NO161340B - PROCEDURE AND MEASURES FOR REDUCING FLUID LOSS IN Aqueous BROWN CONTROL FLUIDS AND SUCH BROWN CONTROL FLUIDS. - Google Patents

Description

The present invention relates to a method

and a means for increasing the viscosity and reducing the fluid loss in aqueous well operating fluids as well as such well operating fluids.

Aqueous media, especially those containing oil field salt solutions, are commonly used as well operating fluids such as drilling fluids, overhaul fluids, completion fluids, packing fluids, well treatment fluids, treatment fluids for underground formations, spacing fluids, hole plugging fluids, etc. Such well operating fluids must if they must be efficient and economically attractive and exhibit low fluid loss. It is known to add certain hydrophilic polymeric materials to well operating fluids to control fluid loss. It is e.g. known to use starch and cellulose products, e.g. corn starch and potato starch derivatives, as additives to well operating fluids for fluid loss control. The degree of fluid loss control or regulation exhibited by such materials is largely dependent on the composition of the well operating fluid. Thus, it is known that the presence of high concentrations of calcium or zinc ions, common components of heavy salt solutions, makes fluid loss control more difficult.

Viscosity increase in aqueous well operating fluids is also necessary in many applications. Here too, starch and cellulose derivatives have been used to achieve

such viscosity increase, but their effectiveness is also affected by the presence, in certain heavy salt solutions, of high concentrations of calcium or zinc ions.

The purpose of the present invention is therefore to provide a method and a means for synergistically increasing the viscosity and reducing the fluid loss in aqueous well operating fluids. Another purpose of the invention is to provide a well operating fluid with such properties.

According to the present invention, a method is thus provided for reducing the fluid loss in aqueous well operating fluids by dispersing in said fluid a hydroxyethyl starch and a hydroxyethyl cellulose, and this method is characterized by using a cross-linked hydroxyethyl starch which has a hydroxyethyl side chain with a degree of substitution (DS ) of 0.15-0.8, the weight ratio of the hydroxyethyl starch to the hydroxyethyl cellulose being from 10:90 to 90:10.

Furthermore, according to the invention, it is also provided

an agent for increasing the viscosity and reducing the fluid loss in aqueous well operating fluids, comprising a hydroxyethyl starch and a hydroxyethyl cellulose, and this agent is characterized in that the hydroxyethyl starch is cross-linked and has a hydroxyethyl side chain with the degree of substitution indicated above, and where the weight ratio for the hydroxyethyl starch of the hydroxyethyl cellulose is as indicated above.

According to the invention, there is also provided a well operating fluid comprising an aqueous medium and a hydroxyethyl starch and a hydroxyethyl cellulose, characterized in that the hydroxyethyl starch is as stated above, and the weight ratio for the aforementioned two components is as defined above.

The two polymeric components in the present agent

is, as mentioned, cross-linked hydroxyethyl starch and hydroxyethyl cellulose (HEC). The HEC polymers are usually solid, particulate materials which are water-soluble or water-dispersible and which, upon dissolution or dispersion in an aqueous medium, increase the viscosity of the system. HEO polymers are typically high-yield, water-soluble, nonionic materials prepared by treating cellulose with sodium hydroxide followed by reaction with ethylene oxide. Each anhydroglucose unit in the cellulose molecule has 3 reactive hydroxy groups. The average number of moles of ethylene oxide that are attached to each anhydroglucose unit in cellulose is called moles of combined substituent. In general, the greater the degree of substitution, the greater the water solubility. It is preferred to use HEC polymers which have as high a molar substitution level as possible.

The HEC polymers useful in the present invention, depending on the method of preparation of the well operating fluid, may either be in the form of a dry powder, substantially untreated, or may be an "activated" HEC material. With the term "activated" used

means an HEC polymer which will substantially hydrate or solubilize in a salt solution which has a density greater than approx. 1.70 kg/litre without the need for mixing, such as in roller processing, at elevated temperatures. Examples of such activated HEC polymers are set forth in US-

patents 4,330,414 and 4,392,964. As described in said patents, HEC polymers that have been activated will be made soluble in salt solutions without the necessity for roller treatment or other forms of mixing at elevated temperatures. In general, any HEC polymer that will be solubilized in a salt solution having a density above about 1.70 kg/litre at room temperature is considered an "activated" HEC. However, it should be understood that the present invention is not limited to the use of such activated HEC polymers. Depending on the mixing conditions and composition of the aqueous well operating fluid, unactivated or dry powdered HEC polymers are compatible with the aqueous well operating fluids used in the present invention. The term "compatible" used means that the HEC polymer can be solvated or solubilized in a given aqueous solution using mixing techniques such as roller treatment at elevated temperatures. An incompatible system is thus one in which the HEC polymer will not be solubilized in the salt solution regardless of the mixing conditions used.

The second polymeric component in the present compositions is a cross-linked hydroxyethyl starch. Such hydroxyethyl starches are produced by introducing non-ionic hydroxyethyl side groups into the polymer chain in starch followed by well-known cross-linking techniques such as e.g. those described in US Patent No. 2,500,950; 2929 811; 2,989,521 and 3,014,901. The cross-linked hydroxyethyl starches useful in the present invention are generally those in which the degree of substitution (DS) in the hydroxyethyl side chain is 0.8, preferably 0.25-0.6. A particularly useful cross-linked hydroxyethyl starch is known as "BOHRAMYL CR", a cross-linked potato starch derivative. "BOHRAMYL CR" which is a coarse-grained, white, flake-shaped material, has a bulk density (kg/m 3 ) of about 325 and a DS value of about 0.4.

As stated above in connection with the HEC material, the cross-linked hydroxyethyl starch can be used either in the form of a dry powder or in flake form, substantially untreated, or can be an "activated" starch, where the term "activated" has the same meaning as stated above in connection with the mention of activated HEC. Methods for activating the cross-linked hydroxyethyl starch are described in US patent 4,330,414. It should be understood that the present invention is not limited to the use of

activated hydroxyethyl starch. It is a real feature of the present invention that both HEC and hydroxyethyl starch

■:an is used in dry form for the production of well operating fluids that exhibit excellent rheological properties and low

fluid loss. In certain salt solutions, however, activation or pre-solvation of the HEC material and/or the hydroxyethyl starch may be desirable to reduce mixing times and the degree of hardness of the mixing conditions.

The polymer composition according to the invention which can be used to reduce fluid loss and increase the viscosity of aqueous well operating fluids comprises an effective amount of HEC and an effective amount of the cross-linked hydroxyethyl starch. It has been found that when these two polymeric materials are added to aqueous well operating fluids, depending on the nature of the fluid, a synergistic improvement in viscosity and/or fluid loss is achieved. The particular amount of each of the polymeric components present in the additive composition will vary with the nature and composition of the aqueous well operating fluid with which the additive is to be mixed. As mentioned, the agent according to the invention will contain a weight ratio of hydroxyethyl starch to HEC from 10:90 to 90:10, and preferably from 33:67

to 75:25. The agent according to the invention can either be in the form of a dry mixture of HEC material and hydroxyethyl starch or, if preferred, it can be in the form of solvated or activated forms of the polymers. Thus, e.g. The HEC material and the hydroxyethyl starch can be activated, and these activated solutions can be mixed together to produce the new polymeric compositions used herein.

The new well operating fluid according to the invention comprises an aqueous medium and an effective amount of a cross-linked hydroxyethyl starch and an effective amount of a hydroxyethyl cellulose. The relative amounts of the hydroxyethyl starch and the HEC material mixed with the aqueous medium are such that a synergistic reduction of the fluid loss to the aqueous medium is provided. Here too, the exact quantity of each of the polymeric components used will depend on the nature of the aqueous well operating fluid.

In general, however, the weight ratio of hydroxyethyl starch to HEC in the well operating fluid will be from 10:90 to 90:10, more preferably from 33:67 to 75:25.

Generally, the well operating fluids containing the polymer components in amounts from 0.71 to 14.27 kg/m 3 HEC

and from 1.43 to 14.27 kg/m 3 of hydroxyethyl starch.

The aqueous medium used in the present well operating fluids can vary from fresh water to heavy salt solutions that have a density of over 2.28 kg/litre. In general, well operating fluids such as e.g. those used in completion and overhaul operations, produced from aqueous media containing soluble salts such as e.g. a soluble salt of an alkali metal, is alkaline earth metal,

a group Ib metal, a group Ilb metal, as well as water soluble

salts of ammonia and other cations. The mixed HEC/crosslinked hydroxyethyl starch compositions are particularly useful in the preparation of heavy salt solutions with low fluid loss, i.e. aqueous solutions of soluble salts of polyvalent ions, e.g. Zn and Ca.

The preferred heavy salt solutions which are useful for forming the well operating fluids according to the invention are those which have a density of over approx. 1.32 kg/litre, especially those with a density of over 1.80 kg/litre. Such heavy salt solutions include aqueous solutions of salts selected from the group consisting of calcium chloride, calcium bromide, zinc chloride, zinc bromide and mixtures thereof.

In US patent 4,420,406 it is shown that in certain heavy salt solutions containing zinc bromide in a concentration of less than approx. 20% by weight, HEC is incompatible, i.e. it will not solvate in such salt solutions to effectively increase viscosity. By adding the synergistic combination of HEC and the cross-linked hydroxyethyl starch, salt solutions in which the content of zinc bromide is from 0.5 to 20% by weight and the density is from 40.52 to 46.37 kg/m 3 can be viscosity regulated.

If desired, bridging agents can be added to the well operating fluids to help control fluid loss. Thus somewhat lower filtrates are obtained when they are used. It is, however, a distinct and unexpected feature of the invention that

a "bridging" agent is not necessary to achieve low fluid loss values in aqueous salt solutions. By using the present invention, it is thus possible to obtain clear, heavy salt solutions which have low fluid loss characteristics and, at low concentrations of HEC, have low rheological characteristics.

In the present method, the mixed HEC/cross-linked hydroxyethyl starch can be added to the aqueous well operating medium either in dry form or in activated form as discussed above. In the method, the polymeric components are dispersed in the aqueous medium using suitable mixing techniques.

To more fully illustrate the present invention, the following examples are given. Unless otherwise stated, all measurements for physical properties were made according to the test methods specified in STANDARD PROCEDURE FOR TESTING DRILLING MUD API RP 13B, 7th Edition, April 1978. The HEC polymer used was an HEC material designated "Natrosol 250 HHR" if nothing else is specified. The hydroxyethyl starch used was, unless otherwise stated, "BOHRAMYL CR".

Example 1.

To show the effect on viscosity and fluid loss obtained by mixing HEC and cross-linked hydroxyethyl starch, 5.71 kg/m<3> HEC and either 0, 5.71 or 11.41 kg/m<3> "BOHRAMYL CR" were added to an aqueous salt solution containing 85.60 kg/m<3 >calcium chloride and mixed for 30 minutes in a "multimixer". Then the samples were roll treated at 65.6°C for 16 hours, cooled, stirred for 5 minutes and the API rheology and fluid loss determined. The obtained data given in Table I below show that "BOHRAMYL CR" effectively lowered the fluid loss in the saline solution in the presence of HEC.

Example 2.

This example demonstrates the use of solvated HEC compositions. The following samples were prepared:

Try to.

124.5 parts isopropanol and 0.5 parts "CAB-O-SIL M5"

(colloidal silicon dioxide) was mixed for 10 minutes in a "multimixer". 50 parts by weight of HEC were then added and mixing continued for a further 3 minutes. After this, 75 parts by weight of ethylene glycol were added and mixing was carried out for a further 5 minutes.

Sample B.

A solution of 0.5% by weight "Klucel H" (hydroxypropyl cellulose) in isopropanol was prepared. To 55 parts by weight of this isopropanol solution, 20 parts by weight of HEC and 25 parts by weight of ethylene glycol were added.

The salt solution used to evaluate the samples was 1.92 kg/liter CaB^/ZnB^ solution. The brine samples were prepared and evaluated using the following method: 1. The amounts of HEC, "BOHRAMYL CR" and "BARACARB" (CaCO-j "bridging" agent) indicated in Tables 2, 3 and 4 were added to 1.92 kg/litre salt solution and mixed in a "multimixer" for 15 minutes.

2. The API rheology was then obtained.

• 3. The samples were then aged overnight at room temperature and the API rheology and fluid loss obtained. 4. The samples were then rolled overnight at 65.6°C and the API rheology and fluid loss were obtained after the samples had cooled to room temperature.

Table 2 indicates data obtained using sample A; Table 3 provides data obtained using sample B; and Table 4 sets forth data obtained for "BOHRAMYL CR" alone.

The obtained data shown in Tables 2,3 and 4 show that the HEC material and "BOHRAMYL CR" combine to synergistically increase the viscosity and decrease the fluid loss of the aqueous salt solution. The fluid loss results in the absence of the "bridging" agent ("BARACARB") are particularly outstanding. The aqueous salt solutions were completely clear after hot rolling when all the polymers had dissolved.

Example 3.

To demonstrate the effect of HEC and "BOHRAMYL CR" on heavy salt solutions containing less than 20% by weight ZnBr2, the following method was carried out: a 1.83 kg/liter CaBr,,/ CnBr2 solution containing 15.7% ZnBr2 and 43 .9% CaBr ^ was prepared by mixing a 2.30 kg/liter solution containing 57 wt% ZnBr2 and 20 wt% CaBr2 and a 1.70 kg/liter solution containing 53 wt% CaBr2 in a volume ratio of 0.78pp^^^ferespectively. Three separate portions of this salt solution were d«|t in a "multimixer" for 15 min. including the following":""^ 1. 0.36 kg/liter "BOHRAMYL CR"

2. 2.85 kg/m<3> HEC

3. 8.56 kg/m<3> "BOHRAMYL CR" and 2.8 5 kg/m<3> HEC.

The viscosity values when measured with a "Fann V-G" meter were then obtained and after rolling the solutions for 3 hours overnight at 65.6°C. The data obtained are shown in table 5.

The given data clearly show the synergistic results obtained by adding both "BOHRAMYL CR" and HEC to the saline solution. It was noted that the HEC material would not hydrate and disperse the salt solution when added without "BOHRAMYL CR".

Example 4.

In this example, two samples of non-crosslinked hydroxyethyl starch were evaluated and compared to "BOHRAMYL CR". The HEC sample used was sample A of Example 2. The samples were evaluated using the method set forth in Example 2. The two non-crosslinked hydroxyethyl starches had degrees of hydroxyethyl substitution of 0.29 and 0.83. The data given in table 6 clearly show that the hydroxyethyl starch must be cross-linked in order to cooperate with the HEC material in order to synergistically reduce the fluid loss to the salt solution.

Example 5.

This example demonstrates the synergistic effect and the viscosity and fluid loss in fresh water and in saline solutions of lower density (1.39 kg/liter CaCl,,). The samples used were prepared as follows:

1.39 kg/ liter of salt solution

The indicated amounts of "BOHRAMYL CR" and HEC were added to the salt solution and mixed for 15 minutes in a "multimixer". After obtaining the API viscosities, the samples were rolled at 65.6°C for 16 hours, cooled to 23.3°C, and the API viscosities and API fluid loss were obtained.

Tap water

These samples were prepared and scored as in the case of the 1.39 kg/liter brine except that 1.00 kg/m 3 magnesium oxide was added to each sample to raise the pH and decrease the hydration time of the HEC material.

Data shown in Table 7 clearly indicate a synergistic increase in both viscosity and fluid loss with fresh water and the 1.39 kg/litre salt solution.

Example 6.

This example compares the effect of cross-linked hydroxyethyl starch and non-cross-linked hydroxyethyl starch in combination with HEC in a 10% NaCl solution. The indicated amounts of HEC and hydroxyethyl starch were added to separate 350 ml portions of a 10% NaCl solution. Rheological data obtained after 25 minutes of mixing and after rolling at 65.6°C, cooling at 23.3°C and mixing for a further 5 minutes are given in Table 8. As the data in Table 8 clearly shows, cross-linked hydroxyethyl starch interacts with HEC in a synergistic way so that the fluid loss is reduced and the viscosity is increased. However, samples with hydroxyethyl starch that are not cross-linked only react synergistically with the HEC material to the extent that the viscosity is increased.

Claims (13)

1. Method for reducing the fluid loss in aqueous well operating fluids by dispersing in said fluid a hydroxyethyl starch and a hydroxyethyl cellulose, characterized by using a cross-linked hydroxyethyl starch which has a hydroxyethyl side chain with a degree of substitution (DS) of 0.15-0.8, the weight ratio of the hydroxyethyl starch to the hydroxyethyl cellulose being from 10:90 to 90:10. 2. Method according to claim 1, characterized in that a weight ratio of 33:67 to 75:25 is used. Method according to claim i, characterized in that the hydroxyethyl starch and the hydroxyethyl cellulose are activated before dispersing in said aqueous fluid. 4. Method according to claim 1, characterized in that an aqueous fluid is used which comprises an aqueous solution of at least one water-soluble salt selected from calcium chloride, calcium bromide, zinc chloride, zinc bromide, and mixtures thereof, the aqueous medium having a density in the range 1, 40-2.30 kg/litre. 5. Method according to claim 1, characterized in that an aqueous salt solution is used which contains 0.5-20% by weight zinc bromide and has a density in the range 1.70-1.95 kg/litre. 6. Means for increasing the viscosity and reducing the fluid loss in aqueous well operating fluids, comprising a hydroxyethyl starch and a hydroxyethyl cellulose, characterized in that the hydroxyethyl starch is cross-linked and has a hydroxyethyl side chain with a degree of substitution (DS) of 0.15-0.8 , the weight ratio of the hydroxyethyl starch to the hydroxyethyl cellulose being from 10:90 to 90:10. 7. Agent according to claim 6, characterized in that said ratio is from 33:67 to 75:25. 8. Agent according to claim 6, characterized in that the hydroxyethyl starch and the hydroxyethyl cellulose are activated. 9. Well operating fluid comprising an aqueous medium and a hydroxyethyl starch and a hydroxyethyl cellulose, characterized in that the hydroxyethyl starch is cross-linked and has a hydroxyethyl side chain with a degree of substitution of 0.15-0.8, the weight ratio of the hydroxyethyl starch to the hydroxyethyl cellulose being from 10:90 to 90:10 . 10. Well operating fluid according to claim 9, characterized in that said ratio is from 33:67 to 75:25. 11. Well operating fluid according to claim 9, characterized in that the hydroxyethyl starch and the hydroxyethyl cellulose are activated. 12. Well operating fluid according to claim 9, characterized in that the aqueous medium comprises a solution of at least one water-soluble salt selected from calcium chloride, calcium bromide, zinc chloride, zinc bromide and mixtures thereof, the aqueous medium having a density in the range 1.40-2.30 kg/l. 13. Well operating fluid according to claim 9, characterized in that the aqueous medium contains 0.5-20% by weight zinc bromide and has a density in the range 1.70-1.95 kg/l.