Process of fracturing subterranean formations

Abstract

Process of fracturing subterranean, oil-and/or gas-bearing formations by injecting an aqueous fracturing fluid into a wellbore at a rate and pressure sufficient to penetrate into the formation, and to initiate or extend fractures in the formation, wherein the aqueous fracturing fluid is obtained by mixing at least an aqueous base fluid and an aqueous polyacrylamide concentrate having a concentration of 3.1 to 14.9% by weight of polyacrylamides, relating to the total of all components of the aqueous polyacrylamide concentrate, and wherein at least a part of the aqueous fracturing fluid injected additionally comprises a proppant.

Claims

1. A process of fracturing subterranean, oil-and/or gas-bearing formations penetrated by at least a wellbore comprising at least the steps of (1) providing an aqueous fracturing fluid (F) comprising at least an aqueous base fluid and a polyacrylamide, and (2) injecting said aqueous fracturing fluid (F) into the wellbore at a rate and pressure sufficient to penetrate into the formation, and to initiate or extend fractures in the formation, wherein at least a part of the aqueous fracturing fluid (F) injected additionally comprises a proppant, and wherein step (1) is carried out by mixing at least an aqueous base fluid, a homogeneous aqueous polyacrylamide concentrate having a concentration of 3.1 to 10% by weight of polyacrylamides, relating to the total of all components of the homogeneous aqueous polyacrylamide concentrate, and—for those parts of the fracturing fluid comprising a proppant—a proppant and wherein the homogeneous aqueous polyacrylamide concentrate is manufactured by a process comprising at least the following steps: [1] radically polymerizing an aqueous monomer solution in the presence of suitable initiators for radical polymerization under adiabatic conditions in a polymerization unit, wherein the aqueous monomer solution comprises at least water and 20% to 50% by weight-relating to the total of all components of the aqueous monomer solution-of water-soluble, monoethylenically unsaturated monomers, wherein said water-soluble, monoethylenically unsaturated monomers comprise at least acrylamide, thereby obtaining an aqueous polyacrylamide gel which is held in the polymerization unit, [2] removing the aqueous polyacrylamide gel from the polymerization unit, and [3] comminuting the aqueous polyacrylamide gel and mixing it with an aqueous liquid, thereby obtaining said homogeneous aqueous polyacrylamide concentrate, and wherein the steps [1], [2], and [3] are carried out at a manufacturing site (“location A”) and the process comprises an additional process step [4] of transporting the aqueous polyacrylamide concentrate from the manufacturing site (location A) to the wellsite (location B) in a suitable transport unit.

2. The process according to claim 1, wherein the homogeneous aqueous polyacrylamide concentrate has a concentration of 4 to 8% by weight of polyacrylamides, relating to the total of all components of the homogeneous aqueous polyacrylamide concentrate.

3. The process according to claim 1, wherein the components of the aqueous fracturing fluid are mixed in a blender.

4. The process according to claim 3, wherein the blender is selected from blenders fixed on a truck, mounted on a trailer or mounted in a skid.

5. The process according to claim 1, wherein the process starts with the injection of an aqueous fracturing fluid (F) not comprising a proppant, followed by the injection of an aqueous fracturing fluid (F) additionally comprising a proppant.

6. The process according claim 1, wherein during the entire process an aqueous fracturing fluid (F) comprising a proppant is injected.

7. The process according to claim 1, wherein the process is a process of slickwater fracturing, wherein the polyacrylamide serves as friction reducer, wherein the concentration of the polyacrylamides in the aqueous fracturing fluid (F) is from 20 ppm to 600 ppm of polyacrylamides relating to the total of all components of the aqueous fracturing fluid except the proppants.

8. The process according claim 1, wherein the aqueous fracturing fluid (F) is a viscosified fracturing fluid (V) in which the polyacrylamide serves as viscosifier, wherein the concentration of the polyacrylamides in the aqueous fracturing fluid (V) is from 0.1% by weight to 2% by weight of polyacrylamides relating to the total of all components of the aqueous fracturing fluid except the proppants.

9. The process according to claim 1, wherein the process the process is a hybrid fracturing process and step (2) of the process comprises two sub-steps (2a) and (2b) of (2a) slickwater fracturing the subterranean formation by injecting an aqueous fracturing fluid (F) in which the polyacrylamide serves as friction reducer, wherein the concentration of the polyacrylamides in the aqueous fracturing fluid (F) is from 20 ppm to 600 ppm of polyacrylamides relating to the total of all components of the aqueous fracturing fluid except the proppants, and (2b) fracturing the formation by injecting a viscosified aqueous fracturing fluid comprising 0.1% by weight to 5% by weight of a viscosifier relating to the total of all components of the aqueous fracturing fluid except the proppants.

10. The process according to claim 9, wherein in course of step (2a) the aqueous fracturing fluid does not comprise a proppant, and in course of step (2b) the viscosified aqueous fracturing fluid comprises a proppant.

Description

EXAMPLES

(1) The invention is illustrated in detail by the examples which follow.

(2) Polyacrylamide friction reducers to be tested:

Example 1

(3) Homogeneous aqueous Polyacrylamide concentrate made by mixing an aqueous polyacrylamide gel with an aqueous liquid

(4) Step 1:

(5) Preparation of an aqueous gel of a copolymer comprising 69.4 wt. % (75.0 mol %) of acrylamide and 30.6 wt. % (25 mol %) of sodium acrylate stabilized with 0.25 wt. % Na-MBT relating to polymer by adiabatic gel polymerization (solids content of 23% by weight relating to the total of the gel)

(6) A 5 L beaker with magnetic stirrer, pH meter and thermometer was filled with 1600 g of distilled water, 702.04 g of sodium acrylate (35% by weight in water), and 1071.69 g of acrylamide (52% by weight in water). Then 10.5 g of diethylenetriaminepentaacetic acid pentasodium salt (Trilon C; 5% by weight in water), and 4 g of the stabilizer sodium 2-mercaptobenzothiazole (Na-MBT; 50% by weight in water) were added. After adjustment to pH 6.4 with sulfuric acid (20% by weight in water) and addition of the rest of the water to attain the desired monomer concentration of 23% by weight (total amount of water 1690.08 g minus the amount of water already added, minus the amount of acid required), the monomer solution was adjusted to a temperature of approx. −3° C. The solution was transferred to a Dewar vessel, the temperature sensor for the temperature recording was inserted, and the flask was purged with nitrogen for 45 minutes. The polymerization was initiated at 0° C. with 21 g of a 10% aqueous solution of 2,2′-azobis(2-methylpropionamidine) dihydrochloride (Wako V-50; 10 h t.sub.1/2 in water 56° C.), 1.75 g of t-butyl hydroperoxide (1% by weight in water) and 1.05 g of a 1% sodium sulfite solution. With the onset of the polymerization, the temperature rose to 54.6° C. within about 63 min. A solid polymer gel block was obtained.

(7) After polymerization, the gel block was incubated 4 hours at 60° C. Then, the block was cut vertically into two pieces. One part was sealed in a plastic bag for use in step 2. The other part was kept for example 2.

(8) Step 2:

(9) Preparation of a Homogeneous Aqueous Polyacrylamide Concentrate (Polymer Concentration: 5 wt. %)

(10) The aqueous polyacrylamide gel obtained in course of step 1 was first chopped to small particles ranging in size from 2 to 5 mm. To give a final concentration of 5.0 weight % polymer, 58.82 g of these chopped particles were then dispersed into a 600 ml beaker containing 241.18 g of distilled water. The gel particles were added while mixing via an overhead mixer with a 75 mm diameter half-moon propeller. The mixing rate was initially set at 300 rpm for the first 5 min, then lowered to 30 rpm for an additional 18 hours.

Example 2

(11) Homogeneous aqueous polyacrylamide concentrate made by mixing polyacrylamide powder with an aqueous liquid

(12) Step 1:

(13) Preparation of an aqueous gel of a copolymer comprising 69.4 wt. % (75.0 mol %) of acrylamide and 30.6 wt. % (25 mol %) of sodium acrylate stabilized with 0.25 wt. % Na-MBT relating to polymer by adiabatic gel polymerization (solids content of 23% by weight relating to the total of the gel)

(14) Step 1 was carried out in the same manner as in example 1. A part of the polymer gel obtained in step 1 of example 1 was used for example 1 and the other part for the present example 2.

(15) Step 2:

(16) Drying the Aqueous Gel

(17) The gel obtained in example 1 was comminuted with a meat grinder. The particles were dried for two hours at 55° C. in a fluid bed dryer. After drying, the dried particles were grinded in a lab mill and filtered with a 1 mm sieve. A polyacrylamide powder with an active content of 94.6% by weight (the remainder being moisture) was obtained.

(18) Step 3:

(19) Preparation of a Homogeneous Aqueous Polyacrylamide Concentrate (Polymer Concentration: 5 wt. %)

(20) An amount of 284.21 g of water was added into a 600 ml beaker while mixing via an overhead mixer with a 75 mm diameter half-moon propeller. The mixing rate was initially set at 300 rpm. Thereafter 15.79 g of the polyacrylamide powder (i.e. the amount to give a final concentration of 5.0 weight % of polyacrylamides in the concentrate) obtained in course of step 2 was slowly added to the vortex over a few seconds to avoid the formation of lumps. After 5 min, the mixing rate was lowered to 30 rpms for an additional 18 hours.

Example 3

(21) Aqueous Polyacrylamide Concentrate Made by Mixing Polyacrylamide Powder with an Aqueous Liquid

(22) Step 1:

(23) Preparation of an aqueous gel of a copolymer comprising 69.4 wt. % (75.0 mol %) of acrylamide and 30.6 wt. % (25 mol %) of sodium acrylate stabilized with 0.25 wt. % Na-MBT relating to polymer by adiabatic gel polymerization (solids content of 30% by weight relating to the total of the gel)

(24) A 5 L beaker with magnetic stirrer, pH meter and thermometer was filled with 1100 g of distilled water, 915.71 g of sodium acrylate (35% by weight in water), and 1397.85 g of acrylamide (52% by weight in water). Then 10.5 g of diethylenetriaminepentaacetic acid pentasodium salt (Trilon C; 5% by weight in water), and 5.2 g of the stabilizer sodium 2-mercaptobenzothiazole (Na-MBT; 50% by weight in water) were added.

(25) After adjustment to pH 6.4 with sulfuric acid (20% by weight in water) and addition of the rest of the water to attain the desired monomer concentration of 30% by weight (total amount of water 1149.05 g minus the amount of water already added, minus the amount of acid required), the monomer solution was adjusted to a temperature of approx. −3° C. The solution was transferred to a Dewar vessel, the temperature sensor for the temperature recording was inserted, and the flask was purged with nitrogen for 45 minutes. The polymerization was initiated at 0° C. with 21 g of a 10% aqueous solution of 2,2′-azobis(2-methylpropionamidine) dihydrochloride (Wako V-50; 10 h t.sub.1/2 in water 56° C.), 1.75 g of t-butyl hydroperoxide (1% by weight in water) and 1.05 g of a 1% sodium sulfite solution. With the onset of the polymerization, the temperature rose to 84.4° C. within about 20 min. A solid polymer gel block was obtained.

(26) After polymerization, the gel block was incubated 4 hours at 80° C. Then, the block was comminuted with a meat grinder. The particles were dried for two hours at 55° C. in a fluid bed dryer. After drying, the dried particles were grinded in a lab mill and filtered with a 1 mm sieve. A polyacrylamide with an active content of 95.0 wt. % was obtained.

(27) Step 2:

(28) Preparation of an Aqueous Polyacrylamide Concentrate (Polymer Concentration: 5 wt. %)

(29) An amount of 284.21 g of water was added into a 600 ml beaker while mixing via an overhead mixer with a 75 mm diameter half-moon propeller. The mixing rate was initially set at 300 rpm. Thereafter 15.79 g of the polyacrylamide powder (i.e. the amount to give a final concentration of 5.0 weight % of polyacrylamides in the concentrate) obtained in course of step 2 was slowly added to the vortex over a few seconds to avoid the formation of lumps. After 5 min, the mixing rate was lowered to 30 rpms for an additional 18 hours.

Comparative Example 1

(30) Inverse Emulsion of Polyacrylamides

(31) Inverse emulsion of a copolymer comprising 69.4 wt. % (75.0 mol %) of acrylamide and 30.6 wt. % (25 mol %) of sodium acrylate stabilized with 0.25 wt. % Na-MBT relating to polymer (solids content 23% by weight relating to the total of the inverse emulsion).

(32) A 600 mL beaker with magnetic stirrer, pH meter and thermometer was charged with 150.44 g of sodium acrylate (35% by weight in water), 128.97 g of distilled water, 229.65 g of acrylamide (52% by weight in water), 0.5 g of diethylenetriaminepentaacetic acid pentasodium salt (Trilon C; 5% by weight in water), and 0.86 g of the stabilizer sodium 2-mercaptobenzothiazole (Na-MBT; 50% by weight in water).

(33) After adjustment to pH 6.4 with sulfuric acid (20% by weight in water), the rest of the water to attain the desired monomer concentration of 23% by weight (total amount of water 138.61 g minus the amount of water already added, minus the amount of acid required) was added.

(34) A high 1 L beaker was charged with 12.2 g sorbitan monooleate (Span® 80) and 189.9 g of a high-boiling dearomatized hydrocarbon mixture (Exxsol® D100) was added and stirred with a spatula.

(35) The beaker with the oil solution was fixed in a Silverson high shear mixer. While mixing the oil solution at 4000 rpm, the aqueous solution was poured in quickly. Then, the Silverson high shear mixer is turned up to 8000 rpm for 2 min 48 sec.

(36) The emulsion was transferred to a double jacketed reactor, stirred at 200 rpm and adjusted to the initiation temperature of 10° C. During this time the emulsion was purged with nitrogen (for 60 minutes). The polymerization was drop-wise initiated with 9 g of a 0.1% sodium bisulfite solution and 5 g of 0.1% t-butyl hydroperoxide solution.

(37) The initiators were added with a squeezing pump, controlled by hand. When the respective 0.1% solutions were empty, the initiators were changed to 9 g of a 1% sodium bisulfite solution and 5 g of a 1% t-butyl hydroperoxide solution. Thereby, the temperature rose 1° C. per minute up to 40° C., from there the temperature was maintained at 40° C. When the second initiator was added completely, the emulsion was stirred for additional 60 minutes at 40° C. The emulsion was then filtered through a 190 μm filter.

(38) Activation of the Inverse Emulsion

(39) The activation was carried out 24 h prior to use in the Friction Loop experiment. For activation, 97.75 g of the inverse emulsion was poured in a glass beaker and stirred with an over-head stirrer at 650 rpm. With a 5 mL plastic syringe, 2.25 g of a commercially available activator was added at once to the vortex of the inverse emulsion. The mixture was stirred for additional 8 minutes.

(40) Friction Loop Apparatus

(41) The friction reduction performance of the friction reducing agent was assessed using a Chandler model M5600 friction loop, which circulates fluid through a section of known diameter pipe to determine the effectiveness and longevity of a friction reducing agent added to a test fluid. Fluid in the loop flows from a ˜37.8 l (˜10 gallon) reservoir through a pump, mass flow meter and then two ˜250 cm (10 feet) long sections of pipe before returning to the reservoir to be recirculated. Pressure drop is measured over the two sections of pipe. One is 1.27 cm outer diameter (½ inch), the other is 1.91 cm outer diameter (¾″ inch), giving different ranges of Reynolds number.

(42) The friction loop was loaded with 37.85 l (10 gallons) of aqueous test fluid (fresh water or brines). The flow rate was set to 37.85 l per minute (10 gallons per minute) and once a stable, initial pressure was recorded. Thereafter, the friction reducing composition to be tested was injected into the vortex of the fluid reservoir using a plastic syringe.

(43) The injection time was taken as the start of the test (time=0 seconds). The subsequent drop in pressure measured the performance of the friction reducing composition. The pressure data from the 1.27 cm pipe is reported, because it reflected a higher Reynolds number than the 1.91 cm pipe.

(44) Pressure data was converted to friction reduction using the formula:
% Friction Reduction (% FR)=Initial Pressure with no FR−Pressure with FR/Initial Pressure with no FR

(45) Friction Loop Tests

(46) 26.08 g of each of the aqueous polyacrylamide concentrates obtained in examples 1 to 3 (each having a concentration of 5 wt. % of polyacrylamides) was used for the friction loop testing. This dosage amount corresponds to a final concentration of 35 ppm polymer once diluted in the friction loop with additional fresh water.

(47) The aqueous polyacrylamide concentrate was added directly to vortex of the friction loop mixing tank at time=0, as mentioned in the above description of the Friction Loop.

(48) 5.67 ml of the activated inverse emulsion sample (comparative example 1) was directly injected into the vortex of the flow loop mixing tank, to achieve also a final concentration of 35 ppm polymer.

(49) All results (percentage of friction reduction vs. time) are summarized in FIG. 17.

(50) Discussion of the Results Obtained

(51) FIG. 17 shows a comparison of the % Friction Reduction of examples 1 to 3 and comparative example 1 as a function of time. Each sample was measured individually under the same experimental conditions and at the same effective dosage concentration.

(52) Both the aqueous gel (example 1) and the powder polyacrylamide samples (examples 2 and 3) yielded higher maximum friction reduction, as well as at a faster rate when compared to the inverse emulsion (comparative example 1). The aqueous gel (example 1) also out performs both powder samples (examples 2 and 3) and inverse emulsion (comparative example 1) over the 10 min interval.

(53) The aqueous polyacrylamide gel (example 1) has the best performance of all samples. The two powder samples (examples 2 and 3) show slightly reduced amount of % Friction Reduction. Without wishing to be bound by theory, this performance loss may be a result of the impact from additional drying and processing needed to generate the powder form.