- Email: [email protected]
The influence of interfacial potential on friction and wear in an aqueous drilling mud
33
Wear, 170 (1993) 33-38
The influence of interfacial aqueous drilling mud N.P. Brandon*
potential
on friction and wear in an
and R.J.K. Wood
BP Research Centre, Sunbury-on-Thames, Middlesex TW16 7LN (UK) (Received
December
15, 1992; accepted
June 17, 1993)
Abstract Aqueous drilling muds, whilst recognised as environmentally benign, give higher coefficients of friction than their oil-based counterparts, leading to increased torque and drag on the drillstring. This work concerns a novel approach to improving the lubricity of aqueous drilling muds, namely by controlling the electrochemical potential at the drillstring-drilling mud interface. A Cameron-Plint high-frequency friction machine, modified to accept an electrochemical cell, has been used to measure friction and wear in aqueous drilling muds for mild steel pins rubbing on mild steel or sandstone wear plates. It was found that the coefficient of friction at metal-rock contacts decreased on application of a cathodic potential, whilst friction at metal-metal contacts decreased when an anodic potential was applied.
1. Introduction Aqueous drilling muds, whilst recognised as environmentally benign, give higher coefficients of friction than their oil-based counterparts, and their use can lead to increased torque and drag, particularly when drilling highly deviated wells. Improving the lubricity of water-based drilling muds therefore offers increased rate of penetration, together with increased reach if drilling becomes torque limited, e.g. in directional drilling where there is extensive contact between the drillstring and the hole wall. This work concerns a novel approach to improving the lubricity of aqueous drilling muds, namely by controlling the electrochemical potential at the drillstring-drilling mud interface; it has been shown previously that the lubricity of aqueous lubricants can be enhanced by this approach [l]. The aim of this work was to study the influence of potential on friction in an aqueous drilling mud for two types of contact; metal-metal (simulating drillstring on casing) and metal-rock (representing open-hole drilling).
2. Experimental details An electrochemical cell (Fig. 1) was designed for use on the Cameron-Plint TE/77 high-frequency friction machine. This instrument allows a wear pin to be *Current address: Rolls-Royce Applied Science Laboratory, Box 31, Derby DE24 8BJ (UK).
0043-1648/93/$6.00
PO
reciprocated against a wear plate under a controlled load, and the friction coefficient measured. The apparatus described previously [l] was modified to include a PTFE-coated stainless steel sump which incorporated a heater in the base, enabling operation at temperatures up to 80 “C. A calomel reference electrode was sited outside the cell and connected via a Luggin capillary placed above the wear plate. En24 mild steel wear pins having a ground end radius of 25 mm were used. The wear plates were of either En24 steel or Stancliffe sandstone (Young’s modulus, 9.8 GPa). Initial tests were carried out using steel plates lapped to a surface finish of R,=0.045km to enable measurement and interpretation of the wear scars. Subsequent work was carried out on sandstone and ground En24 steel wear plates (Raaround 0.5 pm) which better represent the materials encountered in drilling operations. The drilling mud was made up to contain 40 g 1-l (14 ppb) sea salt, 71.5 g 1-l (25 ppb) KCl, 2.9 g 1-l (1 ppb) xanthan gum, 11.4 g 1-l (4 ppb) Catogel plus (National Starch), 5.7 g l-” (2 ppb) Alcomer 120L (Allied Colloids) and 429 g 1-l (150 ppb) barite [2], where ppb = pounds per barrel. The pH was adjusted to 10 using potassium hydroxide; a pH used to minimise corrosion whilst still enabling safe handling of the mud. To allow comparison between water- and oil-based muds, a weighted 70130 BW oil-based mud was also tested without potential control. Drilling mud was pumped through the cell using a Watson-Marlow 502 peristaltic pump at a rate of 10
0 1993 - Elsevier Sequoia. All rights reserved
34
To RE
Fig. 1. Illustration of electrochemical cell fitted to Cameron-Plint machine to enable friction and wear measurements under conditions of controlled potential. 1, mild steel wear pin (working electrode); 2, mild steel (working electrode) or sandstone wear plate; 3, Luggin capillary to reference electrode; 4, platinum counter-electrode; 5, reciprocating head; 6, PTFE-coated stainless steel sump containing drilling mud; 7, drilling mud in; 8, drilling mud out; 9, heater and force transducer set inbase.
ml min-‘. For the elevated temperature tests, mud entering the cell was pre-heated by passing it through a heat exchanger, and the mud temperature controlled by contact with the heated electrochemical cell. After the temperature had equilibrated, a potential was applied and the pin reciprocated against the plate at a frequency of 1 Hz under an applied load of 40 N. The stroke length was 15 mm, and the duration of the experiment 1 h, giving a total sliding distance of 108 m and a mean sliding speed of 0.03 m s-‘. The 40 N load equates to an initial contact stress of 330 MPa, but this fell rapidly due to pin wear, with over 90% of the sliding distance being carried out at a contact of 6-7 MPa, akin to the side loading encountered during drilling. For metal-on-metal tests, the potentials of both pin and plate were controlled using a Wenking PGS81 potentio-galvanostat, while for metal-on-rock experiments only the potential of the pin was controlled. In addition, two tests were carried out on the metal-rock system in which the pin was shorted to zinc foil placed on the walls of the electrochemical cell. The foil acted as a sacrificial anode, and fixed the potential of the pin at about -800 mV vs. the Standard Hydrogen Electrode (SHE). Current, potential and friction were continuously recorded, and wear was determined at the end of the test by measurement of pin scar diameter or plate scar depth. Wear volume was normalised with respect to load and sliding distance to obtain a value in m3 N-’ m -l. Wear scars were also inspected by optical and electron microscopy to indicate the dominant wear process.
3. Results and discussion At 10 “C the 70/30 oil-based mud gave a friction coefficient of 0.14 kO.02 for mild steel rubbing on a ground mild steel plate, and 0.13 $- 0.03 for mild steel on sandstone. The mud was not tested at elevated temperature. Figure 2 shows the effect of potential on friction and wear for mild steel pins on lapped mild steel plates in the aqueous drilling mud at 10 “C. Friction fell at both anodic and cathodic potentials, decreasing from a value of 0.21 at the rest potential ( -430 mV(SHE)), i.e. the potential adopted in the absence of external control, to 0.12 at + 100 mV(SHE) and 0.15 at - 1000 mV(SHE). These values are similar to those of the oilbased mud under the same conditions. The electron micrographs presented in Figs. 3-5 further show that
-A-
Friction
-c
Pin Wear
!O
-10200-900
-600
-300
0
300
Potential (mV v SHE)
Fig. 2. Potential dependence of friction and wear for a mild steel pin on a lapped mild steel plate in an aqueous drilling mud at 10 “C.
N.P. Brandon
and R.J.K.
Wood / Influence
Fig. 3. Electron micrograph of off-scar area of lapped mild steel plate after running at -1000 mV(SHE) in an aqueous drilling mud at 10 “C.
35
of interjacial potential on drilling mud
Fig. 5. Electron micrograph of wear scar on lapped mild steel plate after running at -300 mV(SHE) in an aqueous drilling mud at 10 “C. Direction of sliding, horizontal.
mV v SHE
I_ 4 m
0.2 mm -600
vlest
-430 potential)
Fig. 4. Electron micrograph of wear scar on lapped mild steel plate after running at -1000 mV(SHE) in an aqueous drilling mud at 10 “C. Direction of sliding, horizontal.
the mode of wear was significantly altered by changing the potential at the metal-mud interface. Figure 3 illustrates the off-scar surface of the lapped plate after running at -1000 mV(SHE), and Fig. 4 shows the corresponding wear scar, which was typical of that obtained by abrasion. In contrast, Fig. 5 shows that at - 300 mV(SHE), i.e. at a potential 130 mV anodic of the rest potential, there was corrosive pitting and surface delamination, indicative of corrosive wear. The transition from abrasive to corrosive wear as the potential varied from cathodic to anodic is also shown in the Talysurf profiles given in Fig. 6, which illustrate the move from a rough, scratched wear scar at - 1000 mV(SHE) to a smoother, broader scar at +lOO mV(SHE). The absence of abrasive grooves on the
+100
Fig. 6. Talysurf profiles of the wear scars obtained on lapped mild steel plate as a function of potential after running in an aqueous drilling mud at 10 “C.
corroded -300 mV(SHE) plate could indicate a transition in the wear mechanism from two-body abrasion at - 1000 mV(SHE), to corrosion-assisted three-body abrasion at -300 mV(SHE). Thus corrosion could decrease the surface roughness and produce debris, such that more rolling and sliding of particles occurs in the contact zone, giving a smooth but delaminated surface. This hypothesis is supported by the work of
Misra and Finnie 131, who have shown that two-body abrasion surfaces are abraded, where three-body are smooth with signs of pitting and delaminations. The effect of potential on friction and wear for the ground metal plates is shown in Fig. 7. Friction was higher than that obtained with the lapped wear plate, and did not fall at cathodic potentials. However, a significant decrease in friction was again seen at anodic potentials, the friction coefficient falling from 0.25 at the rest potential (- 360 mV(SHE)) to 0.13 at + 100 mV(SHE), a decrease of 50%. The latter friction coefficient again matched that given by the oil-based mud under equivalent conditions. Another significant difference between the behaviour of the lapped and ground wear plates was that no significant plate wear scar was seen at any potential with the ground plates. This is shown by the optical micrographs given in Fig. 8, which illustrate that the 45” grinding marks were still evident 4
200 -A-
Friction
I
I
-Loo
-900
-600
-300
0
3oo”
Potential (mV v SHE)
Fig. 7. Potential dependence of friction and wear for a mild steel pin on a ground mild steel plate in an aqueous drilling mud at 10 “C.
within the wear scar after running for one hour at -1000 mV(SHE), with only faint evidence of surface wear. The different behaviour given by the two types of wear plate may be explained in terms of the action of the barite particles, since the tendency of the barite (and wear debris) particles towards either sliding or rolling motion in the contact zone controls the wear mechanism on both pin and plate surfaces. For the smoother, Iapped plate the drop in pin wear, accompanied by increased plate wear (seen at both anodic and cathodic potentiaIs), is typical of that seen in threebody abrasion when particle rolling occurs in the contact [3-4]. Thus it appears that the smoother plate allowed the barite particles to roll in the contact region. It has been reported that this type of three-body wear increases with corrosion [5], which could account for the increase in wear seen in Fig. 2 at f 100 mV(SHE). For the ground plate, the pin wear scars were typical of twobody abrasive wear [6]. Hence it appears that the relatively rough ground finish C&=0.5 pm) was sufficient to limit particle movement relative to the plate, with the barite particles acting as a protective layer over the plate, but as a highly abrasive layer to the sfiding pin. Even though the barite was softer than the steel (the hardness of barite is 150 Knoop whereas mild steel is around 200 Knoop), wear could still occur, since it has been shown that wear takes place even when the wearing surface is harder than the abrading particle [4]. Wear plates with a ground surface were used to investigate the effect of mud temperature on friction and wear. Figure 9 gives the results obtained at rest potential over the temperature range IO to 80 “C. The friction coefficient increased from 0.25 to 0.33 as the temperature was raised. The increase in friction and pin wear with temperature may be related to the
.4
. 200 t
Friction
0
0 0
20
40 Temperature
Fig. 8. Optical micrograph of the wear scar on a ground mild steel plate after running at -1000 mV(SHE) in an aqueous drilling mud at 10 “C. Direction of sliding, horizontal.
60
80
100
(“C)
Fig. 9. Temperature dependence of friction and wear for mild steel pins on ground mild steel plates at rest potential in an aqueous drilling mud.
37
N.P. Brandon and R.J.K Wood / Infiuence of interfacial potential on drilling mud
accompan~ng decrease in mud viscosity, as viscosity roughly halves over the temperature range 25-80 “C. Figure 10 presents the influence of potential on lubricity for this system at 70 “C. Friction fell from 0.32 at the rest potential (-420 mV(SHE)) to 0.16 at -300 mV(SHE), a decrease of 50%. The current measured at - 300 mV was 3.5 mA cm-‘, corresponding to an extremely high corrosion rate of some 1.3 X lo-’ m s-’ or 1600 mpy (mpy = milli-inches per year) if all the current is taken to lead to corrosion. Reduced friction could also be obtained at potentials of -340 and -375 mV(SHE), where the anodic current, and thus the calculated corrosion rate, were less (Table 1). These results suggest that it is the corrosion rate that influences friction, possibly through the lubricating action of the corrosion products, or through the formation of layers of varying mechanical strength on the wearing surfaces. Figure 11 describes the effect of potential on metal-rock friction in the aqueous mud at 18 “C, which was very different from that seen for metal-metal friction. At, or cathodic of, - 600 mV(SHE) the friction coefficient fell to half of its mean rest potential (-420 mV(SHE)) value of 0.24 whereas, anodic of the rest potential, friction either remained unchanged or was increased. Two tests carried out at - 800 mV by shorting
.7
I
+
Friction
-+
Pin Wear
I
L””
-
I
150
I -Loo
-900
-600 -300 Potential (mV v SHE)
0
. E
3E ‘” 0 E
3oo”
Fig. 10. Potential dependence of friction and wear for mild steel pins on ground mild steel plates in an aqueous drilling mud at 70 “C. TABLE 1. The Muence friction at 70 “C Potential (mV (SCE))
-600 -375 - 340 -300
Current (mA cm-‘)
-0.1 + 0.4 + 1.1 t3.5
of potential
Corrosion
on ‘corrosion
rate*
(m SK’)
(mpy)
0 0.2x 0.4x 1.3x
0 200 500 1600
lo+ lo+ 10-F
rate’ and
Coefficient of friction
0.32 0.22 0.17 0.16
h-
Pin Wear
8
d? n
-i200
-900
-600 Potential (mV v SHE)
-300
0
0
Fig. 11. Potential dependence of friction and wear for mild steel pins on sandstone plates in an aqueous drilling mud at 18 “C.
the pin to zinc foil gave identical friction coefficients of 0.12. Thus at cathodic potentials the lubricity of the mud was brought down to that measured in the oilbased mud. Pin wear was invariant with potential. The reason for the different potential-dependent behaviour of the metal-rock contacts is not clear. The absence of friction reduction at anodic potentials may be related to the smaller area of exposed metal with the metal-rock contact, so that the quantity of lubricating corrosion product is reduced. The reduction in friction at cathodic potentials may in part be due to electrostatic repulsion between the metal pin and the sandstone. At pH=9 the sandstone would be negatively charged [7], while the metal pin would be expected to be negatively charged at potentials cathodic of its potential of zero of charge, reported to be around - 400 mV(SHE) for iron f8-91 and -42.5 mV(SHE) for stainless steel ilO]. Thus, as the potential of metal pin is made cathodic of -400 mV, repulsion between it and the sandstone increases, possibly leading to a reduced coefficient of friction. These results suggest that it may be possible to achieve significant improvements in lubricity when drilling in open hole by maintaining a cathodic potential at the drillstring. However, care would have to be taken not to induce hydrogen embrittiement of the steel [ll]. Such cathodic potentials could be imposed by attaching alloys of zinc, aluminium or magnesium to the drillstring to fYixits potential at a negative value, as demonstrated previously by attaching zinc foil to the metal pin.
4. Conclusions The lubricity of aqueous drilling muds can be enhanced by controlling the electrochemical potential at the metal-mud interface, such that it is possible to obtain friction coefficients equivalent to that measured
38
N.P. Brandon and R._I.R Wood i ~n~uence of ~nte~acia~potential on d~lii?~~mud
in an oil-based mud. It was found that the coefficient of friction at metal-rock contacts decreased on application of a cathodic potential, whilst friction at metal-metal contacts decreased when an anodic potential was applied. The potential also influenced the mode of wear. Evidence suggests that there was a transition from twobody abrasion at cathodic potentials to corrosion-assisted three-body abrasion as the potential became anodic of the rest potential of the steel.
2 3
4 5
6
Acknowledgments The authors would like to thank N. Bonanos, M.N. Mahmood, A.J. Moore, P. Lurie, 3. Martin and P.O. Fogarty for helpful discussions, and BP Research for permission to publish this work.
7
8 9 10
References 11 1 N.P. Brandon, N. Bonanos, P.O. Fogatty, M.N. Mahmood, A.J. Moore and R.J.K. Wood, Inffuence of potential on the
friction and wear of mild steel in a model aqueous lubricant. J. App. Electrochem., 23 (1993) 456. Putents: GB-A-2245294, P.R?CJD,June 14,199l; G&A-2245292, l?R.E?D, May 22, 1991. A. Misra and I. Finnie, A classification of three-body abrasive wear and design of a new tester, Wear, 6U (1980) 111. A. Misra and I. Finnie, An experimental study of three-body abrasive wear, Wear, 85 (1983) 57. A.W. Batchelor, G.W. Stachowiak, Predicting synergism between corrosion and abrasive wear, Wear, 123 (1988) 281. E. Rabinowicz, F&ion and Wear of Materials, Wiley, New York, 196.5. G.A. Parks, Aqueous surface chemistry of oxides and complex oxide minerals: isoelectric point and zero point of charge, in R.F. Gould (ed.), Equilibrium concepts in natural water systems, Advances in Chemistry Series no. 67, American Chemical Society, Washington, 1967. T.N. Voropaeva, B.V. Deyagin and B.N. Kabanov, IN. Akad. Nuuk. SSSR, 257 (1963) 151 (in Russian). E.O. Ayazyan, Doki. Akad. Nauk. S&U?, 100 (1955) 473 (in Russian). R.J. Brigham, Crevice corrosion initiation and the potential of zero charge, Corros. Sci., 29(S) (1989) 995. Des&n and operat~naI guidance on cathodic protection of oj+?.shorestructures, subsea installations and pipelines, Marine Technical Directorate, London, 1990.
Wear, 170 (1993) 33-38
The influence of interfacial aqueous drilling mud N.P. Brandon*
potential
on friction and wear in an
and R.J.K. Wood
BP Research Centre, Sunbury-on-Thames, Middlesex TW16 7LN (UK) (Received
December
15, 1992; accepted
June 17, 1993)
Abstract Aqueous drilling muds, whilst recognised as environmentally benign, give higher coefficients of friction than their oil-based counterparts, leading to increased torque and drag on the drillstring. This work concerns a novel approach to improving the lubricity of aqueous drilling muds, namely by controlling the electrochemical potential at the drillstring-drilling mud interface. A Cameron-Plint high-frequency friction machine, modified to accept an electrochemical cell, has been used to measure friction and wear in aqueous drilling muds for mild steel pins rubbing on mild steel or sandstone wear plates. It was found that the coefficient of friction at metal-rock contacts decreased on application of a cathodic potential, whilst friction at metal-metal contacts decreased when an anodic potential was applied.
1. Introduction Aqueous drilling muds, whilst recognised as environmentally benign, give higher coefficients of friction than their oil-based counterparts, and their use can lead to increased torque and drag, particularly when drilling highly deviated wells. Improving the lubricity of water-based drilling muds therefore offers increased rate of penetration, together with increased reach if drilling becomes torque limited, e.g. in directional drilling where there is extensive contact between the drillstring and the hole wall. This work concerns a novel approach to improving the lubricity of aqueous drilling muds, namely by controlling the electrochemical potential at the drillstring-drilling mud interface; it has been shown previously that the lubricity of aqueous lubricants can be enhanced by this approach [l]. The aim of this work was to study the influence of potential on friction in an aqueous drilling mud for two types of contact; metal-metal (simulating drillstring on casing) and metal-rock (representing open-hole drilling).
2. Experimental details An electrochemical cell (Fig. 1) was designed for use on the Cameron-Plint TE/77 high-frequency friction machine. This instrument allows a wear pin to be *Current address: Rolls-Royce Applied Science Laboratory, Box 31, Derby DE24 8BJ (UK).
0043-1648/93/$6.00
PO
reciprocated against a wear plate under a controlled load, and the friction coefficient measured. The apparatus described previously [l] was modified to include a PTFE-coated stainless steel sump which incorporated a heater in the base, enabling operation at temperatures up to 80 “C. A calomel reference electrode was sited outside the cell and connected via a Luggin capillary placed above the wear plate. En24 mild steel wear pins having a ground end radius of 25 mm were used. The wear plates were of either En24 steel or Stancliffe sandstone (Young’s modulus, 9.8 GPa). Initial tests were carried out using steel plates lapped to a surface finish of R,=0.045km to enable measurement and interpretation of the wear scars. Subsequent work was carried out on sandstone and ground En24 steel wear plates (Raaround 0.5 pm) which better represent the materials encountered in drilling operations. The drilling mud was made up to contain 40 g 1-l (14 ppb) sea salt, 71.5 g 1-l (25 ppb) KCl, 2.9 g 1-l (1 ppb) xanthan gum, 11.4 g 1-l (4 ppb) Catogel plus (National Starch), 5.7 g l-” (2 ppb) Alcomer 120L (Allied Colloids) and 429 g 1-l (150 ppb) barite [2], where ppb = pounds per barrel. The pH was adjusted to 10 using potassium hydroxide; a pH used to minimise corrosion whilst still enabling safe handling of the mud. To allow comparison between water- and oil-based muds, a weighted 70130 BW oil-based mud was also tested without potential control. Drilling mud was pumped through the cell using a Watson-Marlow 502 peristaltic pump at a rate of 10
0 1993 - Elsevier Sequoia. All rights reserved
34
To RE
Fig. 1. Illustration of electrochemical cell fitted to Cameron-Plint machine to enable friction and wear measurements under conditions of controlled potential. 1, mild steel wear pin (working electrode); 2, mild steel (working electrode) or sandstone wear plate; 3, Luggin capillary to reference electrode; 4, platinum counter-electrode; 5, reciprocating head; 6, PTFE-coated stainless steel sump containing drilling mud; 7, drilling mud in; 8, drilling mud out; 9, heater and force transducer set inbase.
ml min-‘. For the elevated temperature tests, mud entering the cell was pre-heated by passing it through a heat exchanger, and the mud temperature controlled by contact with the heated electrochemical cell. After the temperature had equilibrated, a potential was applied and the pin reciprocated against the plate at a frequency of 1 Hz under an applied load of 40 N. The stroke length was 15 mm, and the duration of the experiment 1 h, giving a total sliding distance of 108 m and a mean sliding speed of 0.03 m s-‘. The 40 N load equates to an initial contact stress of 330 MPa, but this fell rapidly due to pin wear, with over 90% of the sliding distance being carried out at a contact of 6-7 MPa, akin to the side loading encountered during drilling. For metal-on-metal tests, the potentials of both pin and plate were controlled using a Wenking PGS81 potentio-galvanostat, while for metal-on-rock experiments only the potential of the pin was controlled. In addition, two tests were carried out on the metal-rock system in which the pin was shorted to zinc foil placed on the walls of the electrochemical cell. The foil acted as a sacrificial anode, and fixed the potential of the pin at about -800 mV vs. the Standard Hydrogen Electrode (SHE). Current, potential and friction were continuously recorded, and wear was determined at the end of the test by measurement of pin scar diameter or plate scar depth. Wear volume was normalised with respect to load and sliding distance to obtain a value in m3 N-’ m -l. Wear scars were also inspected by optical and electron microscopy to indicate the dominant wear process.
3. Results and discussion At 10 “C the 70/30 oil-based mud gave a friction coefficient of 0.14 kO.02 for mild steel rubbing on a ground mild steel plate, and 0.13 $- 0.03 for mild steel on sandstone. The mud was not tested at elevated temperature. Figure 2 shows the effect of potential on friction and wear for mild steel pins on lapped mild steel plates in the aqueous drilling mud at 10 “C. Friction fell at both anodic and cathodic potentials, decreasing from a value of 0.21 at the rest potential ( -430 mV(SHE)), i.e. the potential adopted in the absence of external control, to 0.12 at + 100 mV(SHE) and 0.15 at - 1000 mV(SHE). These values are similar to those of the oilbased mud under the same conditions. The electron micrographs presented in Figs. 3-5 further show that
-A-
Friction
-c
Pin Wear
!O
-10200-900
-600
-300
0
300
Potential (mV v SHE)
Fig. 2. Potential dependence of friction and wear for a mild steel pin on a lapped mild steel plate in an aqueous drilling mud at 10 “C.
N.P. Brandon
and R.J.K.
Wood / Influence
Fig. 3. Electron micrograph of off-scar area of lapped mild steel plate after running at -1000 mV(SHE) in an aqueous drilling mud at 10 “C.
35
of interjacial potential on drilling mud
Fig. 5. Electron micrograph of wear scar on lapped mild steel plate after running at -300 mV(SHE) in an aqueous drilling mud at 10 “C. Direction of sliding, horizontal.
mV v SHE
I_ 4 m
0.2 mm -600
vlest
-430 potential)
Fig. 4. Electron micrograph of wear scar on lapped mild steel plate after running at -1000 mV(SHE) in an aqueous drilling mud at 10 “C. Direction of sliding, horizontal.
the mode of wear was significantly altered by changing the potential at the metal-mud interface. Figure 3 illustrates the off-scar surface of the lapped plate after running at -1000 mV(SHE), and Fig. 4 shows the corresponding wear scar, which was typical of that obtained by abrasion. In contrast, Fig. 5 shows that at - 300 mV(SHE), i.e. at a potential 130 mV anodic of the rest potential, there was corrosive pitting and surface delamination, indicative of corrosive wear. The transition from abrasive to corrosive wear as the potential varied from cathodic to anodic is also shown in the Talysurf profiles given in Fig. 6, which illustrate the move from a rough, scratched wear scar at - 1000 mV(SHE) to a smoother, broader scar at +lOO mV(SHE). The absence of abrasive grooves on the
+100
Fig. 6. Talysurf profiles of the wear scars obtained on lapped mild steel plate as a function of potential after running in an aqueous drilling mud at 10 “C.
corroded -300 mV(SHE) plate could indicate a transition in the wear mechanism from two-body abrasion at - 1000 mV(SHE), to corrosion-assisted three-body abrasion at -300 mV(SHE). Thus corrosion could decrease the surface roughness and produce debris, such that more rolling and sliding of particles occurs in the contact zone, giving a smooth but delaminated surface. This hypothesis is supported by the work of
Misra and Finnie 131, who have shown that two-body abrasion surfaces are abraded, where three-body are smooth with signs of pitting and delaminations. The effect of potential on friction and wear for the ground metal plates is shown in Fig. 7. Friction was higher than that obtained with the lapped wear plate, and did not fall at cathodic potentials. However, a significant decrease in friction was again seen at anodic potentials, the friction coefficient falling from 0.25 at the rest potential (- 360 mV(SHE)) to 0.13 at + 100 mV(SHE), a decrease of 50%. The latter friction coefficient again matched that given by the oil-based mud under equivalent conditions. Another significant difference between the behaviour of the lapped and ground wear plates was that no significant plate wear scar was seen at any potential with the ground plates. This is shown by the optical micrographs given in Fig. 8, which illustrate that the 45” grinding marks were still evident 4
200 -A-
Friction
I
I
-Loo
-900
-600
-300
0
3oo”
Potential (mV v SHE)
Fig. 7. Potential dependence of friction and wear for a mild steel pin on a ground mild steel plate in an aqueous drilling mud at 10 “C.
within the wear scar after running for one hour at -1000 mV(SHE), with only faint evidence of surface wear. The different behaviour given by the two types of wear plate may be explained in terms of the action of the barite particles, since the tendency of the barite (and wear debris) particles towards either sliding or rolling motion in the contact zone controls the wear mechanism on both pin and plate surfaces. For the smoother, Iapped plate the drop in pin wear, accompanied by increased plate wear (seen at both anodic and cathodic potentiaIs), is typical of that seen in threebody abrasion when particle rolling occurs in the contact [3-4]. Thus it appears that the smoother plate allowed the barite particles to roll in the contact region. It has been reported that this type of three-body wear increases with corrosion [5], which could account for the increase in wear seen in Fig. 2 at f 100 mV(SHE). For the ground plate, the pin wear scars were typical of twobody abrasive wear [6]. Hence it appears that the relatively rough ground finish C&=0.5 pm) was sufficient to limit particle movement relative to the plate, with the barite particles acting as a protective layer over the plate, but as a highly abrasive layer to the sfiding pin. Even though the barite was softer than the steel (the hardness of barite is 150 Knoop whereas mild steel is around 200 Knoop), wear could still occur, since it has been shown that wear takes place even when the wearing surface is harder than the abrading particle [4]. Wear plates with a ground surface were used to investigate the effect of mud temperature on friction and wear. Figure 9 gives the results obtained at rest potential over the temperature range IO to 80 “C. The friction coefficient increased from 0.25 to 0.33 as the temperature was raised. The increase in friction and pin wear with temperature may be related to the
.4
. 200 t
Friction
0
0 0
20
40 Temperature
Fig. 8. Optical micrograph of the wear scar on a ground mild steel plate after running at -1000 mV(SHE) in an aqueous drilling mud at 10 “C. Direction of sliding, horizontal.
60
80
100
(“C)
Fig. 9. Temperature dependence of friction and wear for mild steel pins on ground mild steel plates at rest potential in an aqueous drilling mud.
37
N.P. Brandon and R.J.K Wood / Infiuence of interfacial potential on drilling mud
accompan~ng decrease in mud viscosity, as viscosity roughly halves over the temperature range 25-80 “C. Figure 10 presents the influence of potential on lubricity for this system at 70 “C. Friction fell from 0.32 at the rest potential (-420 mV(SHE)) to 0.16 at -300 mV(SHE), a decrease of 50%. The current measured at - 300 mV was 3.5 mA cm-‘, corresponding to an extremely high corrosion rate of some 1.3 X lo-’ m s-’ or 1600 mpy (mpy = milli-inches per year) if all the current is taken to lead to corrosion. Reduced friction could also be obtained at potentials of -340 and -375 mV(SHE), where the anodic current, and thus the calculated corrosion rate, were less (Table 1). These results suggest that it is the corrosion rate that influences friction, possibly through the lubricating action of the corrosion products, or through the formation of layers of varying mechanical strength on the wearing surfaces. Figure 11 describes the effect of potential on metal-rock friction in the aqueous mud at 18 “C, which was very different from that seen for metal-metal friction. At, or cathodic of, - 600 mV(SHE) the friction coefficient fell to half of its mean rest potential (-420 mV(SHE)) value of 0.24 whereas, anodic of the rest potential, friction either remained unchanged or was increased. Two tests carried out at - 800 mV by shorting
.7
I
+
Friction
-+
Pin Wear
I
L””
-
I
150
I -Loo
-900
-600 -300 Potential (mV v SHE)
0
. E
3E ‘” 0 E
3oo”
Fig. 10. Potential dependence of friction and wear for mild steel pins on ground mild steel plates in an aqueous drilling mud at 70 “C. TABLE 1. The Muence friction at 70 “C Potential (mV (SCE))
-600 -375 - 340 -300
Current (mA cm-‘)
-0.1 + 0.4 + 1.1 t3.5
of potential
Corrosion
on ‘corrosion
rate*
(m SK’)
(mpy)
0 0.2x 0.4x 1.3x
0 200 500 1600
lo+ lo+ 10-F
rate’ and
Coefficient of friction
0.32 0.22 0.17 0.16
h-
Pin Wear
8
d? n
-i200
-900
-600 Potential (mV v SHE)
-300
0
0
Fig. 11. Potential dependence of friction and wear for mild steel pins on sandstone plates in an aqueous drilling mud at 18 “C.
the pin to zinc foil gave identical friction coefficients of 0.12. Thus at cathodic potentials the lubricity of the mud was brought down to that measured in the oilbased mud. Pin wear was invariant with potential. The reason for the different potential-dependent behaviour of the metal-rock contacts is not clear. The absence of friction reduction at anodic potentials may be related to the smaller area of exposed metal with the metal-rock contact, so that the quantity of lubricating corrosion product is reduced. The reduction in friction at cathodic potentials may in part be due to electrostatic repulsion between the metal pin and the sandstone. At pH=9 the sandstone would be negatively charged [7], while the metal pin would be expected to be negatively charged at potentials cathodic of its potential of zero of charge, reported to be around - 400 mV(SHE) for iron f8-91 and -42.5 mV(SHE) for stainless steel ilO]. Thus, as the potential of metal pin is made cathodic of -400 mV, repulsion between it and the sandstone increases, possibly leading to a reduced coefficient of friction. These results suggest that it may be possible to achieve significant improvements in lubricity when drilling in open hole by maintaining a cathodic potential at the drillstring. However, care would have to be taken not to induce hydrogen embrittiement of the steel [ll]. Such cathodic potentials could be imposed by attaching alloys of zinc, aluminium or magnesium to the drillstring to fYixits potential at a negative value, as demonstrated previously by attaching zinc foil to the metal pin.
4. Conclusions The lubricity of aqueous drilling muds can be enhanced by controlling the electrochemical potential at the metal-mud interface, such that it is possible to obtain friction coefficients equivalent to that measured
38
N.P. Brandon and R._I.R Wood i ~n~uence of ~nte~acia~potential on d~lii?~~mud
in an oil-based mud. It was found that the coefficient of friction at metal-rock contacts decreased on application of a cathodic potential, whilst friction at metal-metal contacts decreased when an anodic potential was applied. The potential also influenced the mode of wear. Evidence suggests that there was a transition from twobody abrasion at cathodic potentials to corrosion-assisted three-body abrasion as the potential became anodic of the rest potential of the steel.
2 3
4 5
6
Acknowledgments The authors would like to thank N. Bonanos, M.N. Mahmood, A.J. Moore, P. Lurie, 3. Martin and P.O. Fogarty for helpful discussions, and BP Research for permission to publish this work.
7
8 9 10
References 11 1 N.P. Brandon, N. Bonanos, P.O. Fogatty, M.N. Mahmood, A.J. Moore and R.J.K. Wood, Inffuence of potential on the
friction and wear of mild steel in a model aqueous lubricant. J. App. Electrochem., 23 (1993) 456. Putents: GB-A-2245294, P.R?CJD,June 14,199l; G&A-2245292, l?R.E?D, May 22, 1991. A. Misra and I. Finnie, A classification of three-body abrasive wear and design of a new tester, Wear, 6U (1980) 111. A. Misra and I. Finnie, An experimental study of three-body abrasive wear, Wear, 85 (1983) 57. A.W. Batchelor, G.W. Stachowiak, Predicting synergism between corrosion and abrasive wear, Wear, 123 (1988) 281. E. Rabinowicz, F&ion and Wear of Materials, Wiley, New York, 196.5. G.A. Parks, Aqueous surface chemistry of oxides and complex oxide minerals: isoelectric point and zero point of charge, in R.F. Gould (ed.), Equilibrium concepts in natural water systems, Advances in Chemistry Series no. 67, American Chemical Society, Washington, 1967. T.N. Voropaeva, B.V. Deyagin and B.N. Kabanov, IN. Akad. Nuuk. SSSR, 257 (1963) 151 (in Russian). E.O. Ayazyan, Doki. Akad. Nauk. S&U?, 100 (1955) 473 (in Russian). R.J. Brigham, Crevice corrosion initiation and the potential of zero charge, Corros. Sci., 29(S) (1989) 995. Des&n and operat~naI guidance on cathodic protection of oj+?.shorestructures, subsea installations and pipelines, Marine Technical Directorate, London, 1990.