Polyampholytes - High Temperature Viscosifiers for High Ionic Strength Drilling Fluids

A new family of viscosification agents based on terpolymers of N-vinyl-2-pyrrolidone-sodium styrene sulfonate-methacrylamidopropyltrimethylammonium chloride is described as an improved viscosity control additive for water-based drilling muds. The present invention relates to these terpolymer materials which function as viscosification agents when added to water-based muds which are fluids used to maintain pressure, cool drill bits and lift cuttings from the holes in the drilling operation for oil and gas wells. the terpolymers have about 40 to 98 mole % N-vinyl-2-pyrrolidone units, about 1 to about 50 mole % sodium styrene sulfonate units and about 1 to about 50 mole % methacrylamidopropyltrimethylammonium chloride units. Normally, the latter two units comprise less than 60 mole% of the total polymer composition. A soluble, low molecular weight acid, base or salt can be added to the aqueous mud solution, wherein the rheological properties of the drilling fluid is markedly enhanced.

The drilling muds formed from these polymeric materials exhibit improved low and high temperature rheological properties as compared to drilling muds formed from homogeneous-charged polymers, i.e., polyelectrolytes. The improved high temperature performance of these polymers, especially in acidic environments, is due to the hydrolytic stability of the N-vinyl-2-pyrrolidone units.

In the field of drilling in the exploration for oil and gas, an important component is that of the formulation of drilling muds. Drilling muds are the fluids which are used to maintain pressure, cool drill bits and lift cuttings from the holes, and vary in composition over a wide spectrum. Generally, drilling muds are based on aqueous formulations or oil-based formulations.

A conventional water-based drilling mud formulation is comprised of basically the following ingredients: water, a clay such as bentonite, lignosulfonate, a weighting agent such as BaSO.sub.4 (Barite), and a caustic material such as sodium hydroxide and a caustic material such as caustic barite, to adjust the pH of the drilling mud to a pH of about 10 to about 10.5.

In addition to cooling the drill bit and sweeping out the drilling fines from the vicinity of bit, the muds are capable of imparting a substantial positive pressure on a formation through its high density. This latter feature is due to the addition of high concentrations of insoluble, solid, high density particulates (i.e., weighting agents) such as barite. However, these particulates inhibit the drilling rate and possibly damage a variety of underground formations. This problem becomes even more acute as the drilling fines are "introduced" into the mud. Therefore, there has been a substantial need for a homogeneous, high density drilling mud which exhibits good performance at both high temperature and high ionic strength.

As alluded to previously, a very desirable change in the formulation of a drilling fluid would be the elimination of all added particulates. One practical approach to this problem is to formulate a drilling fluid that is clear, homogeneous, dense, single phase and possesses the appropriate viscosity requirements (in general, 40 to 50 cps). Therefore, a water-based mud containing principally a polymeric viscosifier in a high concentration brine (weighting agent) could meet the above-stated requirements. Such a fluid would be quite economical since some processing steps (and materials) are eliminated. For instance, brine can be obtained directly at the drill site.

However, it should be pointed out that the ability of macromolecules to effectively viscosify a high ionic strength solution is generally poor, since the dimensions of the polymer chains tend to collapse under these conditions. This is especially true for polyelectrolytes (i.e., homogeneous-charged polymers). A collapse in the dimensions of the chain results in significant loss in viscosity. Therefore, it is imperative for successful use of polymers in high ionic strength solutions that chain expansion rather than contraction should take place. Polymeric materials composed of N-vinyl-2-pyrrolidone (NV2P), sodium styrene sulfonate (SSS) and methacrylamidopropyltrimethylammonium chloride (MAPTAC) were observed to enhance the viscosity of aqueous solutions containing high levels of salt, acid or, base. these materials meet the requirements for producing a homogeneous, single phase, high density, water-based drilling mud. The N-vinyl-2-pyrrolidone units impart a substantially improved high temperature stability to the drilling fluid due to their own intrinsic hydrolytic stability.

There has been substantial need for a water-based drilling fluid which would exhibit good performance at high temperature. Previous experience has shown that most polymeric viscosifiers are effective in salt-free (i.e., fresh water) systems; however, they lose their effectiveness upon the addition of salt. As the temperature is increased, the viscosity loss becomes even more pronounced. There is need, therefore, for a polymeric viscosifier which can maintain viscosity and gel strength in high ionic strength, weighing agent-free (or at low concentrations), water-based muds up to high temperatures (exceeding 300.degree. F.). These needs are not adequately met by the current viscosifiers.

This invention describes an approach to viscosification of water-based drilling muds which permits the substitution of N-vinyl-2-pyrrolidone-based polyampholyte terpolymers for amine clays and barite (weighting agent). The resulting polymer-modified drilling fluid displays rheological properties which are in a desirable range for drilling mud applications, based on tests conducted for 16 hours at a variety of temperatures.

The types of N-vinyl-2-pyrrolidone-based polyampholytes that are envisioned in the present invention include N-vinyl-2-pyrrolidone as the nonionic monomer unit and the following anionic and cationic species:

Anionic: 2-acrylamido-2-methylpropane sulfonic acid, sodium styrene sulfonate, (meth)acrylic acid, 2-sulfoethylmethacrylate and the like.

Cationic: Methacrylamidopropyltrimethylammonium chloride, dimethyldiallylammonium chloride, diethyldiallylammonium chloride, 2-methacryloxy-2ethyltrimethylammonium chloride, trimethylmethacryloxyethylammonium methosulfate, 2-acrylamido-2-methylpropyltrimethylammonium chloride, vinylbenzyltrimethylammonium chloride and the like.

These monomers possess the appropriate water solubility so that polymerization can take place.

The preferred species of the instant invention is low to moderate charge density N-vinyl-2-pyrrolidonebased polyampholytes with approximately 1 to about 60 mole % ionic groups. A 1:1 molar ratio of anionic and cationic is not required for effective utilization of this polymer. It is found that these terpolymers are soluble (low charge density) or readily dispersible (moderate charge density) in fresh water systems. Homogeneous, clear solutions form with the addition of soluble acid, base, or salt showing that the polymer is readily soluble in these solutions. In addition, the viscosity increases with the addition of these solutes. As a consequence, these polymers are extremely effective viscosifiers in a high ionic strength, water-based mud, even at relatively low levels. Moreover, the hydrolytic stability of the N-vinyl-2-pyrrolidone moieties imparts a substantially improved high temperature stability to the water-based drilling fluid.

A new family of viscosification agents based on terpolymers of N-vinyl-2-pyrrolidone-sodium styrene sulfonate-methacrylamidopropyltrimethylammonium chloride is described as an improved viscosity control additive for water-based drilling muds. The present invention relates to these terpolymer materials which function as viscosification agents when added to water-based muds which are fluids used to maintain pressure, cool drill bits and lift cuttings from the holes in the drilling operation for oil and gas wells. The terpolymers have about 40 to 98 mole % N-vinyl-2-pyrrolidone units, about 1 to about 50 mole % sodium styrene sulfonate units and about 1 to about 50 mole % methacrylamidopropyltrimethylammonium chloride units. Normally, the latter two units comprise less than 60 mole % of the total polymer composition. A soluble, low molecular weight acid, base or salt can be added to the aqueous mud solution, wherein the rheological properties of the drilling fluid is markedly enhanced.

The present invention describes a new class of viscosification agents for water-based drilling muds which are used during operation of gas and oil wells, wherein these viscosification agents are intermolecular complexes, i.e., polyampholytes containing primarily N-vinyl-2-pyrrolidone with low to moderate concentrations of anionic and cationic groups. These latter two units are not necessarily present in a 1:1 molar ratio. Typically, the cationic monomer unit is methacrylamidopropyltrimethylammonium chloride (MAPTAC) and the anionic monomer unit is sodium styrene sulfonate (SSS). However, many water soluble anionic and cationic monomer units can be substituted for MAPTAC and SSS units. It is the placement of these oppositely-charged species onto the polymer chain that imparts substantially different physical properties to these materials, as compared to homogeneous-charged macromolecules, i.e., polyelectrolytes.

The water-based drilling muds of the instant invention minimally comprise, but can also include other additives, if necessary, an aqueous liquid such as fresh water or salt water; a clay such as bentonite; lignosulfonate as a viscosifier; a weighting material such as barite (BaSO.sub.4), and a caustic material such as a sodium hydroxide or lime added to adjust pH to about 10.0 to 10.5. In general, the specific gravity is about 7 pounds per gallon to about 20 pounds per gallon, more preferably about 10 to 18, and most preferably about 12 to about 16.

A typical, but non-limiting example of a caustic material which can be readily employed is sodium hydroxide or lime.

A typical, but non-limiting example of a suitable clay additive is bentonite.

A typical, but non-limiting examples of a weighting agent which can be readily employed is barite or a barium sulfate, which may optionally be surface treated with a variety of other cations, such as calcium.

The terpolymers are formed by a free radical copolymerization process. The principal monomer used in the free radical aqueous solution copolymerization process is N-vinyl-2-pyrrolidone monomer, which is copolymerized with an anionic monomer (typically, sodium styrene sulfonate) and a cationic monomer (typically, methacrylamidopropyltrimethylammonium chloride).

A typical water-based drilling mud, as envisioned by the instant invention, comprises water or salt water; weighting material necessary to give the desired mud density; about 0.25 to about 5 lb/bbl. of the N-vinyl-2-pyrrolidone/sodium styrene sulfonate/methacrylamidopropyltrimethylammonium chloride intrapolymer complex; and sufficient concentration of base to adjust the pH of the water-based drilling mud to about 10.0 to about 10.5. Higher levels of the sulfonated polymer can be employed but it is not normally economically attractive. The drilling mud may also contain a clay such as Bentonite, at a concentration level of about 4 to about 30 lb/bbl., wherein the clay is added to the drilling mud to promote circulation and improve hole stability and cleaning. A lignosulfonate, which is a deflocculation agent, can be added to the drilling mud at a concentration level of about 1 to about 30 lb/bbl.

Alternatively, a typical water-based drilling mud, as envisioned by the instant invention, comprises water in which sufficient salt (such as iron-chloride, iron bromide and calcium bromide) is dissolved to give the desired mud density, and about 0.25 to about 5 lb/bbl. of the N-vinyl-2-pyrrolidone-MAPTAC-SSSterpolymer. Higher levels of the terpolymer can be employed, but, it is not economically attractive. The mud may contain a sufficient concentration of base to adjust the pH of the water-based mud to its appropriate level (normally 10.0 to 10.5). The attractive feature of this mud is the elimination of high concentrations of insoluble, solid, high density particulates (example, weighting agents). In some instances, these particulates inhibit the drilling process through damage of the underground formation and reduction in the drilling rate.

The terpolymers of the instant invention are formed by a free radical terpolymerization process in an aqueous medium of an N-vinyl-2-pyrrolidone monomer, a sodium styrene sulfonate monomer and a methacrylamidopropyltrimethylammonium chloride monomer. The resultant water soluble terpolymer has the formula: ##STR1## wherein x is about 40 to aout 98 mole %, more preferably about 50 to about 95, and most preferably about 80 to about 90, y is about 1 to about 50 mole %, more preferably about 2 to about 20 and most preferably about 5 to about 10, and z is about 1 to about 50 mole %, more preferably about 2 to about 20, and most preferably about 5 to about 10, wherein y is z are less than 60 mole %, and M is an amine, or a metal cation selected from the group consisting of aluminum, iron, lead, Groups IA, IIA, IB and IIB of the Periodic Table of Elements.

The molecular weight as derived from intrinsic viscosities, for the terpolymers of N-vinyl-2-pyrrolidone/sodium styrene sulfonate/methacrylamidopropyltrimethylammonium chloride is about 10.sup.3 to about 5.times.10.sup.6, more preferably about 10.sup.4 to about 2.times.10.sup.6 and most preferably about 10.sup.5 to about 10.sup.6. The means for determining the molecular weights of the water soluble terpolymers from the viscosity of solutions of the terpolymers comprises the initial isolation of the water soluble terpolymers, purification and redissolving the terpolymers in water to give solutions with known concentrations. The flow times of the solutions and the pure solvent were measured in a standard Ubbelholde viscometer. Subsequently, the reduced viscosity is calculated through standard methods utilizing these values. Extrapolation to zero polymer concentration leads to the intrinsic viscosity of the polymer solution. The intrinisic viscosity is directly related to the molecular weight through the well-known Mark Houwink relationship.

The water soluble terpolymers of N-vinyl-2-pyrrolidone/sodium styrene sulfonate/methacrylamidopropyltrimethylammonium chloride are formed by a conventional free radical terpolymerization in an aqueous medium which comprises the steps of forming a reaction solution of N-vinyl-2-pyrrolidone monomer, sodium styrene sulfonate monomer and methacrylamidopropyltrimethylammonium chloride monomer (50 wt. % solution in water) in distilled water, wherein the total monomer concentration is about 1 to about 40 grams of total monomer per 100 grams of water, more preferably about 5 to about 30, and most preferably about 10 to about 20; purging the reaction solution with nitrogen; adding base to the reaction solution to adjust the pH of the reaction solution to about 8.0 to 9.0; etc. sufficient acid to the reaction solution to adjust the pH of the reaction solution to about 4.5 to 5; heating the reaction solution to at least 55.degree. C. while maintaining the nitrogen purge, adding sufficient free radical initiator to the reaction solution at 55.degree. C. to initiate terpolymerization of the N-vinyl-2-pyrrolidone monomer, the sodium styrene sulfonate monomer, and the methacrylamidopropyltrimethylammonium chloride monomer; terpolymerizing said monomers of N-vinyl-2-pyrrolidone, sodium styrene sulfonate and methacrylamidopropyltrimethylammonium chloride at a sufficient temperature and for a sufficient period of time to form said water soluble terpolymer; and recovering said water soluble terpolymer from said reaction solution.

In general, the N-vinyl-2-pyrrolidone, anionic and cationic monomers are dissolved in a water phase in the presence of an initiator, wherein the temperature is sufficient to initiate polymerization. The resultant terpolymer is added to the drilling mud formulation at about 0.5 to about 20 lb/bbl.

The total concentration of monomers in the water is about 1 to about 40 grams of total monomer per 100 grams of water, more preferably about 5 to about 30, and most preferably about 10 to about 20. Terpolymerization of the N-vinyl-2-pyrrolidone monomer, sodium styrene sulfonate monomer, and methacrylamidopropyltrimethylammonium chloride monomer is effected at a temperature of about 30 to about 90, more preferably at about 40 to about 70, and most preferably at about 50 to about 60 for a period of time of about 1 to about 24 hours, more preferably about 3 to about 10, and most preferably about 4 to about 8.

A suitable method of recovery of the formed water soluble terpolymer from the aqueous reaction solution comprises precipitation in acetone, methanol, ethanol and the like.

Suitable free radical initiators for the free radical terpolymerization of the N-vinyl-2-pyrrolidone monomers, the sodium styrene sulfonate monomer, and the methacrylamidopropyltrimethylammonium chloride monomer are selected from the group consisting of potassium persulfate, benzoyl peroxide, hydrogen peroxide, azobisisobutyronitrile and the like. The concentration of the free radical initiator is about 0.001 to about 2.0 grams of free radical initiator per 100 grams of total monomer, more preferably about 0.01 to about 1.0, and most preferably about 0.05 to about 0.1.

It should be pointed out that neither the mode of polymerization (solution, suspension, or emulsion polymerization technique and the like) nor the initiator is critical, provided that the method or the product of the initiation step does not inhibit production of the polyampholyte or chemically modify the initial molecular structure of the reacting monomers .

The following examples illustrate the present invention, without; however, limiting the same hereto.

A representative example for the synthesis of these terpolymers is outlined below.

Into a 1-liter, 4-neck flask add:

6.64 g MAPTAC (50% solution)

30 g N-vinyl-2-pyrrolidone

3.1 g sodium styrene sulfonate

300 ml. distilled water

1.0 ml. ammonium hydroxide

We should emphasize at this time that the anionic and cationic monomers were added to the aqueous phase without attempting to form ion-pair comonomers that do not possess nonpolymerizable counterions.

The solution was purged with nitrogen gas for one hour to remove dissolved oxygen. As the nitrogen gas purging began, the solution was heated to 55.degree. C. At this point, 0.05 g azobisisobutyronitrile was added to the solution. After 24 hours, the polymer was precipitated from solution with acetone. Subsequently, the resulting polymer was washed several times with a large excess of acetone and dried in a vacuum oven at 60.degree. C. for 24 hours.

Presented in Table I are representative data on the rheological properties of NV2P-SSS-MAPTAC terpolymer composed of approximately 90 mole % NV2P, 5 mole % MAPTAC and 5 mole % SSS dissolved in a variety of salt and acid environments. In all instances, the properties were measured initially at room temperature. Subsequently, the solutions were heated in a bomb at 150.degree. F. for four hours, cooled to room temperature and the properties measured again. This procedure is repeated again at 400.degree. F. The results show that this polyampholyte is very effective at enhancing the rheological properties of high ionic strength aqueous fluids at room temperature, 150.degree. F. and with relatively modest change upon the 400.degree. F. heat treatment. Undoubtedly, this N-vinyl-2-pyrrolidone based polyampholyte is both chemically and thermally stable in these solution environments. Furthermore, marked improvement in the rheological properties of these water-based mud systems is obtained through moderate increases in either (or both) charge density and molecular weight of the polyampholyte. In any case, these properties (see Table I) compared favorably with conventional (and commercially available) oil-based muds where amine-treated clays are used as the viscosification agent.

A representative example for the synthesis of a NV2P-MAPTAC-(2-acrylamido-2-methylpropanesylfonic acid) AMPS terpolymer is outlined below.

Into a 1-liter, 4 neck flask add:

3.1 g AMPS (acid form)

0.6 g sodium hydroxide

30 g n-vinyl-2-pyrrolidone

6.6 g MAPTAC (50% solution)

150 ml distilled water

The NV2P was vacuum distilled at 56.degree. C./0.6 mm Hg and MAPTAC was purified through addition of activated charcoal (3% by weight) to the as-received monomer solution. The charcoal was filtered from solution via conventional separation methods prior to addition to the reaction flask.

Again we emphasize that the anionic and cationic monomers were added to the aqueous phase without attempting to form ion-pair comonomers that do not possess nonpolymerizable counterions.

The solution was purged with nitrogen gas for one hour to remove dissolved oxygen. As the nitrogen gas purging began, the solution was heated to 50.degree. C. At this point, 0.1 g azobisisobutyronitrile (AIBN) was added to the solution. AIBN was dissolved directly into a small quantity of NV2P prior to addition to the reactant mixture. After 24 hours, the terpolymer was precipitated from solution with acetone. Subsequently, the resulting polymer was washed several times with a large excess of acetone and dried in a vacuum oven at 60.degree. C. for 24 hours.

This particular terpolymer is used in the subsequent examples.

A variety of aqueous solutions containing a NV2P-MAPTAC-AMPS terpolymer (example 3) were formed at 1 g/dl polymer concentration. These solutions were heated for four (4) hours at a particular temperature, cooled to room temperature and the viscosity was measured in a standard Brookfield viscometer. This procedure was continued up to 90.degree. C. The results are presented in Table II.

The results show that this polyampholyte terpolymer is very effective at enhancing the rheological properties of high ionic strength aqueous fluids at room temperature and elevated temperatures. The viscosity of these solutions remains essentially invarient to temperature changes. Undoubtedly, this terpolymer is both chemically and thermally stable in high ionic strength environments. Furthermore, these polymeric materials are especially useful as a high temperature viscosifier for water-based drilling fluids where the weighing agent is the dissolved salt or acid.