Friction modifier for drilling fluids

This invention relates to improved lubrication in drilling and completion fluids used in subterrane drilling for oil and gas. The addition of zinc dialkyl dithiophosphate (ZDDP) acts as a lubricant, an enhancer to other lubricants and rate of penetration enhancer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the invention related to friction modifiers for use with non-sulfurized lubricants used in drilling fluids.

2. Description of the Related Art

During the drilling of an oil and gas well, a specialized fluid referred to as a drilling fluid or alternatively a “mud” is circulated through the drill pipe and bit. The principal functions of a drilling fluid include: stabilizing geological formations, providing hydrostatic pressure, cooling the drill bit, and carrying drill cuttings beneath the bit to transport them up to the surface for separation. The other key function of a drilling fluid is to act a lubricant between the drill pipe and the borehole and/or metal casing. The drilling fluid also acts as a lubricant for the drill bit.

Drilling fluids can be categorized as being either water-based or oil-based. In general, water-based drilling fluids are lower cost and have much better health, safety and environmental performance than oil-based drilling fluids. However, oil-based fluids offer excellent shale stabilization and provide for lower co-efficient of friction. Certain water based drilling fluids such as sodium and potassium silicate based drilling fluids can match the shale inhibition properties but not the coefficient of friction (CoF) of oil based drilling fluids.

The lubricity of a drilling fluid is an important property as it determines the torque (rotary friction) and drag (axial friction) in the wellbore. There are numerous economic and technical reasons for wanting to lower the coefficient of friction of the drilling fluid. Reduction in torque and drag results in:faster drilling rates and, therefore, reduced costwells of greater depth and lengthmore complex well profilessubstitution of oil-based drilling fluids for water-based drilling fluids

Given the numerous benefits of lower torque and drag, it is very common to add a chemical or mechanical lubricant to a drilling fluid to lower the CoF. There is extensive prior art on chemical and mechanical lubricants for lowering CoF in drilling fluids. Examples of commonly used chemical lubricants include hydrocarbons, synthetic oils, esters, fatty acids, natural oils, and surfactants as well as other compounds.

A common problem encountered in drilling is accretion which occurs when partially hydrated drill cuttings stick to the drill string. Preferably, a lubricant will have anti-accretion properties. Anti-accretion properties are desirable in a lubricant because they can provide further reductions in friction. Anti-accretion additives are also referred to as rate of penetration enhancers.

Zinc dialkyl dithiophosphate (ZDDP) is a commonly used additive for motor oils. ZDDP functions as an anti-wear additive by reacting with a metal surface under conditions of temperature and/or pressure. ZDDP decomposes under high temperature and/or rubbing to form a polyphosphate layer that acts as an anti-wear film. This film accommodates and redistributes applied load which reduces wear of the underlying surface. Ancillary properties of ZDDP in motor oil include anti-corrosion.

Although primarily a motor oil additive, ZDDP has seen extremely limited use in drilling fluid lubricants. U.S. Pat. No. 3,712,393 to Sheldahl, et. al. describes the addition of ZDDP to a drilling fluid lubricant composed of sulfurized lard oil, mineral oil and halogenated paraffin. Corrosion inhibitors, wear inhibitors, oxidation stabilizers and odor stabilizers can also be added to the lubricant. ZDDP functions as a wear inhibitor in this composition. The added ZDDP does not function to improve lubricity since, as shown below, sulfurized based lubricants represent one of the few classes of lubricants that do not show a lower CoF with the addition of ZDDP.

U.S. Pat. No. 4,064,056 to Walker, et al. describes a lubricant composition containing from 13 to 15% sodium salts of petroleum sulfonic acids and from 70 to about 82% petroleum oil carrier. ZDDP, among other additives, is added to the mineral oil at a concentration of from 0% to about 1.1% to improve anti wear and antioxidant properties. The presence of sulfurized compounds in the lubricant composition would not allow for ZDDP to impart improvements in lubricity.

BRIEF SUMMARY OF THE INVENTION

It has been discovered that torque and drag in a drilling fluid can be further reduced when a minor amount of ZDDP is added to the drilling fluid or used in combination with other lubricants. Because of its surface chemical properties, ZDDP preferentially films on a metal surface and prevents clay adhesion. The ZDDP film has lubricant properties but it can also act as a coupling agent for other lubricants. Results are most dramatic in drilling fluids that contain additives that compete for metal surface sites and/or disrupt the film forming properties of traditional lubricants. Further, the ZDDP film helps minimize the sticking of drill cuttings onto the drill string. Reduction in co-efficient of friction is particularly evident when the invention is applied in a silicate based drilling fluid.

ZDDP consists of zinc bound to diphosphordithioic acid with alkyl or alkaryl ester substituent groups. The alkyl groups are saturated hydrocarbons that vary in length from C3-C12. The basic chemical structure of ZDDP is shown below. The chemical category of ZDDP can be divided into twelve products that share similar structure types. Substantially any ZDDP could be used in a drilling fluid.

It is anticipated that improvements in lubricity could also be achieved with monothiophosphates and polythiophosphates of cadmium, tin, iron, cobalt, nickel, vanadium, chromium, manganese, molybdenum, tungsten, titanium and zirconium.

DETAILED DESCRIPTION OF THE INVENTION

Alkali silicate -based drilling fluids were selected as the initial drilling fluid since they are known to have a high CoF and are often used by the industry as an environmentally friendly alternate to oil based drilling fluids. The high CoF of silicate based drilling fluids is the result of silicate adhesion to metal. The efficacy of ZDDP was also tested in other families of water-based drilling fluids known for shale inhibition. These family of water-based drilling fluids include; glycol-based, amine-based and formate-based fluids. It is anticipated that the efficacy of this invention would be observed in other water based drilling fluids.

ZDDP could be added to any commonly used class of drilling fluid lubricants, the notable exception being sulfurized based lubricants. Table 1a, lists the different lubricants tested in combination with ZDDP.

In the search for oil and gas, the industry trend is towards wells of greater length and depth. Oil-based drilling fluids are being challenged to provide lower CoF. ZDDP was tested as an extreme pressure lubricant in oil-based drilling fluids.

The lubricity of the drilling fluid and lubricant was measured using Extreme Pressure Lubricity Tester (i.e. surface to surface drag test). This is a common lubricity test that measures co-efficient of friction between a steel block and a rotating steel ring immersed in a drilling fluid. The standard test involves the application of 150 in-pounds of torque applied to the test block. The ring rotates at 60 rpm. The lubricity tester is allowed to run for at least 5 minutes at which point a friction coefficient reading is taken. The drilling fluid samples are sheared at high speed for 5 minutes prior to testing.

Given the wide range of drilling fluid types as well as lubricants, it was not possible to illustrate the efficacy of ZDDP under every combination. Other aspects, object and advantages of the present disclosure will become apparent to those skilled in the art from this disclosure and claims.

Synergistic Effect of ZDDP with Other Lubricants

ZDDP was added to a broad cross-section of drilling fluid lubricants. The ZDDP-enhanced lubricant was prepared by adding 5.0 g of ZDDP into 95 g of lubricant and stirring. The ZDDP readily mixed into the lubricant at room temperature. In the case of graphite, 20 g of ZDDP was mixed into 80 g of graphite.

Reduction in CoF was measured on a potassium silicate based drilling fluid prepared in the lab according to Table 1b. Lubricants were tested in the drilling fluid at a concentration of 2% wt/wt (i.e. 10 g lubricant into 500 g drilling fluid). A 2% lubricant loading was chosen as a reasonable concentration for initial testing. Lubricant and drilling fluid were shear mixed and then hot rolled for 16 hrs@120 F. Table 1c indicates the friction readings and % reduction in coefficient of friction. Notably, no reduction in the CoF was achieved by adding ZDDP to NXS, a sulfurized lubricant.

TABLE 1bBase Drilling FluidWater920mlPotassium Silicate (PQ Corp. EcoDrill ® 317)80mlXanthan gum2gStarch2gPAC2gRev Dust30gchemical components were added under agitation at room temperature. The drilling fluid was then aged by hot rolling for 24 hrs at 120 F.

Reduction in Friction in a Sodium Silicate-based Drilling Fluid

Reduction in CoF was measured on a sodium silicate-based drilling fluid. The drilling fluid was formulated in a similar manner as Table 1b, except potassium silicate was substituted for sodium silicate (PQ Corporation, N® grade). Lubricants were mixed with ZDDP at a ratio of 9:1. Lubricants were added to the drilling fluid at a concentration of 2% wt/wt (i.e. 10 g of lubricant was added 500 g of drilling fluid). Lubricant and drilling fluid were shear mixed and then hot rolled for 16 hrs@120 F. Table 2 shows that the addition of ZDDP to the lubricant resulted in a reduction in the CoF of the lubricant.

Lubricant Concentration vs. Coefficient of Friction

A sample of potassium silicate based drilling fluid was obtained from a well site in Western Canada. The drilling fluid contained 6.5% potassium silicate by volume and remainder being water, polymers and drill solids. Estadril was blended 19:1 with ZDDP on a wt/wt basis. Lubricant was added to the drilling fluid on a 2% wt/wt basis. Table 3 shows that a 0.5% loading of ZDDP enhanced lubricant had a similar CoF as a 2% loading of lubricant.

Order of Addition

ZDDP does not have to be pre-mixed into a base lubricant prior to adding to the drilling fluid. A comparison was made between ZDDP that was blended into a lubricant vs. ZDDP and lubricant added separately into a drilling fluid. Blended lubricants have a ratio 9 parts lubricant to 1 part ZDDP. Lubricants were tested in a potassium silicate-based drilling fluid containing 8% potassium silicate by volume and a sodium silicate-based drilling fluid containing 8% sodium silicate.

Glycol Based Drilling Fluids

Glycol-based drilling fluids represent a class of water based drilling fluids that provide shale inhibition. A glycol based drilling fluid was formulated with 8% v/v polyethylene glycol (PEG 300), polymers, water and simulated drill solids. Lubricity testing was done on ZDDP by itself as well as lubricant blended with ZDDP (9:1). Table 5 shows a reduction in the CoF when ZDDP is added to the lubricant.

Improved Lubricity in Amine-based Drilling Fluids

Amine-based drilling fluids represent another class of inhibitive water based drilling fluids. An amine based drilling fluid was formulated with 0.5% hexadiamine v/v, polymers, water and simulated drill solids. pH of the drilling fluid was adjusted to 9.6. Lubricity testing was done on ZDDP by itself as well as lubricant blended with ZDDP (9:1). Table 6 shows a reduction in the CoF of the drilling fluid with the addition of ZDDP.

Improved Lubricity in Formate-based Drilling Fluids

Formate-based drilling fluids represent another class of inhibitive water based drilling fluids. A formate-based drilling fluid was formulated with 5% potassium formate v/v, polymers, water and simulated drill solids. Lubricity testing was done on ZDDP. The results show that the addition of ZDDP lowers the CoF of the formate-based drilling fluid.

Improved Lubricity in Oil-based Drilling Fluids

Oil-based drilling fluids have naturally low CoF but deeper and extended drilling is creating the need for further reductions in torque and drag. An oil based drilling fluid was formulated according to the drilling fluids

ZDDP was tested as a standalone lubricant. Table 8 shows that ZDDP lowered the CoF of the drilling fluid.

TABLE 8CoF for an Oil Based Drilling FluidCoF in a mineral oil based invert150 lb/in300 lb/in400 lb/in500 lb/in600 lb/inwater0.37control0.030.080.140.240.32(nolubricant)+0.2%0.030.060.110.180.25ZDDP+2% ZDDP0.030.060.110.180.25

Improved Lubricity in Completion Fluids

A completion fluid is a solids-free liquid placed that is sometimes used at the final stages of well completion. Completion fluids are typically brines (chlorides, bromides and formates). The fluid is meant to be chemically compatible with the reservoir formation and fluids, and is typically filtered to a high degree to avoid introducing solids to the near-wellbore area. ZDDP was tested in a saturated solution of potassium formate (75% active) and potassium formate diluted 1:1 with water. Table 9 shows that the CoF of the completion fluid was reduced with the addition of ZDDP.

Anti-accretion Properties of ZDDP

Shale accretion was measured by observing the adhesion of clay onto a metal pipe. Oxford shale was sized through 6-8 mesh screen. Oxford shale is noted for its “stickiness” and is a standard shale for measuring accretion. 20 g of sized shale was placed in a 500 ml steel aging cell. A ¾″×6″ steel rod was cleaned and placed in the aging cell. 350 ml of water, water and 0.2% ZDDP and water and 2.0% ZDDP were added to the aging cells. Samples were hot rolled for 16 hrs at 120 F. Steel rods were observed for clay particles sticking to the steel. The rod that was rolled in water was covered in a fine film of clay.FIG. 1shows the effect of the addition of ZDDP to water in reducing the extent of clay adhesion to the steel rod. Increasing amounts of ZDDP showed a stark reduction in the extent of clay adhesion on the rod.

Inclusion of a document in this specification is not an admission that the document represents prior invention or is prior art for any purpose.