the preparation necessitates using two mixing tanks, viz. one tank for mixing those components which should constitute the continuous hydrophobic phase (the oil phase), and one tank for mixing those components which should consti- tute the discontinuous phase (the aqueous phase). This has been considered to be necessary in order to maintain the poly- saccharide separated from the crosslinking agent until the

time of the desired reaction between them, usually after said emulsion having been forced through the nozzles of the drill bit in the bore hole. Such production process requiring two mixing tanks will require more space than would be desirable, e.g. on an offshore oilrig.

It has now been found that the same effect of keeping the polysaccharide and the crosslinking agent separated from one another until a reaction between them is desired can be achieved by using a different method requiring less space and equipment.

Thus, the invention now provides a method for preparing a plugging liquid for plugging of a zone of a subterranean formation around a bore hole, consisting of an emulsion com- prising a continuous phase based on a hydrophobic liquid and a discontinuous phase based on a hydrophilic liquid, which emulsion contains an emulsifier, a polysaccharide and a cross- linking agent for said polysaccharide. The method is charac- terized by the steps of: introducing the hydrophilic liquid and the hydrophobic liquid into a mixing tank without agitation, introducing the emulsifier into said mixing tank and starting agitation, introducing in any desired order the polysaccharide and the crosslinking agent into the mixture in the tank, each of them pre-wetted by a hydrophobic liquid, and agitating the whole mixture until an emulsion is obtained.

The hydrophilic liquid of the emulsion, which is usually water, will acquire a small content of polysaccharide. This results in the emulsion developing some viscosity which con- tributes to the forming of a stable emulsion. For illustration purposes, the use of water as a hydrophilic liquid will be referred to, although any suitable hydrophilic liquid may be used instead of water.

In the new method only one mixing tank is used, instead of the two mixing tanks utilized in the previously known method. This

makes the new method less space and equipment requiring and less exposed to pollution from conduit systems, pumps, etc., than the method disclosed in NO 931954.

When the emulsifier has been introduced into the mixing tank and the agitation has been started, which results in the water, the hydrophobic liquid and the emulsifier being mixed together, an emulsion of the oil-in-water type is formed in the mixing tank. Such emulsion is not the type of emulsion which is desired for the final plugging liquid, viz. an emul- sion of the water-in-oil type. However, after addition of the polysaccharide and the crosslinking agent, each of them pre- wetted with a hydrophobic liquid, and agitation of the resul- ting mixture, an inversion of the formed oil-in-water emulsion takes place, whereby an emulsion of the water-in-oil type is formed, which emulsion contains a continuous phase on the basis of the hydrophobic liquid, the emulsifier, the polysac- charide and the crosslinking agent for the polysaccharide, and a discontinuous phase based on the water.

As already mentioned, the prepared plugging liquid is stable and is normally not affected by being pumped with a centri- fugal pump or a mud pump for several hours. However, extended agitation/pumping should be avoided beyond the point where a stable emulsion has been formed, to avoid an unintended thick- ening of the emulsion in Condition 1 as a result of the cross- linking agent coming in direct contact with the polysaccha- ride, whereby crosslinking takes place.

On the other hand, when the emulsion is subjected to high shear forces, such as when forced through the nozzles of a drill bit at a pressure drop of 40 to 120 bars, e.g. about 60 bars, the emulsion thickens markedly after 2 to 10 minutes. If desired, however, the thickening time can be made shorter.

The hydrophobic liquid of the plugging liquid of the invention may advantageously be selected from mineral oils, vegetable oils, esters and ethers. It is selected on the basis of con- ventional criteriae such as viscosity, stability, compatibili-

ty with the drilling mud, environmental leniency and avail- ability.

For the purpose of obtaining a stable emulsion a suitable emulsifier is used. The emulsifier may be selected from a broad range of commercially available emulsifiers. The emulsi- fier should preferably have an alkali resistance such that it is stable in the pH range of 11 to 13 at typical bore hole temperatures. Emulsifiers on a triglyceride basis are very suitable for use in the plugging liquid. Based on conventional criteriae which will be well known to those skilled in the art, such combinations of hydrophobic liquid and emulsifier are selected which will provide the desired pH stability and the desired emulsion strength for handling and storing the emulsion, but which result in a breaking of the emulsion when the emulsion is pumped for instance through the nozzles of a drill bit down in a bore hole or through a port in a comple- tion string.

The polysaccharide serving to achieve the desired hardness of the emulsion in Condition 2 may for instance be selected from any of the polysaccharides previously utilized for well con- trol. The currently most preferred polysaccharides are xan- thanes, alginates and carboxymethyl cellulose, due to their combination of good crosslinking properties and viscosity characteristics.

Any of the commonly used cross linking agents for polysaccha- rides may be used, especially such crosslinking agents that are stable at pH values in the range of 11 to 13. A preferred class of crosslinking agents is constituted by those contain- ing divalent or trivalent metal ions as the active moiety.

Examples of such crosslinking agents are Ca(OH)2, CaSO4 and Al2(S04)3. Another class of preferred crosslinking agents is constituted by aldehydes, such as glutaric aldehyde.

In addition to the above-mentioned main constituents the plug- ging liquid of the invention may contain a water-absorbing material. Similarly as the polysaccharide and the crosslinking

agent, the water-absorbing material must be wetted with a hydrophobic liquid before being introduced into the mixture.

The water-absorbing material is thereby incorporated into the continuous phase, i.e. the "oil" phase. As water-absorbing material, a clay mineral is preferred, in particular bento- nite. By being incorporated in the "oil" phase, the water- absorbing material will be kept separated from the water contained in the aqueous phase until the emulsion is broken by being subjected to high shear forces. The water-absorbing material will then come into contact with the water contained in the aqueous phase of the emulsion and will absorb excess amounts thereof, whereby said material will undergo swelling, as explained in US Patent No. 4,663,366. However, in the plugging liquid prepared by the method of the invention, the function of the water-absorbing material is not primarily to provide the thickening of the liquid aimed at in said US patent but to absorb free water after the breaking of the emulsion and thus to prevent a shrinking of the material in Condition 2.

In Condition 1 the plugging liquid should be maintained under gentle agitation and at a liquid temperature below 40"C. In preferred embodiments the plugging liquid contains no environ- mentally harmful or noxious constituents.

As mentioned above, the plugging liquid is usually not affec- ted by being pumped with a centrifugal pump or mud a pump for several hours. However, when the emulsion is subjected to high shear forces, it thickens markedly after 2 to 10 minutes. The thickening time can be predetermined through an appropriate selection of the amounts of crosslinking agent and emulsifier, especially of the amount of emulsifier. An increased amount of emulsifier increases the thickening time, whereas an increased amount of crosslinking agent reduces the thickening time.

In the method of the invention, the constituents are prefer- ably mixed in such amounts as to form an emulsion which per 100 parts by volume of hydrophobic and hydrophilic liquid contains 5 to 50 parts by volume of hydrophobic liquid and 95

to 50 parts by volume of hydrophilic liquid, which emulsion further contains 0.5 to 100 ml/l of emulsifier, 3 to 50 g/l of polysaccharide, and 1 to 200 g/l of crosslinking agent for the polysaccharide, based on the total amount of emulsion.

More preferably, the constituents are mixed in such amounts as to form an emulsion which per 100 parts by volume of hydro- phobic and hydrophilic liquid contains 10 to 30 parts by volume of hydrophobic liquid and 90 to 70 parts by volume of hydrophilic liquid, which emulsion further contains 1 to 50 ml/l of emulsifier, 3 to 30 g/l of polysaccharide, and 2 to 50 g/l of crosslinking agent for the polysaccharide, based on the total amount of emulsion.

The plugging liquid prepared by the new method may be given a content of a weighting material in addition to the consti- tuents already mentioned, whereby the plugging liquid may be used for inhibiting or preventing undesired blow-outs into or from a well bore. The purpose of using a weighting material in the plugging liquid is to increase the density thereof so as to put the plugging liquid in a better condition to resist the blow-out pressure in the well bore. The weighting material must be pre-wetted with a hydrophobic liquid and is added in a last stage of the method of the invention.

Any weighting material can be used in the weighted plugging liquid. Exemplary weighting materials are barite, ilmenite, hematite, steel balls and calcium carbonate. A particularly suitable weighting material is barite.

The other constituents of the weighted plugging liquid, i.e.

the hydrophobic liquid, the emulsifier, the polysaccharide, the crosslinking agent for the polysaccharide, and the hydro- philic liquid, are selected as for the non-weighted plugging liquid, as has been described above in connection with the preparation of the non-weighted plugging liquid.

When the plugging liquid is to be used for permanent closing of a bore hole, the plugging liquid may if desired contain an

additive to prevent circulation loss, such as crushed nut shells or mica folium. This additive is added at the end, optionally together with the weighting material, if such material is also used.

The mud system employed in well and formation treatment opera- tions consists of a number of units, each of which exerts shear stress to a greater or lesser extent on the flowing liquid. The shear stress is closely related to the pressure drop in each individual unit. The highest pressure drop and thus the highest shear stress occurs e.g. during the passage of a port in a completion string or the nozzles in a drill bit. The use of the prepared plugging liquid is based on the condition that only flow restrictions down in the well should produce a sufficiently high shear stress for the reactants (polysaccharide and crosslinking agent) to come into contact with one another, with a resulting crosslinking of the poly- saccharide and a setting of the liquid.

The invention is shown in more detail in the following exam- ples.

ExamPle 1 For preparation of about 1 litre of an emulsion of the water- in-oil type, 100 ml of a linear a-olefin and 900 ml of water were introduced into a 2 1 beaker without agitation, whereupon 2.2 ml of a fatty acid emulsifier were added to said beaker and agitation was started. Under continuous agitation, 6 grams of Ca(OH) 2 were added and then 6 grams of xanthane gum, both pre-wetted with a hydrophobic liquid (linear a-olefin). After the admixing, the formed emulsion was stabilized by being stirred for 5 minutes.

The obtained stabilized emulsion was pumped at a pressure of 50 bars through a nozzle and down into a cup, wherein it set in the course of 2 minutes. After 24 hours the shear stress was measured to be 6000 Pa.

Example 2 For preparation of about 1 litre of an emulsion of the water- in-oil type, 250 ml of a linear a-olefin and 750 ml of water were introduced into a 2 1 beaker without agitation, whereupon 2.5 ml of a fatty acid emulsifier were added to said beaker and agitation was started. Under continuous agitation, 25 grams of bentonite were added, then 6 grams of Ca(OH) 2 and finally 10 grams of alginate, all pre-wetted with a hydropho- bic liquid (linear a-olefin). After the admixing, the formed emulsion was stabilized by being stirred for 5 minutes.

The obtained stabilized emulsion was pumped at a pressure of 50 bars through a nozzle and down into a cup, wherein it set in the course of 0.5 minute. After 24 hours the shear stress was measured to be 9000 Pa.