Salt-tolerant, thermally-stable rheology modifiers

1. A salt-tolerant, thermally-stable rheology modifier comprising a terpolymer of acrylamide, 2-acrylamido-2-methyl-propanesulfonic acid (AMPS) and a long-chain alkyl acrylate having a chain length for the alkyl group of from 12-25 wherein the long-chain alkyl acrylate is stearyl acrylate, wherein said terpolymer is made through a dispersion polymerization process and a saturated KCl or NaCl solution containing 1% by weight of the terpolymer has a Brookfield viscosity of about 400 cps or more at 20° C.

2. A salt-tolerant, thermally-stable rheology modifier in accordance with claim 1 wherein the modifier comprises from about 30-70 wt. % acrylamide, 70-30 wt. % AMPS, and 0.01-5 wt. % stearyl acrylate.

3. A salt-tolerant, thermally-stable rheology modifier in accordance with claim 1 wherein the molecular weight (Mw) of the modifier is over 1,000,000 daltons.

4. A salt-tolerant, thermally-stable rheology modifier in accordance with claim 1 wherein the molecular weight (Mw) of the modifier is from about 2,000,000 to 20,000,000 daltons.

5. A salt-tolerant, thermally-stable rheology modifier in accordance with claim 1 wherein the modifier is suitable for High Pressure/High Temperature (HP/HT) drilling operations at >300° F. temperature.

6. A salt-tolerant, thermally-stable rheology modifier in accordance with claim 1 wherein the modifier comprises from about 40-60 wt. % acrylamide, 60-40 wt. % AMPS, and 0.01-3 wt. % long-chain stearyl acrylate.

7. A salt-tolerant, thermally-stable rheology modifier in accordance with claim 1 wherein a mud formulation containing 1% of the rheology modifier maintains at least 40% of its measured values for plastic viscosity, yield point and gel strength after the mud formulation is subjected to aging up to 350° F. for 16 hours wherein the mud formulation has the composition set forth below:

Mixing

Chemical Name

CAS#

SG

g

Order

Fresh water

7732-18-5

1.000

260.00

Soda Ash

497-19-8

2.500

0.53

1

NaCl

7647-14-5

2.390

49.00

2

Cellulosic Fluid Loss

unknown

1.600

2.00

4

Additive

Modified Starch Fluid Loss

unknown

1.600

4.99

5

Additive

BaSO 4 1-4 μM

7727-43-7

4.200

187.25

6

Lubricant

unknown

1.040

10.92

7

Total

514.69

and the rheology modifier is added into the formulation after NaCl in an amount sufficient to provide a concentration of polymer in the mud formulation of 1% based on active rheology modifier in the total mud formulation and each of the components in the formulation is mixed to a fully hydrated state.

8. A wellbore fluid comprising an aqueous base fluid and the rheology modifier of claim 1 .

9. A wellbore fluid in accordance with claim 8 wherein the aqueous base fluid is present in the wellbore fluid in an amount in the range of from about 30 to 99% by weight of the fluid.

10. A wellbore fluid in accordance with claim 8 wherein the rheology modifier is present in the wellbore fluid in an amount in the range of 0.5 to 50 g/l on an active basis.

11. A wellbore fluid in accordance with claim 8 wherein the wellbore fluid is a drilling fluid, a completion fluid, a workover fluid, a fracturing fluid, or an acidic treatment fluid.

12. A method of preparing a high molecular weight dispersion polymer having a bulk Brookfield viscosity of from about 100 to about 25,000 cps at 25° C. comprising:

a) adding an initiator to an aqueous mixture comprising: i. from about 10 to about 40 weight percent of a mixture comprising 10-45 mole percent of Na AMPS, 55-90 mole percent of acrylamide and about 0.01-5 wt % based on the weight of Na AMPS and acrylamide of a long-chain alkyl acrylate having a chain length for the alkyl group of from 12-25; ii. from about 0.1 to about 10 weight percent based on the total weight of the dispersion polymer solids of a stabilizer, (based on polymer solids) wherein the stabilizer comprises polyacrylamidomethylpropane sulfonic acid and iii. from about 10 to about 40 weight percent based on the weight of the dispersion of a water soluble salt selected from the group consisting of ammonium, alkali metal and alkaline earth metal halides, sulfates, and phosphates; and

b) polymerizing the monomers.