BACKGROUND

The present invention relates to (a) hydraulic fracturing techniques, (b) hydraulic fracturing fluids, and (c) hydraulically fractured subterranean formations.

A common problematic occurrence is the formation of scale proximate and/or within the casing of a production well as well as within equipment handling aqueous fluids produced from a subterranean formation. The scale can reduce, and even totally block, the production of oil or gas or geothermal fluid from the production well. In addition, in subterranean formations containing a high level of a naturally occurring radioactive material (NORM) , such as ²²⁶ Ra and ²²⁸ Ra, the scale formed in the casing and equipment can reach levels which require the scale- containing casing and equipment to be handled and disposed of as a radioactive waste.

Some aqueous subterranean fluids contain both scale-forming constituents and NORMs, while other aqueous fluids essentially contain only one of these adverse constituents.

SUMMARY OF THE INVENTION

According to one aspect of the invention, there is provided a fracturing method comprising the step of injecting through a production well and into at least a portion of a subterranean formation under hydraulic fracturing conditions a fracturing fluid comprising a carrier fluid and a proppant, characterized in that the fracturing fluid further comprises a nucleating agent in a sufficient concentration to reduce the concentration of one or more ingredients selected from ^' the group consisting of

scale-forming ingredients, natural occurring radioactive materials (NORMs) , and mixtures thereof present in an aqueous subterranean fluid produced from the production well. According to another aspect of the invention, there is provided a fracturing fluid comprising a carrier fluid and a nucleating agent capable of reducing the concentration of an ingredient selected from the group consisting of scale-forming ingredients, natural occurring radioactive materials (NORMs) , and mixtures thereof present in an aqueous fluid.

The present invention provides a method for retarding the formation of scale and NORMs proximate and/or within the casing of a production well as well as within equipment handling aqueous fluid produced from a subterranean formation. More particularly, the method of the invention entails injecting through a production well and into at least a portion of a subterranean formation under hydraulic fracturing conditions a fracturing fluid comprising a carrier fluid and a nucleating agent capable of reducing the concentration of scale-forming ingredients and NORMs present in the aqueous subterranean fluid. The nucleating agent is present in the fracturing fluid in a sufficient concentration to reduce the concentration of one or more scale-forming ingredients and/or one or more NORMs present in the aqueous subterranean fluid produced from the production well. Optionally, an additional constituent selected from the group consisting of proppants, friction- reducing additives, fluid-loss-control additives, gelling agents, bactericides, and scale stabilizers is present in the fracturing fluid.

In addition, the invention provides a fracturing fluid and a natural resource system. The fracturing fluid comprises the carrier fluid, the nucleating agent, and optionally an additional constituent selected from the group consisting of proppants, friction- reducing additives, fluid-loss-control additives, gelling agents, bactericides, and scale stabilizers.

The natural resource system comprises a subterranean formation, a natural resource (e.g., oil, natural gas, and/or a geothermal fluid) present in at least a portion of the subterranean formation, and a well penetrating at least a portion of the subterranean formation. In one embodiment, the natural resource system further comprises a fracture formed from the method of the present invention. In another embodiment, the natural resource system also comprises a pipe present in at least a portion of the well and one of the fracturing fluids of the present invention present in at least a portion of the pipe.

Without being bound by the following theoretical explanation, it is believed that the nucleating agent provides a nucleation site for scale-forming and NORM components in the aqueous subterranean formation fluids. These nucleation sites are very attractive to chemically similar ions in the aqueous fluids. For wells with scaling tendencies, the nucleating agents encourage the removal of scale-forming constituents present in the aqueous subterranean fluids as well as the deposition of such scale-forming constituents deep in the fracture rather than in the near wellbore region. The deposition of scale deep in the fracture is less damaging to production than scale deposited near the wellbore.

Similarly, for wells producing a NORM- containing aqueous subterranean fluid, the nucleating agents encourage the removal of the NORMs present in the aqueous subterranean fluids as well as the deposition of such NORMs in the reservoir or subterranean formation

(where they are not an environmental or safety problem) , rather than in well casing, tubing, or surface facilities handling the produced aqueous subterranean fluids (where they tend to be an environmental and/or safety problem) .

Hence, for aqueous subterranean fluids containing a scale-forming constituent, the presence of the

nucleating agent in the fracturing fluid lowers the concentration of the scale-forming ingredient in the produced aqueous subterranean fluids. Likewise, for NORM- containing aqueous subterranean fluids, the presence of the nucleating agent in the fracturing fluid reduces the NORM concentration in the produced aqueous subterranean. In addition, for aqueous subterranean fluids containing both a NORM and a scale-forming constituent, the presence of the nucleating agent in the fracturing fluid decreases the NORM and scale-forming ingredient concentrations in the produced aqueous subterranean fluids.

BRIEF DESCRIPTION OF THE DRAWINGS

The reduction in the concentration of scale- forming constituents and NORMs present in aqueous fluids produced from a subterranean formation by a nucleating agent, as well as other features, aspects, and advantages of the invention will become better understood with reference to the following description, appended claims, and accompanying drawings wherein:

Figure 1 is a graphical plot of data obtained from the comparative experiments of Examples 1-2, below, where, in Example 1, a brine containing about 15 ppm barium ions was passed through an experimental core containing a proppant and a scale nucleating agent, and, in Example 2, a similar brine was passed through a control core containing just the proppant and no scale nucleating agent; and

Figure 2 is a graphical plot of data obtained from the comparative experiments of Examples 3-4, below, where, in Example 3, a brine containing about 40 ppm barium ions was passed through an experimental core containing a proppant and a scale nucleating agent, and, in Example 4, a similar brine was passed through a control core containing just the proppant and no scale nucleating agent.

DETAILED DESCRIPTION OF THE INVENTION

The fracturing fluid employed in the present invention comprises a carrier fluid and a nucleating agent for reducing the concentration of scale-forming constituents and/or naturally occurring radioactive materials (NORMs) in aqueous fluids produced from a subterranean formation. Any carrier fluid capable of being employed in a hydraulic fracturing procedure can be employed in the present invention. Exemplary carrier fluids are discussed in Howard et al., Hydraulic

Fracturing. Society of Petroleum Engineers of the American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc., New York, NY (1970) (hereinafter referred to as "Howard"), chapter 5, as well as in Allen et al., Production Operations. Oil & Gas Consultants, Inc, Tulsa, OK (1989) (hereinafter referred to as "Allen"), volume 2, chapter 8, Howard and Allen being incorporated herein in their entireties by reference. Typical carrier fluids include, but are not limited to those listed in the following Table I.

TABLE I

Carrier Fluids

Class Species

Oil-Base Fluids Napalm gels (e.g., gasoline, kerosene, diesel oil, or crude oil gelled with Napalm)

Viscous refined oils

Lease crude oils

Gelled lease oils

Water-Base Fluids Linear aqueous gels

Crosslinked aqueous gels Alcohol-containing fluids Foamed fluids Thickened aqueous fluids

Acid-Base Fluids Gelled acids

Acid emulsions Chemically retarded acids

Exemplary nucleating agents used in the present invention include, but are not limited to, alkaline earth metal sulfates and alkaline earth metal carbonates. The preferred alkaline earth metals are magnesium, calcium, strontium, and barium, with barium being the most preferred. Typical alkaline earth metal sulfates and alkaline earth metal carbonates include, barium sulfate, barium carbonate, calcium sulfate, calcium carbonate, magnesium carbonate, and magnesium sulfate.

The fracturing fluid optionally comprises any proppant, friction-reducing additive, fluid-loss- control additive, gelling agent, bactericide, and/or any other ingredient (e.g., scale stabilizer) employed in a hydraulic fracturing fluid. Proppants that can be employed in the fracturing fluid of the present invention are

discussed in chapter 6 of Howard and chapter 8 of Allen, these references having already been incorporated herein in their entireties by reference. More particularly, exemplary proppants include, but are not limited to, sand, aluminum, glass beads, nutshells, plastics, sintered bauxite, urea, and sodium bisulfate.

Friction-reducing additives, fluid-loss- control additives, gelling agents, bactericides, and scale stabilizers are discussed in chapter 5 of Howard and include the materials listed below in Table II.

TABLE II

Optional Fracturing Fluid Additives

Additive Examples

Friction-Reducing Fatty acid soap-oil gel High-molecular weight polymers

(e.g., polyacrylamides, partially hydrolyzed polyacrylamides) Guar gum Cellulose derivatives

Fluid-Loss-Control Gas

Guar gum

Hydroxyethyl cellulose Polyacrylamides Silica flour

Gelling Agent Guar gum

Hydroxyethyl cellulose Polyacrylamides

Bactericide Quaternary amines Amide-type materials

Halogenated phenols

Scale Stabilizer Polyphosphates (e.g., sodium, magnesium, and calcium phosphates, and complexes thereof)

The fracturing fluids of the present invention are prepared by processes analogous to those used to prepare fracturing fluids employed in the prior art, the main difference being the incorporation of the nucleating agent into the fracturing fluid. When a proppant is also present in the fracturing fluid, the nucleating agent can

be mixed with the proppant prior to combining the proppant with the carrier fluid. Alternatively, the nucleating agent and the proppant are separately added to the carrier fluid during the preparation of the fracturing fluid.

In general, when a proppant is employed in the fracturing fluid, the fracturing fluid generally comprises at least about 0.01 volume percent nucleating agent based on the volume of the proppant employed in the fracturing fluid. Preferably, the fracturing fluid comprises about 0.01 to about 15, more preferably 0.1 to about 10, and most preferably about 1 to about 5, volume percent nucleating agent based on the proppant volume in the fracturing fluid.

When a proppant is not employed in the fracturing fluid, the fracturing fluid generally comprises at least about 0.01 volume percent nucleating agent based on the total volume of the fracturing fluid, i.e., the combined volume of the carrier fluid, the nucleating agent, and all other ingredients (e.g., friction-reducing additive, fluid-loss-control additive, gelling agent, bactericide, and scale stabilizer) present in fracturing. Preferably, in this embodiment of the invention, the fracturing fluid contains about 0.01 to about 20, more preferably about 0.1 to about 10, and most preferably about 1 to about 5 volume percent nucleating agent based on the total volume of the fracturing fluid.

The nucleating agent employed in the present invention usually is capable of passing through a No. 3 U.S. Standard Sieve. Typical size ranges for the nucleating agents include, but are not limited to, 6/12, 8/16, 12/20, 16/30, 20/40, 30/50, 40/70, and 70/140, wherein a minimum of about 90 percent of the tested nucleating agent sample falls between the designated U.S. Standard Sieve size numbers. Usually, not over about 0.1 percent of the total tested nucleating agent sample is larger than the numerator U.S. Standard Sieve size and not

over about 1 percent of such sample is smaller than the denominator U.S. Standard Sieve size. In general, when a proppant is employed in the fracturing fluid, the nucleating agent preferably has a maximum particle size less than or equal to the maximum particle size of the proppant.

The fracturing fluids of the present invention are usually injected into a subterranean formation using procedures analogous to those disclosed in U.S. Patent 4,488,975, U.S. Patent 4,553,601, Howard, and chapter 8 of Allen, the patents being incorporated herein in their entireties by reference (Howard and Allen having already been incorporated herein in their entireties by reference) . Typically, the nucleating agent is substantially uniformly distributed throughout the injected fracturing fluid. However, in certain situations, a nonuniform nucleating agent distribution is desirable. For example, when an aqueous subterranean fluid has a low scale forming ingredient concentration, it generally is desirable to include the scale nucleating agent in just the latter portion (e.g. , about the last half or about the last quarter) of the injected fracturing fluid. Alternatively, in those instances where it is desired to maximize near wellbore conductivity, the nucleating agent concentration in the injected fracturing fluid is preferably gradually decreased during the hydraulic fracturing procedure.

EXAMPLES

The following examples, which are intended to illustrate and not limit the invention, demonstrate that the method of the present invention reduces the concentration of scale-forming constituents in water produced from a subterranean formation.

EXAMPLE 1

Dynamic Test At Reservoir Temperature And Pressure

15 ppm Ba ⁺⁺ In Feed

A stainless steel container (about 2" x about 14") capable of testing at conditions similar to typical oil reservoirs was filled with a mixture comprising about 97 volume percent 20/40 U.S. Standard Sieve Sizes sand and about 3 volume percent 20/40 U.S. Standard Sieve Sizes barium sulfate crystals.

The cation-containing brine and anion- containing brine shown below in Table A were both made using deionized water and were filtered through 45μm HA Millipore brand filters.

Equal volumetric rates (10 ml/minute each) of the cation- and anion-containing brines set forth above in Table A were pumped to a T-fitting where they were combined to form a scaling brine having a pH of about 7.07. The scaling brine was introduced into the filled, stainless steel container, the container being maintained at a temperature of about 65.6°C (about 150°F) and an internal pressure of about 3.549 pascal (Pa) (about 500 psig) . The barium (Ba ⁺⁺ ) concentration in the combined sample employed in this Example 1 was about 15 ppm. An effluent sample was taken every half hour and checked for pH. An aliquot of each effluent sample (about 5 ml) was then placed in a bottle with about 20 ml of a nitric acid solution (about 2 volume percent nitric acid and also containing about 2,000 ppm KCl) to form a stabilized sample. Each stabilized sample was tested for Ba ⁺⁺ concentration using atomic absorption. Data obtained from the experiment are set forth in the following Table B and plotted in Figure 1.

TABLE B

Scale Nucleating Agent-Containing Core 15 ppm Ba ⁺⁺ In Feed

Pressure, Cation ¹ Anion ² Time, pH, pH, pascal (psig) Brine Brine Ba++. ppm hrs Feed Eff ³ Inlet Outlet Lt ⁴ . ml Lt. ml Feed Eff

0.5 N/D ⁵ 7.5 4.238 4.238 3750 3500 N/D 5.5

(600) (600)

1.0 N/D 7.5 4.273 4.307 3500 3250 N/D 7.75 (605) (610)

1.5 N/D 7.5 4.293 4.307 3300 2900 N/D 6.0

(608) (610)

2.0 7.5 7.5 4.293 4.307 2850 2650 15.5 6.0

(608) (610) 2.5 N/D 7.5 4.293 4.307 2685 2350 N/D 6.0

(608) (610)

3.0 N/D 7.5 4.293 4.307 2400 2150 N/D 6.5

(608) (610)

3.5 N/D 7.5 4.293 4.307 2050 1800 N/D 6.0 (608) (610)

4.0 7.5 7.5 4.293 4.307 1750 1500 15.5 5.0

(608) (610)

4.5 N/D 7.0 4.293 4.307 1500 1250 N/D 5.0

(608) (610) 5.0 N/D 7.0 4.293 4.307 1200 850 N/D 5.0

(608) (610)

5.5 N/D 7.0 4.300 4.307 900 650 N/D 4.5

(609) (610)

TABLE B (continued)

Scale Nucleating Agent-Containing Core 15 ppm Ba ^"1"1" In Feed

Pressure, Cation ¹ Anion ² Time, pH, pH, pascal (psig) Brine Brine Ba++. ppm hrs Feed Eff ³ Inlet Outlet Lt ⁴ . ml Lt. ml Feed Eff 6.0 7.5 7.5 4.307 4.307 650 450 16.3 4.3

(610) (610)

1. The cation-containing brine had a Ba++ concentration of about 31.13 ppm.

2. The anion-containing brine had a Ba ⁺⁺ concentration of about 0.0 ppm.

3. "Eff" denotes effluent. 4. "Lt" denotes left or remaining.

5. "N/D" denotes not determined.

EXAMPLE 2

Control Dynamic Test At Reservoir Temperature And Pressure 15 ppm Ba ^"1"1" In Feed

The procedure of Example 1 was repeated with one major modification, namely, the stainless steel container was only filled with 20/40 U.S. Standard Sieve sand. The data obtained from this control experiment are listed in the following Table C and plotted in Figure 1.

TABLE C

Core Devoid Of Scale Nucleating Agent 15 ppm Ba ⁺⁺ In Feed

Pressure, Cation ¹ Anion ² Time, pH, pH, pascal (psig) Brine Brine Ba++. ppm hrs Feed Eff ³ Inlet Outlet Lt . ml Lt. ml Feed Eff

0.5 N/D ⁵ 7.5 4.376 4.548 3125 3125 N/D 15.5

(620) (645)

1.0 N/D 7.5 4.376 4.383 2800 2800 N/D 16.0 (620) (621)

1.5 N/D 7.5 4.376 4.383 2550 2550 N/D 16.0

(620) (621)

2.0 N/D 7.5 4.376 4.383 2250 2250 N/D 16.0

(620) (621) 2.5 N/D 7.5 4.376 4.383 2000 2000 N/D 16.5

(620) (621)

500 ml added to each brine

3.0 N/D 7.5 4.376 4.383 2300 2300 N/D 16.5

(620) (621) 3.5 N/D 7.5 4.376 4.383 1900 1900 N/D 15.5

(620) (621)

4.0 N/D 7.5 4.376 4.383 1600 1600 N/D 16.5

(620) (621)

4.5 N/D 7.5 4.376 4.383 1350 1350 N/D 16.0 (620) (621)

5.0 N/D 7.5 4.376 4.411 1100 1100 N/D 15.5

(620) (625)

5.5 N/D 7.5 4.376 4.411 800 800 N/D 15.5

(620) (625)

TABLE C (continued)

Core Devoid Of Scale Nucleating Agent 15 ppm Ba ⁺⁺ In Feed

Pressure, Cation ¹ Anion ² Time, pH, pH, pascal (psig) Brine Brine Ba++. ppm hrs Feed Eff ³ Inlet Outlet Lt ⁴ . ml Lt. ml Feed Eff 6.0 7.5 7.5 4.376 4.411 500 500 5.5 15.5

(620) (625)

1. The cation-containing brine had a Ba++ concentration of about 30.5 ppm.

2. The anion-containing brine had a Ba ⁺⁺ concentration of about 0.0 ppm.

3. "Eff" denotes effluent. 4. "Lt" denotes left or remaining.

5. "N/D" denotes not determined.

The data set forth above in Tables B and C and plotted in Figure 1 indicate that the mixture containing a proppant (namely, sand) and a scale nucleating agent (i.e., barium sulfate crystals) substantially reduced the concentration of a scale-forming constituent, namely, barium sulfate, in the scaling brine passed through the mixture. In contrast, the data also show that there was little, if any, reduction in the concentration of the scale-forming constituent when the scaling bring was passed through just the sand proppant.

EXAMPLE 3

Dynamic Test At Reservoir Temperature And Pressure

40 ppm Ba ^"1""1" In Feed

The procedure of Example 1 was repeated with one major modification, namely, the concentration of the Ba ⁺⁺ in the feed water to the stainless steel container was increased to about 40 ppm. The data obtained from the experiment of this Example 3 are listed in the following Table D and plotted in Figure 2.

TABLE D

Scale Nucleating Agent-Containing Core 40 ppm Ba ⁺⁺ In Feed

Pressure, Cation ¹ Anion ²

Time, pH, pH, pascal (psig) Brine Brine Ba++. ppm hrs Feed Eff ³ Inlet Outlet Lt ⁴ . ml Lt. ml Feed Eff

0.5 N/D ⁵ 7.5 4.238 4.273 3650 3500 N/D 6.5

(600) (605)

1.0 N/D 7.5 4.293 4.307 3400 3250 N/D 6.5

(608) (610) 1.5 N/D 7.5 4.300 4.307 3050 2900 N/D 6.5

(609) (610)

2.0 7.5 7.5 4.300 4.307 2750 2600 38.5 5.0

(609) (610)

2.5 N/D 7.0 4.293 4.307 2500 2300 N/D 4.5 (608) (610)

3.0 N/D 7.5 4.300 4.307 2250 2100 N/D 6.0

(609) (610)

3.5 N/D 7.5 4.300 4.307 2000 1850 N/D 6.5

(609) (610)

TABLE D (continued)

Scale Nucleating Agent-Containing Core 40 ppm Ba ⁺⁺ In Feed

Pressure, Cation ¹ Anion ² Time, pH, pH, pascal (psig) Brine Brine Ba++. ppm hrs Feed Eff ³ Inlet Outlet Lt ⁴ . ml Lt. ml Feed Eff

4.0 N/D 7.5 4.300 4.307 1650 1500 N/D 9.0

(609) (610)

4.5 7.5 7.5 4.300 4.307 1350 1250 38.0 5.0 (609) (610)

5.0 N/D 7.5 4.300 4.307 1050 950 N/D 5.0

(609) (610)

5.5 N/D 7.5 4.300 4.307 825 650 N/D 6.0

(609) (610) 6.0 7.5 7.5 4.300 4.307 490 350 38.5 5.0

(609) (610)

1. The cation-containing brine had a Ba++ concentration of about 80.0 ppm. 2. The anion-containing brine had a Ba ⁺⁺ concentration of about 0.0 ppm.

3. "Eff" denotes effluent.

4. "Lt" denotes left or remaining.

5. "N/D" denotes not determined.

EXAMPLE 4

Control Dynamic Test At Reservoir Temperature And Pressure

40 ppm Ba ⁺⁺ In Feed

The procedure of Example 3 was repeated with one major modification, namely, the stainless steel container was only filled with 20/40 U.S. Standard Sieve sand. The data obtained from this control experiment are listed in the following Table E and plotted in Figure 2.

TABLE E

Core Devoid Of Scale Nucleating Agent 40 ppm Ba ⁺⁺ In Feed

Pressure, Cation ¹ Anion ² Time, pH, pH, pascal (psig) Brine Brine Ba++, ppm hrs Feed Eff ³ Inlet Outlet Lt ⁴ . ml Lt. ml Feed Eff

0.5 N/D ⁵ 7.5 4.438 4.445 N/D N/D N/D 32.2

(629) (630)

1.0 N/D 7.5 4.459 4.507 3400 3350 N/D 23.3 (632) (639)

1.5 N/D 7.5 4.479 4.514 3100 3025 N/D 18.5

(635) (640)

2.0 7.5 7.5 4.493 4.514 2750 2700 40.5 17.5

(637) (640) 2.5 N/D 7.5 4.500 4.514 2500 2450 N/D 17.5

(638) (640)

3.0 N/D 7.5 4.500 4.514 2250 2150 N/D 18.0

(638) (640)

3.5 N/D 7.5 4.500 4.514 1850 1800 N/D 18.5 (638) (640)

4.0 7.5 7.5 4.486 4.507 1650 1550 37.5 17.0

(636) (639)

4.5 N/D 7.5 4.459 4.500 1300 1275 N/D 14.0

(632) (638) 5.0 N/D 7.5 4.459 4.500 1025 1000 N/D 14.0

(632) (638)

5.5 N/D 7.5 4.459 4.507 750 700 N/D 14.0

(632) (639)

TABLE E (continued)

Core Devoid Of Scale Nucleating Agent 40 ppm Ba ⁺⁺ In Feed

Pressure, Cation ¹ Anion ² Time, pH, pH, pascal (psig) Brine Brine Ba-H-. ppm hrs Feed Eff ³ Inlet Outlet Lt ⁴ . ml Lt. ml Feed Eff 6.0 7.5 7.5 4.459 4.507 500 450 38.5 14.0

(632) (639)

1. The cation-containing brine had a Ba++ concentration of about 87.5 ppm.

2. The anion-containing brine had a Ba ⁺⁺ concentration of about 0.0 ppm.

3. "Eff" denotes effluent. 4. "Lt" denotes left or remaining.

5. "N/D" denotes not determined.

The data set forth above in Tables D and E and plotted in Figure 2 indicate that the mixture containing a proppant (namely, sand) and a scale nucleating agent (i.e., barium sulfate crystals) substantially reduced the concentration of a scale forming constituent, namely, barium sulfate, in the scaling brine passed through the mixture. The data also show that there is a smaller degree of reduction in the concentration of the scale forming constituent when the scaling brine passed through just the sand proppant. The latter reduction is believed to be due to the extremely high Ba ⁺⁺ concentration in the scaling brine, rendering the brine very unstable.

Although the present invention has been described in detail with reference to some preferred versions, other versions are possible. Therefore, the spirit and scope of the appended claims should not necessarily be limited to the description of the preferred versions contained herein.