Calcium Tolerant Deflocculant for Drilling Fluids

The invention relates to a method of preventing the flocculation of drilling fluids which contain high amounts of calcium.

In the drilling of oil wells and other deep weels by rotary drilling processes, it is now the practice to recirculate an aqueous fluid called "drilling mud" down through the hollow drill pipe, acrossthe face of the drill bit, and upward through the drill hole. The drilling mud serves to cool and lubricate the drill bit, to raise the drilling cuttings to the surface of the ground, and to seal the sides of the well to prevent loss of water and drilling fluids into the formation through which the drill hole is being bored. After each passage through the well, the mud is passed through a settling tank or trough wherein the sand and drill cuttings are separated, with or without screening. The fluid is then again pumped into the drill pipe by a mud pump.

In the formulation of drilling fluids, it is common to add a material known as a deflocculant. These deflocculants are usually materials capable of neutralizing positive edge charges of clay particles. Interparticle attractions due to positive edge charges are considered to be the cause of the flocculation of the active components of the fluid.

Common deflocculants used to treat drilling fluids are low molecular weight polyacrylic acids. These polymers are good as deflocculants when the amount of calcium in the drilling flid is relatively low. When the amount of calcium exceeds 100 ppm, polyacrylic acid or other deflocculants such as sodium acid pyrophosphate (SAPP) are not too successful in preventing flocculation.

The invention, therefore, is directed to a method of preventing the flocculation of drilling fluids which contain high amounts of calcium, e.g. greater than 100 ppm.

The invention comprises an improved high calcium containing drilling fluid which contains a major portion of drilling fluid and a deflocculating amount of a water-soluble copolymer or terpolymer of acrylic acid which contains from 5-50 mole percent of:

Sulfoethyl acrylamide; or

Acrylamide and Sulfoethyl acrylamide; or

Ethyl acrylate and Sulfoethyl acrylamide; or

Acrylamide and Sulfophenyl acrylamide; or

Acrylamide and Sulfomethyl acrylamide, or a mixture thereof.

In a preferred embodiment of the invention, the copolymer or terpolymer of acrylic acid contains from 5-20 mole percent of one of the above.

Generally, 0.1-2 pounds of polymer per barrel of drilling fluid is sufficient to prevent flocculation of the additives in the drilling fluid, although 0.1-10 pounds per barrel may be needed. The amount of polymer needed is generally related to the solids content of the drilling fluid, so that increases in the solids content of the drilling fluid generally increase the polymer requirements. The dosage may also be regulated so as to provide between 0.1--up to about 2 ppm of polymer per each part of calcium present in the fluid.

The molecular weight of the polymers can vary between as little as 1000 up to as much as 50,000. A preferred molecular weight range is about 4,000 to 20,000.

The polymers of the invention are useful in preventing flocculation of a variety of drilling fluids. These include:

(1) lime muds;

(2) gypsum muds of "gyp muds";

(3) sea water muds;

(4) fresh water muds contaminated with cement or other sources of soluble calcium.

The following table lists polymers actually used by sample number, composition, percent actives and weight average molecular weight:

To illustrate the invention, the following is presented by way of example:

A representative mud system was used for testing these polymers. The composition of the base fluid of Example I was as follows:

300 pounds per barrel (ppb) tap water

20 ppb bentonite clay

30 ppb Kaolinite

276 ppb barite (weighting agent)

pH 10 with sodium hydroxide

1.25 ppb gypsum

The compositions of all the examples were prepared in a standard 350 milliliter laboratory barrel. The examples provided were actually performed on a laboratory scale. All compositions specified are presented on the basis of a real barrel (42 gallons). The calculations were made assuming 1 gram per milliliter of water, 1 ppb=1 gram per 350 milliliter in a laboratory barrel.

The base fluids of each of the examples was dosed with sample polymer to provide the number of pounds of sample per barrel of drilling fluid indicated in the Tables. With no aging, the following results were obtained with the base fluid of Example I:

The composition of the base fluid of Example 1 was aged at 250.degree. F. for sixteen hours. The following results were obtained:

The following mud system was used for testing these polymers. The composition of the base fluid was as follows:

299 pounds per barrel (ppb) tap water

20 ppb bentonite clay

30 ppb Kaolinite

276 ppb barite (weighting agent)

pH 10 with sodium hydroxide

1.25 ppb gysum

After aging at 250.degree. F. for 16 hours, the following results were obtained:

In this example, the composition of the base fluid was as follows:

299 pounds per barrel (ppb) tap water

20 ppb bentonite clay

30 ppb Kaolinite

276 ppb barite (weighting agent)

pH 10 with sodium hydroxide

1.5 ppb gypsum

Without aging, the following results were obtained: