NO874066L - PROCEDURE FOR SEALING BASIC FORMS ON A SEA. - Google Patents

Description

The present invention relates to an improved method for sealing or sealing selected zones in an undersea foundation formation. More specifically, this invention relates to an improved method of sealing such subsea zones by successive injections therein of an aqueous solution of a water-soluble silicate material, an aqueous backfill material and an aqueous hydraulic cement slurry.

During the drilling, completion and installation of oil and gas wells, it is known that selected zones in a subsea foundation formation can be sealed or sealed by injecting an aqueous sodium silicate solution into it. In connection with such a treatment, solidification of said sodium silicate solution can be effected by bringing said solution into contact with a suitable salt solution at the desired subsea location or by adding to said solution, before injecting it into the base formation, a reagent which causes a controlled, gradual gelation in said sodium silicate solution.

It is also known that the previously mentioned sodium silicate gel formation can be followed by injection of a hydraulic cement slurry. However, since hydraulic cement slurries rapidly solidify and harden (i.e., bind) upon contact with such sodium silicate material, it is common practice to use a water fill material to separate the cement slurry and the silicate solution during injection of these into the subsea bedrock formation of interest.

Although the use of a water filler during injection to separate the cement slurry from the silicate solution can be quite effective in preventing premature mixing of these, it is nevertheless still possible for the materials to prematurely come into contact with each other and solidify in an unwanted place during the injection, such as e.g. in the borehole itself or in an unplanned position within the subsea foundation formation. Accordingly, it would be highly desirable to have a means of controlling (eg retarding) the rate of solidification exhibited by a mixture of such materials, thereby ensuring or achieving continued mobility or pumpability of such a mixture for a limited period of time after the individual silicate solution and cement slurry ingredients thereof come into direct contact with each other.

It has now been discovered that if an aqueous silicate solution and a hydraulic cement slurry are mixed together in the presence of an effective amount of a water-soluble, polybasic inorganic phosphate salt, solidification and curing of the resulting mixture is significantly delayed (eg, for a period of up to several hours), compared to the almost instantaneous solidification (e.g. in just a few seconds) which otherwise occurs in the absence of said phosphate salt.

The discovery lies in one aspect of the present invention in the form of an improved method for sealing a zone in an undersea foundation formation by continuously injecting an aqueous solution of water-soluble silicate material followed by an aqueous filling material and an aqueous hydraulic cement slurry, characterized by mixing in said silicate solution , said aqueous backfill material or said hydraulic cement slurry, of a water-soluble, inorganic polybasic phosphate salt to reduce the rate of gelation that occurs when said silicate material, backfill material and hydraulic cement are mixed.

In another aspect, the present invention is a method for retarding gel formation in an aqueous mixture of a water-soluble silicate material and having in said mixture a water-soluble, polybasic inorganic phosphate salt in an amount sufficient to substantially reduce the solidification rate of said mixture .

The aforementioned method and improved process is particularly useful in those cases where it is desirable to seal or seal a selected zone of a subsea foundation formation such as, for example, where it is desirable to seal a lost circulation zone during a well drilling or cementing operation; to reduce or eliminate unwanted salt flow in the base formation of a producing well; to block areas of high formation permeability in an injection well in connection with an enhanced oil collection operation; beer..

Silicate materials useful in the use of the present invention include those water-soluble silicate materials whose aqueous solutions are already known in the art to produce relatively stiff gels by contacting or mixing with concentrated sodium-based saline solutions or with calcium-containing saline solutions or by having their pH adjusted to a rapid solidification region by the addition of an acid or an acid-forming material. Such silicate materials include the various alkali metal salts of silicic acid such as e.g. lithium, sodium and potassium salts thereof.

A particularly preferred water-soluble silicate material for use in the application of the present invention is an aqueous sodium silicate solution which has a total content of solid dissolved substances of 38% by weight and has a weight ratio between silicon dioxide (SiO 2 ) and sodium oxide (Na 2 0 ) of approx. 3.2:1. However, sodium silicate solutions having different SiO 2 :Na 2 O ratios (e.g., ranging from 0.5:1 to 4:1) may be used instead if such should prove desirable from a practical standpoint such as cost, availability, etc. Similarly, the silicate material can initially be obtained in the form of aqueous solutions with different solids contents or in solid form. However, the latter option is typically less desirable as special equipment is required to transfer the solid material into an aqueous solution before it is injected into the subsea bedrock formation of interest.

When the previously mentioned aqueous silicate solution is used in the application of the present invention, it is typically diluted before injection into the base formation to a silica content of from 5 to 25% by weight. Preferably, the SiO 2 content of the solution during the injection is from 10 to 20% by weight.

In the present invention, it is typically desirable to cause the soluble silicate solution to solidify after it has reached the desired location in the subsea foundation formation, and then to utilize the resulting silicate gel to cause the subsequently injected aqueous hydraulic cement slurry to be controlled, delimited and/or placed in a special, desired place within said basic formation. Any commonly known technique can be used to start the initial solidification of the silicate solution. For example, if the zone of interest already contains relatively concentrated sodium-based salt solution or a calcium-containing salt solution, the desired solidification will occur spontaneously when the silicate solution comes into contact with said salt solution. Alternatively, injection of the silicate solution may follow injection of a quantity of a suitable, naturally occurring or synthetic salt solution. In the latter case, of course, a fill material of fresh water will typically be pumped between said salt water injection and said silicate solution to prevent mixing and accompanying solidification of these until after said materials exit the borehole and enter the base formation area of interest.

As yet another alternative, the initial solidification of the injected silicate solution can be initiated by mixing it in (i.e. at the soil surface prior to pumping or injection)

of a suitable reagent to effect solidification in a sufficiently gradual manner to permit application of said material prior to solidification or curing thereof. E.g. a strongly acidic material such as hydrochloric acid can be used to adjust the pH

in the aqueous silicate solution (which is normally in a pH range from 11.5 or above) to a range of e.g. 0.5 to 1.5, wherein the solidification time is sufficiently long to permit placement thereof in the desired subsea location. (See, e.g., U.S. Patent No. 3,375,872). Alternatively, a reagent which gradually releases a gel-forming material (eg, polyvalent metallic or alkaline earth metal ions or a weakly acidic material) can be mixed into the aqueous silicate solution to effect gradual solidification thereof after its placement in the desired subsea location. (See, e.g., U.S. Patent No. 2,208,766, Canadian Patent No. 1,070,936; U.S. Patent No. 3,435,899 and U.S. Patent No. 3,294,563).

Hydraulic cement materials suitable for use in the present invention include those which are usually used for cementing work in oil and/or gas wells. Portland cement is preferred for this purpose and, of these, API Class A and API Class C Portland cement are most preferred for use in the present invention.

Production and pumping of the hydraulic cement slurry in the present invention is usually according to common practice for the production and treatment of cement slurries for oil and gas burners. E.g. can common additives to cement such as dispersants, liquid loss additives, etc. used, if desired. Correspondingly, the water contents in the cement top slurries can conveniently be within the usual range used, but can vary somewhat in quantitative sizes depending on various factors such as the particular type of cement used, etc.*. For example, when an API Class A Portland cement is used, it is preferred to use it in the form of a slurry having a water content of about 46% by weight of the cement. On the other hand, when an APO Class H Portland cement is used, it is preferred to use a slurry containing approx.

38% water by weight in relation to the weight of the cement.

When an aqueous filling material is used to separate

the cement slurry from the silicate solution during the continuous injection of these, it is preferable to use a filling material of fresh water. Otherwise, high concentrations of sodium ions and/or the presence of significant calcium ion concentrations in said filling material could induce solidification of the silicate solution at the interface between the silicate solution/filling material during the injection through the borehole. However, it is of course possible to use a salt solution as filling material if desired, as long as its nature (e.g. salt concentration and/or calcium content) is such that premature silicate drying is avoided during the injection.

Regardless of whether the aqueous filler material is based on fresh water or dilute saline solution materials, it may further contain various common filler materials such as polymeric thickening materials and the like. after wish.

As previously noted, a principal feature of the present invention includes the discovery that certain polybasic inorganic phosphate salts are capable of substantially delaying or retarding the otherwise very rapid (eg, in just a few seconds) solidification or curing that occurs when a cement slurry and an aqueous silicate solution are added to each other or mixed. Suitable inorganic phosphate salts for this purpose include the various known water-soluble, polybasic inorganic phosphate salts such as e.g. the water-soluble dibasic or tribasic alkali metal or ammonium phosphate salts. Preferably, the alkali metal phosphate salts or the alkali metal/ammonium phosphate salt mixtures are used to eliminate or reduce the formation or release of ammonia during use. Particularly preferred phosphate salts for use here are disodium phosphate and trisodium phosphate.

The aforementioned water-soluble polybasic inorganic phosphate salt may be incorporated into the soluble silicate/cement slurry system in any convenient manner so long as it is present during the period of time when said cement slurry and silicate solution are initially contacted or mixed together. For example, the phosphate salt can be mixed into the cement slurry itself, the silicate solution itself or both. Alternatively, an aqueous filling liquid having the phosphate salt dissolved in it can be used to separate the cement slurry from the silicate solution during the continuous injection of these. Most preferably, an aqueous filling liquid is used in the present invention, and at least a portion of the required inorganic phosphate salt is dissolved in this.

The minimum amount of water-soluble polybasic inorganic phosphate salt used herein corresponds to an amount sufficient to reduce the rate of gelation upon addition or admixture of the hydraulic cement slurry and the silicate solution. The usual solidification time when mixing such components in the absence of the aforementioned phosphate salt typically corresponds to only a few seconds. Preferably, the aforementioned phosphate salt is used in the present invention in an amount sufficient to give an extended solidification time of at least approx. 1/2 hour, and preferably at least approx. 1 hour, after the initial mixing of the cement slurry and silicate dissolution components therein.

Expressed quantitatively, the aforementioned phosphate salt will typically be used in an amount varying, on the basis of an anhydrous salt, from 0.5 to 5 (preferably from 0.75 to 2)% by weight based on the total weight of the cement slurry, the aqueous filling fluid (if any) and silicate solution. In those cases where an aqueous filling liquid is used, and in which the phosphate salt is introduced into the system by dissolution in the filling liquid, the phosphate salt will typically make up from 2 to 20 (preferably from 2 to 10 and most preferably from 2.5 to 5) weight-% of said aqueous filling fluid.

The present invention is further illustrated by references to the following working examples.

Example 1

A 40° Baume aqueous sodium silicate solution having a weight ratio of 3.22:1 SiO to Na 2 0 is diluted by mixing 42.6 parts by weight of this with 57.4 parts by weight of water.

E(1) part by weight of the resulting dilute sodium silicate solution is simultaneously mixed with two (2) parts by weight of an API Class A Portland cement slurry containing 46% water based on the weight of the cement, and with one (1) part by weight of a variety of aqueous filling mixtures such as various amounts of trisodium phosphate or disodium phosphate dissolved in it.

The content of phosphate salt in the different filling mixtures, together with the setting time for the corresponding mixtures prepared with them, is summarized in table I below.

EXAMPLE 2

Large amounts of salt solution are formed in an oil well. Conventional perforating and cement injection operations in the water zone allow the cement to enter the weaker water zone without sealing channels between the water and the oil zone.

A batch of 1.89 m<3> (500 gallons) of calcium chloride salt solution is prepared containing 9% by weight CaCl 2 on a total weight basis. Two 3.785 m<3> (1000 gallon) batches of an aqueous sodium silicate solution are also made. Each batch of 3.785 m<3>(1000 gallons) is made by adding 2.42 m<3>(640 gallons) of water to a tank adding 1.32 m<3>(350 gallons) of 40°Be liquid sodium silicate top solution (glass of water containing approx. 38% solids, weight ratio between SiO2 and Na2O 3.22) while stirring.

The well is perforated in the water zone and a pressure packer is inserted between the perforations in the water zone and the producing perforations. The batch of 1.89 m<3>(500 gallons) of CaCl2 solution is pumped down the pipe into the water zone followed by 0.757 m<3>(200 gallons) of fresh water which in turn is followed by 7.57 m<3>(2000 gallons) of the sodium silicate mixture and new 0.757 m<3> (200 gallons) of water containing 10% trisodium phosphate dodecyl hydrate and then of an aqueous slurry containing 100 sacks (4268 kg) of Portland cement containing 4% calcium chloride to accelerate setting.

The first sodium silicate solution forms a gel when mixed with CaCl2 salt solution in the first flow areas in the base formation, thereby diverting the remaining liquid to other areas. The Portland cement solidifies quite quickly on contact with the sodium silicate solution and thus strengthens the weak area and forces the remaining cement into channels leading to the production section. The solidification of the Portland cement is, however, significantly delayed compared to what it would have been in the absence of the trisodium phosphate component in the above-mentioned aqueous filling mixture.

While the present invention is described herein by reference to certain specific designs and illustrative examples thereof, these must not be considered or interpreted as in any way limiting the scope of the present patent-claimed invention.

Claims (10)

1. Method for sealing a zone in an undersea foundation formation by continuous injection of an aqueous solution of a water-soluble silicate material followed by an aqueous filling material and an aqueous hydraulic cement slurry, characterized by incorporating said aqueous filling material into said silicate solution or said hydraulic cement slurry, of a water-soluble, inorganic, polybasic phosphate salt to reduce the rate of gelation that occurs when said silicate material, backfill material and hydraulic cement are mixed. 2. Procedure according to claim 1, characterized in that the water-soluble phosphate salt is a dibasic or tribasic alkali metal or ammonium phosphate salt, or a mixture of these. 3. Procedure according to claim 1, characterized in that the water-soluble phosphate salt is trisodium phosphate or disodium phosphate. 4. Procedure according to claim 1, characterized in that the water-soluble phosphate salt is incorporated into the aqueous filling material in an amount, on an anhydrous basis, of from 2 to 20% by weight of the aqueous filling material. 5. Procedure according to claim 4, characterized in that the water-soluble phosphate salt constitutes, on an anhydrous basis, from 2 to 10% by weight of said aqueous filling material. 6. Procedure according to claim 4, characterized in that the water-soluble phosphate salt constitutes, on an anhydrous basis, from 2.5 to 5% by weight of said aqueous filling material. 7. Method according to any of claims 1 to 4, characterized in that the water-soluble phosphate salt is used in an amount that is sufficient to provide an extended solidification time of at least approx. 0.5 hours after mixing the aforementioned water-soluble silicate, cement and filling materials. 8. Method according to claim 1 or 7, characterized in that the water-soluble phosphate salt is used in an amount, on an anhydrous salt basis, of from 0.5 to 5% by weight based on the total weight of the cement slurry, the silicate solution and, possibly, the the aqueous filling material. 9. Procedure according to claim 1, characterized in that the water-soluble silicate consists of silicon dioxide and sodium oxide in a ratio of from 0.5 to 4 parts by weight of silicon dioxide per part by weight of sodium oxide. 10. Method according to claim 1, which is further characterized in that, before the continuous injection of the aqueous silicate solution, filler and cement slurry, an aqueous calcium chloride solution is continuously injected followed by an aqueous filling liquid.